Repair of Tektronix FG502 11 MHz Function Generator

[xrunner]

Just arrived. A little rough - advertised as "Used". Plugged it into a power module - like the PG501 no light on this one, I think we have another burned out lamp. 

Need to gather the manual from the BAMA (Boat Anchor Manual Archive) -

Boat Anchor Manual Archive

Don't know what awaits inside.

Developing ...

[Johnny10]

I'm going to pull mine out.
Have a broken one also and wanting to watch a pro work out the bugs!

[xrunner]

Have a broken one also and wanting to watch a pro work out the bugs!

Nothing like working without pressure. 

Do you remember what seems to be wrong with yours? I might want to compare symptoms.

[Johnny10]

This is your thread ..

But I did pull my unit out, haven't opened it yet.

Plugged in it shows no response to frequency adjustment.
Shows same waveform  on all settings and only adjusting between 20mv and 40mv amplitude. If amplitude higher or lower.. waveform disappears.
No Trigger output.

But we are following your repair.

Besides I need time to read the theory of operation.

[xrunner]

This is your thread ..

Oh I can share. 

I'm kickin' back with a beer but I did go into the lab and hook it up to my scope. Ugh ... nasty output. Sine - triangle - square - all bad. Badness varies with the amplitude adjustment but the frequency seems to change OK

Will post screen shots in the morning.

[Rerouter]

Ill kick you the schematics this afternoon if you havent found them, i have 3 of these, though i wish i knew what part caused the frequency shift when the output loading varies

[Rerouter]

http://w140.com/smb/FG502_SM.pdf

[xrunner]

http://w140.com/smb/FG502_SM.pdf

Thanks. Here's the screen shots from the output of the FG502. Pretty bad. Shown is sine and triangle selections. Trigger out is better. Looks like the negative half of the waveform is coming out.

Also as I said the lamp is burned out - will convert to LED but I'll tackle that easy job later on.

For now, time to crack it open and see what a visual inspection reveals.

[Kleinstein]

The waveform from xrunners unit looks like a broken output amplifier.

As circuit seems to use no special parts, it should be possible to fix.

If loading the output has an influence on the frequency, the problem is likely with the power supply. The frequency depends slightly on the supply and there seems to be no really good supply stabilization. Anyway I would not expect a very stable (e.g. with temperature) frequency.

[xrunner]

The waveform from xrunners unit looks like a broken output amplifier.

As circuit seems to use no special parts, it should be possible to fix.

Looks very possible. 

Visual inspection reveals two burned resistors. One in the center of the pic and the other one all the way at the back.

Time to heat up the iron and locate them on the schematic ... be back later with some documentation. 

[bitseeker]

xrunner, you're doing a good job making me want to start down this road. 

[xrunner]

xrunner, you're doing a good job making me want to start down this road. 

Come on in the TM500 pool - the water's fine.  (that is of course after you have repaired the entire pool filtering system  )

I pulled the two resistors, as shown on the schematic they are 10 ohm rated @ .25 W. burnt to a crisp. One measures 24 ohms the other is an open. I also pulled the two output transistors - Q540 and Q542. As you can see in the pic they look like they got overheated BUT they test OK in the transistor checker with an hFE of 137 and 145 respectively. I cross referenced them as follows according to the Tektronix part number on the case -

Q540 - 151-0103 --> NPN 2N2219A
Q542 - 151-0134 --> PNP 2N2905A

These are very common transistors so even though they check out I'm going to replace them, they may misbehave at high temperatures. Also will replace the resistors of course but I'm going to go with a .5 W size just for good measure. Have to order these parts. I ordered plenty of these transistors in case the previous stages require replacement. Also will provide socket pins too because I don't want to stress this old board out with any more heat in that area, as you can see they appear to have been replaced at least once already.

I have inspected the entire area; with a mirror I can see underneath Q530 & Q532 and do not see signs of overheating, also junctions read properly so I don't think these are bad. Next will go backwards and make sure the incoming waveforms are being generated properly prior to entering the output stage.

So guys any speculation as to what happened? Remember people do some very dumb things with test equipment up to and including connecting the output of one unit into the output of another ... 

[MarkL]

...
So guys any speculation as to what happened? Remember people do some very dumb things with test equipment up to and including connecting the output of one unit into the output of another ... 

I'm not sure why none of the 33ohm resistors wouldn't be toast too.

I'd make sure the +20V and -20V rails are actually that.  And check the C536 and C546.  And when the 10ohm resistors are replaced, check the bases and emitters of Q540, Q542, Q530, and Q532 to see if they are reasonable voltages.  Note that Q530/Q540 and Q532/542 and paralleled so they should share the current load to the output.

[xrunner]

I'm not sure why none of the 33ohm resistors wouldn't be toast too.

Just checked them per your suggestion; look pristine and check out OK.

I'd make sure the +20V and -20V rails are actually that. 

They are.

And check the C536 and C546. 

Checked in circuit not shorted. I could pull 'em and replace since I have plenty on hand.

And when the 10ohm resistors are replaced, check the bases and emitters of Q540, Q542, Q530, and Q532 to see if they are reasonable voltages.  Note that Q530/Q540 and Q532/542 and paralleled so they should share the current load to the output.

Will do ... 

[bitseeker]

So guys any speculation as to what happened? Remember people do some very dumb things with test equipment up to and including connecting the output of one unit into the output of another ... 

Without looking, my first thought was an overload into the output. However, glancing at the schematic, all those undamaged resistors on the output make it seem unlikely.

I'm still contemplating what would've caused the transistors to open up so far and long enough to fry. Could the open resistor have (unusually) failed as a short before blowing open?

[MarkL]

And check the C536 and C546. 

Checked in circuit not shorted. I could pull 'em and replace since I have plenty on hand.

Quickie in-circuit check is fine.  If a cap was shorted I guess it doesn't explain why both 10ohm R's are toasted.

There's always a possibility that a shorted pass transistor (for the +/-20V) in a former TM500 cooked it.

[bitseeker]

Ah, yes, I forgot about the original chassis that it had come from. It could very well have had over-voltage rails.

[xrunner]

There's always a possibility that a shorted pass transistor (for the +/-20V) in a former TM500 cooked it.

Ah, yes, I forgot about the original chassis that it had come from. It could very well have had over-voltage rails.

Yep, the power module is the random variable for sure. I've personally used these systems in my career, (as the lab user not the repair/cal person), and I can tell you for a fact the equipment gets abused and slammed in the chassis etc. It's very possible this FG502 got tangled up with a bad MoJo chassis. 

[David Hess]

The FG502 is my favorite function generator although it is also my fastest which probably explains that.  The other function generators in this series are slower (except the double wide FG504) but have some very useful features.

So guys any speculation as to what happened? Remember people do some very dumb things with test equipment up to and including connecting the output of one unit into the output of another ... 

It looks like a typical output short to me; I have repaired several function generators with similar class-AB output stages and similar symptoms.

Usually if the output stage transistors have not outright failed they are damaged so that should be checked.  They might have been damaged from thermal runaway resulting in high leakage and/or low Vce and the resulting high collector current damaged the resistors.  If you have a curve tracer, then this is a good place to use it.  Or test Icbo and Iebo.

Unfortunately there is a dearth of modern medium power complementary VHF transistors to replace TO-39 packaged parts.  I am cheap so I often Vbe match pairs of 2N4401s and 2N4403s and put them in parallel for each TO-39 part.

I'm still contemplating what would've caused the transistors to open up so far and long enough to fry. Could the open resistor have (unusually) failed as a short before blowing open?

Thin film resistors usually fail short before failing open so this is likely.

The puzzle is why the 33 ohm emitter resistors show no signs of damage or overheating.  My guess is that high transistor temperatures allowed enough collector-base leakage so there were *two* current paths; one through the emitter resistors and one through the 10 ohm resistors and biasing diodes.

[Kleinstein]

The one 10 Ohms resistor reading 24 Ohms now might indicate that those resistors could drift upwards when they get hot and old. As these resistors are seeing a more or less constant maximum current (in case of a short at the output), it could be just continued aging with a final run away once the resistance go too high. So the rest of the circuit might be still OK.

A slightly higher temperature (poor ventilation) a higher supply, or a short towards an external voltage could have also be a reason.

The transistors look slightly strange with the dark color. But this should be the collector pin, thus must be something from the outside - maybe flux of something during manufacturing the transistors. If they are actually 2N2905 / 2N2219 one could replace them of cause.

[tautech]

Check those three series'd diodes, if one is open the output stages may have suffered from too much drive.

[xrunner]

It looks like a typical output short to me; I have repaired several function generators with similar class-AB output stages and similar symptoms.

Usually if the output stage transistors have not outright failed they are damaged so that should be checked.  They might have been damaged from thermal runaway resulting in high leakage and/or low Vce and the resulting high collector current damaged the resistors.  If you have a curve tracer, then this is a good place to use it.  Or test Icbo and Iebo.

Yea I'll be replacing at least two of the transistors, but the way it's going I might have quite a pile of components to replace.

The transistors look slightly strange with the dark color. But this should be the collector pin, thus must be something from the outside - maybe flux of something during manufacturing the transistors. If they are actually 2N2905 / 2N2219 one could replace them of cause.

I don't think it's flux - it doesn't come off with alcohol, plus I would think it would be on the other pins as well, and the other two transistors I looked under, but it's only on these two and only the collector pins. I dunno ... 

Check those three series'd diodes, if one is open the output stages may have suffered from too much drive.

Checked and OK.

Update:

I decided to pull C536 & C546 - 10 uF caps to check, good thing. In the "transistor checker" the report on both is "damaged". With the Rigol DMM one reads no capacitance and 54k ohms, the other reads 0.7 nF. Will replace with new parts. Now I'm on the hunt for bad caps.

The pile is growing. 

[Radio Tech]

Looks like another great mystery novel. Will be watching this one.

[MarkL]

Update:

I decided to pull C536 & C546 - 10 uF caps to check, good thing. In the "transistor checker" the report on both is "damaged". With the Rigol DMM one reads no capacitance and 54k ohms, the other reads 0.7 nF. Will replace with new parts. Now I'm on the hunt for bad caps.

The pile is growing. 

Interesting find.  You can connect the caps up to a power supply and see if they will withstand at least 20V with no appreciable current.  Maybe they're breaking down at a low voltage and that's what killed the 10R's?

If they do start drawing a lot of current, look out for exploding tantalums!

[xrunner]

Interesting find.  You can connect the caps up to a power supply and see if they will withstand at least 20V with no appreciable current.  Maybe they're breaking down at a low voltage and that's what killed the 10R's?

If they do start drawing a lot of current, look out for exploding tantalums!

Well I'm going to replace with aluminum electrolytics - anybody disagree?

[David Hess]

I decided to pull C536 & C546 - 10 uF caps to check, good thing. In the "transistor checker" the report on both is "damaged". With the Rigol DMM one reads no capacitance and 54k ohms, the other reads 0.7 nF. Will replace with new parts. Now I'm on the hunt for bad caps.

I should have mentioned it at the time but if the tantalum decoupling capacitors shorted, then those resistors would be destroyed; I have seen this happen a few times where Tektronix used an RC or RLC decoupling network.  However this suggests another possibility; if the pass transistors in the mainframe were shorted as MarkL suggested, then the 25 volt capacitors would likely draw enough current to destroy the resistors.

The pile is growing. 

Capacitors are inexpensive.  I would take a close look at every other solid tantalum capacitor and series resistor on the +/-20 volt supply lines though.

Well I'm going to replace with aluminum electrolytics - anybody disagree?

Their high frequency performance will not be quite as good but that is unlikely to make a difference and if it does, then additional low value ceramic or film decoupling capacitors can be added.  Use a capacitance of 2 to 4 times the solid tantalum's value to make up for their greater dissipation factor.

If you do use solid tantalum replacements, derate them to 35 volts instead of the 25 volts Tektronix used.

[tecman]

Pretty well known that the TM500 vintage was famous for tant caps that fail shorted.  I have had FG, SC, PS and DC modules all suffer bad tant cape fate.

Paul

[Johnny10]

What should you replace the tantalums with?

[David Hess]

Pretty well known that the TM500 vintage was famous for tant caps that fail shorted.  I have had FG, SC, PS and DC modules all suffer bad tant cape fate.

Paul

I think this was just because of the timing.  400 and 7000 series oscilloscopes of the same era suffer from the same problem with shorted solid tantalum capacitors.  Besides likely poor quality of the solid tantalum capacitors, Tektronix trusted the marketing which said that there was no requirement for them to be voltage derated.

[xrunner]

What should you replace the tantalums with?

For the two original 10 uF @ 25V I'm using aluminum electrolytics 10 uF @50V. Also there are four 22 uF @35V I'm replacing with 22 uF @ 50V. Note on the original caps the green vertical line indicates the positive side (not negative)

I did check the sine wave coming into the amp and it looks OK, despite needing some tweaking, but that comes later.

[tautech]

Well I'm going to replace with aluminum electrolytics - anybody disagree?

I'd be happy with modern 50V Tants there but if you must swap them for electrolytics parallel them with some ceramics, 0.1, 0.33, those sort of values. 

[xrunner]

I'd be happy with modern 50V Tants there but if you must swap them for electrolytics parallel them with some ceramics, 0.1, 0.33, those sort of values. 

I'm just gonna go ahead and order the tantalums then.

How about these 750 uF @ 40V, if I want to re-cap for good measure - I've looked on Ebay, Mouser, and Digikey and can find nothing axial in that value (which is not standard) or the next higher 820 uF axial @40v or more. I can find 820 uF @ 40V and higher in radial. Would I be naughty to use a radial if I can get the leads to reach? 

[tautech]

How about these 750 uF @ 40V, if I want to re-cap for good measure - I've looked on Ebay, Mouser, and Digikey and can find nothing axial in that value (which is not standard) or the next higher 820 uF axial @40v or more. I can find 820 uF @ 40V and higher in radial. Would I be naughty to use a radial if I can get the leads to reach? 

Pull them and measure.
Can't see if they're part of the timing circuits or on supply rails. 
If they're on supply rails the value is not critical, go to 820 or 1000uF if they fit.
Caps on rails usually are under more strain.

[bitseeker]

If they're just filtering the power rails, go with a larger available capacity axial. A higher voltage would also be OK, if no matching ones are available. I wouldn't do radial unless there's no other choice. It'll look weird and move around more.

[xrunner]

OK I found an 800 uF axial @40V on Digikey.

And inspecting the board I found a pot that was so loose it couldn't be in good shape; I turned it a few times and it just fell apart.

Has anyone ever seen the movie "The Money Pit"? 

[David Hess]

How about these 750 uF @ 40V, if I want to re-cap for good measure - I've looked on Ebay, Mouser, and Digikey and can find nothing axial in that value (which is not standard) or the next higher 820 uF axial @40v or more. I can find 820 uF @ 40V and higher in radial. Would I be naughty to use a radial if I can get the leads to reach? 

If you have to order parts then I would change them but there is a much better selection of radial parts and they will be less expensive.  Just fold one lead back along the capacitor body to mount it in the original spot.  The value is not particularly critical so 680, 820, or 1000 will work.

[xrunner]

If you have to order parts then I would change them but there is a much better selection of radial parts and they will be less expensive.  Just fold one lead back along the capacitor body to mount it in the original spot.  The value is not particularly critical so 680, 820, or 1000 will work.

I just found 10 820 uF @50V radial for $6 on Ebay. I have an idea how to do the mounting, but I have to make sure I do the soldering really really nicely so Tautech doesn't chew me out like my last project. 

[Johnny10]

What are the diodes CR140,145, 170, 175 making the current bridge?

[David Hess]

What are the diodes CR140,145, 170, 175 making the current bridge?

That is one way to make a fast current switch to drive the switched integrating capacitor shown on the right side of schematic 3.

The straightforward design uses a Miller integrator where the capacitor is part of a feedback loop but the bandwidth of the amplifier limits high frequency performance.  Instead in this case, current sink U175/Q175 and current source U140/Q140 both drive the diode bridge and the switched voltage at the left side of the diode bridge determines which current sink or source drives the integrating capacitor.  Since currents are involved, the diode forward voltage drops are irrelevant and diodes switch very fast and have very low capacitance.

I have an idea how to do the mounting, but I have to make sure I do the soldering really really nicely so Tautech doesn't chew me out like my last project.

I am following that thread and will try to post a photograph of my TM501 repair so Tautech has more to complain about.

[tautech]

 
Someone has to keep an eye on what you butchers are doing to nice Tek equipment.   

[bitseeker]

You guys crack me up.

[Kleinstein]

The large 800 µF caps may not be that critical. At only 120 Hz they usually don't get that hot as new ones in SMPS. Also the larger ones are not that sensitive to electrolyte drying out slowly. So they may very well live another 50 years. It might be enough to test them.  I would test them before replacing - if they are still good, they could be more reliable than a new one, especially when low cost from an obscure source.

[xrunner]

The large 800 µF caps may not be that critical. At only 120 Hz they usually don't get that hot as new ones in SMPS. Also the larger ones are not that sensitive to electrolyte drying out slowly. So they may very well live another 50 years. It might be enough to test them.  I would test them before replacing - if they are still good, they could be more reliable than a new one, especially when low cost from an obscure source.

I will consider that thanks.

Someone has to keep an eye on what you butchers are doing to nice Tek equipment.   

I'm devastated - no one has ever accused me of being a test equipment butcher!

[tautech]

Probably not the best choice of description but the kindest that came to mind at the time. 

Where do you source axial caps mostly ?
Do you know of Tedss ?
http://www.tedss.com/Capacitors

[xrunner]

Where do you source axial caps mostly ?
Do you know of Tedss ?
http://www.tedss.com/Capacitors

Never heard of Ted's I checked there and they have basically the exact capacitor at 820 uF instead of 750 uF. What concerns me is the pricing - there ain't any on that one - it just says to "request pricing. A sangamo on the same page at a voltage rating of 75V goes for $50 EACH! I'm afraid his prices at least on these would be just ridiculous.

But perhaps I shouldn't bother, the caps test OK and I've been advised, although they are original, to not worry about them if they test good. I did get the transistors today so the parts are coming in. Due to Memorial day no more parts till at least Tue.

I did get a sweet DC503A in to play with in the mean time (which actually works) 

[tautech]

Yep saw your post of you new acquisition. 
You have to win the odd one occasionally. 

I've only used Tedss a couple of times looking for obscure values and at the time they were the cheapest source but not for one offs. 


I guess there won't be any closeup pics now of your soldering repairs. 

[xrunner]

I guess there won't be any closeup pics now of your soldering repairs. 

I can see this one slip up isn't going away, so I opened it back up and cleaned up around the --> empty <-- holes

{where no components were anyways}

but in any case this should be acceptable. 

Note to Audience: This started from my repair of a different unit a TM501 but it's following me around the forum ... 

[xrunner]

While waiting on parts I can do the LED retrofit. Again the original power light was a grain of wheat bulb - unlike the PG501 this one is specified as an 18V lamp @ 26 mA through a 200 ohm R, hung off the +20V rail. We'll use the same PCB holes and simply substitute a new resistor value to give a not-so-bright light using something less than 20 mA. The green top hat will be glued on the LED also.

[xrunner]

Decided to run the LED at ~9 mA, which gives enough light. Running at 20 mA would be way too bright and would have resulted in .34 W dissipation on the resistor, requiring going with a .5 W resistor. 9 mA gives .15 W so can use the same size resistor (1/4 W) (that's what the .25W means on the schematic)

Gah I hate trying to draw with a mouse, sorry for the 6 year oldish looking notation on the schematic, but I have remedied that I ordered a graphics tablet/pen this afternoon. 

[David Hess]

Gah I hate trying to draw with a mouse, sorry for the 6 year oldish looking notation on the schematic, but I have remedied that I ordered a graphics tablet/pen this afternoon. 

Back in the Amiga days, drawing with a mouse was compared to drawing with a bar of soap.

[xrunner]

Back in the Amiga days, drawing with a mouse was compared to drawing with a bar of soap.

It's funny, it seems effortless to use the mouse to move and point on the screen accurately, but when it comes to drawing a line with one it's total FAIL.  Just for a test I'm going to re-draw that notated schematic properly!

*****

The only thing I did today (no mail today) is replace R250 pot. I had ordered the replacement which I determined from the parts listing was 1k, but when I unsoldered it today the pot had "500" marked on it (this marking was hidden from view also). I went back to the parts list and it did have two listings for R250 - 1k for certain serial numbers below "X", and 500 for serial numbers above "X". I wasn't paying close enough attention when I ordered the 1K pot. But ... not to be deterred, I just substituted an on-hand 500 ohm, 25 turn pot for the 500 ohm, 1 turn pot because I don't want to order another one. It'll be OK, since it just has to be set one time. 

[Rerouter]

The golden trick to drawing with a mouse is use the line tool and hold shift, makes perfectly straight lines.

[Johnny10]

xrunner

Wondering if you could use your oscilloscope to show the square waveform at the collector of Q290.

My unit was molested before I started my repair.

[xrunner]

The golden trick to drawing with a mouse is use the line tool and hold shift, makes perfectly straight lines.

Yea I know in a drawing program you can do that but I'm using a screen annotation app called Epic Pen -

http://epic-pen.com/index.html#features

It's just a quick and dirty pen/highlighter for the screen. It's pretty convenient and with the simple tablet I ordered I can draw a lot more accurately on these old scanned-in schematics.

Wondering if you could use your oscilloscope to show the square waveform at the collector of Q290.

Sure thing Johnny but at the moment it's out of commission till I get some parts, but looks like USPS tracking says I will get some today. Yay.  ALso after it's working and calibrated I'm going to post a lot more scope shots like I did for the PG501, so in the future this thread can be a resource for others trying to fix this unit.

My unit was molested before I started my repair.

I hope someone reported that to the police! 

[David Hess]

Wondering if you could use your oscilloscope to show the square waveform at the collector of Q290.

My unit was molested before I started my repair.

What exactly is it that you want to see?  Levels?  Edge speed?  Transient response?

[Johnny10]

I realize if I can't define it I am not understanding the circuit.

I was thinking I should see a square wave at this point.

[xrunner]

Got parts today - should have something cooking - I mean working - by tomorrow (bad choice of words!) 

I have a bin full of brand new BNC jacks and was going to replace the originals since the center pin female are tarnished quite a bit. Problem is I don't have & can't locate the one specialized BNC, the one that has an insulated ground (see pics). I can find ones on Ebay but they are not made the same and go into a "normal" size hole (this hole is much bigger). I suppose I could use washers but does anyone know where to find the original part?

[tautech]

That is one unusual BNC socket, never seen the likes of one before. 
Washers and a bushing seems the simplest remedy to me.

[xrunner]

Well the trigger buffer is attached. I have two other plug-ins - PG501, FG501A, both have trigger output BNC on the front and neither is an insulated jack. As you can see the trigger out is routed to the backplane but that one has a direct ground reference.

I think when the transistor conducts there would be about 17V at the emitter. But I have no idea what the reasoning is for the insulated ground design. It goes to ground through a 1.5 ohm resistor. Maybe David can shed some light? 

[David Hess]

I think the unusual trigger output circuit and isolated BNC keep the fast edge from the trigger from getting into the function generator output which is a problem on my B&K and Beckman function generators.  This would be important when the trigger output goes directly to an oscilloscope used for measurements causing a ground loop.

There are other subtle design features in the TM500 series instruments for preventing ground loop problems.

[MarkL]

I think when the transistor conducts there would be about 17V at the emitter.
...

The emitter should be about 5.4V when on.  The base is driven to a high of 6.1V as determined by R290, CR264, and R262.

[xrunner]

The emitter should be about 5.4V when on.  The base is driven to a high of 6.1V as determined by R290, CR264, and R262.

Yea I forgot about a 50 ohm load.

Check my calculations -

If you connect a 50 ohm terminated load to the trigger output BNC you have 50 + 24 = 74 ohms in parallel with the 1 k. That gives 69 ohms. I calculate when the transistor is on it will pass ~72 mA given the above resistance plus the other resistors. 72 mA through 69 ohms gives 4.97V. if you look at the screen shot I posted at the beginning of this repair that's about what I measured at the trigger output with a 50 ohm termination at the scope. The 1.5 ohm has ~ 0.11 V across it.

[MarkL]

The emitter should be about 5.4V when on.  The base is driven to a high of 6.1V as determined by R290, CR264, and R262.

Yea I forgot about a 50 ohm load.

Check my calculations -

If you connect a 50 ohm terminated load to the trigger output BNC you have 50 + 24 = 74 ohms in parallel with the 1 k. That gives 69 ohms. I calculate when the transistor is on it will pass ~72 mA given the above resistance plus the other resistors. 72 mA through 69 ohms gives 4.97V. if you look at the screen shot I posted at the beginning of this repair that's about what I measured at the trigger output with a 50 ohm termination at the scope. The 1.5 ohm has ~ 0.11 V across it.

Which other resistors?

I calculated 5.4V without a 50ohm load.

I didn't notice the amplitude on your screenshot before.  I would expect the trigger output to be less than that with a 50ohm termination given R280 and the output impedance of emitter follower Q268.

The service manual for the FG502 says the trigger out level should be >=5V for 1Mohm, and >=2.4V for 50ohm.  I can't argue with something that's working, but what amplitude do you get if the trigger output is unterminated?


EDIT: Added a little more detail to schematic snippet.

[xrunner]

Thanks for your analysis. You're probably right but how this started was why they used an insulated BNC for the trigger output. I have two other Tektronix plug-ins that have trigger outputs and they do not have an insulated ground. Why did they design this one differently? I'd like to know your thoughts on that.

But today I have enough parts to get it going, assuming it doesn't have any more problems, then we can make some more measurements for the record. That's what I've been doing for the other units I've fixed, for the record, so that someone in the coming years can find correct waveforms for these units and fix them. Measurements and waveforms from internal test points that just aren't available anywhere else, nit even in the troubleshooting section of these manuals. That's my legacy to leave here for whatever it's worth. 

[xrunner]

More problems ...

First I replaced the 4 22 uF caps and tested without the two output transistors - it worked OK got a good clean output trigger.

I then replaced the two output transistors, the two 10 ohm resistors (R536 & R546), and the 10 uF caps (C536 & C546) - after a few seconds one of the 10 ohm res. started to smoke. So it should appear obvious to the most casual observer that there is more to this picture. No, I didn't put the tantalums in backwards (beat you to it  )

I checked the 10 ohm resistors and they still read 10 ohms, so after pulling the new final transistors Q540 & Q546 (see that's why I installed sockets ...) it does put out a very nice amplified signal at all switch positions indicating Q530 & Q532 operating well. So, there's an issue in the final circuit still which I will dive into today.

[Kleinstein]

Are the "new" transistors still ok or are they broken.
One possible cause could be trouble with the 1 µF caps C540,C542 and corresponding.
It is also possible that the replacement transistors were too slow (e.g. other source for the 2N2905) and this way made the amplifier oscillate at high frequency.

There might also be a problem with load sharing: if Q530 and Q540 are too different in U_BE one of them would get most of the load current.

[MarkL]

Ch1 (yellow) looks like you have some distortion going high.  It's flattening out.

Plus you have Ch1 AC coupled.  What does it look like DC coupled?  The output could be way out of balance.

I would scope E, B, and C of Q530 and Q532 (without Q540 and Q542 installed).

Also note that you have FOUR output transistors: Q530 and Q540 share the load equally from the +20V rail and Q532 and Q542 share the load equally from the -20V rail.  It's interesting it works with only Q530 and Q532 installed.  Another way to narrow down the issue is to see if it works with only Q530 and Q542 installed, or only Q532 and Q540.

I'm still wondering why the 33ohm resistors aren't cooking.  Are they even hot?

[xrunner]

Are the "new" transistors still ok or are they broken.

Still OK.

Ch1 (yellow) looks like you have some distortion going high.  It's flattening out.

Yea that due to the fact it needs a complete re-alignment

...

I'm still wondering why the 33ohm resistors aren't cooking.  Are they even hot?

Nope.

But good news - I believe it to be fixed and working good as new (but it needs to have the cal procedure completely done). 

What was the final issue? Let me eat lunch and I'll type it up - but it should prove to be interesting and will go to show that you shouldn't take anything for granted in this hobby. There was nothing wrong with the transistors or resistors, and I made no installation mistakes like reversing leads and such ... if you want to guess go ahead. 

[David Hess]

There might also be a problem with load sharing: if Q530 and Q540 are too different in U_BE one of them would get most of the load current.

Through 33 ohm emitter resistors?  Not a chance, the excess Vbe from the triple diodes produces more current than any mismatch.  The bypass capacitors in parallel with the 33 ohm resistors should be checked though.

Also note that you have FOUR output transistors: Q530 and Q540 share the load equally from the +20V rail and Q532 and Q542 share the load equally from the -20V rail.  It's interesting it works with only Q530 and Q532 installed.  Another way to narrow down the issue is to see if it works with only Q530 and Q542 installed, or only Q532 and Q540.

Exactly, there are 4 output transistors in two parallel pairs and having either pair or a complementary transistor from each pair installed will produce the output.

I'm still wondering why the 33ohm resistors aren't cooking.  Are they even hot?

I would look for oscillation and the bypass capacitors carrying the DC current.  I would also change all four transistors at least temporarily to 2N4401s and 2N4403s until the problem is figured out rather than risking more expensive TO-39 parts.

[xrunner]

David see my post above - 

[MarkL]

Thanks for your analysis. You're probably right but how this started was why they used an insulated BNC for the trigger output. I have two other Tektronix plug-ins that have trigger outputs and they do not have an insulated ground. Why did they design this one differently? I'd like to know your thoughts on that.

David's explanation of reducing ground-loop currents is what I what I would have assumed also.

The SG502 also has this type of connector on the trigger output, but there's no detail in the schematic of how it's connected.

If you're up for an experiment, you could try shorting R284 to see if it has any noticeable effect on the signal output waveform (such as increased glitches at the trigger transitions).

[MarkL]

What was the final issue? Let me eat lunch and I'll type it up - but it should prove to be interesting and will go to show that you shouldn't take anything for granted in this hobby. There was nothing wrong with the transistors or resistors, and I made no installation mistakes like reversing leads and such ... if you want to guess go ahead. 

Ok, I'll take a wild guess:  The problem was mechanical.  Somehow, the heatsinks for the output transistors Q540 and Q542 got shorted together.  That would short their collectors together and put quite a bit of current through both 10ohm resistors.

[xrunner]

So remember I had ordered new parts - the transistors, the 10 ohm resistors, and 22 and 10 uF tantalum caps. The original failure burned the two 1/4 W 10 ohm resistors and also I found the 10 uF tantalum caps had failed, but I didn't find any other failures even the two output transistors checked out good.

The first things I replaced were four 22 uF tantalum caps, and checked it at that point and it worked OK. I then replaced the rest of the components in the output stage I ordered and turned it on and the magic smoke started coming out of the 10 ohm resistors. I turned it off before they were damaged too much to continue troubleshooting. I then pulled the last two transistors from the sockets and turned it back on and it worked OK even without the last two transistors.

At this point I was befuddled. I studied the schematic again and couldn't see what else it could be since I had checked most everything else in that area. I then inspected the board again and I noticed that the two new 10 uF tantalums I installed were discolored. I pulled them and found they were now bad. They both measured something like 45 - 55 ohms and failed the capacitance test.

So did I install them backwards? Nope. So what happened? I turned my attention to the rest of them in the bag I bought - 10 total. About half of the "new" caps tested bad! Bad as in they had no capacitance only resistance. So this is very very odd, that the original failure was bad tantalums and the following failure was the same using what was supposed to be new tantalum caps.

I decided I wouldn't use any of the 10 uF tantalums I bought, even though half of them tested OK. I installed two 10 uF electrolytics and the thing is humming along now like a finely tuned engine for the last hour up to 10.9 MHz. 

I will now write a message to the seller of those 10 uF tantalums asking for a refund.  Also I'm now confident this part is done and will shorten up the transistors leads and re-re-install two new 10 ohm resistors.

Now I can move on to the calibration procedure and lastly the cosmetics/cleanup.

OK johnny10 - your turn.  You can use this thread if you need any help or measurements.

[xrunner]

xrunner

Wondering if you could use your oscilloscope to show the square waveform at the collector of Q290.

Here's your waveform. CH 1 is the collector of Q290, CH 2 is the trigger output for reference.

[MarkL]

But why, when you removed "Q540 & Q546" (I assume you meant "Q540 & Q542"), did it work fine without smoking the 10ohm resistors?

The newly installed bad capacitors would have still been in place, if I understand your sequence correctly.

Did you ever go back and test the original capacitors with a power supply to make sure they were bad?  A little post-repair failure analysis can never hurt for confirmation, especially if it's easy.

[xrunner]

But why, when you removed "Q540 & Q546" (I assume you meant "Q540 & Q542"), did it work fine without smoking the 10ohm resistors?

Yes I meant those. Because the deed was done by then - the caps shorted out or nearly so burning the new resistors and then they may have changed properties a little or a lot as they cooled down when I turned it off, and then didn't act the same way again. When I turned it back on the second time after pulling the socketed transistors I only flicked the power on very shortly and then back off because I was not confident in the situation.

I can't explain the physics of what went on inside them. But I can tell you that half of the new ones left in the bag are bad and after replacing them with electrolytics it's working just fine.

Did you ever go back and test the original capacitors with a power supply to make sure they were bad?  A little post-repair failure analysis can never hurt for confirmation, especially if it's easy.

With a power supply? No why should I when they tested bad with a capacitance meter and a DMM on the ohms range?  No there is enough evidence they went bad. But as I said it's purring along like a kitten now for several hours, all is well.

If you're up for an experiment, you could try shorting R284 to see if it has any noticeable effect on the signal output waveform (such as increased glitches at the trigger transitions).

Yes I will try that this evening if possible. 

[MarkL]

Here's your waveform. CH 1 is the collector of Q290, CH 2 is the trigger output for reference.

I realize this wasn't directed at me, but I see that the trigger output is about 2.7Vpp.  I assume you have the 50ohm terminator on.  This is the level I'd expect.

And it also shows that the signal driving the Q268 base circuit is about 5.7V, which is close enough to 6.1V considering that there's additional loading on Q268's emitter when the trigger output is terminated.

I'm only pointing it out since you don't usually want the voltage on your trigger output BNC's to be at a level that could cause damage to TTL circuits when they are unterminated.  By some of your previous posts I was concerned about that, which is why I was digging into it.  But everything looks ok.

Did you ever go back and test the original capacitors with a power supply to make sure they were bad?  A little post-repair failure analysis can never hurt for confirmation, especially if it's easy.

With a power supply? No why should I when they tested bad with a capacitance meter and a DMM on the ohms range?  No there is enough evidence they went bad. But as I said it's purring along like a kitten now for several hours, all is well.

You tested them as bad caps, but the test did not show they were shorted or low-resistance under voltage, which would have to be the case in order to fry the 10ohm resistors.

I'm only suggesting confirmation.  I like to do it when possible.  Certainly your call.

[xrunner]

And it also shows that the signal driving the Q268 base circuit is about 5.7V, which is close enough to 6.1V considering that there's additional loading on Q268's emitter when the trigger output is terminated.

I'm only pointing it out since you don't usually want the voltage on your trigger output BNC's to be at a level that could cause damage to TTL circuits when they are unterminated.  By some of your previous posts I was concerned about that, which is why I was digging into it.  But everything looks ok.

Yes you are correct thanks.

You tested them as bad caps, but the test did not show they were shorted or low-resistance under voltage, which would have to be the case in order to fry the 10ohm resistors.

I'm only suggesting confirmation.  I like to do it when possible.  Certainly your call.

I still have all the bad parts in a cup so let me see what I can find out. A little more smoke in the room won't hurt. 

[MarkL]

I have a bin full of brand new BNC jacks and was going to replace the originals since the center pin female are tarnished quite a bit. Problem is I don't have & can't locate the one specialized BNC, the one that has an insulated ground (see pics). I can find ones on Ebay but they are not made the same and go into a "normal" size hole (this hole is much bigger). I suppose I could use washers but does anyone know where to find the original part?

[I forgot to press "Post" on this from yesterday.  Maybe it's still useful...]

This is in the parts list, KC79-67.  A search on "bnc kc79-67" shows that it's commonly available, amazingly enough.

[Johnny10]

Actually, I have just changed that C546 and R546 as they were burned and shorted.
I did not change transistors Q540,542,530 or 532.
With the diode test they checked OK.
Funny seeing that pic of yours, I thought hey that looks familiar.

I rebuilt the Current Switch CR140 etc and new transistors Q140 and Q175. There was a lot of flux in this area from the previous owner and when I touched the connection between the diodes and resistor it just fell apart.


Replaced resistor R284 which was burned and cracked.

Checking Pin 6 output on U140 and U175 varying voltage while moving the Frequency knob.
The timing capacitors check out fine.
Yet, I have never had a Frequency output waveform or an output trigger waveform.
The Trigger Output is dead.

The main output has voltage which can be varied by the amplitude switch of near 0 - 1.3 Vdc.

I am still working my way through since changing those parts.
But I imagine I have some dead transistors somewhere.

I still get no waveform at Q290.

Will have to work on this tonight.

[xrunner]

This is in the parts list, KC79-67.  A search on "bnc kc79-67" shows that it's commonly available, amazingly enough.

Oh cool - I just found that part on Ebay - 2 for $14.99, ordered. Who would have thought the same part was available? Thanks. 

I tested the old 10 uF caps again. One measured a complete open, the other around 45 Mohms. Tested under a voltage the open one didn't break down up to 30V, but the other one tripped the OCP (set to 3 amps) on my supply at around 12 VDC.

I am still working my way through since changing those parts.
But I imagine I have some dead transistors somewhere.

I still get no waveform at Q290. Will have to work on this tonight.

OK let me know if you want a comparison waveform or voltage anywhere. I got a good one now to play with. Going to burn it in all evening. Proceeding here to calibrate and clean up, and replace the BNCs! 

[xrunner]

I just went into the lab to turn the FG502 on for a few hours - and nothing came out of the signal or trigger output. 

I immediately turned it off and put my nose close to the board, and thankfully didn't smell that smell. When I turned it over I saw that a wire on the extender cable had come off. It was one of the 25 VAC wires. I soldered it back on and we're back in business.

I hate it when that happens! 

[xrunner]

Actually, I have just changed that C546 and R546 as they were burned and shorted.
I did not change transistors Q540,542,530 or 532

....
I still get no waveform at Q290. Will have to work on this tonight.

Here's a couple of waveforms for your consideration, might help I don't know.

Had a similar issue with the last repair I did - the FG501A. There were no time-varying signals whatsoever anywhere in the thing. It tuned out that an output from a comparator (which had failed) was required to make the charging circuit switch states. There was nothing wrong with the basic triangle generator in that one.

Although that's not the same circuit as this one has, I think you know that it's just going to sit there until it can switch states. So keep looking in that circuit area. You might try to capture an initial state from as many point as you can (you have a 4 channel scope I assume?) when power is applied up to the point where it stalls, then narrow it down to where a signal is missing that should be there. For example when power is applied it should charge in one direction until it can't go the other way.

In the attached screen shot, the signals are DC coupled centered on Zero. You can see the triangle charge and discharge as the square wave changes.

[MarkL]

I tested the old 10 uF caps again. One measured a complete open, the other around 45 Mohms. Tested under a voltage the open one didn't break down up to 30V, but the other one tripped the OCP (set to 3 amps) on my supply at around 12 VDC.

Well, that certainly fully explains one of the dead resistors.  The other one, not so much.

I threw together a quick simulation of the output amp.  With a 50ohm load and the output fully high (or low), there is 0.257W being dissipated in the 10ohm resistor.  Since the FG502 operates at a 50% duty cycle, the average power dissipation in each of the 10ohm resistors is 1/2 that.  If the high or low time is long in relation to the thermal resistance, it could still get pretty hot.

However, if the output is shorted, the fully-on dissipation is 0.69W.  Even with 50% duty cycle, it's still going to be more than the rating of 1/4W.  Maybe driving a short or very low impedance is what killed it.

I would probably replace those resistors with 1/2W or 1W.  Lab equipment is supposed to be as bullet proof as possible under unexpected conditions.

(Ok, I think I'm done beating this one to death.)

[xrunner]

Well, that certainly fully explains one of the dead resistors.  The other one, not so much.

I threw together a quick simulation of the output amp.  With a 50ohm load and the output fully high (or low), there is 0.257W being dissipated in the 10ohm resistor.  Since the FG502 operates at a 50% duty cycle, the average power dissipation in each of the 10ohm resistors is 1/2 that.  If the high or low time is long in relation to the thermal resistance, it could still get pretty hot.

However, if the output is shorted, the fully-on dissipation is 0.69W.  Even with 50% duty cycle, it's still going to be more than the rating of 1/4W.  Maybe driving a short or very low impedance is what killed it.

I would probably replace those resistors with 1/2W or 1W.  Lab equipment is supposed to be as bullet proof as possible under unexpected conditions.

Well we're thinking the same thing, I guess you missed it but I had decided to upgrade the 10 ohm resistors to 1W a while back. Check the pic out you can see the blue resistors installed, they just fit into the holes, and the transistor sockets and 10 uF electrolytic caps. I'm half way through the service alignment and I haven't noticed any ill effects from using electrolytics.

(Ok, I think I'm done beating this one to death.)

Yea but now we can beat Johnny10's to death!

[Johnny10]

I am taking a methodical path through the parts.
I have found a few out of spec Caps.
Now I need part numbers or substitution numbers for FET Q200, Q202 one is missing leg.

Says matched pair !

 

[xrunner]

I am taking a methodical path through the parts.
I have found a few out of spec Caps.
Now I need part numbers or substitution numbers for FET Q200, Q202 one is missing leg.

Says matched pair !

Try this site out -

Tektronix to Generic Transistor
Cross Reference Database


Transistor Part Number 151-1042-00 has the following equivalent generic types:

2N5454   FET, N-CHANNEL

There's some on Ebay, buy a few and get as close as you can. 

[Kleinstein]

The exact JFET type should not that critical. Also matching is not that critical. One will need to adjust the DC level anyway. A good matching will likely also keep offset drift small.

[Johnny10]

I see that the part number you recommended is available as dual package.
Already matched.

[David Hess]

I am taking a methodical path through the parts.
I have found a few out of spec Caps.
Now I need part numbers or substitution numbers for FET Q200, Q202 one is missing leg.

Says matched pair !

They are Tektronix part number 151-1042-00 which is type 2N4416 and matched for an Idds (1) of 0.5mA between 10 and 15mA at 15 volts which is trivial to test with a power supply and current meter.

The 2N5486 (8 to 20mA) would be best but the 2N5485 (4 to 10mA) is almost as good.  I am sure the 2N5484 (1 to 5mA) will work but the other two will be better at the same cost and are closer to the original specification.

I think a J112 or BF256B (Mouser has them) would be acceptable also at half the cost.

(1) Drain current with the gate shorted to source or Vgs=0.

[bitseeker]

I just went into the lab to turn the FG502 on for a few hours - and nothing came out of the signal or trigger output. 

I immediately turned it off and put my nose close to the board, and thankfully didn't smell that smell. When I turned it over I saw that a wire on the extender cable had come off. It was one of the 25 VAC wires. I soldered it back on and we're back in business.

I hate it when that happens! 

That's a feisty function gen. Glad to see that it's finally behaving. Congrats!

[xrunner]

That's a feisty function gen. Glad to see that it's finally behaving. Congrats!

Yea they can be - Johnny10 has a rascal on his hands too. 

So cool beans - it made it through the entire calibration with flying colors. Just threw in a pic of my bench, I like to use an iPad with the manual PDF on the iCloud right there. Also shown is the test checking the 2nd and 3rd harmonics when set to 11 MHz. Should be -30 dB or better - they are.

[Johnny10]

I guess I know why I have no ramp waveforms.
After finding bad Q200.
And after pulling a few tantulum caps I now have voltage at Trigger Output.

Just need to get those FET transistors back in my unit.

And maybe I can take pictures like xrunner ?

[xrunner]

I guess I know why I have no ramp waveforms.
After finding bad Q200.
And after pulling a few tantulum caps I now have voltage at Trigger Output.

Just need to get those FET transistors back in my unit.

When you get the new tantalums check 'em first. When I got my new parts (the transistors, resistors, and tantalum caps) I checked the transistors and resistors, then figured it all had to be good. It all wasn't good. I'm messaging the seller and trying to get a refund.

And maybe I can take pictures like xrunner ?

Maybe? You gotta post pics dude - we all love pics. Speaking of which I'm not done yet, I'll have more to post, pretty pics with new BNCs and spit-shined knobbies. If she's choochin¹ she's gotta look the part.




1. Chooch - a term used by the vlogger AvE to signify an item is working.

AvE Speak

[xrunner]

I wanted to clean the PCB switch as with some of the other TM500 plug-ins. There are two of them in this unit. When I took the cover off one, I saw a tiny piece of wire just laying there. Just goes to show you can't take anything for granted. 

I gently clean under the contacts with some very fine 600 grit paper soaked in alcohol. Just wipe it a few times is all that's needed. Now I'm proceeding to remove all the controls and BNCs from the front panel so I can take it off, there's a lot of old sticker residue that has a date with some Goo Gone.

[David Hess]

I gently clean under the contacts with some very fine 600 grit paper soaked in alcohol. Just wipe it a few times is all that's needed.

You used sandpaper on gold plated switch contacts?  See photograph below.

Non clay bearing paper like vellum or card stock soaked in alcohol is more than enough and will not destroy the switch's contact surfaces.

[xrunner]

Remain calm - you misunderstand - I didn't use it like I was trying to refinish an old dresser. I just slid it under and back out very gently 2 times wit no pressure soaked in alcohol - nothing is damaged.

[David Hess]

Remain calm - you misunderstand - I didn't use it like I was trying to refinish an old dresser. I just slid it under and back out very gently 2 times wit no pressure soaked in alcohol - nothing is damaged.

[xrunner]

Yea well ... moving on ...

[xrunner]

So not only do I want it to work as new I also like to restore these to as new-looking as possible, because it's a challenge and I enjoy it. 

Removing the front face requires removing the knobs, shaft nuts, and all BNCs. The BNCs will be replaced including the original insulated BNC used for the trigger which I was afraid was unobtanium but MarkL found it was still available (thanks).

Once you get all that off the front face comes off. You can see where the grain of wheat bulb was mounted in the little white holder (will be replaced by an LED). I've also found on these that the corners crack over many years, due to years of pressure by the four corner screws. Some corners even crack completely off (I have a few like that). On this design you have to get the front panel off to remove the top and bottom aluminum frame pieces. The other design they came up with is easier to take apart.

The upper left corner you can see has a small horizontal crack but it's easy to fix with plastic welder (MEK). The front aluminum panel is cleaned with Goo Gone, will need to soak for several hours. It doesn't appear to be scratched so it should look really nice.

[xrunner]

Just cosmetics updates. Front panel cleaned, dial polished, cracks repaired, aluminum panels cleaned. All knobs cleaned in ultrasonic cleaner. New LED installed (no it isn't nearly as bright as the camera makes it look).

Put a few of the pieces on the panel - looks like a new one coming off the assembly line. 

[tautech]

Real nice job on the front panel xrunner, it came out like new. 

Looking forward to seeing finished.

[Rerouter]

Your tempting me to rip my own ones down and make them look good as new,

dark times are ahead, I've already collected too many modules

my poor free time xrunner, you've doomed it.,
 

[Johnny10]

Are those BNC connectors new?

  Great Job,  I can see you are making me look bad already!

[xrunner]

Are those BNC connectors new?

Yep sure are. Got one installed (for the main output), but the rest are on hold until I get the insulated ground BNC. This because the nut can't be easily tightened on that one with the other parts in the way.

Great Job,  I can see you are making me look bad already!

I'm sure you will be able to out-class me Johnny10. No pressure, no pressure. Did you get the parts on order that you required?

[bitseeker]

Looking great there, xrunner! Any tips for restoring brushed aluminum face plates where some monster scratched it cross-grain trying to remove calibration stickers with a screwdriver ?

[David Hess]

If it is brushed aluminum then you can brush it again.  These Tektronix faceplates are silk screened and then anodized.

[xrunner]

Looking great there, xrunner! Any tips for restoring brushed aluminum face plates where some monster scratched it cross-grain trying to remove calibration stickers with a screwdriver ?

No not really. If it's got silk screened labels forget it. If it was clean aluminum you could brush it again with any number of materials but it wouldn't look exactly like the other panels, which may or may not matter.

The thing about those labels (cal stickers especially) is that you have to be patient with the removal (I know you know this). For example, on this project several places were so old that the old adhesive was more like petrified wood - rock hard. It literally took all night for the stuff to get soft, and then it took more than one application. But it will all come off eventually. If you get in a hurry then SCRACTH!  Fortunately the goo gone does not harm faceplates with painted labels.

Other things happen, for example on the FG 501A I wanted to remove the plastic dial for cleaning behind it, and it's held on by two little screws, just like this one. But one of the screws would not come out. It's steel into brass and they had been there from day one. I tried everything including penetrating oil but no go. I had to resort to drilling it out, and I was very nervous, but I succeeded.

I don't like the design of this older TM 500 plug-in frame, the second pass design is better. This one holds the front face plate to the frame with two screws each - top and bottom - going in from the front. Problem is the screws, once tightened, are then made inaccessible once the front face plate is on. If the plastic relaxes and makes it feel loose, you can't tighten it unless you take everything holding the face plate off which can be somewhat of a PITA. There's also a small screw that holds the locking mechanism for the frame once it's slid into the mainframe hidden behind there. So once you realize this you think man I need to make sure they are tight so I don't have to take it all apart again some day. Yea, but if they are too tight the plastic cracks, which I've seen now on most of them. So they have a fine line on how tight to screw them in.

And, the side shields are pressed in on the older design (the newer one has a securing screw and does not get warped trying to remove it). They can be warped because people have pulled on them when stuck, pulling too hard the wrong way trying to remove them. Instead of pulling up with your hand from the back try to pop each side out with a tool under the channel going along each side. A few I've gotten have side panels warped so bad I had to go into the garage with the panels and do unspeakable things to them with a hammer and block of wood. But it all comes out OK in the end

[David Hess]

The thing about those labels (cal stickers especially) is that you have to be patient with the removal (I know you know this). For example, on this project several places were so old that the old adhesive was more like petrified wood - rock hard. It literally took all night for the stuff to get soft, and then it took more than one application. But it will all come off eventually. If you get in a hurry then SCRACTH!  Fortunately the goo gone does not harm faceplates with painted labels.

I find that the same heat gun I use for heat shrink tubing does wonders for removing old labels even when the adhesive has dried out.

Other things happen, for example on the FG 501A I wanted to remove the plastic dial for cleaning behind it, and it's held on by two little screws, just like this one. But one of the screws would not come out. It's steel into brass and they had been there from day one. I tried everything including penetrating oil but no go. I had to resort to drilling it out, and I was very nervous, but I succeeded.

I keep some of the appropriately sized hex wrenches *without* ball tips (1) and cut the ends off when they get dull so they fit tightly.  If the set screw is worn so the wrench cannot fit tightly, then I take the next larger size hex wrench and grind the flats down to make one which fits very tightly.

(1) This was learned watching someone else break a ball tip off inside of a hex screw making it wisdom instead of experience.

[xrunner]

The insulated BNCs were to arrive today, all I got was this from tracking -

June 5, 2017, 10:59 am    Notice Left (No Secure Location Available)

 

The mailperson must be on crack. That's an insane message. There is no secure location? How about the  ... mailbox?

A notice? Must have put it up in a tree in my backyard, yea let me get a ladder and look fot it way up high in a tree.

Unreal.

[David Hess]

How was it shipped?  Nobody except the US Postal Service is legally allowed to use your mailbox.

[xrunner]

How was it shipped?  Nobody except the US Postal Service is legally allowed to use your mailbox.

I said mailperson, what do you think that means?

[David Hess]

How was it shipped?  Nobody except the US Postal Service is legally allowed to use your mailbox.

I said mailperson, what do you think that means?

That is just weird then unless the package would not fit into your mailbox although that has never stopped them here.

[bitseeker]

It is unfortunate that the plastic part of the front plates are held on so tightly with screws that they end up fracturing. Alas, manufacturers weren't really considering what would happen to their gear this long after they were made. It, like former-owner sticker damage, can aggravate the OCD. 

[xrunner]

That is just weird then unless the package would not fit into your mailbox although that has never stopped them here.

No excuses for them David. They continually leave mail too big for my mailbox (yes I really do have a secure mailbox) on the porch. They simply ring the bell and run back to the truck. I was sitting right in my lab right by the window. Nobody came to the door, nobody left a "notice". A bag with two connectors in it will fit in my mailbox as has much bigger things for many years.

Most seem incompetent and/or like they'd rather be cleaning porta potties than doing the job they have. They've proven it time and again. They had the deliveries mixed up and instead of notating that in the tracking someone put in a false and irrational claim that no secure location was available and a false claim that a notice was left. That's a bold lie. That's total and complete BS. It was in another mailperson's route. Somebody just wanted to go home for the day and not drive out of their route to deliver it. But they don't want to actually say that. All those USPS commercials you see on TV, claiming they are the best delivery for business, don't believe it for a second.

They just don't have their act together. I'll have to go to the post office and get it (who knows if it's actually there), and present them with the tracking notes from their own system that are just non-sensical. All I'll get from the clerk is a blank stare because they don't care what happened, and they don't care if it happens again.

End of ranting.

It is unfortunate that the plastic part of the front plates are held on so tightly with screws that they end up fracturing. Alas, manufacturers weren't really considering what would happen to their gear this long after they were made. It, like former-owner sticker damage, can aggravate the OCD. 

Yes, although it was top quality test gear in the day it certainly wasn't considered especially valuable or "collectable". As I said I used this exact Tek gear in the aerospace industry and it got used & abused, that's just the way it was. It was just expendable equipment supplied by the Company, if it broke, just send it back to the cal lab for repair or salvage, go get another one from the storage room. It wasn't like "Man I really want to have one of these 20 years from now!". 

Now that I think about it I have an idea for reinforcement of that screw area. I have some very thin plastic strips for modelling, the same kind of plastic that melts together with MEK. What I should have done (and will do next time) is make a reinforcement for each of the four screw areas. Like a square with a hole in the middle. Then glue it to the back of each screw hole. I bet that would really help. Let me see if I have another old case type that I can get the front off of easily. Maybe I'll make a thread showing that kind repair, but I've put too much back together on this one for now.

[David Hess]

It is unfortunate that the plastic part of the front plates are held on so tightly with screws that they end up fracturing. Alas, manufacturers weren't really considering what would happen to their gear this long after they were made.

The plastic bezels on the TM500 series are a known weak point of their design.  The plastic becomes brittle with age and broken off corners are common.

[xrunner]

Picked 'em up at the post office. I took the tracking info and asked the clerk to read it. She was rather befuddled. She said "That doesn't make any sense".

 

I said "Yea, I know that it doesn't make any sense, since I was sitting right there at that time, there was no note on my door, and there is a secure mailbox at the street, can you go back there and see if you can find it?"

She came back with it. She said "Oh it just arrived here today!"

"What do you mean? It was out for delivery from this post office yesterday."

I said "Look I have a repair thread on the EEVBlog that's been stopped in it's tracks because this part is MIA. This is important! 

I just quit asking questions at that point and left. But it's the right part, NOS made in 1989. 

[bitseeker]

Yeah, who does the postal service think they are, suspending a repair thread like that!

[xrunner]

Yeah, who does the postal service think they are, suspending a repair thread like that!

LOL. 

Well I have to tell ya - this little gem is lookin' really good. I still have a few more things to put back on, but I expect to have an unveiling in the morning. 

[xrunner]

Here it is all put back together. Project is basically done except for some additional data I will be posting, which will include scope screen captures from selected test points and associated schematics with notations. Perhaps somebody in the future will be able to make use of the data for their own repair project.

[Johnny10]

Beautiful unit!

I am out of town so won't be working on mine again till next week.
But I don't see a way for any better looking unit from me.

Glad you are posting waveforms.

[bitseeker]

xrunner, now you've done it. I'm afraid it looks too good to use!

[kayvee]

Nice job xrunner.  Love those shiny BNCs!  That FG is looking somewhat shabby next to it.

I scored two of these babies in pristine condition in a batch of modules I was after.

This thread reminds and inspires me that I need to make a repair attempt on the FG 504 that was in the batch.

[xrunner]

Thanks for the kind comments. I'm working on a set of scope captures and associated schematics. I also got a little graphics pad I'm going to try out to notate the schematics. Takes some getting used to as compared to a mouse. I'm going to highlight the adjustment points in addition to inputs/outputs and active devices.

I also have an idea for a way to repair/reinforce the plastic corners of these older units, where it cracks at the screw attachment points. I'll put it in a little thread by itself (if I'm satisfied it's viable). I do have a guinea pig available ... 

[xrunner]

Waveforms - Triangle Generator

Comments:

Serial # B037759. Some alignment instructions pertain to certain serial number ranges, consult your particular S.N.

Note step 8 "Adjust Triangle Peak", C287 had very minimal effect on this unit.

[xrunner]

Waveforms - Sinewave Generator Set A

[xrunner]

Waveforms - Sinewave Generator Set B

[xrunner]

Waveforms - Output Amplifier

10 MHz Sinewave Harmonics