Author Topic: Repair of Tektronix FG502 11 MHz Function Generator  (Read 35142 times)

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Offline David Hess

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #25 on: May 26, 2017, 03:23:33 pm »
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.

Quote
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.
 
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Offline tecman

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #26 on: May 26, 2017, 04:43:30 pm »
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
 

Offline Johnny10

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #27 on: May 26, 2017, 04:46:34 pm »
What should you replace the tantalums with?
« Last Edit: May 26, 2017, 04:54:12 pm by Johnny10 »
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Offline David Hess

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #28 on: May 26, 2017, 04:56:23 pm »
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.
 

Online xrunnerTopic starter

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #29 on: May 26, 2017, 06:21:32 pm »
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.
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Offline tautech

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #30 on: May 26, 2017, 06:43:46 pm »
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.  ;)
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Online xrunnerTopic starter

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #31 on: May 26, 2017, 09:46:53 pm »
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?  :-//
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Offline tautech

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #32 on: May 26, 2017, 10:00:13 pm »
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.
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Offline bitseeker

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #33 on: May 26, 2017, 10:13:46 pm »
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.
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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #34 on: May 26, 2017, 10:25:20 pm »
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"?  :-DD
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Offline David Hess

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #35 on: May 26, 2017, 11:06:37 pm »
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.
 

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #36 on: May 26, 2017, 11:45:23 pm »
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.  :-DD
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Offline Johnny10

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #37 on: May 26, 2017, 11:56:48 pm »
What are the diodes CR140,145, 170, 175 making the current bridge?

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Offline David Hess

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #38 on: May 27, 2017, 12:17:41 am »
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.
 
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Offline tautech

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #39 on: May 27, 2017, 12:24:42 am »
 :-DD
Someone has to keep an eye on what you butchers are doing to nice Tek equipment.  :scared:  :P
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Offline bitseeker

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #40 on: May 27, 2017, 04:30:30 am »
You guys crack me up. :-DD
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Offline Kleinstein

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #41 on: May 27, 2017, 09:25:26 am »
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.
 

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #42 on: May 27, 2017, 11:31:55 am »
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.

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

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Offline tautech

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #43 on: May 27, 2017, 10:37:45 pm »
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
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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #44 on: May 27, 2017, 11:10:06 pm »
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)  :clap:
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Offline tautech

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #45 on: May 27, 2017, 11:22:30 pm »
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.  ;D
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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #46 on: May 28, 2017, 12:21:44 am »
I guess there won't be any closeup pics now of your soldering repairs.  ;D


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.  :P

Note to Audience: This started from my repair of a different unit a TM501 but it's following me around the forum ...  :palm:
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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #47 on: May 28, 2017, 08:01:59 pm »
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.
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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #48 on: May 29, 2017, 12:10:02 am »
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.  :-+
« Last Edit: May 29, 2017, 12:11:58 am by xrunner »
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Offline David Hess

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Re: Repair of Tektronix FG502 11 MHz Function Generator
« Reply #49 on: May 29, 2017, 12:35:10 am »
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.
 


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