HP 3561A Dynamic Signal Analyzer repair

[ckm]

Well, I've got +/-12.3vdc on the two test points, so the 12v power supply is OK.   I think it's likely CR100, the TO3 rectifier, has gone south, maybe along with C210, the 5v filter cap.

Still curious about CR302 & CR304, but one thing at a time...

[Cubdriver]

I just re-read what you'd posted earlier and noted you saying that CR100 appears to be shorted.  Based on your readings, that sounds likely.  I just measured CR100 on mine; forward biased both sides of it read 0.201 on the diode check setting of my Fluke 77-II.  Reverse biased, they show as open.

And just to confirm, you went from each pin to the body of the rectifier, correct?  Pin to pin measures the transformer secondary; if I measure that I see 0 on the diode check setting and 0.2 ohms on resistance.

-Pat

[ckm]

Yeah, I just pulled it out (easy when you have a Hakko 470 desoldering station ;-). 



Attached are some photos of resistance measurements in forward & reverse flow, seems broken on one leg....






Did a diode test as well, one leg is ok, the other one measured the same voltage forward & reverse (about 0.00004v IRC).

[slbender]

Sir - regarding your comment of a dead TO-3 regulator, I believe this set only has TO-220 regulators, the TO-3 parts include a dual Shottkey diode and two Mosfets.


-Steven

[slbender]

Pat - Its about 3 years later....

I just recently scored a similar HP-3561A unit, in more or less, the same condition (cosmetically). However, my set has two, or three, or four problems. I pulled the power supply circuit boards and checked the diodes zeners, regular, and switching - none seemed defective on my Fluke DMM using the "Diode Test".  Replacing those boards, and applying power - all the front panel leds light up, but do not extinguish (frozen).

I have not checked the +/- 12 volt Bias test points yet, as I could not locate any 12 volt regs, in case they are bad, so I will do that test on next power-up situation.

Pulling the PS boards again, I did see that R203 seemed to be slightly darker at the solder connections, so maybe I will resolder it... That getting hot, makes me suspect the CR-100 part (the VSK-231 pair of 3 Amp Shottkey diodes in the TO-3 can) of being shorted, leaky, or otherwise or bad, meaning no +5 volts at that point. I noted that the resistance between what would normally be the Base and Emitter pins (if that TO-3 part had been a transistor) appeared to be shorted together.  If you know that these pins should be infinite, or shorted in this configuration, that would be a helpful bit of news!

Since it looks like the two diodes do have a common connection on the schematic, I can't yet compare the part with the circuit board and the schematic, but anyway, that is my first guess.  The only fast shottkey diodes I have are small modern ones that look like 1N4004 diodes, but are rated at 100 Volts and 1 Amp, less than what the circuit requires.  So, I plan to parallel pairs of three together to get it up to 3 Amps and remove the TO-3 and sub those in its place.  If the set comes to life (unlikely, but you never know :-). and it doesn't blow it up, I'll add a 4th parallel diode and leave that in until I see smoke, or other symptoms, in other places.

I also see what looks like a fleeting error message (garbled) fly by on the CRT - but I can't read it.

None of the 4 overload red led's lights up, meaning???

But another board has a red and a green led pair that cycle on and off rapidly - in succession. Can anyone guess what that means?

No smoke, but yet there are several problems to solve.

Thanks for any help or suggestions,


-Steven

[slbender]

Some further tests - I checked C210, which is on the motherboard, with an LCR meter and it read 1.09 ohms, which is a value that is OK for a 25V cap < 200uF.  C210 is a fairly large cap, but the markings are oriented so I can't read them (of course).  It seems like it might be too low for such a large cap.

Also checked the CR100 rectifier, it reads the same resistance forward & reverse (about 0.4 ohms), which also seems wrong.  That might explain why R203 got hot, I'd have to check the schematics (which I don't yet have).

Also, I double checked the two white wires, I don't think wire '1' shorted.  It doesn't seem to have burned through the insulation, just charred the insulation.   Also the wire was only touching the resistor 'jacket' not the bare leg.

I spotted a couple of other diodes which I am suspicious of, namely CR304 & CR302.  Testing them in circuit gives me 18 kohms on both.  I'll have to check where they go when I get the schematics.

But right now, I think the 5v rectifier has gone south....

I could not find that Orion thread, did you correctly put the URL in there?  I'll have to check some of the motherboard and other tantalum caps in my set also.

-Steven

[Cubdriver]

Steven - a quick reply between trips to the garage to replace a wheel bearing in a friend's car - the 'base' and 'emitter' leads of that TO-3 dual Schottky diode are the anodes; the common cathode is the case.  I doubt you'll get an accurate reading with it in circuit.  If it's good, 'base' to 'emitter' should be open circuit as you'd be looking at back to back diodes in series, with one forward biased and the other reversed.  The fly in the ointment for in circuit testing as that you wind up measuring the transformer secondary winding, as it normally feeds to the input of the rectifier.

I don't have the instrument open to look right now and can't recall how it's mounted, but if you can take out the screws and slip paper or something between the dual diode case and cathode connection on the board, that might enable you to check the diodes somewhat in isolation (though as I think about it, the transformer secondary would still be tying their anodes together).  It would at least let you know if they're shorted, as at worst they'd act like they were in parallel.

Can you tell me which board has the blinking LEDs?  Knowing would give me an idea where to look in the manual and perhaps provide some insight.

-Pat

[slbender]

Steven - a quick reply between trips to the garage to replace a wheel bearing in a friend's car - the 'base' and 'emitter' leads of that TO-3 dual Schottky diode are the anodes; the common cathode is the case.  I doubt you'll get an accurate reading with it in circuit.  If it's good, 'base' to 'emitter' should be open circuit as you'd be looking at back to back diodes in series, with one forward biased and the other reversed.  The fly in the ointment for in circuit testing as that you wind up measuring the transformer secondary winding, as it normally feeds to the input of the rectifier.

I don't have the instrument open to look right now and can't recall how it's mounted, but if you can take out the screws and slip paper or something between the dual diode case and cathode connection on the board, that might enable you to check the diodes somewhat in isolation (though as I think about it, the transformer secondary would still be tying their anodes together).  It would at least let you know if they're shorted, as at worst they'd act like they were in parallel.

Can you tell me which board has the blinking LEDs?  Knowing would give me an idea where to look in the manual and perhaps provide some insight.

-Pat

Hi Pat,  the long board with the yellow pull to remove, which has the big 68000 and the ROM chips (12 of them) mine doesn't have all possible spaces filled and it doesn't have the UV erase types. Parts CR1 green and CR2 red crazy blinkin'!!!

Thanks,

Steven

[Cubdriver]

Hi Pat,  the long board with the yellow pull to remove, which has the big 68000 and the ROM chips (12 of them) mine doesn't have all possible spaces filled and it doesn't have the UV erase types. Parts CR1 green and CR2 red crazy blinkin'!!!

Thanks,

Steven

Hi Steven -

I'll be of limited help at the moment as it's nearly four in the morning and despite my best intentions I'm still up. 

Mine too has some blank spots on the processor board.  Here's a photo of it so you can compare it to yours:


I just excavated it, pulled the cover and fired it up.  The green LED is marked 'run', and with the instrument powered up and successfully booted, it blinks at about a 1 or 2 Hz rate.  The red LED is marked 'halt', and it is dark.  Sounds like the processor on yours is running, but something is repeatedly stopping it.  I haven't the foggiest where my manual is hiding, but will look at some point on Saturday.

-Pat

[slbender]

Hi Pat, I pulled the VSK231 part and it seems to measure OK on diode test 0.195 and .0.210 on my Fluke 79, anode to anode is infinite. The nearby R203 (22 ohm) measures about 23 ohm, but it does have a crack in the case and signs of overheating. I now suspect the +12 volt reg. could be bad. It measures between left and center pins 0.091 (also on diode test) while the -12 volt reg is more in the range of .485 and .700 (depends on direction) since I don't have any +12 volt regs to compare readings with, I have ordered both types, half a dozen of each.  I measured a half a dozen +15 volt reg. Of several brands and yes, they seem to be 0.537 +/- so vastly different.  I think the +12 volt reg is toasted, despite having the thermal switch on its heat sink....

Steven

[slbender]

My CPU / ROM board looks identical to yours, even the same jumpers jumped.

Steven

[Cubdriver]

The 12V regulator does sound suspicious, but when comparing it to the -12V one, bear in mind that IIRC the positive and negative ones have different pinouts.  That reading sounds like an internal short, though, so it may well be toast.

Is that an in circuit or out of circuit test, by the way?

-Pat

[slbender]

That part seemed a bit difficult to remove, so that is on the board, yes there is a big 2000 uf cap in the circuit, so that may be affecting the reading, I guess I should pull one lead of that cap and take another reading...  it was 4 AM and I was only half awake  !

Steven

[slbender]

The 12V regulator does sound suspicious, but when comparing it to the -12V one, bear in mind that IIRC the positive and negative ones have different pinouts.  That reading sounds like an internal short, though, so it may well be toast.

Is that an in circuit or out of circuit test, by the way?

-Pat

Boy, this set is giving me lots of grief!

Without that big electrolytic cap in the circuit, I read the same 0.091 volts in both directions, on "diode test". That is unlike the about .5xx to 1.2xx on the 15 volt 7815's, so now it looks like the 12 volt reg. is still a short.  So yes, that 0.091 reading was not caused by the big cap in the circuit.  I still don't know why R203 was cracked and burning the board or anything... but it seems I do need a new +12 volt reg. at this point I can't justify such a low reading by anything else in the circuit.

Steven

[slbender]

The 12V regulator does sound suspicious, but when comparing it to the -12V one, bear in mind that IIRC the positive and negative ones have different pinouts.  That reading sounds like an internal short, though, so it may well be toast.

Is that an in circuit or out of circuit test, by the way?

-Pat

Pat, Tonight I was looking through the remains of a circa 1995 Sencore Video tester of which I dismembered some months back, turns out among the twenty-five or so assorted regs and transistors on the main board, there was a single nice looking 7812. As a result of finding this part, I may not have to wait another two weeks or so for my 7812CV and 7912 TO-220 V regs, ordered on June 1st to show up from hongkong or china. Readings on the Fluke 79 on "diode test", for this part appear to be very close to those seen on the previous 7815's I tested, so unlike the original part seen inside  the 3561A, so maybe tonight I will get a chance to try and fix that +12 Volt Bias supply board, and see if that results in the 3561A default screen showing up, or some other error messages, hopefully ones that might be readable on the CRT screen this time.

Steven

[Cubdriver]

<fingers crossed>

-Pat

[slbender]

<fingers crossed>

-Pat

HELP  HELP  HELP!!!!   

Before inserting the 7812C (U100) onto the A71 Board, I checked it and also a 7805C on my Fluke meter "Diode Test" for a comparison.  Well, the one I pulled off the board was rather unlike either of the two fixed regs I had for comparison, and it didn't seem shorted either when out of the circuit.
 
Boy am I glad I didn't turn this thing on...  As you stated - the <+> and <-> regs have different pinouts.

There are also different pinouts for the variable type regs, compared to the fixed ones!  I guess I just assumed the U100 part was a 7812C...

First of all... This is why I HATE House Numbered Parts ... You need the Service Manual to know what anything actually is, and finding that in a .pdf file often takes hours. 

AS IT TURNS OUT, that the 7812CV I put on the A71 PS Board at part U100 should not be a 7812CV after all !!!

At this point I'm not sure what it should be 

I seemed to recall that the middle pin on a <+> reg goes directly to Ground. Yet the 3561A schematic in the SM shows a 1.07K resistor to ground on part U100.    There is also the same resistor present on the U001.    So I started saying... WHAT 

I double checked the 3561A Service Manuals Vols 1 and 2, and found conflicting information.
According to the Parts List, the SM says that part U100 should be: (1826-0527) is an LM337T.  And part U1 should be: (1826-0393) which is an LM317T!

HOWEVER! On my 3561A, present on the A71 Board, U100 is clearly marked (1826-0393) which is an LM317T, and part U1 is (1826-0527) which is an LM337T!

Now, did some dumber than me tech replace and switch these two parts at some point in the last 33 years, Or - Is the HP Service Manual WRONG? 

Again, now I have to pull that 7812 and insert the right part which - according to all that I hold truthful, probably should be: the LM317T (unless HP went out of their way to reverse these two parts, for no good reason). So Most Likely, the SM is WRONG and my A71 Board is RIGHT!

So Pat, its quite easy to see which part on A71 is U1, its not blocked by a big C102 as the U100 is on the heatsink.

So, can you please verify that on your A71 Board you have (1826-0527) for U1 (LM337T) in which case I need to find an LM317T, or put the original part back (but not sure I trust it, I would like to compare it with a few other 317T's first, in which case, if the readings don't match, I'll use a different 317T part).

Thanks,

-Steven

[Johnny10]

I have a couple of these units and use one for swapping boards.
Aren't they the most beautiful boards with all that gold plate?

I bought the electronic service manual from Q service

[Johnny10]

I had some garbled fast error messages on the screen.
I used my iphone in slo-mo video mode, recorded the screen as message went by and then looked at each pic individually to make out error message.

[CopperCone]

I hate repairs that seem to span the decades. I had a DSA made by some company in the late 80's and after 3 years of getting pissed off and quitting I threw it out.  There was so much fucking logic in that thing I don't think I would have gotten anywhere even if I had like 5 extension cards to bring it all out on one testable plane.

Total crapshoot without the schematic. And the analog front ends were so shit.

[Johnny10]

U101


Service Manual #2 Page 7-207
Section 7-30 Troubleshooting the Power Supply

If none of the LED's light. there is a problem in the main power transformer (A71T300) or the Rectifier circuits.
Disconnect A20 before troubleshooting power Supply

[slbender]

Thanks Johnny10 for the photo confirming the 337 part at U1.

Yes lovely Gold plated circuit traces.

I also replaced R203 with a wire wound part, 22.4 ohms 3W., since the original part was cracked all around the body and had started burning the circuit board.  I'm still unsure why R203 went south.  I put it's replacement up on 1/2 inch stilts that I made from the cut down plastic hollow part of a "Q Tip" that I used to clean the board after putting MX-4 thermal grease on both the CR-100 and on the U100 Vreg. that I put on its heat sink.

I found another few LM317T's compared the numbers on my Fluke 79, and even though the original had almost identical numbers as the other five, I installed a new part as U100.  Then I resoldered C102 back in, checked the board over and put it back in. Powering it up... Exactly the same as before. Frozen with all front panel leds lit, garbled error screen flashing by quickly.  Hey, at least I cleaned the fan, and it still works 

Guess next step will be pull all the boards and check items on the motherboard. (Tantalum caps, Rectifiers & etc.)

-Steven

[Johnny10]

Have you tried the Fault Isolation procedures in Service Manual 1 ?
Starting on 6-11

Not sure why you think problem is in power supply if the unit has screen activity.
Unit starts then fails some test that is shown onscreen lights all front panel LEDs and locks-up.

Manual says that is normal part of start-up test 0.

I would try and find error on screen before taking anything else apart.
The service manual is quite complete with over 100 tests available, test points, waveforms, all this really aids in troubleshooting this unit.

And it is a complicated unit.

[slbender]

Hey guys,

Johnny, I suspect that since the screen output isn't stable, therefore, there is still a gremlin or two to be hunted down and excised in the power supply.  Personally, I hope its Tantalum Caps, as opposed to anything else.

So this afternoon, I took another stab at the 3561A power supply.  This time I checked the nine 78xx/79xx regs all in a row on the side of the unit.  The 12V VA and 12 VB both come from a common point on the Slave PS Filter board, which is the Cathode of Zener diode CR40. Since the circuitry of 12 VA and 12VB are similar, I expected the readings in both circuits to be the same.  They did not seem to give similar readings.

Across CR200 (on 12VA) I was seeing 0.022 volt on diode test, while on the 12VB I get "normal" readings like 0.586.  I guess I've been fooled before, but I think there is a problem here.  I pulled out C200, C201, CR200, and U200 (the 7812 on 12 VA). Still getting 0.022 across the CR200 holes or 29.3 ohms when the Fluke is set to Resistance range. Next, I pulled the "Slave PS Filter" Cap/Coil/Zener board which contains all those big blue caps, the coils and the Pre-regulator Zeners including: CR40, the 17.4 volt Zener, which feeds both 12VA and 12VB.

Still no change.  CR40 does not appear to be shorted.  With the "Slave PS Filter" board gone, I think we can also rule out the T300 transformer secondaries, and the power rectifiers as being effectively out of the circuit.

Having pulled C200, C201, CR200, and U200 out of the circuit and the Slave PS Filter Cap/Coil/Zener board (meaning L3, C32, C25, and Zener CR40 are out of the circuit) across the CR200 holes, I still get the same 0.022 diode test reading in either direction, or 29.3 ohms on R; using the C201 holes, I get 0.198 on diode test.  Frankly, I do not see anything else that could possibly be giving those abnormally low readings other than - there is a short in the motherboard ?

Last thing today, I'm going to pull all of the plug-in boards, maybe the short is actually on one of those.  If pulling out any other board changes the motherboard reading, or returns it to an "open", or a vastly different "normal" type reading, then maybe the short is actually a bad tantalum or something else on that plug-in board.  I sure hope so, if this short is on one of the "other" boards, then that will make my life a lot easier   

Any suggestions as to what to look at next guys?  Comments always welcome.

-Steven

[slbender]

Well lucky me, I didn't think any board would try to pull a lot (12 divided by 29.3) from that 1 Amp 7812 regulator.  The video display board, the last one I pulled, caused that motherboard short to disappear.  OK. 

Now maybe those power transistors (2N3054) are shorted, it could be that one had already been replaced. Seems odd that both are the same part number, can someone verify that they both should be?  I would have expected a complementary pair, but finding their ratings in the SM has eluded me so far.  Again, I'll have to pull them to test, I have some modern ones with much better ratings, higher current and higher voltage, which may be "better than" subs, I suspect any TO-66 parts are going to be difficult and expensive , and not well matched these days (if they are bad in some way - leaky, low gain, shorted, etc.).

Also, those weird high voltage caps with the cuts in top seem to give odd and inconsistent readings, at least in circuit.  Have to pull them too and test.  More when I know more.

So at least its not the Motherboard.... 

-Steven