HP 3561A Dynamic Signal Analyzer repair

[Cubdriver]

I snared this little beastie from the 'bay; it arrived today.  It was advertised as 'For parts or not working', with a note that it wouldn't power up and that the fuse was good.  For the price, I figured it was worth a shot:


As noted, it doesn't power up - plug it in, press the power switch and from all external appearances, a whole lot of nothing happens.  No lights, no cooling fan, no nothing.  As advertised, the fuse checked out as good.


Externally, it looks almost brand new - the front panel is in beautiful condition, and other than a missing HP emblem on the carrying handle and a bent fan filter retaining screw it looks great.  The missing emblem makes no difference to me, and the bent fan retainer was fixed in about 30 seconds with some gentle persuasion from a pair of lineman's pliers.  This is as it came out of the box - no cleaning:





Removing the top cover (cake - 4 captive quarter-turn fasteners and off it came, with no loose hardware to lose) reveals the usual solid construction typical of HP instrumentation.  There is beryllium copper fingerstock along the sides and top to bond the cover to the chassis, and the boards are further shielded with sheet metal covers:



This was a surprise - I was expecting a long electrostatically deflected CRT like what you find in a scope; instead it has a short little magnetically deflected one.  It's so cute.  I wonder, if you feed it, will it get bigger?:



There's similar bonding fingerstock for the bottom cover which is unsurprisingly retained by the same quarter-turn fasteners as the top, and the underside of the mainboard is basically a giant plane layer for shielding.  It's protected by a sheet of Plexiglas, not readily evident at a glance in the photo:



Removing a metal cover exposed the power supply section:



Linear regulators on the main board.  I suspect that the blue electrolytics on the removable card immediately above them are the filters:



The initial troubleshooting procedure I found in the manual on Keysight's site began with checking a pair of 12V bias supplies, which were operational and within spec.



The following step was to check the indicator LEDs on another board in the PS section.  The lighted one shown indicates an overcurrent fault. (surprise, surprise!!)



That fault was then isolated to the main part of the instrument by pulling the board out of the edge connector to disconnect it from the array of linear regulators on the mainboard (it feeds them).  When power was reapplied, the overload LED was off.  This came as no great surprise.



Of course THAT troubleshooting sequence is in part II of the manual, which is not readily available online as a PDF that I've found, so further misadventures in troubleshooting will have to wait a bit until I get my hands on a hard copy of the two volume troubleshooting and repair manual.  Off to evilBay...

-Pat

[crispy_tofu]

Wow great score! 
Beautiful condition, hopefully it can get fixed! 

[Vgkid]

Nice pick up.

[Cubdriver]

Thanks, I hope so too!

-Pat

[VK5RC]

Love that attention to detail, keep the photos coming, is this thread better in the "Repair" listing?

[AF6LJ]

Good to see it was complete, nice construction as usual.
Keep us posted on what you find.

If it were me I would go on with troubleshooting for as far as I could get without the second half of the troubleshooting guide.

[SeanB]

Being HP, I would guess a shorted tantalum capacitor, probably on one of the 15V rails. Replace with a 35V part, and for good measure look on the board for all the relatives and do them as well on that rail and other highish voltage rails.

[Cubdriver]

My hope is that it will turn out to be a shorted cap on one of the rails, too, at which point I'll likely shotgun similar ones too to try to head off the next one crapping out and causing grief.

I expect to order a copy of the two volume service manual later today, but in the meantime will perhaps poke about further at it tonight.  Troubleshooting is complicated a bit by the fact that the instrument's schematics are contained in the volume of the repair guide that I don't have access to (and of course the scanned versions would likely be fuzzy and frustrating to read anyway).

In hindsight, yes, this probably should go in the 'repair' section.  I'll see if I can have it moved.

-Pat

[free_electron]

My 2 cents : one of the two TO-3 regulators is dead.  i;ve sene that happen before. They conk out and the circuitry behind gets to eat unregulated power so it overloads.

[Cubdriver]

Did a touch more troubleshooting tonight, and also ordered a copy of the service manual (both parts).  Whatever ails it seems to be in the main body of the instrument - I pulled all but the power supply cards and it still indicates the overload.  I made some resistance measurements in the vicinity of the row of linear regulators, and found one tantalum cap that seemed a bit suspicious (29 ohms where one other was in the neighborhood of 500-600 ohms, and the rest well north of that).  It's the one on the bottom right of this picture, next to the CR201 silkscreen.  Lifting one side of it was no help; it appears to be fine and even with it out of the circuit I still had my overload.  I then noticed that a big electrolytic on the backplane board smack in the middle of the power supply (it lives between the two power supply cards when they're plugged in) looks a bit lumpy near the bottom.  That'll be the next thing to investigate when I get more time.  It's barely visible at the top edge of this image, slightly to the left of center.



They put a seriously physically huge microprocessor chip in this thing.  Apparently they needed a lot of pins, so they just made a huge DIP to encase it.  From what I've learned online, it appears to be a custom labeled Motorola MC68000L; a 32 bit processor with a 16 bit bus.  It looks like a supercarrier sailing among a fleet of frigates.




My misadventures will continue when I get more time and/or when the manuals arrive.

-Pat

[PA2HK]

Nice machine, I also have one. The CPU is indeed a good old Motorola 68K, pretty standard for hardware of this vintage. What makes the HP3561A special is the optional usage of magnetic bubble memory that was offered as an option (option 001 - my unit has that option), with that memory option a 3561A can store waveforms. Bubble memory is a non volatile memory technology of the eighties that was going to be a big thing before the advant of flash memory technologies. Well, the 3561A is the only test equiment model HP used bubble memory in, guess it was not that great.

The fault in your unit is most likely a shorted tantalum, unfortunately there a quit a of these caps that can go bad

[Cubdriver]

After weeks of procrastination, laziness, lack of time due to business travel and generally having too much on my plate, I finally got around to digging further into this now that I have both volumes of the service manual.  The issue causing the power supplies to go into shutdown turned out to be a shorted diode (CR207) in the half wave rectifier feeding the +8V supply rail.  It's an HP 1901-0050 (specs according to the manual are diode, switching, 80V, 200mA, 2nS recovery, DO-35 package); according to what I found online it crosses to a 1N4150 which seems to have slightly different specs, but as it's a rectifier I can't imagine that it's super critical.  Since I don't have any 1N4150s kicking around, I installed a 1N4148 in its place for the time being to check things out.  The replacement rectifier brought it to life immediately.  I plan to get the proper replacement on order shortly, and will then wring it out and see if there are any other issues.

The failed diode and its location in the circuit:


A detail of the cleverly disguised short circuit:


The 'temporary' fix installed (CR208 will likely be changed, too, as it's gotten a bit toasty; its location is not shown in the schematic image, but it's the top right diode of the bridge, directly above CR204, and was likely loaded by CR207 as it was failing):


Amazingly, with the power supply once again doing its thing, the instrument comes to life:


And finally a detail of the display:


I'll dig into it further once I get the proper part installed, and post some more detailed images of the various boards and whatnot as I go through it.  It's an impressive little box, with a lot of stuff packed inside.  I don't recall if I mentioned this earlier, but based on the serial number it appears to be a late 1985 manufactured unit.

-Pat

[BFX]

Congratulation 
Nice unit and repair story

[crispy_tofu]

Fantastic, congrats! 

[ckm]

I have one of these as well - it's throwing an error on boot, but the error is only listed in Volume 2 of the manual, which isn't available online - I'm trying to get my hands on a paper copy, but it's pretty rare....

I should probably open it up & check the rails/status lights even with only Volume 1 of the manual.....  Haven't gotten to that point yet, too many other projects.

[Cubdriver]

What error is it throwing?  Perhaps I can look it up and try to at least point you in the right direction.

-Pat

[ckm]

What error is it throwing?  Perhaps I can look it up and try to at least point you in the right direction.

Thanks.  I brought it home today to try to troubleshoot over the weekend and now it's not powering up.   

I was looking at the same power supply board you found the burnt diode on and discovered that one of the two transformer wires (labeled "1") was touching R203.  The leg of R203 closest to the transformer looks like it got really hot (it's almost black) and the part of wire "1" that was touching R203 is chard - so is about 1mm sq of the case of R203 (not enough to affect the resistor AFAIK, it looks like only the paint). 

Not sure what either wire '1' or R203 do, but it doesn't seem normal that R203 would have gotten so hot.   Maybe an inductive heating of some sort?  R203 seems to be connected to a transform output, while wire '1' seems to be an input.  Without a schematic, it's hard to tell....  FWIW, the diodes where you had the heat discoloration are mint on my unit, no discoloration.

I found a copy of both sections of the service manual and have ordered them.   In the meantime, if you have any thoughts, love to hear....

Thx.

Chris.

[Cubdriver]

R203 and C300 are in series and form a snubber network across the 5V supply windings of T300.  It's a center-tapped full wave bridge setup (shades of an old vacuum tube circuit).  Based on the drawings, I'm assuming that the wire that was shorted to the resistor was connected to the board in the area to the right of R203 where there are three tied in - numbered 1, 2 & 3.  On the drawing, 1 & 3 are labeled 'red' and 2 is labeled 'yel'.  If this is the wire that was shorted against the resistor, then I'm not surprised it overheated - 1 & 3 are the ends of the 5V winding, and 2 is the center tap.  If 2 was shorted to the resistor, it would have been either a dead short to one end of the winding, or, at best, 22 ohms based on R203's value.

Separating them would be a good start.  The resistor should be 22 ohms; the cap you won't be able to measure without lifting one side as it will be in series with the resistor and the transformer secondary winding.

Hope this helps a bit.

-Pat

[Cubdriver]

The full wave rectifier for that 5V supply is CR100; that's the TO-3 packaged device on the heat sink above the transformer.  It's a common cathode rectifier; I assume the pins are the anodes and the case is the cathode, but don't presently have the energy to dig out and open the 3561A to check.  Perhaps tomorrow, if it's necessary.

-Pat

[ckm]

R203 and C300 are in series and form a snubber network across the 5V supply windings of T300.  It's a center-tapped full wave bridge setup (shades of an old vacuum tube circuit).  Based on the drawings, I'm assuming that the wire that was shorted to the resistor was connected to the board in the area to the right of R203 where there are three tied in - numbered 1, 2 & 3.  On the drawing, 1 & 3 are labeled 'red' and 2 is labeled 'yel'.  If this is the wire that was shorted against the resistor, then I'm not surprised it overheated - 1 & 3 are the ends of the 5V winding, and 2 is the center tap.  If 2 was shorted to the resistor, it would have been either a dead short to one end of the winding, or, at best, 22 ohms based on R203's value.

Separating them would be a good start.  The resistor should be 22 ohms; the cap you won't be able to measure without lifting one side as it will be in series with the resistor and the transformer secondary winding.

Hope this helps a bit.

That helps, thanks.  The wire that was shorted was not one of the red-yellow-red wires, it was a white one coming from just to the left of the diode you replaced - part of a pair of two white wires, one with a label on the wire marked '1'.  Goes to the center bottom of the transformer, I assumed it was the input, comes from CR206 & 207.

I haven't tried re-installing the board after separating the wires, that's next.

Edit - corrected the wire descriptions, this is what I get for working from memory

[ckm]

Just want to put a link to http://www.orionsword.net/Electronics/TestEquipment/HP3561A/HP3561A.html in the thread as it has some very helpful tips about repairing these.

[ckm]

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....

[ckm]

Forgot to mention, I re-installed the 5v board, same issue.  Strangely, measuring the 5v test point (which doesn't have post on my unit) shows way less than 5v, something like 0.6 ish, but very unstable.

[Cubdriver]

That helps, thanks.  The wire that was shorted was not one of the red-yellow-red wires, it was a white one coming from just to the left of the diode you replaced - part of a pair of two white wires, one with a label on the wire marked '1'.  Goes to the center bottom of the transformer, I assumed it was the input, comes from CR206 & 207.

I haven't tried re-installing the board after separating the wires, that's next.

Edit - corrected the wire descriptions, this is what I get for working from memory

The '1' is what confused me - the terminals the red and yellow wires go to are numbered 1-3.  I didn't realize it was on the wire itself.  I popped the unit open and took a look at the board, and now see more clearly. 

[Cubdriver]

Those two white-sleeved wires are taps on the transformer secondary winding that provides +/-12V to the grounded section of the supply.  The entire winding feeds a full wave bridge that, combined with the grounded center tap of the transformer, forms a pair of two diode full wave rectifiers providing unregulated +/- 14ish volts that is regulated down to +/- 12V on the motherboard.  The sleeved white wires are taps in the winding that are full wave rectified to provide about 12V unregulated that's then knocked down to +8V by the linear regulator (U203) on the motherboard.

C210 is a 3300uF, 25V electrolytic.

C300, R203 and CR100 are in the pink section at the top, as is C210.  C210 is also bypassed by a 22 ohm resistor (I'm not sure I trust the R7 designator shown, as I can't find an A99R7 in the replacement parts list; A99R300 is, however, listed as a 20 ohm 2W metal film, so there might be an error in the diagram...)

The sleeved white wire is in the grey rectangle below the pink one; the pair of them feed CR206 & 207.  On mine, the #1 wire goes to terminal 6 on the board, and feeds CR206's anode.

Hopefully this sheds some further light:



-Pat

[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

[slbender]

Well, so far Q200 and Q201 on the analog video board, did turn out to be a TO-66 single diffused 4 Amp / 60 Volt complementary pair.  They were fairly well matched in terms of hfe, at 99 for the NPN and 67 for the PNP part, at low currents.  Both of these older parts seemed to be working and measured OK when off the board.  I'll add some MX-4 compound on them when they get resoldered.  Maybe I should check the smaller transistors too, as something is making the video garbled.  The power resistors seem fine, no burn or overheat spots on the board.  I pulled a few of the 10uF. 'Lytic caps but they too seemed healthy, and above their rated values.

While the scan of the Service Manual had put the polarities of Q200 and Q201 in doubt, since I could not see clearly which way the arrows were pointing, at this point, they do not seem to be the problem.  The "better than" 8 Amp / 150 volt pair of TO-220 parts, while they would have fit nicely, even onto the original heatsinks, the replacements, it seems were not needed.  So its back to square one - again.

Any suggestions?

-Steven

[Cubdriver]

Unfortunately I don't currently have access to the analyzer or manual, but will try to offer further suggestions in a few days.

In the meantime, perhaps take a look at any caps on the power supply rails on that particular board?  I don't know if there are any dipped tantalums on there, but they are infamous for failing shorted (and sometimes imitating matches when they do; yours if present obviously haven't or you'd know it by now). 

-Pat

[slbender]

I've hit so many brick walls with this unit you'd think I'd give up....  Nope, gluten for punishment, I bought a second HP-3561a which has different problems, and it is also a later unit.  DUT #2 is about several thousands newer according to the serial number, and features the CMOS memory board (A42) Option 1 instead of the Bubble Memory board (A65) Option 1.

The video comes up nicely, but it comes up with a 1.2.02 error, and at the default screen you don't see the random noise spectrum, so it has either a noise source failure or an FFT failure of some sort.

Switching the 'known working' Analog Display board into the first set - again, no change, still garbled video.  SO, that rules out that board as the source of the video problem.  Once I get some sleep, I'll try switching some of the set #1 boards into DUT #2 to see where and when that random noise on the default screen returns - if it does.

I'll also try to post pictures of the A42 Circuit Board.

-Steven

[slbender]

Sunday morning- lacking in luck!  I switched in three boards into unit #2 and I got even more errors.... :-

Now with the 03561-66510 Front end, 03651-66515 (Analog to Digital??) board and 03561-66520 FFT boards switched - it seems functionality is even worse...  Now DMA error detected, Front End error, 1.1.00 and now the Start, Auto, Half and Over LEDs are flashing continuously.

OK so that did not help, I will again sleep on this, I suspect mismatched levels, but still no noise displayed on the default screen, guess I was hoping for too much, the boards probably are individually matched to each other, still I thought I'd try and give it a shot...

-Steven

[Floopy]

Reviving an old thread...

I acquired an HP 3561A. Upon power up (after checking caps) I was greeted with a screen shrunk on the vertical axis. After about 20 seconds or so the device shutoff and the "Primary over current" LED turned on. After some time I isolated the problem inside the (S] (T] (U] parts of the power supply (all located in A71). A quick check on the diodes I don't see any of them that are shorted. Although CR100 is not living up to it's expectations. It's a VSK231 and the datasheet says that it should be positive on the case and the two leads should be the cathodes.

Where can I get a replacement for it?
Also my main processor board is different. Most of the parts have a date stamp of 1987.

[Floopy]

More measurments

[Floopy]

FYI:
service manual website, it has both.
http://www.ko4bb.com/getsimple/index.php?id=manuals
Type 3561 in the search bar.

[gabor.dani]

Looks like every one of us have a blown CR100.

I just got my unit for 200$, it is in perfect -I mean absolutely 100% pristine- condition. Not even a scratch. I don't know where it was stored, who used it, but people really did not touch this unit in years.
It did not power on. CR100 was shorted. I replaced it with a 60A dual schottky diode, not it works. Best deal ever.

[unknownsoldier90]

Digging out an old thread again…

I have the same DSA, but mine powers up without errors, but the display isn’t working properly:
https://www.eevblog.com/forum/repair/hp-3561a-dsa-repair-crt-double-deviation-in-x-axis/

Btw: R203 is cracked, but has the correct value of 22 ohms 😬