HP3325A Troubleshooting - 03325-66502 Rev. C

[No.Mad]

Hi all,

Recently I obtained HP3325A with Option 001. Initial examination revaled blown fuse, unit was not powering up. When fuse was replaced and cord connected fuse was blowing up, even when not powered (power button on Standby). Further investigation showed (after opening enclosure) that voltage was set at 120VAC. Uh-oh.

Fuse replaced (good I bought bulk), voltage set to correct one (240VAC), button pushed - nothing. I took a peek at PCB - no massive damage visible (pic.1)

That was the time for service manual - I followed steps for power section debug: disconnected W22 from A2P5 and grounded pin 10. Voltage rails for +15V and -15V showed +15.043V and -15.091V. Nice.
+5V rail was sitting on -0.6V. OK, problem sits with 5V regulator section. I checked Q1, Q2, Q3 and Q10. They seemed fine, no short or open circuit.

Then I noticed open circuit on track (pic.2). Fixed it with solder wick. I found almost invisible discolouration on substrate where big polarised caps are located. Desolered showed another problem: open circuit right next to vias. It was fixed with copper foil (in progress) and Milimax gold plated pin sockets (pic.3)

I will continue to debug it and report it here for your entertainment and my sanity 

Here is the time for small cry for help:
1) Does anyone have schematic for Rev.C of 03325-66502? I spent las week searching for it in interwebs but no luck. I have Rev. F, but layout changes don't help xD
2) Could somebody help me identify Q1, Q2, Q3? Again, information is scarce, some posts indicate Q1 as TIP36. I would like to know beforehand part numbers so I could source spare parts.

[Lorenzo_1]

I have an old Rev E board that stopped working. Q1 is marked 3-453 A8101 Logo not recognized but looks to be map of Texas with indecipherable letter/s inside.  Q2 is 4-800 8810 - looks like Motorola logo. Q3 is K 713 3-450 also Motorola.  These are original parts from look of solder/heat shrink.  Can’t help with Rev C schematic. My manual only has Rev F.

[No.Mad]

Hi Lorenzo_1,

Thanks for reply. Indeed, Q2 and Q3 are Motorola brand, TO-220 package. That would indicate MJExxxx part number (M - Motorola, JE - power plastic case). I found MJE800, which could be Q2 one, but it's Darlington. That got me thinking, if it wouldn't be drawn on schematic as such, not as single transistor.

There is also MJ450, but that's totally different case: TO-3. The problem with these two is if naming was done in accordance to JIS standard or Motorola internal standard.

Q1 is Texas Instruments, so by their nomenclature (naming things for non-english crowd  ), part number should be TIPxxx (Texas Instruments Power).

This guessing game is giving me a headache. I will ask senior engineers at workplace today as they probably worked with these transistors back in a day.

EDIT: Added photo of transistors mentioned, from left to right: Q1, Q3, and Q2

EDIT2: Corrected misidentifed Q2 and Q3 with help from wn1fju and Lorenzo_1

[Lorenzo_1]

That's quite different from this set - photo attached. Perhaps this will be useful.

[Lorenzo_1]

I think I misunderstood your post. I thought you were just looking to identify what ICs were fitted but seems you want to work out what they are and where to source them. Apologies for that.  Attached what data I could find from part numbers in manual for Rev F board and cross-references to NSN numbers. Q1 and Q2 references look as though they might be reliable. Q3 data looks unreliable. Slim pickings.

[wn1fju]

A marking like 3-450 usually means it is HP part number 1853-0450.  I guess HP suppressed the leading 185.  Anyway, from the readily downloadable HP Cross Reference list, I get the following:

1853-0450 = MJE371K
1853-0453 = TIP36A

I found a listing for 1854-0800 on Keysight's part's page.  They say it was replaced by 1854-0456.  Again the HP Cross Reference List gives

1854-0456 = TIP41A

Hope this helps.

[No.Mad]

Ah, yes. Some HP3325A when sent for repair or calibration were "upgraded" to HP3325B state-like. Rev. C had sockets for pass transistors, where in later revisions they soldered all of them like Q1 for increased reliability. Also mine when opened had no sign of usage or dust at all. I suspect it was working, blew the fuse, then it was packed into storage. For years probably.

Therefore I think it never had chance to be updated.

[RaymondMack]

I had a similar problem with a 3325B where the +-15V supplies were fine but the 5V supply was shorted. Turned out the unit was dropped and that jared the mounting screw just enough to bridge the mounting tab to chassis since HP used a TO-220 insulator that was too short to completely isolate the tab. Replacing with a longer insulator fixed the problem, but I still replaced the 5V pass tansistor with a new TIP36C (same specs with higher breakdown voltage) since it had gotten extremely hot while shorted out.

https://www.eevblog.com/forum/repair/hp-3325b-repair-(shorted-5v-supply)/msg3606808/#msg3606808

Any luck with replacing the 5V pass transistor?

[No.Mad]

This week I had a guest but still got some work done. Out of spite I just redraw schematic as it wasn't in best resolution. Also I did some reverse engineering just in case if whole assembly is busted. It will take some more work, but if somebody is in need of Eagle files or gerbers I can include them in next post.

I had to wait for epoxy glue to dry, but at last PCB is fixed. This week I will do some testing and report back. In attachment PDF siwth schematic (without varistor, where the hell is this guy?), .txt file with part lists (I chose some current replecment due to obsolence) and layout draft.

BTW, did anybody noticed how on Fig. 8-49 there are two resitors with R15 designators? Next to C9 (big cap). Is it an error in drawing or print?

Also, any feedback, pointers, comments or corrections to attachments are welcomed 

I had a similar problem with a 3325B where the +-15V supplies were fine but the 5V supply was shorted. Turned out the unit was dropped and that jared the mounting screw just enough to bridge the mounting tab to chassis since HP used a TO-220 insulator that was too short to completely isolate the tab. Replacing with a longer insulator fixed the problem, but I still replaced the 5V pass tansistor with a new TIP36C (same specs with higher breakdown voltage) since it had gotten extremely hot while shorted out.

https://www.eevblog.com/forum/repair/hp-3325b-repair-(shorted-5v-supply)/msg3606808/#msg3606808

Any luck with replacing the 5V pass transistor?

Not yet. I checked out this topic before posting, got some good pointers 

EDIT: attached correct .txt file. Previous one was Work In Progress and is missing some entries. Parts listed there were searched parametrically on Mouser and are equivalent or better than ones from original BOM

[trobbins]

My unit had a poor C16 (I think) which caused noise to upset front panel switches.  The +5V power supply pass transistor and diodes were certainly the hottest parts.  I reset Q1 with thermal paste and slightly improved the thermal path from that main heatsinking bar.  I vacillated over ways to lower the CR1-4 diode temps - like paralleling each MR751 with a 31DQ10, and soldering on a small tab to each part to improve convection cooling - but in the end just left them alone.  The main fan was noisy and needed replacement, so that may also have been a benefit.

[No.Mad]

Ladies and gentlemen, we got him!

Fanspin present, voltage rails back to normal. In attached photos there are measurements before and after adjustment. Pardon poor quality, as it was taken with one hand, measuring with the other and at the same time trying not to get electrocuted.

To summirize what I have done: fixed trace supplying 9VAC, replaced C5 and C8 with 1000uF/50V capacitors (Nichicon, UVZ1H103MRD for curious), replaced C14 (Tantalum, 6.8uF/15V, PN: T322B685K15AT), replaced C7 (Tantalum, 4.7uF/20V, PN: T322B475K020AT), fixed lifted pads of C5 & C8. Changed isolation pads and washers of Q1, Q2 and Q3 as suggested by RaymondMack. Replaced CR10 (SCR). I plan on replacing pass transistors (Q1, Q2, Q3) and electrolytic caps at later opportunity as Mouser takes a bit of time to arrive.

When following troubleshooting described in service manual I measured all voltage rails going around unit. And lo and behold! Our journey is not yet done.

A6 03325-66506 have no +5VDC present. Bummer. P52 9VAC is present (reading 9.28 VAC). Regulator looked wierd (with black oxidation? cupric oxide from copper?) so I replaced it with two additional electrolytic caps. That side of PCB now has +5VDC rail.

Next I checked isolated side with custom HP hybrid uC. No +5VDC again. Resistance to ground is at approx. 1.3k ohms. I did voltage injection @5V /50mA from PSU, hooked up to pins P5.5 (GND) nad P5.7 (+5V). Instantly hit current limit and voltage dropped to 1.05V. Then I isolated rail by desoldering one lead of L1 and injected again. Voltage recovered so the issue is downstream. I already isolated C36, C37 and C7 (electrolytics and tantalum, biggest suspects)

My question to people fixing these: have you ever encountered similar fail mode? I have no visible damage so a hint where to look would be tremendous help.

EDIT: reduced size of photos

[trobbins]

It can certainly be onerous to track down over-current faults within a pcb that has a lot of distribution paths.  Divide and conquer by making pcb trace cuts at convenient spots is often resorted to unless you have some fancy dc current sensing probe or a thermal camera and a bit of time for the bad part to heat itself up. 

I had a few faulty parts to track down on my unit, but I doubt there is much consistency between faults.  My faults were mainly bad LM310's (U19 and U21).  The remaining issues for me are above spec sinewave harmonics, and twitchy front panel pushbutton operation.  The varactor controlled crystal is a little 'drifty' by a few Hz around whatever R30 trims it to, so if you want rock steady frequencies then use a GPSDO 10MHz ref input (https://www.eevblog.com/forum/projects/budget-gpsdo-a-work-in-progress/50/).

Unplugging/plugging any and all loom connectors is worthwhile, especially for loss of voltage on a particular pcb.

[Wallace Gasiewicz]

I repaired my 3325 years ago.
Just a couple suggestions.
I think you already did this but if there are any caps left on the questionable board, replace them. Sometimes the little ones near ICs fail under power and test OK on most meters.
Are there any other small ICs on the problem board? Maybe other volt reg? Remove them one by one and put them in sockets for easier diagnosis further on.
What is the custom IC??

[RaymondMack]

The 7805 from A6 likely has a sliver alloy plating. Older ICs often had exotic platings (gold, palladium and silver). So I wouldn't worry about surface oxidation too much. Unless there are dendritic growths, it's purely cosmetic.

I do have a spare A6 board that I believe works. If you need an unobtainum part off it let me know and we can work something out. I also have most of the other boards too, if needed.

My methodology is to unplug everything not needed and start from the power supply forward. Once the PSU has been established to be in working order I start plugging in boards one at a time while keeping an eye on the supply voltages. This lets you isolate faulty boards, but as you've seen, you still need to find the bad parts on them.

One way that has worked in the past is to have a working unit on hand to compare IC temperatures with, either using a thermal camera or just a finger. I had a Wavetek 90 that had a whole host of failures: the unobtainium PC288 gate array, the uC, the multiplexed ADC and many logic ICs were toast. (Basically anything connected to the digital 5V supply.) I used a working 288 to compare temps and isolated many bad parts that way. The ADC on the 90 was much hotter than on the 288, but it wasn't scorching hot. None the less, it ended up being faulty. I basically had to part out the 288 to get the 90 working again.

[No.Mad]

So, after long debugging session with A6  - HP-IB and Control board I changed all electrolytic caps and one tantalum toghether with sus regulator. Then I applied 5VDC from linear PSU. In idle state this assembly was drawing approx. 750mA @ 5VDC. Attached thermal images for interested.

After that I connected everything, following service procedures. Unit came to live, was displaying 1MHz (I think, didn't pay attention then) and after 10 min, nothing blew up or died. I noticed some missing lights on panel buttons, so I thought that would be next.

Before progressing further I changed Q1, Q2 and Q3 on power supply PCB as follows:

Q1 - TIP36CW (https://www.mouser.co.uk/ProductDetail/onsemi/TIP36AG?qs=xZq1yRCsb1fF63jvF9TIew%3D%3D)
Q2 - TIP41AG (https://www.mouser.co.uk/ProductDetail/onsemi/TIP41AG?qs=xZq1yRCsb1eIo%2FaUEqzl0w%3D%3D)
Q3 - TIP42AG - I thought why not, it's complementary to TIP41 (https://www.mouser.co.uk/ProductDetail/onsemi/TIP42AG?qs=xZq1yRCsb1egVgVfgh8sgQ%3D%3D)

Done some capacitor replacement: C5, C8, C7, C14, C16, C17, C18. Also, I added some wires with spade terminals, as soldering and de-soldering filter wires and et al, was fun till 4th time.

When I wanted to test everything, no load and so on, to see if it's all in order, well....

Let's say, that this week was disappointing. Now I have, like it seems, fully unregulated voltage on regulated rails.
Measurements in reference to ground, PCB mounted in unit, no load, P5-5 and P5-10 shorted (connector P5 from service manual):

====== Q1 (+5V) ======
Base (Pin 1) = 10.44V
Collector (Pin 2) = 7.75V
Emitter (Pin 3) = 11.04V 

====== Q2 (-15V) ======
Base (Pin 1) = -23.87V
Collector (Pin 2) = -23.22V
Emitter (Pin 3) = -25.20V

====== Q3 (+15V) ======
Base (Pin 1) = 22.15V
Collector (Pin 2) = 21.65V
Emitter (Pin 3) = 23.88V

I tried to adjust -15V reference with potentiometer. Nothing. I tried to return to original parts (which is why it looks awful on photo). Same values. Just. Awesome.   

As of now, I measured some sections of PCB, but there is no short, no obvious open circuit, Zeners are supplying desired voltages and impedance is correct in regards to schematic and components markings.

A marking like 3-450 usually means it is HP part number 1853-0450.  I guess HP suppressed the leading 185.  Anyway, from the readily downloadable HP Cross Reference list, I get the following:

1853-0450 = MJE371K

Now I think it's wierd. From schematics Q2 is a NPN BJT, but from MJE371 datasheet it seems it's PNP: https://www.mouser.co.uk/datasheet/2/308/1/MJE371_D-2316041.pdf -> EDIT: I was having a brain fog, mistaking Q2 and Q3 xD

EDIT: Grammar. I wrote it like I had a stroke.

[No.Mad]

What is the custom IC??

It's a white ceramic uC made by HP, I wrongly identified it as hybrid. By first initial power up, it looks like it's working as it booted up

[trobbins]

After that I connected everything, following service procedures. Unit came to live, was displaying 1MHz (I think, didn't pay attention then) and after 10 min, nothing blew up or died.

Are you indicating that the unit booted and passed cal and likely working ok at that point in time?  And then you swapped out power supply bjt's and had problems?

[No.Mad]

After that I connected everything, following service procedures. Unit came to live, was displaying 1MHz (I think, didn't pay attention then) and after 10 min, nothing blew up or died.

Are you indicating that the unit booted and passed cal and likely working ok at that point in time?  And then you swapped out power supply bjt's and had problems?

Yes, exluding cal test as I wasn't looking (tiny workbench) so I can't say for sure. It is not regulating after I changed to pass transistor mentioned and situation is the same after soldering back old, original parts. I checked all of them with transistor tester - all good. In previous reply I posted measured voltages before re-caping and pass transistors replacement - it worked fine.

At the beginning I thought it was due to a bit different characteristics of new Q1 .. Q3 transistors. Tried adjusting, voltage doesn't change. My suspicion is that there si something wrong with -15V regulation - maybe it's not turning on properly, making pass transistor to go full saturation. Also, whole regulator is referenced to -15V, so if it's wrong, all voltages are wrong. I plan on testing Q8, Q11, Q4, Q5 and U4 now. On top of that I will recheck all measurments once again, making sure it's referenced to PSU ground (I did reference it properly, but who knows, maybe oxide layer made poor contact or my multimeter is wrong or test lead are bad).

[No.Mad]

So... the battle continues.

After another week of troubleshooting in my free time, countless probing and circuit analysis I found a problem. It was a f***ing S1. Here in UK we have 230V/50Hz. I previously set S1 and S2 in BC configuration (240VAC). When transporting PCB to work to use microscope, so I can fix the track, this little bugger had to tilt into A position setting itself to 220VAC. My suspicion (somebody correct me if I'm wrong) is that 220VAC fed more current to circuit putting Q1 .. Q3 in saturation, therefore constantly fully open. Lesson learned the hard way.

PSU PCB fixed and modified (photo attached), back to business.

So back to where I was, P5 disconnected, all good, rails are within specs, instrument boots up. Self test passed, LED for square wave not lighing up - I can fix that later. There is tiny problem though - no output. I thought it was wierd but I disconnected jumper between A6 STBY and A6 ground. Now I get scrambled hierpglyphs on display - see photos attached. Sometimes I got message OSC FAIL. Seems to be on random basis.

On top of that, when instrument is not powered but in standby mode I have few display LEDs constantly on. Seems like problem is still downstream.

To summirize:

• A6-STBY shorted to A6-GND: A2 and A6 voltage rails within specs, display works and is responsive, no output
• Power button STBY (OFF) position (default, no ingeretion, as designed): Some dispaly LEDs are on (randomly), +15VDC rail sits at +2.435VDC, other voltages are fine
• Power button ON position (default, no ingeretion, as designed): Voltage rails are all good, display shows random hieroglyphs or "OSC FAIL" message, buttons are unresponsive

I narrowed +15V STBY behaviour to A14 Function assembly, by disconnecting A6-J2 and isolating it. When disconnected, +15VDC on STBY was sitting at -0.79V.

I'm moving to troubleshooting A6 (Control, Service group C) and A14 (Function assembly, Service groups I J K L N). Any hints are welcomed.
Additionaly I would appreciate if somebody in possesion of working unit could measure voltage rails and resistances of rails in regards to ground on A2, A6 and A14 on STBY and ON. This way there would be direct comparision not only for me but anybody in the future.

I will be back with another update so stay tuned! 

[trobbins]

All I can say is to battle on with using the troubleshooting sections, even though I found them quite onerous to navigate through to determine which sections were the root causes for my issues, as there seem to be control loops that span across a few sections - print-outs of schematics may help as hopping between PC screens was a pain.  Apart from confirming supply voltages are correct and stable and have no hum throughout the many sections, the control actions I ended up focussing on were the two LM310N (U20, U24) that are used in Service Group I for sample and hold latches (Fig 8-42 schematic 8-I-6), but I think you have a few other demons to contend with.

[Lorenzo_1]

Here's some voltages and resistances on rails - is this what you were after? Reference grounds on A2, A6 & A14 respectively.

[SilverSolder]

[...]  I noticed some missing lights on panel buttons [...]

Mine had a few bad solder joints for the panel button LEDs themselves - wasted a lot of time looking for more "advanced" problems! 

[Lorenzo_1]

Was bothered by negative resistances in previous readings - not encountered this before so recorded as found,  but bit of reading suggested likely caused by residual voltages in circuit.  Remeasured with unit off for a few days allowing capacitors to discharge.  New resistance measurements in this table should be more meaningful.