HP8662A: power supply fault - FIXED - Page 1
Forum section: electronics / repair

author: Bill Woodbridge
author_rank: Contributor
timestamp: July 16, 2025, 04:50:42 pm
content: I’d be very grateful please for any clues to this frustrating fault on the rather notorious 8662A switching power supply.Briefly, the headline symptom is a power on-off cycling at about 0.5Hz as soon as the unit is switched on (ie switched out of ‘standby’). Everything tries to get going, then collapses again, then tries to come back up again etc. Accompanied by a hot smell from the region of the power cards.I’ve traced it, I think, to a problem with the two negative unregulated rails from the A7A3 inverter board, labelled in the diagram as -13V and -45V. With the regulator board removed, these rails are roughly double what they should be (-33V, -108V). The +23V unregulated rail however is OK (read 26V). (These three rails are loaded, albeit lightly, by resistors on the power supply motherboard so I don’t think lack of load is the problem – I read about 2-diode rectifiers possibly turning into voltage doublers due to stray capacitance under no-load conditions.) These doubled voltages would easily account for the high input detectors on the regulator board to trigger and shut things down (and the hot smell?), thus starting the power cycling. The rest of the inverter board and its A7A2 control board combination seemed superficially to be operating OK – the main 5.2V rail (which is the only one that the switching logic regulates) is rock-solid and its green LED is lit.After that I temporarily switched to looking for bad components, particularly diodes and capacitors, but I couldn’t find anything amiss on the inverter board. The control board, when run without the inverter board present (the control board runs from an auxiliary supply) gives perfect switching pulses for the +/-160V control lines, so I’ve OKd that board (I’m sure the PWM logic is correct too, since the 5.2V is solid).I went back to the inverter board and looked at the outputs of the 2-diode rectifiers (before the LC filters) on the secondary windings of T3. The 23V rectified signal looked just like the manual picture (56V pk-pk pulse train, around 50% duty cycle, 38kHz switching frequency). The -45V rectified signal however consisted of a DC level of -108V and huge 136V pk-pk ringing on top of that, starting at each falling edge of the 23V signal then becoming damped.My final measurement was at the primary of T3. The positive-going pulses looked OK, but the negative was sick – reduced voltage compared to the primary, and huge ringing again.I’m a bit stuck as transformer-coupled thingies, especially when floating, are out of my comfort zone:1 The primary side of T3 and its associated switch drive circuitry is only ‘indirectly’ coupled to earth via 18kohm resistors and 15uF capacitors. So measurement-wise it’s almost as bad as floating, since there’s massive 240V pickup everywhere so the only measurements I can make are differential. This was fairly obvious, and successful, when going across the primary of T3, but the manual shows other waveforms to look for in the switch drive circuitry and it doesn’t explain how to reference differential measurements there.2 I don’t understand if the bad waveform I see on the primary is some sort of reflection back from a fault in the secondary circuits, or whether it’s a symptom of a fault in the primary drive circuits.3 The only real clue I’ve got is that the two negative rails are bad, and the one positive rail (or two if you include the +5.2V) is good – this gives me a vague feeling that there’s a problem with the negative-going (-160V) primary drive which messes up the negative rectifiers, but the positive rectifiers (+25V and 5.2V) don’t mind.4 It’s damn awkward making all these measurements without extender boards, and also obeying the strict rule of instrumenting everything before power-up and staying well away from poking about with the power on!Further details, pictures of waveforms etc available on request! Many thanks if anyone can help.

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 16, 2025, 08:24:20 pm
content: Hi!Termal imager can help you to localize the heating zone.If "everything tries to get going, then collapses again" then it can be overcurrent protection or luck of input energy.I had similar problem with degraded PFC bulky capacitor.Do you have any circuit diagrams or high quality SMPS photos?

author: KE5FX
author_rank: Super Contributor
timestamp: July 16, 2025, 08:43:50 pm
content: You've checked ESR on all caps, including the large ones on the motherboard? If one of those is open, the power supply will try to run, but the crest factor will be sufficiently screwy that the protection circuit will trigger.The capacitors in the voltage divider driving the power transistors on that board need to be be changed pre-emptively. Very likely that one or both has gone high-ESR.And yes, even a cheap cellphone IR camera attachment is a godsend when dealing with these things.

author: Bill Woodbridge
author_rank: Contributor
timestamp: July 16, 2025, 10:36:13 pm
content: Many thanks to you and Harry_22 for the suggestions.ESR: not yet, but an ESR meter is now on order! Re. pre-emptive change anyway, do you mean the voltage divider on the inverter board (A7A3 C1 & C2, 15uF) or the one on the power motherboard (A7A4 C2 & C3, 600uF)? I'll certainly do the former in any case, but the latter are more difficult and expensive to tackle so I'd prefer to prove there's bad ESR before embarking on those. Perhaps I've spent too much time trying to figure out waveforms and not enough thinking about likely component failures!I don't have a thermal imager either and without extender boards it could be a bit awkward to use. I could switch off, quickly whip the board out onto the bench and look for residual heat (but I don't fancy finding residual voltage too!). I'll try the ESR route first.I have the full service manual so there are copious full schematics of everything. I've attached a clip of the power motherboard stuff followed by the inverter board switch drivers, which includes the two voltage dividers mentioned above. The main transformer (T3) primary is just off the picture to the right.Thanks again!

author: KE5FX
author_rank: Super Contributor
timestamp: July 17, 2025, 04:44:37 am
content: Yep, C1 and C2 on A7A3, 15 uF.The ones on the motherboard can be left alone unless they are actually defective. I don't replace those reflexively, just the 15 uF parts.

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 17, 2025, 09:10:14 am
content: You will find some info here:https://whatever.sdfa3.org/hp8662a-synthetized-signal-generator-part-1.htmlDon't forget to check RT1 thermistor.

author: Bill Woodbridge
author_rank: Contributor
timestamp: July 20, 2025, 06:45:31 pm
content: I think I’ve finally located the fault.I replaced A7A3 C1 & C2 with new capacitors (admittedly only 10uF as that’s what I had, and as another repairer noted the board actually had 22uF fitted rather than the 15uF shown in the schematic) which made no difference. The capacitance of the old removed components was pretty much spot-on, and the ESR for both (out of circuit of course) was around 1.5 ohms – about the same as the replacements. I’ll order 22uF to finish the job properly but clearly the fault lay elsewhere.I tested all the passives and the diodes on the inverter board (admittedly most of them in-circuit) and couldn’t find anything amiss, including ESR. The big motherboard electrolytics A7A4 C2 & C3 both tested OK out of circuit with about 2 ohms ESR. The RT1 thermistor was also OK.Finally I extracted the four transistors on the inverter board for testing. Q1 and Q2 in the switch driver circuits looked identical, and OK. Q3 and Q4, the actual switching transistors, finally revealed an anomaly – one read hFE of 5, the other of 70. Stupidly, I muddled up which was which and I don’t understand the driver circuit anyway, but I hope such a gross difference accounts for the asymmetric behaviour I’m seeing at the primary of T3 and I won’t need to do any further head scratching – I’m assuming perhaps the faulty one isn’t turning off/on hard enough, or maybe fast enough.There’s quite a bit of talk online about replacement equivalents for Q3 and Q4, but mine have a Motorola part number MJ13091 (they might be replacements of the originals themselves of course) which seems to be still available, albeit not cheaply. The data sheet quotes minimum hFE of 8 which doesn’t sound too far away from my value, but I’ve only got a toy transistor tester and hopefully a proper curve tracer would confirm that it’s actually too sick to function properly.I’ll replace them as a pair of course, and I’ll add a quick (I hope) update for the record once that’s done. Thanks again for everybody’s help.

author: Bill Woodbridge
author_rank: Contributor
timestamp: August 09, 2025, 06:26:35 pm
content: Fixed! In short, fitting a new pair of MJ13091 power switching transistors Q3 & Q4 restored normal operation. I got them from the US seller dalbani-global on eBay.But two things are still puzzling me:1. Both new transistors read hFE of zero on my transistor tester. It’s only a toy one (picture) but is there some reason why such a tester won’t work for a power switching transistor? Is it measuring at some voltage/current region where there is no hFE? I almost sent them back to the seller straight away as I was reluctant to risk trying them out in the unit if they were dud in case of further consequential damage, but I’m glad I gritted my teeth and pressed on. A proper curve tracer has moved higher up the list of old test equipment to add to the electronics workbench!2. The near-doubling of the negative unregulated outputs from the inverter board (labelled in the schematics as -13V and -45V) was still there even with the new transistors, at least when the regulator itself wasn’t installed to load them. Once the regulator board is plugged in, they come down (or rather up) to their nominal values. I thought the inbuilt loads for these rails on the power motherboard would be sufficient to make the inverter board outputs behave properly even without the proper loading from the regulator board, but clearly not. Since I took the doubled unregulated outputs as a sign of failure (without bothering to reinsert the regulator board each time I tested, as I didn’t want to keep stressing the rest of the machine in case the power up/down cycling was still present), I’m now wondering if replacing the capacitors C1 & C2 had actually fixed the problem all along, and I didn’t need to change the transistors? On the other hand, and regardless of the hFE behaviour of the tester, the old transistors read very differently to each other (hFE of 5 & 70) which can’t be healthy for a pair that are supposed to behave as perfectly symmetrically switches once in circuit.I guess the moral of the story is to change well-known failure suspect components first before worrying too much about the theory. Thanks again for everyone’s help.

author: KE5FX
author_rank: Super Contributor
timestamp: August 09, 2025, 09:36:38 pm
content: Beta may read low at both very high and very low collector currents, and at low Vce. Never seen one read zero, but then these parts are only specified for hFE=8 to begin with.These little guys are great, if you ever find yourself wanting to upgrade from the Fnirisi. They will do basic curve tracing, albeit obviously at low voltages and currents compared to a real curve tracer.

author: microvia
author_rank: Regular Contributor
timestamp: August 10, 2025, 01:14:41 pm
content: Quote from: Bill Woodbridge on August 09, 2025, 06:26:35 pmI also own this tester but never use it for transistors. It once gave the same Hfe for a few transistors which in realty were close to each otherm but not as to register the same value on a good tester.If you want to make good tests, the Peaktech DCA75 Pro is a very good one. It can perform tests with variable voltages of up to 12V, and can trace curves through USB (not fast, but sufficient if you don't need that too often).From what you say, the bad transistor could to be the one with highest gain, probably due to an internal leak. The other with Hfe of 5 may be ok if the Hfe is within specs. But Measure it with a good tester.

author: Bill Woodbridge
author_rank: Contributor
timestamp: August 10, 2025, 01:56:19 pm
content: Thanks both for the suggestion. I was in a hurry to get something cheap in order to progress the faultfinding and I mistakenly assumed that all modern ‘one button’ testers were all about the same without reading up properly, so I just went for the cheapest. I suppose at least it told me that my original pair were behaving very differently, and therefore were probably the cause of the fault.Having now had a look at a Youtube demo, I’ll probably invest in a DCA75 Pro although (with no offence to the capabilities of what I’m sure is an excellent unit) I’m a bit reluctant to spend £200 on something that looks as if it came out of a Christmas cracker. A Tek 576/577 would be more in the spirit of my vintage test equipment vibe, but the whole world wants one of those it seems (and no doubt it would need repairing itself).

author: KE5FX
author_rank: Super Contributor
timestamp: August 10, 2025, 05:27:43 pm
content: I have a DCA75 and a nice 577. They aren't really competitors. While the DCA75 does feel a bit cheap-and-cheerful for the price, that impression goes away the second you turn it on. It is exactly what I would have built, if I were looking to build a pocket-sized semiconductor tester.Meanwhile, the 577's plastic knobs are starting to crack and fall apart, and it's cumbersome to use at best. It is more of an educational aid than a practical bench instrument. It would certainly be interesting to put the two transistors you removed on the 577, but when it comes to identifying, sorting, and quick-checking parts, the DCA75 is the undisputed champ.What would be great is if the DCA75 designer came back with a more full-featured benchtop instrument, designed with the same ethos in mind. I'd camp out in line for that.