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Old 2nd Aug 2023, 3:58 pm   #501
SiriusHardware
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Default Re: Commodore PET 4032/8050/3022

If you look at post #109 of Pudwink's thread linked to earlier, the recommendation he was given was to replace them with Metal-Film 1/4 watt types.

Don't just get two, there may be a reason that these have failed and they may fail again.... but I have seen cases where they were replaced, and that's all.
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Old 2nd Aug 2023, 4:02 pm   #502
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Default Re: Commodore PET 4032/8050/3022

Ah ok - I await advice then.

Colin.
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Old 2nd Aug 2023, 5:00 pm   #503
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Default Re: Commodore PET 4032/8050/3022

Just buy a handful of (eg, about five) 56R 1/4 watt Metal-Film resistors. If you have spares immediately to hand, then you almost certainly won't need them. If you only get two, that's asking for trouble. You might be tempted to go for higher wattage (1/2 Watt, 1 Watt, etc) on the grounds that they would surely be more robust but the idea is that these resistors are supposed to sacrifice themselves if need be, so don't overrate them.

I'm curious as to why these do, clearly, fail so often. In the case of a real electronic fault you would expect that only one of the resistors, whichever one is protecting the area which goes faulty, would burn out. I'm honestly wondering if the resistors just go bad if you do nothing more than leave them in a damp environment for decades, same goes for all of these inexplicable, unrelated chip deaths we seem to get from one end of the machine to the other, as though the chips have been attacked chemically (by the elements, especially damp) rather than electrically.

After all, why would there be faults on the machine AND the monitor otherwise, surely if you get a genuine run-time failure it's just going to be in one unit or the other. It's as though they have both been slowly killed by nothing more than sharing the same environment for decades.

Last edited by SiriusHardware; 2nd Aug 2023 at 5:10 pm.
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Old 2nd Aug 2023, 5:29 pm   #504
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Default Re: Commodore PET 4032/8050/3022

OK - ordered. Waiting for the postman again now for a few days.

Colin.
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Old 2nd Aug 2023, 5:41 pm   #505
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Default Re: Commodore PET 4032/8050/3022

Yep, at ease.

I'm glad the 6845 'gamble' seems have paid off, anyway.
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Old 2nd Aug 2023, 6:41 pm   #506
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Default Re: Commodore PET 4032/8050/3022

There do seem to have been some steps forwards today.

Colin
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Old 2nd Aug 2023, 11:31 pm   #507
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Default Re: Commodore PET 4032/8050/3022

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Originally Posted by ScottishColin View Post
I've tried typing ? CHR$(7) and PRINT CHR$(7) to no avail.

I've also tried plugging in datasette drives - on power up, a datasette plugged into port one, the motor starts and doesn't stop. On port 2, the motor doesn't start. So I've got some fun coming there too later on.

RUN/STOP with a BASIC 4 ROM tries to load the first program off a disk drive rather than the datasette so I haven't tried that. I guess I could pug in my 4040 and see what happens. Also, I could try the Tynemouth board set to BASIC 2 to see if anything happens with datasettes, but with the motor not stopping spinning, I doubt it.

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Originally Posted by ortek_service View Post
I did also have another thought about Colin's 4032 PET:
- If it is now producing a 'Chirp', so seems to be booting at least (albeit it with no display), is it possible to do a <Ctrl>+<G> etc. from the keyboard to get it to produce a 'Bell' etc. sound?

Or type a BASIC command (Not sure if <Shift> / <CBM Key> ? + <RUN/STOP> does this - that IIRC did on the C64) to turn-on an attached cassette deck Motor?

May also be able to blindly type some POKE's, to set certain PIA etc. bits / expansion port lines.

This would at least give some confidence that the problem is mainly confined to 'just the display circuitry', and the main computer circuitry is actually running OK.

- Of course this does rely on the Keyboard still working, which may well need disassembling and giving a good clean etc. after a long period of no use / less than ideal storage conditions.

Just as a matter of interest, were those attempts at the Keyboard done with the original 6545 CRTC IC fitted, or had you fitted the new UM6845 by then, (Which its good to hear now seems to be working somewhat better, as regards video H & V drives) ?

I just wondered if an issue with the original CRTC IC could stop the PET responding correctly in other ways (the different cassette motor enables does also seem odd, but I recall that has been seen on the first PET you were repairing, so may be a similar issue on this one).

It does seem that the computer is running if it generates the Chirp, and now generating H & V drives signals. But I do expect the keyboard itself to be much more likely to be an issue (I do recall having to take out lots of screws on a PET keyboard PCB, to clean all the (Gold?) contacts / the conductive rubber at the end of the key plungers which that particular used, as most weren't working)
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Old 3rd Aug 2023, 12:23 am   #508
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Default Re: Commodore PET 4032/8050/3022

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Originally Posted by SiriusHardware View Post
Just buy a handful of (eg, about five) 56R 1/4 watt Metal-Film resistors. If you have spares immediately to hand, then you almost certainly won't need them. If you only get two, that's asking for trouble. You might be tempted to go for higher wattage (1/2 Watt, 1 Watt, etc) on the grounds that they would surely be more robust but the idea is that these resistors are supposed to sacrifice themselves if need be, so don't overrate them.

I'm curious as to why these do, clearly, fail so often. In the case of a real electronic fault you would expect that only one of the resistors, whichever one is protecting the area which goes faulty, would burn out. I'm honestly wondering if the resistors just go bad if you do nothing more than leave them in a damp environment for decades, same goes for all of these inexplicable, unrelated chip deaths we seem to get from one end of the machine to the other, as though the chips have been attacked chemically (by the elements, especially damp) rather than electrically.

After all, why would there be faults on the machine AND the monitor otherwise, surely if you get a genuine run-time failure it's just going to be in one unit or the other. It's as though they have both been slowly killed by nothing more than sharing the same environment for decades.


Although higher resistance values carbon resistors are known for going rather higher with age / high voltage across these, I wouldn't expect this to occur with much lower 56ohm ones - especially metal film ones (which too can have problems with higher-resistance ones going open-circuit, but usually when they've got >> 100V across them, so could have their max voltage rating exceeded / manufacturers often put upto four in series, to reduce the voltage across each, to try to improve reliability).

So much-more likely to have burnt-out, due to a fault causing too much current flowing through them for a short while (where there may be a single burnout-mark on them, or may be nothing at all, if failed very-quickly) - or just these running hot for a very long time, if it was on for most of the day for several years (Where they may well end-up looking blackened all over)
Although having all the faults on the mainboard occur at the same time does seem less likely.

It is unusual that the part number for R752+R753 is marked on the schematic. And even more unusual that it seems RN 1/4(W) series ones are still being made after all this time - Although maybe by different manufacturers, as there seems to be at least three and not all spec's are the same (like voltage rating, although that shouldn't matter here as won't be much voltage drop across a 56R for it to dissipate < 0.25W).

https://www.megastar.com/Content/pdfs/RN.pdf
https://pccomponents.com/datasheets/SEI-RN.PDF
http://www1.futureelectronics.com/do....4K-1%25-R.pdf


I doubt Commodore originally used any special 'fusible' resistors (often grey-coloured body), as 'Safety' parts weren't common-place back then (And normally these would be marked on PCB, Schematic with an 'S' in a diamond box - plus the parts list description etc. highlighting these).

Although I note that some of the ones I found did have 'Flameproof' specs (Plus standing them off the PCB, would also help prevent the PCB catching fire - as well as getting blackened due to heat from these).
I thought Metal film resistors could run much-hotter than the +70degC limit for standard carbon film. But you do still have to de-rate the RN-series above +70C (whereas standard '1/4W size' MRS25 metal film resistors were rated to a much-higher 0.6W to start-with, upto +70C: https://www.vishay.com/docs/28724/mrs16m25.pdf )

So I presumed that Commodore were specifying Metal-Film ones here, rather than Carbon-Film, as they offered better high-temperature specifications. But (modern at least) carbon-film resistors seem to have the same max temperature / de-rating-graph, as metal-film:
https://www.farnell.com/datasheets/1716725.pdf (Also available in flame-proof types)
https://docs.rs-online.com/0c4b/A700000008919910.pdf


From what I've found in some of the following discussion threads on vcfed, it seems these resistors often burn-out on the 12" monitor version. And that the capacitors these feeds are suspected as breaking-down / becoming leaky? at high-voltage (not too easy to test with normal capacitance meters), so causing the resistors to overheat and fail.

Therefore it may have been an idea to have ordered some replacements capacitors as well (assuming Colin's found resistor supplier also did these) to save possibly needing to do another order for these as well / may be advisable to just change them as a precaution, if prone to eventually breaking down.


These all reference R752 (although some not too much, so need to search webpage) / some the capacitors etc. that may be also be faulty:

https://forum.vcfed.org/index.php?th...d-issue.74565/
https://forum.vcfed.org/index.php?th...-repair.79319/
https://forum.vcfed.org/index.php?th...g.73212/page-9
https://forum.vcfed.org/index.php?th...e.35405/page-5
https://forum.vcfed.org/index.php?th...1242852/page-3

Plus discussions on converting to 80-col:
https://forum.vcfed.org/index.php?th...2.25609/page-4

And some more-general fault-diagnosis info on the 4032:
https://forum.vcfed.org/index.php?th...o-video.80583/

Also, might also be some useful info on fixing PET's, here:
https://www.commodore.ca/manuals/Tes...t_computer.htm

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Old 3rd Aug 2023, 8:00 am   #509
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Default Re: Commodore PET 4032/8050/3022

Quote:
From what I've found in some of the following discussion threads on vcfed, it seems these resistors often burn-out on the 12" monitor version. And that the capacitors these feeds are suspected as breaking-down / becoming leaky?
Thanks for collating all of the info, Owen.

I did notice that in some of those past cases the cause of resistor failure has been deemed to be associated with ageing or failure of some of the capacitors. That's another reason why I suggested not just buying the two resistors needed, but a few spares as well.

It may well turn out to be the case that one or more of the capacitors on the downstream side of the resistors will need to be replaced.
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Old 3rd Aug 2023, 9:37 am   #510
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Default Re: Commodore PET 4032/8050/3022

While the monitor PCB is out, are there any helpful tests I can do on the capacitors?

Colin.

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Originally Posted by SiriusHardware View Post
Quote:
From what I've found in some of the following discussion threads on vcfed, it seems these resistors often burn-out on the 12" monitor version. And that the capacitors these feeds are suspected as breaking-down / becoming leaky?
Thanks for collating all of the info, Owen.

I did notice that in some of those past cases the cause of resistor failure has been deemed to be associated with ageing or failure of some of the capacitors. That's another reason why I suggested not just buying the two resistors needed, but a few spares as well.

It may well turn out to be the case that one or more of the capacitors on the downstream side of the resistors will need to be replaced.
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Old 3rd Aug 2023, 10:13 am   #511
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Default Re: Commodore PET 4032/8050/3022

You can check C753 and C752 to make sure they are not obviously short-circuit, also check D752 and ... what seems to be labelled D253.

Where the capacitors have failed in past cases the thinking seems to be that they usually break down under realistic operating conditions (high voltage etc) so even if they pass a simple resistance check with a meter (which only outputs a few volts on the ohms ranges) they could still be faulty.

If you don't find anything obviously wrong with those components my pragmatic approach would be to fit a pair of resistors and switch on, while keeping an eye on the resistors.
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Old 3rd Aug 2023, 11:10 am   #512
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Default Re: Commodore PET 4032/8050/3022

No shorts there - I couldn't find a D253 so I assumed D753.

By the way, having got the PCB out and cleaned, I can confirm it is 321448 by comparing it with the layout in this document.

http://www.zimmers.net/anonftp/pub/c...lay_321448.pdf

Colin.

Quote:
Originally Posted by SiriusHardware View Post
You can check C753 and C752 to make sure they are not obviously short-circuit, also check D752 and ... what seems to be labelled D253.

Where the capacitors have failed in past cases the thinking seems to be that they usually break down under realistic operating conditions (high voltage etc) so even if they pass a simple resistance check with a meter (which only outputs a few volts on the ohms ranges) they could still be faulty.

If you don't find anything obviously wrong with those components my pragmatic approach would be to fit a pair of resistors and switch on, while keeping an eye on the resistors.
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Old 3rd Aug 2023, 11:58 am   #513
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Default Re: Commodore PET 4032/8050/3022

Quote:
No shorts there - I couldn't find a D253 so I assumed D753.
I suspected that would be the case but there are some D2xx diodes elsewhere in the circuit so I gave it the benefit of the doubt. There are a surprising number of labelling errors in the circuit for this particular PET, I don't remember coming across any when we were looking at your 3016.

Useful to know. If / when the resistors arrive, put a couple in and turn it on - if they start steaming, that will be their little way of showing that there are further problems downstream.

I think it is worth just trying the resistors to begin with because in many cases (including Pudwink's, referred to earlier) that has been enough to get the monitor going again.

If it works you can, if you wish, 'service' the monitor in the near future by replacing any likely suspect capacitors but that has potential pitfalls as well - in high frequency switching circuits like this the electrolytic capacitors will usually be specific high temperature low-ESR types and not just any old common or garden electrolytics, so it is actually possible to make things worse by recapping for the sake of it. That is why I don't really want to go down that avenue unless it actually needs to be done.
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Old 3rd Aug 2023, 6:51 pm   #514
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Default Re: Commodore PET 4032/8050/3022

The schematics haven't been 100% have they. Perhaps this thread may be of use to someone else in the future.

With regard to swapping components out, I'd rather leave as much original in there unless it's actually broken. I'm slightly tempted once this is working to remove sockets and solder some chips back in - ROMs, 6520s - that kind of thing. However, there's something to be said about leaving them alone to show what got done - almost like archeology. I'll think on it.

Colin.
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Old 3rd Aug 2023, 7:20 pm   #515
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Default Re: Commodore PET 4032/8050/3022

Postulating on a possible reason for the unreliability of these resistors, it occurred to me while looking around for suitable replacements that the rated voltage of C753 is 350V, whereas nearly all of the 1/4 watt 56R metal-film resistors I have found when looking around so far are 250V rated. As Owen reasoned, with the combination of low current through and low resistance of R753 the normal voltage across the resistor won't be anything close to 250V.

However, at switch-on, C753 is completely discharged so it will look like a momentary short to GND until such time as it has charged up enough to reduce the voltage across R753.

For that brief moment at switch-on, R753 will bear the full weight of whatever voltage is at the #1 terminal of the Line Output Transformer, plus a much higher current.

If I was worried about that, I would replace both of the 56R resistors with 2x 27R resistors in series for a total of 54R, which would be near enough and would spread the voltage drop at start-up equally across the two resistors.

(Colin, I'm not suggesting you do this. I'm just going off on one).
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Old 3rd Aug 2023, 7:25 pm   #516
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Default Re: Commodore PET 4032/8050/3022

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I'm slightly tempted once this is working to remove sockets and solder some chips back in
You seem sufficiently skilled now to get away with that, so why not, but I would let it run for a while first, remember the old saying '...never let a PET know that you think you have fixed it'.

And... I guess we'd better get it working first.
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Old 3rd Aug 2023, 10:47 pm   #517
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Default Re: Commodore PET 4032/8050/3022

Here's a close up of R753. I think we can agree that the magic smoke got let out at sometime.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
Postulating on a possible reason for the unreliability of these resistors, it occurred to me while looking around for suitable replacements that the rated voltage of C753 is 350V, whereas nearly all of the 1/4 watt 56R metal-film resistors I have found when looking around so far are 250V rated. As Owen reasoned, with the combination of low current through and low resistance of R753 the normal voltage across the resistor won't be anything close to 250V.

However, at switch-on, C753 is completely discharged so it will look like a momentary short to GND until such time as it has charged up enough to reduce the voltage across R753.

For that brief moment at switch-on, R753 will bear the full weight of whatever voltage is at the #1 terminal of the Line Output Transformer, plus a much higher current.

If I was worried about that, I would replace both of the 56R resistors with 2x 27R resistors in series for a total of 54R, which would be near enough and would spread the voltage drop at start-up equally across the two resistors.

(Colin, I'm not suggesting you do this. I'm just going off on one).
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Old 4th Aug 2023, 12:16 am   #518
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Default Re: Commodore PET 4032/8050/3022

It might not be a bad idea to fit the first pair 'ugly' style, just standing up above the PCB on long leads. If you shape, dress, install and solder them meticulously so that you can't even tell they've been changed, they will probably last about half a second.
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Old 4th Aug 2023, 3:13 am   #519
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Default Re: Commodore PET 4032/8050/3022

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Quote:
I'm slightly tempted once this is working to remove sockets and solder some chips back in
You seem sufficiently skilled now to get away with that, so why not, but I would let it run for a while first, remember the old saying '...never let a PET know that you think you have fixed it'.

And... I guess we'd better get it working first.
It's upto you Colin, but personally I'd recommended leaving the sockets in rather than risk further PCB damage, removing those (especially the larger 24 & 40pin ones, that were often fitted originally). And it does make it rather easier to fault-find / repair in future , if (/ when with these!) there's a future problem. Plus also allows various expansion / conversion modules to be plugged into some of these (especially the ROM's).

Retaining as many of the original IC's as possible (even with the odd leg-repair), so the date-codes look period-correct, is probably more-important.
As those do tend to distinguish it from a more-recently built replica, which may be able to make the PCB very-similar (Although probably a lot more glossy finish, without using some matt-finish solder resist etc)

If I do have to replace IC's in rarer / more vintage equipment, I do try and use 'New Old Stock' devices, with similar date-codes. But that can be difficult, without having accumulated a good collection of most of the IC's used in these, over the years.

I do now also try to use some period / matching type to originals IC sockets, as well (especially after acquiring a large box of 100's of most-sizes from a rally a few years ago, with many 'Augat' etc. ones that BBC computers etc. had).
And I try to avoid turned-pin ones, even if they are often regarded as being more-reliable (although they can be handy if a top-side PCB track has broken, as easier to solder directly to top side pin-sockets - but I have recently done some 'surgery' with v.thin solid-core tinned copper wire formed to follow the track underneath the socket, whilst inserting it over that).
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Old 4th Aug 2023, 3:51 am   #520
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Default Re: Commodore PET 4032/8050/3022

Quote:
Originally Posted by SiriusHardware View Post
You can check C753 and C752 to make sure they are not obviously short-circuit, also check D752 and ... what seems to be labelled D253.

Where the capacitors have failed in past cases the thinking seems to be that they usually break down under realistic operating conditions (high voltage etc) so even if they pass a simple resistance check with a meter (which only outputs a few volts on the ohms ranges) they could still be faulty.

If you don't find anything obviously wrong with those components my pragmatic approach would be to fit a pair of resistors and switch on, while keeping an eye on the resistors.

Quote:
Originally Posted by SiriusHardware View Post
Quote:
No shorts there - I couldn't find a D253 so I assumed D753.
>>

Useful to know. If / when the resistors arrive, put a couple in and turn it on - if they start steaming, that will be their little way of showing that there are further problems downstream.

I think it is worth just trying the resistors to begin with because in many cases (including Pudwink's, referred to earlier) that has been enough to get the monitor going again.

If it works you can, if you wish, 'service' the monitor in the near future by replacing any likely suspect capacitors but that has potential pitfalls as well - in high frequency switching circuits like this the electrolytic capacitors will usually be specific high temperature low-ESR types and not just any old common or garden electrolytics, so it is actually possible to make things worse by recapping for the sake of it. That is why I don't really want to go down that avenue unless it actually needs to be done.

I think in this case, the monitor is really quite similar to old CRT TV's back then, with max 15.625kHz line-frequency. And I don't recall those ever using any particularly-special Low-ESR / High-Frequency etc. (that weren't very-common back then) electrolytics.
Although I would recommend always using at least +105degC types, rather than usually 'standard' back then +85degC types. And also try to use 'long-life' types, rated for closer to 10,000hrs etc. than the 'standard' only 1,000hrs life at that temperature. Plus also ensure they have a good ripple-current rating, if on main DC-output rails.

I do recall it being said that BBC Computer (Astec-made) SMPSU's did require special Low-ESR / High-Frequency electrolytics, and wouldn't work if you used standard ones. But from recently fixing a few of these, I managed to find datasheets on the originally fitted ones / measured the ESR on some still-working ones, and found that all the +105degC types rated for at least 2,000hrs at that temperature in Farnell etc. had at least as good max. ESR spec. (as well as a similar / higher ripple-current rating, for larger ones on main output rails).


'High-Frequency' types, usually just means 'low-ESR' (inc. upto 100kHz etc) / a good ripple-current rating (inc. after derating at higher-frequencies). But I measured any I removed and any replacements I used also measured less / similar ESR than those that still-worked in that position (I usually only replaced the existing ones, if their ESR had shot-up /their capacitance had reduced a lot).
I did use an HP/Agilent/Keysight LCR-meter, so could measure these all the way upto > 100kHz, but I have found those LCR+ESR+Transistor tester PCB-Modules, using an ATMega uC (originally designed by a German Radio Amateur) that you can get for around £10 from Far-East marketplace sellers, usually gives quite-similar readings / accurate-enough to find faulty-ones. And would recommend these to anyone doing repairs involving electrolytics, as sometimes you find that capacitance still reads OK on DMM Capacitance ranges, but ESR has shot up to sometimes > 1kohm.

I have also bought some quite cheap +105degC, Long-Life, Low-ESR - often 'High-Frequency described types' / 'Dark-Green' coloured taller ones that were common on PC Motherboards, when replacing many failed ones on of those, cheap from Far East sellers on the usual marketplaces. And these too have measured OK for low-ESR - at least when new - rather better than the failed ones (that often had their top bulged, so had rather self-overheated due to increased ESR)

Plus all the BBC SMPSU's have worked OK afterwards. (Although I doubt these run at 'really high' frequencies - probably < 100kHz - compared to some more-recent low-voltage DC-DC Converters that operate at upto 2MHz to be able to use smaller ceramic etc. capacitors)

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