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Old 4th Aug 2023, 4:29 am   #521
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Default Re: Commodore PET 4032/8050/3022

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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).
I did have another thought about what could be happening with the original high-voltage electrolytic. If it hasn't been powered for many years, then HV Electrolytics can lose their Aluminium-oxide dielectric / insulation. /they will normally 'self-reform', once they have a polarising voltage applied, but this needs to be current-limited while they reform, to prevent a high dc-current flowing across the broken-down insulation and destroying them / damaging the circuit. So I'm wondering if there was enough of a surge to take out one of the 56R resistors / this didn't limit the current enough to prevent this, the first time it was powered-up?
- I couldn't really tell from the photo, if that resistor had blackened over-time, or there had just been a fairly-instant failure due to a large overload.

Although the Electroytic isn't that large (0.47uF?), so not sure if these do suffer from the same reforming-issues as much-larger HT-reservoir ones for
Plus it looks like the other capacitor isn't an electrolytic, so wouldn't really explain that one. (Pity there wasn't room to fit a non-polarised 0.47uF 350V non-polarised type, in place of the electrolytic).

I did wonder if Colin had a 1/4W leaded resistor kit / any similar values, that could be used to make up around 56R as a temporary test?
- Especially as it seems that current Carbon-Film ones have the same temperature rating / de-rating above +70degC to same max. temperature as the original RN-1/4(W) Metal Film ones.
As could do a quick test with those, to see what happened.

I was quite surprised that this monitor seems to run off only around 18Vac, producing a regulated +18Vdc main supply. So the LOPT has to step-up quite a lot more than from the usual >+100Vdc HT supply.
But I imagine that the EHT supply on this 12" Mono CRT is rather-lower than on large colour CRT TV's.
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Old 4th Aug 2023, 8:02 am   #522
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Default Re: Commodore PET 4032/8050/3022

To the above I can only relate a short story - in the 1990s I was a field engineer working on, among other things, town centre CCTV where there was a particularly popular make and model of video multiplexer in use. These would fail from time to time and would have to be swapped out and sent back to the manufacturer, who would sometimes take quite a while to turn them around.

There came a day when we had more failures than we had replacements for, so we decided to look at one of the failed units and decided that many of the electrolytics appeared to have failed so we went to Maplin, then just up the road, and bought a full set of replacement electrolytics. To our relief, replacement of all the electrolytics brought the unit to life, so we took it back and reinstalled it, where it worked well...

..for about a month. When we got it back again we discovered that many of the newly replaced electrolytics were bulging and weeping. That was the first time I became aware that not all electrolytics are equal, and that it sometimes isn't enough just to replace them with parts with the same value and the same or greater voltage rating.

Incidentally the line frequency on these is not 15KHz, but 20KHz as shown by the frequency of the drive signal in Colin's recent measurements from the new CRTC.

Last edited by SiriusHardware; 4th Aug 2023 at 8:08 am.
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Old 4th Aug 2023, 9:20 am   #523
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Default Re: Commodore PET 4032/8050/3022

I do have a box of 1/4w resistors but none that are near half the 56Ω value to spread the load - there's 22Ω, 33Ω and lots of other values above and below that (the box is described as "Metal Film Fixed Resistor Kit 1R-4.7MR Ω ohm 60 Values 1/4w(0.25 w) ±1% Tolerance 0.01 MF ").

What I'm now slightly concerned about is that I've just gone to market and bought some 1/4W 56Ω resistors when there's clearly more to it than that - as you post below and Sirius' post 522, there's electrical components, and then there's electrical components.

The ones i have bought are simply described as "Resistor Metal Film 1/4W 0.25W 1%".

Did I ought to start to hunt down better quality resistors for this problem?

Colin.

Quote:
Originally Posted by ortek_service View Post
I did have another thought about what could be happening with the original high-voltage electrolytic. If it hasn't been powered for many years, then HV Electrolytics can lose their Aluminium-oxide dielectric / insulation. /they will normally 'self-reform', once they have a polarising voltage applied, but this needs to be current-limited while they reform, to prevent a high dc-current flowing across the broken-down insulation and destroying them / damaging the circuit. So I'm wondering if there was enough of a surge to take out one of the 56R resistors / this didn't limit the current enough to prevent this, the first time it was powered-up?
- I couldn't really tell from the photo, if that resistor had blackened over-time, or there had just been a fairly-instant failure due to a large overload.

Although the Electroytic isn't that large (0.47uF?), so not sure if these do suffer from the same reforming-issues as much-larger HT-reservoir ones for
Plus it looks like the other capacitor isn't an electrolytic, so wouldn't really explain that one. (Pity there wasn't room to fit a non-polarised 0.47uF 350V non-polarised type, in place of the electrolytic).

I did wonder if Colin had a 1/4W leaded resistor kit / any similar values, that could be used to make up around 56R as a temporary test?
- Especially as it seems that current Carbon-Film ones have the same temperature rating / de-rating above +70degC to same max. temperature as the original RN-1/4(W) Metal Film ones.
As could do a quick test with those, to see what happened.

I was quite surprised that this monitor seems to run off only around 18Vac, producing a regulated +18Vdc main supply. So the LOPT has to step-up quite a lot more than from the usual >+100Vdc HT supply.
But I imagine that the EHT supply on this 12" Mono CRT is rather-lower than on large colour CRT TV's.
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Old 4th Aug 2023, 9:36 am   #524
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Default Re: Commodore PET 4032/8050/3022

No, they will be fine. I browsed through those threads Owen hunted out and there seems to be a consensus that Metal-Film resistors are the right ones to use.

Since you have them you could use a 22R + 33R (=55R) in series to replace each resistor for a 'see what happens' run, but if you did then each 1/4W resistor would only be handling a percentage of the wattage, so your DIY 55/56R resistor would be slightly overrated wattage wise.

Or, do you have some actual 56R resistors in that resistor kit box? As long as they are rated at no more than 1/4 Watt, the worst that can happen is that they burn out again, indicating a problem further downstream.
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Old 4th Aug 2023, 9:37 am   #525
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Default Re: Commodore PET 4032/8050/3022

No - the lower values are 1, 1.5, 2.2, 3.3, 4.7, 5.1, 10, 20, 22, 33, 47 and 51.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
No, they will be fine. I browsed through
those threads Owen hunted out and there seems to be a consensus that Metal-Film resistors are the right ones to use.

Since you have them you could use a 22R + 33R (=55R) in series to replace each resistor for a 'see what happens' run, but if you did then each 1/4W resistor would only be handling a percentage of the wattage, so your DIY 55/56R resistor would be slightly overrated wattage wise.

Or, do you have some actual 56R resistors in that resistor kit box?
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Old 4th Aug 2023, 9:54 am   #526
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Default Re: Commodore PET 4032/8050/3022

Well then, if you want, try 33R and 22R in series as replacements for the two single 56R resistors. (Remove the original resistor, twist one end of a 33R and 22R together and solder the twisted connection, then drop the leads at the other ends of the resistors into the holes originally occupied by a single 56R resistor and solder them in place).

The end result should be two vertically mounted resistors, leaning towards each other and soldered together at the upper end. Same for the other resistor.
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Old 4th Aug 2023, 11:51 am   #527
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Default Re: Commodore PET 4032/8050/3022

In other words, replace the original resistor and standoff (left) with two series resistors as shown (right).
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Old 4th Aug 2023, 7:08 pm   #528
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Default Re: Commodore PET 4032/8050/3022

One step forwards, one step back. The step forwards is attached (Tynemouth board installed, ROM & RAM configured from the Tynemouth board).

However, the magic smoke was being released from R753 so I powered off quickly.

What might make R753 fail?

Colin.
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Old 4th Aug 2023, 9:32 pm   #529
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Default Re: Commodore PET 4032/8050/3022

First light!

Good that you got something on the screen, anyway. We are going to have to fix the monitor properly first, obviously. Why is this machine fighting us every step of the way?

Chief suspect would be C753 leaking or breaking down under realistic operating conditions. I think that is going to have to be replaced, despite my initial reluctance.
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Old 4th Aug 2023, 9:38 pm   #530
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Default Re: Commodore PET 4032/8050/3022

According to the diagram that one is 0.47uF, 350V rated. Can you take several photos of it?
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Old 4th Aug 2023, 9:54 pm   #531
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Default Re: Commodore PET 4032/8050/3022

It wouldn't be any fun if I bought a PET and it worked first time.

Photos attached.

Colin.

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According to the diagram that one is 0.47uF, 350V rated. Can you take several photos of it?
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Old 4th Aug 2023, 10:08 pm   #532
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Default Re: Commodore PET 4032/8050/3022

Looks a tight fit in that position, can you measure the pin pitch (distance between the pins), approximate height and the diameter across the top, all in mm?
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Old 4th Aug 2023, 10:17 pm   #533
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Default Re: Commodore PET 4032/8050/3022

5mm pitch, c. 16mm height, dia 10mm.

It's marked as 315v 0.47uF. Marked as CE 04W 85 degrees C.

Colin.


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Looks a tight fit in that position, can you measure the pin pitch (distance between the pins), approximate height and the diameter across the top, all in mm?
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Old 4th Aug 2023, 10:30 pm   #534
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Default Re: Commodore PET 4032/8050/3022

Really surprised it is an 85 degree part in that application.

Radial electrolytics of that low value and high voltage seem remarkably scarce. Does anyone know of a source (ideally in the UK?). Obviously needs to be at least 350V rated but I think 400V might be a more commonly available voltage to look for.

I'm wondering if, nowadays, people replace them with non-electrolytic capacitors. I'll look through some threads to see if anything is mentioned about that.

I looked at Cricklewood as they seem to have an uncanny tendency to keep PET parts but drew a blank this time.
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Old 5th Aug 2023, 9:24 am   #535
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Default Re: Commodore PET 4032/8050/3022

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I do have a box of 1/4w resistors but none that are near half the 56Ω value to spread the load - there's 22Ω, 33Ω and lots of other values above and below that (the box is described as "Metal Film Fixed Resistor Kit 1R-4.7MR Ω ohm 60 Values 1/4w(0.25 w) ±1% Tolerance 0.01 MF ").

What I'm now slightly concerned about is that I've just gone to market and bought some 1/4W 56Ω resistors when there's clearly more to it than that - as you post below and Sirius' post 522, there's electrical components, and then there's electrical components.

The ones i have bought are simply described as "Resistor Metal Film 1/4W 0.25W 1%".

Did I ought to start to hunt down better quality resistors for this problem?

Colin.
Quote:
Originally Posted by SiriusHardware View Post
No, they will be fine. I browsed through those threads Owen hunted out and there seems to be a consensus that Metal-Film resistors are the right ones to use.

Since you have them you could use a 22R + 33R (=55R) in series to replace each resistor for a 'see what happens' run, but if you did then each 1/4W resistor would only be handling a percentage of the wattage, so your DIY 55/56R resistor would be slightly overrated wattage wise.

Or, do you have some actual 56R resistors in that resistor kit box? As long as they are rated at no more than 1/4 Watt, the worst that can happen is that they burn out again, indicating a problem further downstream.
Quote:
Originally Posted by ScottishColin View Post
No - the lower values are 1, 1.5, 2.2, 3.3, 4.7, 5.1, 10, 20, 22, 33, 47 and 51.

Colin.
Quote:
Originally Posted by SiriusHardware View Post
Well then, if you want, try 33R and 22R in series as replacements for the two single 56R resistors. (Remove the original resistor, twist one end of a 33R and 22R together and solder the twisted connection, then drop the leads at the other ends of the resistors into the holes originally occupied by a single 56R resistor and solder them in place).

The end result should be two vertically mounted resistors, leaning towards each other and soldered together at the upper end. Same for the other resistor.

It is a little surprising that Colin's kit has 51R (a >= E24-Series value), and one E-series higher than a more-common 56R (>= E12-Series value):
https://en.wikipedia.org/wiki/E_seri...ferred_numbers
https://www.electronics-notes.com/ar...24-e48-e96.php
But it looks like this is a small-subset selection of E12/E24 values, rather than the complete range than many (no doubt more expensive) kits have.

I would have thought that using 51R would have been close enough, as it's < 10% lower (and original resistors may have been 5% tolerance, like most carbon-film types, even though 1% is now most common in metal film).

Using 51R 1/4W would also mean having about the same effective power-rating, as (has been previously stated) putting two nearly-equal values in series (or parallel), effectively doubles the total power handling - Which is normally a desirable thing to have)

But if really not wanting to lower the resistance from 56R, then you could put 4.7R in series with 51R, to give 55.7R total. And as the 51R would dissipate the most power of the two (with < 1/10th of the power dissipated in the 4.7R), then overall max power would still be limited by the 1/4W rating of the 51R and still nearly 90% of power being in that one.

However, I don't really believe there was an intentional for these resistors to fail if there was a fault, as you can't really rely on them failing predicable at just over 1/4W max rating and should really put a fuse (which are rather more predictable to a standard, even though they can take a long time to blow under slight overloads) in series.
Later-on, special 'safety' resistors were developed, that are designed to fail in a more-predictable manner and so can eliminate the need for a fuse (but then not very user-serviceable, and it can be difficult to establish exactly what type/spec. of safety resistor was originally fitted / getting data on these can be difficult).

Plus it is normally bad practice to run components near their max. ratings, so if these do normally run hot because they are dissipating close to 1/4W - they did go to the effort of standing them off the PCB - then they probably should have use 1/2W to start with. I don't see how going to metal-film types helped, if the ones used do have the same max. temperature / derating curve as carbon-film ones of the same size.
Although some manufacturers of metal film resistors had higher power rating in the same '1/4W' case size, like 0.6W rating of (originally Philips) MRS25 ones.

Fortunately, there's rather less parameters to worry about with resistors compared to capacitors etc. And in this application, it's really just the value and the power-rating that is important.

It looks like the resistors are really just there to limit the capacitor-charging 'surge' current through the rectifier-diodes. But will also provide some current-limiting, if there is an overload / short after these.
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Old 5th Aug 2023, 9:41 am   #536
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Default Re: Commodore PET 4032/8050/3022

I had considered the very large+very small value resistor idea but that would have placed most of the >250V voltage drop at switch-on across the larger value resistor, so either way, not ideal.

Any idea where to get a replacement capacitor from, then Owen?
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Old 5th Aug 2023, 9:53 am   #537
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Default Re: Commodore PET 4032/8050/3022

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Originally Posted by SiriusHardware View Post
According to the diagram that one is 0.47uF, 350V rated. Can you take several photos of it?
These parts are axial but would at least prove if it's that one causing the problem.
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Old 5th Aug 2023, 10:12 am   #538
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Default Re: Commodore PET 4032/8050/3022

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To the above I can only relate a short story - in the 1990s I was a field engineer working on, among other things, town centre CCTV where there was a particularly popular make and model of video multiplexer in use. These would fail from time to time and would have to be swapped out and sent back to the manufacturer, who would sometimes take quite a while to turn them around.

There came a day when we had more failures than we had replacements for, so we decided to look at one of the failed units and decided that many of the electrolytics appeared to have failed so we went to Maplin, then just up the road, and bought a full set of replacement electrolytics. To our relief, replacement of all the electrolytics brought the unit to life, so we took it back and reinstalled it, where it worked well...

..for about a month. When we got it back again we discovered that many of the newly replaced electrolytics were bulging and weeping. That was the first time I became aware that not all electrolytics are equal, and that it sometimes isn't enough just to replace them with parts with the same value and the same or greater voltage rating.
>>
Yes, sometimes there are other specifications with Electrolytics etc to consider. But I have seen that just because ones aren't branded as 'Low-ESR' or 'High-Frequency', doesn't mean they aren't suitable - Just have to try and look at the datasheet (which can be difficult for some of the more-obscure makes, often sold on online marketplaces, although some do include data / I have measured the ESR over a wide frequency range of many of these I've bought and they have been OK + worked reliably in circuit. Although some more-major manufacturers don't always provide full data / any ESR limits)

It always was the case that the max. 'Ripple-Current' rating could be important - especially when using larger values for power-rails smoothing (even at low 50/100Hz frequencies). And you also often have to derate the ripple current for higher frequency use (typ. 20% less at 100kHz?)

Whilst recently looking for the best-spec. at reasonable price ones in Farnell, I did recently notice that some moderately-expensive major-brand types did have a much lower (only a 1/4?) ripple-current rating, even though they were the same case-size - It's usual that going to a physically-larger type, then ripple-current rating also increases (as can going to a slightly-higher voltage one, where ESR also often reduces - especially if case size increases). I presume this is because aluminium-foil is thicker on larger ones.

Going for higher-temperature / longer lifetime types, also probably means a better spec. as they have to run less marginal to ensure they carry-on working for a lot longer.
What is normally a bad idea is using 'miniaturised' types, where often the manufacturer does show that this compromises most of the other parameters.
Although modern electrolytics do tend to be smaller than older ones, whilst many specifications can often be better (So low ESR / higher temperature etc. is now common on even most 'standard' ones).

I imagine Maplin only sold +85degC types, although I thought they did mainly sell known good makes. So it was surprising you had ones fail so so bad / soon in those units back then. However if the originals were also failing, then it sounds like maybe there was more of an issue with the original design, and they should have fitted much-larger capacitors (both physically and in capacitance?) to handle larger ripple-currents that were probably causing self-heating in the capacitor's ESR and causing them to fail.

I do recall many in 'Television (servicing)' magazine recommending to put lower-capacitance 100nF Polyester etc. capacitors in parallel with electrolytics being used for high-frequency smoothing, to acts as a low-Z HF bypass and reduce self-heating in the electrolytic, making it more reliable than originally.


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Originally Posted by SiriusHardware View Post
>>
Incidentally the line frequency on these is not 15KHz, but 20KHz as shown by the frequency of the drive signal in Colin's recent measurements from the new CRTC.
Thanks - I'd overlooked that (Although not much-higher - Still nearly half that of the approx 31kHz used on later VGA PC monitors - so still shouldn't really require any particularly specialist capacitors that weren't common in TV's back then).
I wonder why they used a standard Vertical frequency of 50Hz on UK models (rather than using 60Hz on all, like PC's with CGA etc), but then used a rather non-standard line-rate - Even though the 6845 is quite capable of doing 15.625kHz, as many computers using it worked with TV's.
It might be that to get 15.625kHz, you need a slightly-special crystal oscillator frequency, but for a round 20kHz, could get away with a (multiple?) of the system 1MHz frequency, to provide its 'dot-clock' oscillator input.
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Old 5th Aug 2023, 10:36 am   #539
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Default Re: Commodore PET 4032/8050/3022

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One step forwards, one step back. The step forwards is attached (Tynemouth board installed, ROM & RAM configured from the Tynemouth board).

However, the magic smoke was being released from R753 so I powered off quickly.

What might make R753 fail?

Colin.
Well I reckon that's a major step forward ! - The first time it's shown a proper display (albeit it for a brief period), and should be a great help (especially once it's showing a bit more permanent!) in revealing what is / what is still wrong with the computer part (probably helps to explain why no response from the keyboard commands typing)

It looks like most of the PET is now running OK, in order to produce a display like that, and may be a display-RAM fault? (I'm not sure with the Tynemouth board's on-board RAM, if that is just for the processor and can't be accessed by the 6845?)


So it's not really a step backwards, as it does show that monitor actually does display correctly. And despite there being a fault causing R753 to fail, it does at least mean there is a definite cause of its failure and less uncertainty over its original failure mode.

Unfortunately the 321448 monitor schematic yo'd attached isn't a very-clear scan in this area, so difficult to read component part numbers / values etc.

But most likely reason for failure if R753 is C753 breaking down (but not short, so still producing a voltage across it) or D253? being leaky / s/c and letting a large AC current go through the capacitor, so meaning R753 is passing much more than the usual DC capacitor-charging ripple-current.

It's still not clear what caused failure of R752 as well, but maybe it's related to running for a while without R753 working or C752 / D752 being faulty.
However, it seems that the E400 (412Vdc) supply derived via R753/D752, relies on the right voltage being present on C753, as C752 is not referenced to ground but to positive side of C753 (Probably to avoid requiring a much higher voltage for C752 / it tracks the other supply better).
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Old 5th Aug 2023, 11:09 am   #540
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I had considered the very large+very small value resistor idea but that would have placed most of the >250V voltage drop at switch-on across the larger value resistor, so either way, not ideal.
Yes, although if voltage-rating was OK originally for a single resistor (plus it should never have much across it due to v.low value, and probably find winding resistance in LOPT is comparable), then shouldn't need to split this (especially for a quick test, to try and ascertain presence of other faults causing it to burn-out). Hopefully Colin's kit does have a good quantity of these, so can fix it before needing to fit the ordered 56R ones.

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Originally Posted by SiriusHardware View Post
Any idea where to get a replacement capacitor from, then Owen?
I'll have a bit of a search for a suitable capacitor.
First thoughts are CPC (or Farnell / RS), Rapid Electronics
- but will have high handling charges, for small orders

or any 'TV / consumer products-spares' suppliers still around these days
- In the days before the net, I frequently used Grandata, East London Components, Economic Devices / Telepart, as well as (already tried) Cricklewood.

Not sure how many of those are still around / now online - These days I just normally look on online marketplaces, and be prepared to wait a bit (especially for cheapest sellers in the far east where I normally buy several to have spacers for next time I need one).
I have now built-up good stocks of most ones I'll ever need, but not too many high-voltage ones (I think the smallest ones I have around 400V are 10uF, where I've often seen them at radio rallies for around 10p!).

The value isn't really that critical (Plus Electrolytics do have a large tolerance - usually can be -20% and sometimes upto +50%), and normally having a bit extra isn't a problem. So if you find anything upto 1uF or so at the right voltage (or a bit higher) that will fit, it will probably be OK (especially as a temporary measure - I'm often used to improvising something, just to prove it is only that part that is required to make it work, before ordering anything)

Of course, it's still possible the capacitor isn't the (only) problem. But testing it isn't too easy without access to / building capacitor-reformer (or a HV PSU, with adjustable current limit - which I do have, but need to fix!).
So I would be tempted to also look for the diodes, just in case (they shouldn't really be very much - usually < 50p each)

Unfortunately, it's not really easy to isolate the circuitry after C753 into individual paths, without removing lots of other parts. I did think of just removing C752 from the circuit, so that would at least show if that was causing R753 to fail.
But although that could easily go o/c by itself, it may cause other undesirable effects on the operation / potential damage (Although this monitor has probably been run for long-periods with other supplies not being right, unless its supply input was disconnected whilst working on the computer-part itself).

You could in theory mount just R753, D253 and C753 in-series on the rear of the board, as a 'test-circuit' for these components in isolation. And see if the resistor survives a short power-up. It would be nice to measure actual current through it but not always easy at 20kHz and high-voltages (A good DMM with true-RMS AC Voltage range that has >20kHz B/W could be used to measure voltage across it to calculate this). Otherwise would need a 'scope with high input voltage rating (or use rarer 100:1 rather than 10:1 probes).
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