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Commodore 8050: repair/update of parts

lucyferpl

lucyferpl

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May 23, 2022
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Somewhere in the future
Hello,
My station after 5-10 minutes of switching on begins to generate high-pitched sounds typical of a giant transformer.

I decided to replace the capacitor filter in it. Unfortunately I was not able to find a capacitor filter with the same parameters as the original one. So I bought a much bigger one. Can this fact cause any problems?
1aaaa.jpg
Original

1bbbbb.jpg
replacement


3aaaa.jpg
Also, on the motherboard I noticed that cr5 and cr6, which contain 200v and 2A rectifier diodes are blackened (from temperature?) so I decided to replace them with 800v and 3A and here the same question: Can this fact cause any problem?

I also thought about replacing the LM323K transistor from 2A to 3A
4aaaa.jpg


If any of you have any advice, please let me know.
 
In the evening I got such a wiring diagram

I would like to do it right this time, electronics of this station is expensive and hard to get. I do not want to risk another short circuit. I have the impression that the first time I had more luck than sense. If I understand correctly, the current did not even pass through the transformer and electronics of the device but immediately caused a short circuit on the circuit socket - capacitor. Additionally, was replacing the rectifier diodes with more powerful ones a good idea?
 

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Instead of randomly replacing parts I would rather try to find out what's wrong (by measuring voltages, ripple, leaky caps, components getting very hot due to a short somewhere etc.)

The VRs are rated accordingly, I don't think it will help replacing it by a larger one.

Interestingly, my 8050 didn't have an XY line filter capacitor
 
powerlot said:
Instead of randomly replacing parts I would rather try to find out what's wrong (by measuring voltages, ripple, leaky caps, components getting very hot due to a short somewhere etc.)

The VRs are rated accordingly, I don't think it will help replacing it by a larger one.

Interestingly, my 8050 didn't have an XY line filter capacitor
Click to expand...
Because simply someone deleted it after it probably burned early on. There are plenty of posts on the web of people who had their original filter explode and simply removed it. I am looking for a replacement, but it is hard to find something with similar parameters.
 
Just opened up an 8032-SK with the same unit which has exploded too.

They just dont seem to exist.
 
Pretty sure the disk drive was in original condition when I got it, as the computer that came with it still had the (visibly cracked) RIFA filter cap in it. Unless your house has noisy lines I don't think you really need it.

You can probably replace it by a single X1/X2 cap or one of those IEC-plug filters with similar specs (https://uk.rs-online.com/web/p/iec-filters/3864172)

I'm more worried about the condition of the rest of the disk drive and would advise to still check the voltages (and ripple).
 
053747CC-6FD2-4226-BA5D-066F60327CEC.jpeg
35727CFD-382D-4E87-8CEB-5B74ADB63E96.jpeg


I decided to remove the filter. I have no knowledge to find the right one. Now I just want to make sure I have connected it correctly. I have no way to check the values you write about.
 
Looks correct, I would just make sure that the connections are firm, maybe remove those snipped off wires and sleeve the connected ones. It's all potentially dangerous with live current...

I had to replace the line cord and fuse holder in mine, here's what it looked like

8050_transformer.jpg
 
The LM323K is not a transistor, it's a linear regulator and it's actually
rated for 3A.


Can't read properly the marking on the picture, but on the service manual
I found CR5 and CR6 are a couple of 1N5402 rated at 3A.

(www.primrosebank.net/computers/pet/documents/8050_8250_service_manual.pdf)

Going up in voltage is overkill in that part of the circuit,
but it won't hurt. I would just lift them few mm up from PCB bottom and
attach them some kind of heatsink.
 
I checked the voltages on the capacitors and the transformer output. Everything is in accordance with the service manual. Current 8.2v and 18.5v
I also determined that this unbearable sound is generated by the transformer itself
What other transformer can I replace it with and how to find one?
 
The thermal (heat dissipation) in the two rectifier diodes is the product of their forward voltage drop and the rms current.

Looking at your photo, it looks like the factory might have been aware of an issue here and fitted 1N5825 Schottky rectifiers (40V 5A rated) to lower the heat dissipation, instead of the 1N5402 standard silicon that the schematic suggests.

A Schottky rectifier has a low forward voltage drop. That forward drop is nearly half the typical value of a standard silicon rectifier like the 1N5402 & 1N5407 for the same forward current.

Check the numbering on those original diodes closely, it is a little hard to be 100% sure from the photo. If they are the Schottky types, you should replace with the same, as the standard silicon rectifier will run nearly twice as hot. If they are standard silicon types, you could solve much of the heat dissipation issue with fitting a Schottky type, instead.

Also, there are literally dozens of line power filters available from places like RS if you wanted to replace the one you have:

FN2010-10-07 | Schaffner, FN2010 10A 250 V ac 0 → 400Hz, Chassis Mount EMI Filter, Wire Lead, Single Phase | RS

au.rs-online.com au.rs-online.com

However for computers/apparatus with analog supplies there is little call for them, I would just leave it out.
 
Last edited:
Hugo Holden said:
The thermal (heat dissipation) in the two rectifier diodes is the product of their forward voltage drop and the rms current.

Looking at your photo, it looks like the factory might have been aware of an issue here and fitted 1N5825 Schottky rectifiers (40V 5A rated) to lower the heat dissipation, instead of the 1N5402 standard silicon that the schematic suggests.

A Schottky rectifier has a low forward voltage drop. That forward drop is nearly half the typical value of a standard silicon rectifier like the 1N5402 & 1N5407 for the same forward current.

Check the numbering on those original diodes closely, it is a little hard to be 100% sure from the photo. If they are the Schottky types, you should replace with the same, as the standard silicon rectifier will run nearly twice as hot. If they are standard silicon types, you could solve much of the heat dissipation issue with fitting a Schottky type, instead.
Click to expand...
The diodes I removed are marked IN5402 G18250 and I replaced them with IN5407
 
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