Atari SC1435 different HOT/PSreg

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sandord
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Atari SC1435 different HOT/PSreg

Post by sandord »

Haven't been active on this forum for more than a year... I miss this place and all of you! Anyway, here goes.

I'm recapping an Atari SC1435 colour monitor (basically the same as the Commodore 1084S/Philips CM11342) and all of the three service manuals show BUT11AF for part 7432 (PS power regulator) and BU508AF for part 7512 (HOT).

But, what I found in this particular unit was a D1577 for part 7432 and a C3795B for part 7512.

What caught my eye was a significant amount of raisin-like residue at the soldering joints. That makes me believe that someone replaced them before. But I can't be really sure. Maybe this unit is a revision of some sorts that doesn't match my service manual.

Since the specs of the mismatching parts are different, I'm not sure if I can safely put replacement parts in the unit as suggested by the service manual.

Code: Select all

BUT11AF:  Vcbo 1000V, Vceo 450V, Vebo  9V, Lc 5A, Lb  2A
D1577:    Vcbo 1500V, Vceo 600V, Vebo  5V, Lc 3A, Lb  2A

BUT508AF: Vcbo 1500V, Vceo 700V, Vebo 10V, Lc 8A, Lb  ?A
C3795B:   Vcbo 1300V, Vceo 600V, Vebo  8V, Lc 6A, Lb  3A
I don't know enough about electronics to assess whether this was a safe replacement or that this units had different parts from the get-go.

So basically my question is: is it safe to replace 7432 and 7512 with parts that meet the original specs as listed in the service manual?
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Re: Atari SC1435 different HOT/PSreg

Post by Tomswork »

Please be careful the parts your are working with are horizontal output transistors

NPN Si Transistor, HV Horizontal Output - NTE2300


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derkom
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Re: Atari SC1435 different HOT/PSreg

Post by derkom »

sandord wrote: Tue Jun 23, 2020 6:13 pm But, what I found in this particular unit was a D1577 for part 7432 and a C3795B for part 7512.

So basically my question is: is it safe to replace 7432 and 7512 with parts that meet the original specs as listed in the service manual?
When I serviced a dead 1435 I picked up last year, the HOT (which appeared to be original) was a D1577. I replaced it with a BU508AFI and everything is working fine.
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Re: Atari SC1435 different HOT/PSreg

Post by sandord »

Tomswork wrote: Tue Jun 23, 2020 6:16 pm Please be careful the parts your are working with are horizontal output transistors

NPN Si Transistor, HV Horizontal Output - NTE2300

Tom
Well, I have a bunch of BUT11AF and BU508AF parts in my possession so I have no problem meeting the service manual specs. It's just that I'm very curious why there are different parts in this unit. I could simply ignore that but I don't want to blow it up.

Hmm, I'm just starting to realise that if I verify that the other parts are up-to spec and there are no modifications, the parts from the manual SHOULD work...
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Re: Atari SC1435 different HOT/PSreg

Post by sandord »

derkom wrote: Tue Jun 23, 2020 6:17 pm
sandord wrote: Tue Jun 23, 2020 6:13 pm But, what I found in this particular unit was a D1577 for part 7432 and a C3795B for part 7512.

So basically my question is: is it safe to replace 7432 and 7512 with parts that meet the original specs as listed in the service manual?
When I serviced a dead 1435 I picked up last year, the HOT (which appeared to be original) was a D1577. I replaced it with a BU508AFI and everything is working fine.
Well, that's very useful information. Thanks! Apparently there is some headroom to the specs in this case.
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Re: Atari SC1435 different HOT/PSreg

Post by Tomswork »

It could have been serviced in the past or was made on a Friday

The high spec parts should work fine

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Re: Atari SC1435 different HOT/PSreg

Post by sviru »

Will hijack the thread. My 1435 is not working. High pitch sound. I guess flyback is gone. But. My question is about HOT. Mine is D1577.

How to check HOT? When I measure B C E to Ground there is a short. I thought that D1577 is shorted. Desoldered and out of the board D1577 - no short.
The board still has short to gnd from either B C E solder points. \

Is this a "normal" thing when flyback is gone - or something else died? Thanks for ideas.
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Re: Atari SC1435 different HOT/PSreg

Post by exxos »

I haven't touched monitor for over 20 years now, but if I remember rightly, the main switching transistor will likely measure short-circuit because it is across the coils of the flyback transformer anyway.

But considering you have a high-pitched whine, I would suspect that it is actually switching. But of course, it will be better to remove the main driver transistor and check it anyway.
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Re: Atari SC1435 different HOT/PSreg

Post by sviru »

exxos wrote: Fri Apr 12, 2024 7:40 pm I haven't touched monitor for over 20 years now, but if I remember rightly, the main switching transistor will likely measure short-circuit because it is across the coils of the flyback transformer anyway.

But considering you have a high-pitched whine, I would suspect that it is actually switching. But of course, it will be better to remove the main driver transistor and check it anyway.
Desoldered the FLYBACK and there is an internal short inside it. Transistor measures fine.
Since flyback is on its way to me I will replace FLYBACK tomorrow. Fingers crossed!
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