DSP-508A in Atari 1040STE

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BrettRogersUK
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Re: DSP-508A in Atari 1040STE

Post by BrettRogersUK »

exxos wrote: Sun Dec 05, 2021 12:04 am I'm not sure about changing value and types without trying them out. I assume the ones you took out are carbon, you may have to chop it up to see. Going from carbon to wirewound , your adding inductance on a wire wound one, without studying the PSU with both types I don't recommend using wire wound, unless you can confirm they are already used.

BTW
If you run a incandescent lamp in series with the mains, say 40 or 60 watt , then it will act as a current limiter. Normally the lamp will light up like 50% if things are going well, if the PSU has failed, the lamp will go full brightness and gives you a few seconds to turn off before the PSU goes boom. I used to that with all the PSUs I serviced in my old job. Evil things they were. I also used to put a heavy catalog on top of it and turn it on across the workshop ! Did I mention those things were evil ? :lol: :roll:
Cricklewood Electronics do 56R Carbon Film resistors in 2W types. I'm just assuming it's a 2W resistor. That's what it lists on the BOM for the PSU schematic that's available. However, the values and where they are placed do not match with what I've got in front of me on the PSU.

The chared resistor I took out does still read at 54R on the multimeter and has also got continuity as well on either end.

Thanks,

Brett
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exxos
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Re: DSP-508A in Atari 1040STE

Post by exxos »

Looks like it's a feedback resistor which turns on a transistor. I doubt a couple ohms higher would really matter that much.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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BrettRogersUK
Posts: 129
Joined: Wed Sep 15, 2021 12:48 am

Re: DSP-508A in Atari 1040STE

Post by BrettRogersUK »

Hi All,

The Saga continues.......

I've replaced the parts (BU508A, 51R resistor and D667 transistor) and I've plugged the power supply in.

The good news is that the power supply didn't go bang or fizzle and the fuse remained intact.

The bad news is that there isn't any voltage on the output pins. I've checked the bridge rectifier. I'm getting 240v on the AC pins and getting 326v on + and - (DC pins).

There is no output on the 5v or 12v lines. Could it be that the bridge rectifier is knackered and that I will need to replace it?

I do honestly think a new power supply will be needed now.

Regards,

Brett
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Re: DSP-508A in Atari 1040STE

Post by exxos »

If you have around 350VDC on the capacitor then is generally correct. Careful how you measure that voltage though!!

It sounds like it is not actually switching. I think you are pretty much changed every everything other than the Opto now ? It might be worth a go for a last attempt and then call it a day as it is becomes a point where spending lots of time and money on something just isn't worth it anymore.
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
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