DSP-508A in Atari 1040STE

[BrettRogersUK]

Oh FFS, I forgot about checking that. 😭😭😭😭😭😭

How do I go about checking to see if it's shorted and I'll do that now. Sorry, been busy with other things and completely forgot about the BU508.

[exxos]

Check across every combination of pins on resistance and see if any are like a few ohms. Mostly should measure in the megaohm range if anything, and you need to remove it as well.

[BrettRogersUK]

Hi,

Have removed BU508 and transistor labelled Q1 which in my supply is a D667. Removing the D667 I found that the emitter leg has come away from the base, completely removed itself. So the D667 has only two legs attached now, so that needs replacing.

Also removed the BU508A and upon testing I'm getting measurements of the following:

Base to Emitter = 0.7R
Base to Collector = 0.7R
Collector to Emitter = 0.7R

so it would appear that the 508A is shot too. Probably the D667 caused this by having the leg come off and therefore it was this transistor that failed causing the BU508 to fail as well maybe??

EDIT: Further inspection found this resistor (See pic). I think it's labelled R7 (it's a 51R resistor looks rather beefy wattage wise). I would guess it's probably 2W. Upon checking the resistor with multimeter (despite showing burn on one end) it still reading at 54R. For some reason the values do not match with the BOM on the schematic for the PSU.

Regards,

Brett

20211204_174240.jpg

[exxos]

Yep its goneski . if you check the large resistors and other transistors etc and replace them, then you should be working again really.

[BrettRogersUK]

Hi,

Can anyone recommend a replacement transistor for the D667 at all? I've been searching and I wonder if this one would be a suitable replacement:

https://uk.rs-online.com/web/p/bipolar- ... s/1868664/

For things like this I would much more buy from a reputable source than a certain well known auction site or anywhere else.

Thanks,

Brett

[exxos]

Is D667 what's on it ?

[BrettRogersUK]

Is D667 what's on it ?

Hi,

Yes, that's what is written on it.

Regards,

Brett

[exxos]

2SD667 out of stock everywhere :(. I've got a databook at home with such info in somewhere. Will have a look tomorrow when I get back home.

EDIT:
The one you list looks very similar at a glance.

EDIT2:

https://www.cricklewoodelectronics.com/2SD667.html

[BrettRogersUK]

2SD667 out of stock everywhere :(. I've got a databook at home with such info in somewhere. Will have a look tomorrow when I get back home.

EDIT:
The one you list looks very similar at a glance.

EDIT2:

https://www.cricklewoodelectronics.com/2SD667.html

Thanks for that I'll get a couple ordered. Might as well for that price. They also have the resistors as well but not 51R they have 56R 2 Watt carbon film resistors. Would they be an adequate substitute?

EDIT: https://www.cricklewoodelectronics.com/3W56R.html Would these be an adequate substitute? they are the same colour as the one I took out.

Regards,

Brett

[exxos]

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, you're 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: