Recapping a 1000SX & PSU / Speaker Interference

[GrandPoobah]

Hey guys. Hoping to pick your brains real fast.

I've started trying to restore a junk SX I've had kicking around (see my sig for all the missing parts) and I thought I'd gotten it back up and running. However I've run into an issue where after the machine has been up for an extended period, I get what sounds like radio interference over the internal speaker. This seems to happen even when sound is diverted to external speakers. This problem occurs with all cards, devices and drives disconnected. It can be eliminated by turning the speaker pot all the way down, but I would still like to be able to have internal sound when possible. I'm thinking the next step is to just recap the board, and do the PSU while in in there.

Does this sound like the way to go? And before I pull it all apart, does anyone have a cap list for the board and PSU? The machine is a 1000A revision.

thanks

 

[Trixter]

This sounds like the way to go, although I can't help with a parts list, sorry. Also, you may not be able to reduce the interference noise 100%. The original Tandy 1000 has very low/no interference noise on its RCA output jack, but I've never owned an SX so I couldn't tell you if it has the same audio quality.

 

[GrandPoobah]

Yeah. My other SX has interference on the speaker too, but this goes way beyond that. It sounds like somebody is tuning a ham radio and scratching on cardboard. Not the typical whine of poorly shielded audio. Even while idle, the sound is all over the place.

 

[seat safety switch]

I just re-did the caps on my basket-case 1000SX's PSU because the C23 (-12V rail) cap exploded on the initial startup.

Here's my table:

Cap ID Capacitance Voltage Comments
C6 680µF 200V Mains filter cap. BIG.
C15 1uF 50V
C16 2200µF 25V
C17 470µF 25V
C18 1000µF 16V
C19 4700µF 10V
C20 4700µF 10V
C21 4700µF 10V
C22 2200µF 10V
C23 47µF 25V
C24 47µF 25V
C25 1uF 50V

If you're going to test the power supply it needs a minimum load of 1.25A on the 5V rail and 0.15A on the +12V rail as per the service manual. My whole weblog entry on testing the power supply is here.

I still don't have a working machine yet (it flickers white on composite video when I push the power switch and doesn't seem to do anything else but run the fan - voltages are solid both coming out of the PSU and into the motherboard).

My multimeter says the five-volt rail is shorted (continuous) but I was able to bring the board out of current limit with a bench power supply. Waiting on some new VRAM chips as 3/4 of them got really hot, and tested shorted on VCC-GND - then I'll go around the board with a logic probe and see what's up.

 

[GrandPoobah]

Thanks. That’s really helpful.

Btw. I have a parts SX motherboard if you need parts. I’ve already pulled a few chips off, but let me know if you’re looking for anything

 

[seat safety switch]

Thanks for the offer! I'd definitely be interested in the "Light Blue" (U51) custom chip if you've still got it.

Put the oscilloscope to it today, and all the clock voltages seem too low to be working (0.36 - 1.10V).