No chorus and distorted SS voicing after noise mod

MarshallJMP · July 08, 2024, 09:59:45 AM

Quote from: Moulashaker on July 08, 2024, 09:50:26 AM-20V! You might be onto something!

The rest of the amp has also come back to life with only the chorus effect not working.

Also normal, there is no current draw anymore, so according to ohms law no voltage drop between the via and pin 9

Moulashaker · July 08, 2024, 10:06:07 AM

The underside looks cooked from the heat building up. Removed some solder from the end of the capacitor in case it was touching the via but still have the -20V at pin 9.

Moulashaker · July 08, 2024, 10:10:36 AM

With both chips removed I have a resisteance between pin 9 and C129 of around 200 ohms.

MarshallJMP · July 08, 2024, 10:13:15 AM

That's your problem. Give me a minute and I'll tell you what to do to fix this.

MarshallJMP · July 08, 2024, 10:19:58 AM

See pic, cut those 2 traces and put a small wire between pin 9 of U26 and pin 9 of U32 on the underside of the pcb. This should solve your problem.

Also replace both ic's with new ones. Could be those diodes survived but 100mA is alot that went through them. Wouldn't take any chances sinces these are quite cheap.

Moulashaker · July 08, 2024, 10:22:57 AM

Will get onto it and let you know!

Many thanks,

Andy

MarshallJMP · July 08, 2024, 10:25:01 AM

Ok let us know how goes.

MarshallJMP · July 08, 2024, 10:33:22 AM

Quote from: Moulashaker on July 08, 2024, 10:06:07 AMThe underside looks cooked from the heat building up. Removed some solder from the end of the capacitor in case it was touching the via but still have the -20V at pin 9.

You have some carbon buildup due to the burning and this carbon conducts.

Moulashaker · July 08, 2024, 10:52:26 AM

It's working! At least for a few minutes! The chorus sound is back as well!

So it seems I must've done something when installing the capacitor which had perhaps internally shorted that via.

After a few minutes it has cut out and the red display and green lights go a bit crazy. It's now struggling to boot up which has occurred a few times whilst trying to get it sorted.

It trys to go through the presets (C1, C2,...) but cuts out and flashes a few crazy symbols.

If I leave it a few minutes between boot up at attempts it seems to get a little further each time.

Moulashaker · July 08, 2024, 10:53:12 AM

I've given it a good scrub with IPA but might really try and get it cleaned up.

Moulashaker · July 08, 2024, 11:17:29 AM

After leaving it for five minutes it booted up and I've restored the presets and appeared to be back in business with everything sounding great however after about five minutes of playing it suddenly drops sound and once again has some crazy led flashing moments.

Pic attached. At least one problem seems solved! :)

Moulashaker · July 08, 2024, 11:24:16 AM

Definitely strange!

Moulashaker · July 08, 2024, 11:51:09 AM

This has occurred at rare times since owning the preamp, not just since the recent mod and issues. Perhaps a poor connection somewhere?

MarshallJMP · July 08, 2024, 12:32:52 PM

Does it have the 78M05 5 regulator? Both are near the transformer bolted to a heatsink.

MarshallJMP · July 08, 2024, 12:45:00 PM

Check the 5V see if it does funny stuff.

On the left side of the Z80 cpu there is an orange tantalum cap. I saw on your pics that is has some brown dots, are these burnmarks? This cap is over the 5V.

Sometimes one of these small orange caps close to all those digital ic's goes bad causing the 5V to drop, especially with those 78M05's.

Could also be a bad contact like a via

Bad IC

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No chorus and distorted SS voicing after noise mod