DSL100 intermittent total volume fade

[chris101]

Hi folks,

Dont know if anyone on here can help me, I have a couple of DSL100s, one of which has had a problem for a while now. Every now and again, regardless of volume setting, the volume output will drop off to almost nothing, the sound thins out and fades away, then a minute later it fades back in again and works as normal. All tubes light up.

I've swapped all the tubes, both power and preamp, tried different cables and speaker cabs.

Anyone got any clues as what this might be, dont suppsoe its a common or known problem with these amps by any chance? I'll probably change all the electrolytics next week, but if there are any other components prone to overheating or failure I'll check those.

Thanks in advance,

Chris

[52 Bill]

Have you tried plugging in a cable bridging the effects loop? With the symptoms you describe, that would be first on my list.

[chris101]

Hi,

Thanks, I'll try that next, but what does it test for? Is it bypassing something?

[Amp Kat]

It bridges the send and return together. The jacks can get corroded and cause bad connections and the signal passes right thru those jacks during normal operation with nothing plugged into them. Check your plate resistors and cathode current sense resistors also. Is it the model with the bias board in the back or no bias board in the back ? Make sure all 4 tubes are glowing and one isn't red plating which is a common failure with these amps. Tubes are an issue also with this amp and catching them before they go is a plus because once they go it usually takes something out and I've seen different stuff go with different tube shorts.

[52 Bill]

The effects loop jacks have a normally closed switch contact that connects the send jack to the return jack, so that when you are not using the loop, the signal continues on it's path through the amp. If the contact becomes oxidized or dirty, it will break the signal path and shut off the signal.

I've seen dirty jacks cut off the signal completely or act like a dirty pot and cause the volume to randomly fluctuate up and down.

[chris101]

Yeah, I replaced the jacks last year, they were corroded, new ones were cheap direct from Marshall.

Typically the problem seems to have gone for now, had the amp switched on for a few hours this evening and its running perfectly.. damn it! It wont last though.

[Enzo]

You say all the tubes are lighting up. Do the front end tubes STAY lit up WHILE this fade out occurs? Fading in and out, as opposed to sudden changes, points to the heaters coming and going.

In that case, check the rectifier that serves up the DC for the heaters on the first couple tubes. SOme fail, and a lot of them never took solder well when first installed. Pull the bridge out and scrape the legs clean and reinstall, or just replace it.

[chris101]

thanks for all the info so far. Yes unfortunately all tubes stayed lit during the volume drop. I might swap out the BR anyway as I keep hearing about problems with it, is it BR1? If so do you have the component specs or the part number? On my schematic I think the Marshall part number is 'BR62'

[Amp Kat]

Br62 is the type of rectifier. It's a 200 prv,125-IFM rated at 6.0 amps. What I usually see or find in these are the WO5 or WO8 which is a 1.5 amp 600 volt40-IFM. If you follow the trace from the power tube heaters it will take you straight into it. I would get a rectifier as high as you can but watch the package as if you get to big it won't fit thru the holes. Mouser # 583 BR62 will work but I'd try a 583-BR86 or BR810 and maybe even BR1010. They are only $1.50 a piece.

[chris101]

Edit, BR physical size established.