Marshall Bass 12 - Repair Help

barnsleyboy

Last chance saloon on this amp before I finally give up. My last hope is that the source of the hum is within the transformer, and could be related to delamination of the core/ small vibrations in the coils, which could have happened over time.

With the amp on and no speaker connected I can hear the same hum, which makes the chassis resonate. I'm going to remove the transformer and dip the whole thing in clear transformer varnish, let it cure then reconnect it.

Anybody have any views on this? or advice? Like I said, I'll be giving up on this one if this doesn't work. I'll probably chop it about a bit and convert it into a wall mounted key cabinet, occasional table, or footstool!!!

ICBM

barnsleyboy said:

With the amp on and no speaker connected I can hear the same hum, which makes the chassis resonate.

Ha! Well that would certainly explain why it was so baffling to find…

Dipping the transformer might fix it, but if not then - or probably first, since it should be easier - try mounting it on rubber grommets.

barnsleyboy

I've just unbolted it and placed it on a foam pad. Obviously isolating from the chassis gives no resonance, but still sound when the speaker is connected. I'm going to go with the varnish dipping ..... fingers crossed!

robinbowes

I imagine a replacement transformer wouldn't be all that pricey.

R.

barnsleyboy

it has DET-4942 stamped on it. I've tried googling it and I can't find a replacement, or an equivalent whereas I can buy a litre of transformer varnish for 20 quid, and if it works the amp will still be stock. Admittedly, if the varnish doesn't work, I may need to look harder for a replacement!

robinbowes

I'm on my phone so can't check easily, but you'll just need a mains transformer with the correct secodaries and power rating. It's probably 12-0-12 or 15-0-15 or similar. Pretty easy to find.

R.

Keefy

I’m pretty sure I have a 15-0-15 from an old Squier 15W combo - will check when I get home tomorrow.

barnsleyboy

@Keefy thanks for the offer mate. I'm trying to identify what type it is. I think I am definitely on to something with the transformer. Turning it over it looks like the core has started to delaminate, which could definitely explain the hum!

robinbowes

This is an interesting post: https://www.ssguitar.com/index.php?topic=3787.msg30171#msg30171

Basically, up the voltages, upgrade the PSU caps, change a couple of resistors, and you have a 50W amp!

barnsleyboy

Funnily enough, I had read that post this morning, which gave me the lightbulb moment on the varnish dipping option.

Increasing the output to 50w is an interesting concept, but quite frankly 12w is plenty loud enough for me - my poor old ears would probably shrivel up and drop off!!

Keefy

Squier transformer located. It came from a BP15 combo, for which the schematic shows +/-21V supply rails, about what you would get from a 15-0-15V secondary. The Marshall 12W Lead schematic shows a 14.3-0-14.3V secondary - close enough I reckon.

Fixing centres at 70mm. Body 50mm high x 58mm long x 58mm wide.

@barnsleyboy yours for the £2.85 postage if you want it.

barnsleyboy

@keefy Just PM'd