Marshall Bass 12 - HELP I'M AT MY WITS END!!!

[FirebladePilot]

I did think about that, but I'm more than likely going to order up replacement transistors - they are pennies to replace. As I'm not certain what the values should or shouldn't be, I'll assume at the moment that the transistors are duds.

I've set them to one side. I'm thinking that I should remove and test all the capacitors that I haven't replaced to make sure they are in spec. It will be a lot of work with the soldering iron, but if I'm methodical, at least I'll feel like I'm getting closer to my end goal!

Thanks for taking the time to post the spec sheets - much appreciated.

Cheers,

Paul.

Have you got any further with this faults, looks like you have nearly replaced everything?

 

[TassieViking]

5215 Mosfet Reverb power amp hum

Ever since I got my 5215, it has hummed. Shortly after getting it I went into the amp and adjusted the balance trim in the power section, which reduced the hum and reduced the DC offset of the output. There’s still a bit too much hum and still a bit of DC, though, and the trim is maxed out...

www.marshallforum.com

 

[FleshOnGear]

@Barnsley Boy, have you resolved the humming yet? @TassieViking referenced my thread about my amp that was humming.

I had replaced a good amount of components before discovering that it was ripple current on the 0V trace, or some kind of ground loop. There was continuity, but adding another wire between the 0V connection of the reservoir cap and the 0V connection at the input to the power amp board alleviated the hum.

 

[Barnsley Boy]

Sorry @TassieViking & @FleshOnGear haven't logged on for a little while. I've left the amp for a bit, but now about to get my transistors ordered. I need to get my head back into the problem. I'm interested in the soldered wire solution to get rid of the ground hum, but I'm still inclined to think why would the amp leave the factory with a hum issue designed into it? I still think that it could be a failed/ failing component problem. Having said that, this has dragged on for so long that I'm sick and tired of the damn thing. @FleshOnGear would you be able to mark on the schematic from where to where you made the connection? Once I've reassembled the Bass 12 (and if the new transistors don't fix it), I'm going to get my croc clips out and do a bit of investigation.

 

[FleshOnGear]

Sorry @TassieViking & @FleshOnGear haven't logged on for a little while. I've left the amp for a bit, but now about to get my transistors ordered. I need to get my head back into the problem. I'm interested in the soldered wire solution to get rid of the ground hum, but I'm still inclined to think why would the amp leave the factory with a hum issue designed into it? I still think that it could be a failed/ failing component problem. Having said that, this has dragged on for so long that I'm sick and tired of the damn thing. @FleshOnGear would you be able to mark on the schematic from where to where you made the connection? Once I've reassembled the Bass 12 (and if the new transistors don't fix it), I'm going to get my croc clips out and do a bit of investigation.

I’m not sure how much help it will be, since it’s a different amp altogether, but here you go:

Keep in mind that I specifically attached the wire at C4, not at C5. I tried connecting the other end to every component that attached to the 0V trace, and the point shown above gave the most hum reduction. Good luck!

 

[Barnsley Boy]

Nice one mate - many thanks. I'll report back on my findings.

 

[arthur.lowery]

I'm glad to see that you have twisted together the two red leads from the transformer to the rectifier. These carry very "spiky" currents - at the peaks of the sinewave, when the caps are being recharged. That might account for the high harmonics in your sound clip. One step further would be to include the black (centre tap) to the transformer in the twist (3 wires twisted together). The black wire should go close to the centre point of the first two smoothing caps (it almost is in Pete's photo), as it is carrying spiky currents to them. (Top quality caps have 4 terminals - two for the spiky currents and 2 for the smoothed current, for this reason.
It's also a shitty little bridge rectifier (like 1 amp rating). One of the diodes inside it might have failed, giving you half-wave rectification on one rail. That would give some interesting harmonics of 50 Hz.
Maybe look at the spectrum of the hum on your soundclip, if the audio software will let you!
I hope this helps.
Arthur

 

[Barnsley Boy]

It's been a while!

Finally got round to changing the BC184 transistors - a bit of variance in a batch of 25, from 317 to 345 Hfe. Out of the batch 3 were the same value at 321, so swapped out TR1, TR2 and TR6 for those. Switched it on, and hey presto, exactly the same humming problem. Next step is to try @FleshOnGear probing along the 0v rail.

I'm going to connect one end of the wire to chassis, and progressively try at each component along the 0v rail and see what happens. Before I fry myself, the probe will be insulated so I won't be in contact with anything live. Should I switch off the amp between each position? or is it ok to connect and reconnect whilst it's still powered up? Snapshot below shows what I'm planning. The arrow gets moved progressively to the left in a last ditch attempt to resolve this.

 

[FleshOnGear]

It's been a while!

Finally got round to changing the BC184 transistors - a bit of variance in a batch of 25, from 317 to 345 Hfe. Out of the batch 3 were the same value at 321, so swapped out TR1, TR2 and TR6 for those. Switched it on, and hey presto, exactly the same humming problem. Next step is to try @FleshOnGear probing along the 0v rail.

I'm going to connect one end of the wire to chassis, and progressively try at each component along the 0v rail and see what happens. Before I fry myself, the probe will be insulated so I won't be in contact with anything live. Should I switch off the amp between each position? or is it ok to connect and reconnect whilst it's still powered up? Snapshot below shows what I'm planning. The arrow gets moved progressively to the left in a last ditch attempt to resolve this.

View attachment 151908

If you think there’s any chance you’d accidentally short something like a voltage rail, it might be safer to power down while you make sure you have the jumper properly placed. I didn’t do that, but I mapped out where I was going to place the alligator clip ahead of time.

 

[Wahjam]

Hi,

I've the same amp 5501 with the same issue: a hum that decreases when the volume pot is at 7 or 8.

I bought the amp new in 1990, the hum came after along period of storage without using it (~10 years).

I tried changing the filtering caps C18 and C19, and it slightly improved, the hum is still there.

@Barnsley Boy : thank you for all the tests you made. Did you finally get rid of that hum ?

 

[Barnsley Boy]

Hi,

I've the same amp 5501 with the same issue: a hum that decreases when the volume pot is at 7 or 8.

I bought the amp new in 1990, the hum came after along period of storage without using it (~10 years).

I tried changing the filtering caps C18 and C19, and it slightly improved, the hum is still there.

@Barnsley Boy : thank you for all the tests you made. Did you finally get rid of that hum ?

I've left this for quite a while since the transistor swap didn't work. I'm going to order up a replacement BR as @arthur.lowery mentioned. I really can't think of anything else that I can do. It's sat in pieces for so long :-(