Marshall Bass 12 - Repair Help

barnsleyboy

Hi,

Just bought an old 80's Bass 12 off fleabay - really grubby and looked like it hadn't been used in ages. When I switch it on there is a loud humming sound. Pots very crackly, So I have stripped it down, given the cabinet a good going over, given the board a visual inspection to see if there is anything black and charred hanging off it - all looks in order. Pots have had a good drenching with contact cleaner to dislodge any grime.

A more detailed description of the hum:

Amp switched on, no lead attached. volume and gain at zero, amp humming through speaker. Gradually rotate the gain control, the hum quiettens down (but still there), then goes back up to the original volume with gain at 10.

With guitar attached, same thing happens but with the gain at zero there is a sound like shortwave radio interference.

Does anyone have any ideas what it might be? It's a pretty basic looking board, so rather than taking it to an amp tech, I'm wondering if I can sort any component replacement myself to save a few quid (usual disclaimer about high voltages/ possible death etc.) At a guess, I wonder if it needs the caps replacing - it's over 30 years old, so that may be a possibility. While I'm at it I may replace the pots as the previous owner has squeezed the split shaft on each of them and in some cases slightly bent the spindle.

I appreciate that it is difficult to diagnose a problem without seeing the amp, but it sounds like it may be something that is relatively easy to fix. Any suggestions would be much appreciated.

Funkfingers

Photographs usually aid remote diagnosis.

First things first. Have you inspected inside the mains plug? 

springhead

Sounds like a missing ground on the volume pot. Possibly faulty pot or dry joint where it's soldered into the board. I'd be doing a resistance check from chassis to the ground leg of the pot as a starting point.

ICBM

Sounds like it might be a bad ground connection. The gain control is literally the first component in the circuit after the two input resistors, so any noise must be being picked up there.

Check the mains earth before you do anything else as Funkfingers said - not least for safety.

These are excellent little amps and well worth sorting out. They're a bit cleaner and chunkier-sounding than the Lead 12 and take pedals very well. It's possible the caps could need replacing, but don't assume so until you've found the source of the problem.

If you're *very* careful you can often straighten out the bent pot splines, but it's extremely easy to bend them too far and snap them off if you go even slightly beyond the original position. I do it by forcing a tapering knife blade into the gap, that seems to be controlled enough to open them out with the least risk.

barnsleyboy

Doh! Got my knobs mixed up - it's the volume control not the gain knob.

Checked the mains plug, all secure. Here are some pics:

I've desoldered the volume pot and checked it. 0 ohms at min. 1.3 Meg at maximum, so seems to be working ok.

A resistance check from chassis to the ground leg of the pot - I'm assuming that this should read zero i.e no resistance?

ICBM

If it's the volume pot that has a null point for the hum then the chances are it's the filter caps. The gain stages before and after the volume pot are out of phase, so there will be a point where the hum in each cancels out - the remainder is from the power stage.

I did type all that out first since it's the usual answer to this problem, then scrubbed it since you said gain!

barnsleyboy

My bad @ICBM, my apologies for the error. It was the excitement of getting hold of the Bass 12 at a reasonable price, and within a very short space of time having an offer accepted on a Lead 12 too! (The lead 12 was my first amp back in the mid eighties, and as a naiive 17 year old I failed to recognise the awesomeness of it.) You've just gotta love those old solid state British built Marshalls.

I've found what I believe to be some suitable replacements:

https://www.ebay.co.uk/itm/303842006307,

I'll try that and while I'm at it I'll replace the pots too. I did the old prising apart the splines trick on my Master Lead Combo and like you say it is way too easy to overdo it!

barnsleyboy

This is probably a really stoopid question, the new caps arrived today and they are a lot smaller in physical size compared to the originals. They are the same value etc. Are they still ok to fit? or is there something that I'm missing? Also, due to their close proximity, the small gap between the originals is sealed with a bead of silicone, obviously the replacements are a lot smaller, so won't be touching. do I still need to give them a dab of "jollop? or will they be ok just soldered to the board?

ICBM

Caps have generally got physically smaller over time, for the same electrical values. They will work fine. I would sometimes use such a big discrepancy as an opportunity to increase either the value or the voltage rating, but in this case there's probably no point - the schematic says 19V for the supply rails, which is comfortably under the 25V rating.

There is some anecdotal evidence that physically smaller caps don't have quite the same 'heft' to the tone, especially in big bass amps - this may be real, and if so is due to what's called 'equivalent series resistance' which is generally higher in smaller caps - but I doubt it will make a difference in such a relatively low-powered amp.

You don't need to glue them as long as they're tight down to the board and can't vibrate - it's common to run a bead of hot-melt glue along the board directly under the cap before you push it down, if you're worried about it. In fact, you probably don't even need that - just pull the leads through tightly and kink them under the board before soldering.

barnsleyboy

That's awesome @icbm many thanks. I'll be firing up the soldering iron later on tonight. Just as a side question, I don't suppose you know what fuse rating I need to use in the mains plug for these amps? It's got a 13A in it at the moment, But with these older amps I always like to fit the correct fuse just in case. Back in the mid eighties when I was a student a lad got killed through some pretty shonky electrical wiring in a microphone during a band practice, so I'm always cautious about these things (always inspect the mains plug and usually replace it whenever I get an amp).

ICBM

barnsleyboy said:

That's awesome @icbm many thanks. I'll be firing up the soldering iron later on tonight. Just as a side question, I don't suppose you know what fuse rating I need to use in the mains plug for these amps? It's got a 13A in it at the moment, But with these older amps I always like to fit the correct fuse just in case. Back in the mid eighties when I was a student a lad got killed through some pretty shonky electrical wiring in a microphone during a band practice, so I'm always cautious about these things (always inspect the mains plug and usually replace it whenever I get an amp).

3A is best, but it's irrelevant since the amp's internal fuse value is much lower and will always blow first - 160mA, although I'm actually surprised it's that low, I would have guessed 250mA - either way, even a 3A fuse in the plug won't notice.

I always inspect inside the mains plug before I ever plug an unknown amp in for the first time - always! I've seen some really scary stuff over the years, including things that could at an outside chance kill you if there's also a missing earth connection.

(The real worry about dodgy wiring in venues and rehearsal rooms is that your own gear can be in perfect order and properly earthed, and it still won't save you from a fault in a PA or the room wiring if you touch the mic and your guitar at the same time.)

barnsleyboy

Hmm, changed the caps and still have the same problem. Any ideas anyone?

Not sure if it is relevant, but the negative wire is soldered directly to the -ve on the capacitor (under the board)

I compared this to my Lead 12, and the lead 12 has the wire connected to the point I have circled in red.

The tracks appear to be joined, so I'm sure it's not an issue, but it seems a bit strange. I'm wondering if any other caps need changing? Is the volume pot to blame? or should I cut my losses and take it in to an amp tech to get it done properly?

ICBM

It's very unlikely that moving that wire would make any difference. The two points are connected by a wide trace. If anything, it's possible that someone has already moved it in an attempt to fix the problem, by making it a 'star ground'. That should be the only connection between the PCB ground and the chassis. If it isn't - eg if one of the pot casings is connected to the PCB - then that could be the cause.

There are two other filter caps which could also be the cause, C2 and C6 - they're for the lower-voltage rails for the preamp.

barnsleyboy

So I should check that there is no connection between the pot cases and the chassis - I'll get the multimeter out.

If that's all ok, I'll replace C2 and C6 - They don't look as significant as the other 2 - I'm guessing that every little helps!!!

ICBM

Odd... that doesn't correspond with the schematic at all!

C2 and C6 should be 22uF 25V electrolytics.

On the 20W schematic they're labelled as C15 and C16, but in either case they should be smaller versions of the ones you've changed already. It looks like they may be at the top right of your pic.

barnsleyboy

I did think that they looked a bit "pedestrian"!

So it's more likely to be C15 & C16? While I'm at it and just to be on the safe side, should I also change C13 & C14 too? That way all of the electrolytics will have been renewed.C14 looks to be a 100uF 25v, C13 is the same value as C15 & C16.

Good old uncle Jim! There are probably so many variations of these old 80's s/s amps that he probably lost track of his numbering!!

ICBM

Yes, C15 and C16 must be the low-voltage filter caps. It won't hurt to replace the others as well although they're not likely to be the problem, they're coupling caps in the power amp.

Apart from those there should be no other components that will have degraded with age.

Also just spotted that from memory, TR4 is meant to be heatshrinked to the heatsink leg next to it - that's why it's sited there - to provide thermal feedback for the power amp.

barnsleyboy

ICBM said:

Also just spotted that from memory, TR4 is meant to be heatshrinked to the heatsink leg next to it - that's why it's sited there - to provide thermal feedback for the power amp.

Sorry @icbm not sure if I understand what that means..... and if there is something I have to do?

barnsleyboy

Also, can't seem to find replacements that have a lead at each end of the can. I presume that it doesn't matter if the caps stand vertically? i.e. both leads come out of the bottom of the can.

ICBM

barnsleyboy said:

Sorry @icbm not sure if I understand what that means..... and if there is something I have to do?

There should be a piece of heatshrink sleeving holding the transistor against the heaksink leg closest to it. A piece of thin plastic tubing would work too. I'm not sure how important it is though.

barnsleyboy said:

Also, can't seem to find replacements that have a lead at each end of the can. I presume that it doesn't matter if the caps stand vertically? i.e. both leads come out of the bottom of the can.

It doesn't matter, as long as they can't vibrate about. Stick them to the board with a bit of hot glue.

barnsleyboy

Nice one @icbm as always, your wisdom is much appreciated!!