Marshall Bass 12 - Repair Help

barnsleyboy

Cheers mate! Much appreciated!

JMP220478

barnsleyboy said:

Cheers mate! Much appreciated!

pm an email address-I'll send them over- havent got any love for posting websites this late into evening ..

barnsleyboy

robinbowes said:

barnsleyboy said:

robinbowes said:

Try replacing R17 with a 1K resistor. You can simulate that without soldering by connecting a 1.5K resistor in parallel.

Box of goodies arrived in the post today. 1.5k resistor in parallel with R17, no change. Same noises..... Grrrr!!

Bugger...

Now then, I put this to one side and through frustration didn't carry on with trying to find a solution. I've now started up on my quest again. I've replaced the stock pots with some new Alpha ones (slight issue with the bushing size being a bit smaller, so need to order up some additional nuts). The knobs now fit on properly without having to squeeze the splines together. No change to the hum situation, but it can only be an improvement.

I've tried the old R17 trick again, but this time connecting the 1.5k resistor using crocodile clips. Previously I had twisted the wires together, turned the amp on, heard the hum and dismissed it as not working. This time when I disconnect one of the clips there is a definite increase in the volume of the hum, so the change in resistance is definitely reducing the hum, but not eliminating it completely. The caveat to this is that the overall volume of the amp is also reduced, so by knocking down the amp volume it is having a corresponding effect on the hum i.e. amps quieter, therefore hum is quieter. I'm going to replace the 2.7k with a 1k as a temporary fix, but then it has got me thinking, could I nudge the resistor down even lower than 1k to further reduce the hum? Would this put strain on other components in the circuit and ultimately lead to something more catastrophic failing? Or is all this not getting to the root of the problem.

robinbowes said:

FYI, you might find this interesting: http://guitarder.fc2web.com/other2/o2-28-bass12.html

The website above is a very thorough investigation into what I believe is the same problem that I am having. He managed to narrow the problem down to the power amp stage by running a Lead 12 pre-amp through the Bass 12 power amp, and vice versa. He then compared the two circuit diagrams and saw that the only difference between the lead 12 and bass 12 power amp stage is the value of resistor R17 and capacitor C11. Hence his rationale for swapping out R17 for a different value. But, this only partially fixes the problem, and doesn't explain why the amp hums. Is it a design fault?

I have a Lead 12, so I suppose my next job is to open it up and forensically check that what he concluded is correct. I only have the rather scrappy hand drawn circuit diagram found on the internet, so I'm probably going to redraw it in Powerpoint so that I can decipher it.

The other thing I am struggling with is getting any kind of distortion from the amp just using the gain control and not overdriving it with pedals. I thought maybe the tones created by Mr Segborn on his youtube channel were produced by running everything at 10 (he does after all wear ear defenders on most of his videos), but I have seen videos of others producing a lovely overdriven sound without breaking the windows. Sorry to drag this one up again, but it's driving me bananas!

barnsleyboy

At last, I think I've solved it!!!

As with previous posts, R17 seems to be the culprit. The original value is 2.7k. According to my Japanese friend this should be 1k, which is supposed to be the same as the Lead 12. I opened up my own Lead 12 and discovered that R17 is 10k. I tried a 10k, but that didn't work (it hummed pretty loudly), Then a 1k which definitely reduced the hum, but doesn't eliminate it completely. Looking at the Lead 12 schematic though, it shows that R17 is 100 ohms. Hey presto, replacing R17 with 100 ohms, and it all works swimmingly, albeit that the amp is not as loud.

I then thought suppose there is a sweet spot between 100 ohms and 1k which I'm missing. I took out the 100 ohm and carefully connected a 4.7k pot. I then used the pot to dial in the point where the hum just disappears. Disconnected it all then took a reading across the pot. It came out at 110 ohms, so 100 ohms seems to be the correct value.

At last, an end to the saga. But it still begs the question, why? The 2.7k appears to be the wrong value, so was it a design fault? Is R17 perhaps masking another component in the signal chain that isn't working correctly? Why is it that when I google the Bass 12 and humming, nobody else seems to have the same problem? If anyone knows the answer, then please let me know. Many thanks for all your assistance along this rather rocky road.

ICBM

Be careful - those two resistors aren't the same positions. What you have as R17 is R18 in the later Lead schematic - and R16 is R17. R16/17 is 100 ohms in all versions of the circuit, R17/18 is 4K7 in the bass and 1K in the Lead.

Lowering the value of R17/18 increases negative feedback from the speaker output, which is why it reduces the volume - it's effectively reducing gain of the power stage, which is also why the hum stops. Lowering it as far as 100 ohms if the stock value is in the K range is probably not the right solution.

Since these two resistors form a potential divider going through C14 which limits the low frequency NFB, and C14 is electrolytic, it's possible that could have failed. Did you replace it along with the others? It's possible we've been chasing a problem in the wrong part of the circuit.

barnsleyboy

All the eloctrolytics have been replaced. C13, C14, C15 and C16 and the larger ones C18 and C19. After replacing them all, it didn't make the slightest difference.

There doesn't appear to be a Bass 12 schematic anywhere on the internet. The closest to it is the Marshall 20 watt Combo Series dated 30-3-84, which has some different pre-amp permetations. The Lead 12 schematic shows R17 as 100 ohms, but my Lead 12 has a 10k in the R17 position. The schematics are unfortunately all hand drawn (to be expected from the 80's), and the scans are not very good. Do you have access to the actual Bass 12 schematic?

My Japanese chappie (http://guitarder.fc2web.com/other2/o2-28-bass12.html) after a very thorough investigation concluded that it was R17, but like you say cahnging something with a k in it to 2.7k to 100 ohms is a pretty steep jump. I managed to get some pics from @JMP220478 of the innards of his Bass 12, but it was a later model so there were differences.

I don't really know what else to do with it other than leave the 100 ohm in there until a better solution comes up. If i take it in to an amp tech, they'll most probably charge me double what I paid for it and tell me there's nothing wrong with it

ICBM

barnsleyboy said:

There doesn't appear to be a Bass 12 schematic anywhere on the internet. The closest to it is the Marshall 20 watt Combo Series dated 30-3-84, which has some different pre-amp permetations. The Lead 12 schematic shows R17 as 100 ohms, but my Lead 12 has a 10k in the R17 position. The schematics are unfortunately all hand drawn (to be expected from the 80's), and the scans are not very good. Do you have access to the actual Bass 12 schematic?

That’s because of the difference in numbering schemes. I’ve got the schematics, I’ll post them later - although the Bass and Keyboard 12 one is barely legible. I did find them online but I can’t remember where…

barnsleyboy said:

I don't really know what else to do with it other than leave the 100 ohm in there until a better solution comes up.

I’m pretty sure it won’t cause any harm, although it will drastically restrict the output.

barnsleyboy

ICBM said:

That’s because of the difference in numbering schemes. I’ve got the schematics, I’ll post them later - although the Bass and Keyboard 12 one is barely legible. I did find them online but I can’t remember where…

Ah, top man! This is the only one that I can find:

https://el34world.com/charts/Schematics/files/Marshall/Marshall_bass20_20w_5502.pdf 

In terms of the restricted output, It's only for home use, so I can live with that for the time being. It's all just got me foxed on why my one does this! Might drop Marshall HQ an e-mail, but it's probably unlikely that they will be able to assist, but worth a try

ICBM

That’s essentially what I’ve got.  It shows R16 and R17, which are numbered 17 and 18 in the later Lead model. They form a potential divider from the output to ground via the 100-ohm resistor and C14, so the smaller the value of the other resistor, the greater the negative feedback voltage. It really shouldn’t need to be as low as 100 ohms though, that means 50% of the output voltage is being fed back.

barnsleyboy

This is the other schematic that I have:

https://el34world.com/charts/Schematics/files/Marshall/Marshall_lead12_12w_3005_5005.pdf 

This shows that R17 is 100 ohms which is why I tried that value. The thing that is confusing me is not having a specific Bass 12 schematic.

I'm going to have a go at producing one myself based on the board that I have got. I've imported a picture of the underside of the board into powerpoint and flipped it. I'm planning on drawing over the top of this to give me a simplified track layout. Then I'll try to piece it all together. Maybe it will make it clearer in my own head what's going on, although as I've said previously apart from the CSE grade 1 in electronics and some common sense, I'm feeling my way here!

ICBM

barnsleyboy said:

This is the other schematic that I have:

https://el34world.com/charts/Schematics/files/Marshall/Marshall_lead12_12w_3005_5005.pdf 

This shows that R17 is 100 ohms which is why I tried that value.

This is exactly what I've been trying to say! R17 in that version of the Lead 12 is R*16* in your one, so 100 ohms is *not* the value you're looking for. Look at the arrangement of the circuit, not the resistor numbers. R18 is the equivalent of your R17 and is 1K. (The cap is also labelled C15, not C14.)

The actual cause of the hum is somewhere else - it's just that it's being amplified through the power stage, so adding too much negative feedback suppresses it, but you've dealt with the symptom not the cause.

barnsleyboy

Sorry, that's just me being dense! I think for me to fully understand what's going on, I need to do my drawing exercise and work from there. Besides, it will be a good learning experience!

Probably a rookie question to ask, but is it likely that the solution could be a simple cold solder joint? Or do you think I am looking at a failed or failing component?

barnsleyboy

Sorry to resurect this one again, but I'm edging closer to a solution! Using a mix of photographs, powerpoint, DIY Layout creator and the Lead 12 schematic, I have at last managed to piece together the schematic for my Bass 12.

Here is the link to download if anyone is interested (I have left a copy of the Lead 12 schematic to compare)

https://flic.kr/s/aHsmXbkPNU

There are a few differences between the Bass and Lead versions, mostly around component numbering and the line out/ headphones/ DI jacks. I have marked up and extracted a section of the schematic below. This shows the Lead 12 numbering in yellow boxes, ticks against components which are identical and clouds around components whose values are different.

Assuming that the problem is within the power amp stage, there are 3 components whose values don't match the Lead 12 power amp. R17 still seems to be a prime suspect, but C9 and C12 which are both ceramic disc capacitors marked with the number 220, which I believe makes them 22pF are also different. I know that replacing R17 with a 1k makes a difference, and replacing it with 100 ohm makes a massive difference (albeit that it reduces the amps overall output). Could it be that setting R17 as 1k, and replacing C9 & C12 with 220pF is the golden bullet that will make everything hunky dory?

Any comments before I order up a couple of the appropriate ceramic discs would be very much appreciated!

ICBM

Those caps are too small a value to have any possible bearing on hum, they're there to prevent high-frequency instability.

Does the hum stop if you turn the volume control down to zero, or do you still have this 'null' position where it hums above and below but not at the null point? If so the hum is still something to do with the power supply or grounding.

barnsleyboy

The hum is there at zero, gradually reduces as it's rotated clockwise, then is back at 10. I've tried powering the Bass 12 with the Lead 12 power supply, but it was still there.

robinbowes

Those are the Miller capacitors. It wouldn't harm to try replacing with higher values, eg. 220pF as in the Lead 12.

R.

barnsleyboy

I'll order up some new ones and see if it makes a difference. I'll do a bit of reading up on Miller effect too so I can get my head round what it's all about. 

There are a couple of differences in the pre-amp stage too, so I might get a few different values just in case. Although I'm pretty certain that we have narrowed it down to the power amp, so changing these out will probably not make any difference.

Would there be any mileage in using an oscilloscope to check what the signal is doing at various points along the circuit? Maybe this would narrow down the search. 

robinbowes

The Miller capacitance is a function of the gain of the amp stage and the impedance presented to the amp stage. I think you said that the hum goes at certain positions of the volume control - perhaps that's something to do with impedance matching the input of the power amp section. Maybe changing those caps might help? Dunno - I'm speculating a lot here.

barnsleyboy

I'm happy to go with speculation! Another rookie question, does it matter what voltage rating they are? I'm about to order some 220pF rated at 50v.

ICBM

barnsleyboy said:

I'm happy to go with speculation! Another rookie question, does it matter what voltage rating they are? I'm about to order some 220pF rated at 50v.

The absolute maximum voltage in the circuit is 38VDC (between the two +/- 19V power rails) so 50V is adequate anywhere in the amp.