Fender Bassman 200 'Pulsing' Noise

[Sloan]

Hi,
I'm working on a Bassman 200 solid state bass combo amp. After about 90 seconds it begins to produce some noise. I'm pretty sure it's in the power section since it happens even with the preamp pcb disconnected.

I've replaced the two large filter caps and some of the other caps on the power board. Someone had previously changed two 1000uf caps to 47uf caps I guess to try and calm the situation, but it only really obscured it, since now i can actually hear all the noise happening. haha.

Unfortunately I do not have an internet connection at home which makes referencing the schematic for you guys a bit difficult.

I've recorded a clip of the noise, this is with the master volume all the way down and nothing plugged into the amp while it just sits there idle. I've compressed/limited the track to bring the overall volume up so it's easier to hear. This is all coming through the drivers.

I'm wondering if I should go ahead and replace the other few electrolytics on the board, or if this sounds like something completely different. I haven't ever had a pulsating noise problem like this before so I'm kinda stuck.

Any help would be greatly appreciated and thanks in advance.

[Sloan]

Is this the correct forum for this kinda thing, or should it go in guitar amp> trouleshooting?

[Enzo]

This is the place.

Are you sure those 47uf caps were someones substitution? The two main filters are 3300uf, the two low voltage filters are 1000uf, and the two filters after the voltage regulators are indeed 47uf.

Don't just throw parts at it. What if you spend ten hours and a hundred dollars on cap changes only to find out there is a broken ground trace on the board? FIND OUT what is wrong and fix that.

Are your two main power rails up to voltage and even? ABout 57v each ,more or less, and with similar ripple on each? Are both 15v rails up to voltage and even and free of ripple?

I have no way to listen to your noise, but if it is periodic, is there any indication on the 15v rails of its existence?

Plug your guitar into the FX return with the guitar volume control to zero. ANy different? leave it there to isolate the power amp.

Lower left of PA circuit, see D15 there? DO you have the 40v at the test point, and is it steady?

[Enzo]

Here

[Sloan]

All soldering looks solid and i've poked around to see if there was anything loose with no results. I've isolated it to the power amp as described, even running the amp without the preamp board hooked up produces the same results.

The actual signal from a bass guitar is very strong, but the noise obviously interferes since it's kind of a full range thing. The amp works perfect ex

It sounded like it *could* be a filter cap problem so I went ahead and eliminated that possibility. The caps that someone previously replaced were 1000uf in the schematic, but yeah there are two other 47uf caps that i haven't replaced.

The sound wasn't 'pulsing' before I upped those two caps to the correct value, before that it was kind of just some static.


The file I attached is an mp3, so it should play in any common audio player.


I'll go through and check some of those test points then report back when i can. Just kind of see if anyone else had info based on the type of noise to cut down on wasting a bunch of time on this thing.

[Sloan]

Lower left of PA circuit, see D15 there? DO you have the 40v at the test point, and is it steady?

I checked this. It's about 48.8 when quiet, then about 50 when the noise happens.

[Jazz P Bass]

D 15 Test Point

Can you measure any Vac ripple on the test point?
I am thinking it will go up when the Vdc goes higher.

[Sloan]

Thanks for the help everyone. It turned out the problem was a loose Q11. The pcb's in this amp are not really that good and there are traces coming up everywhere, so i had to kinda make some jumpers from Q11 to the adjacent capacitor (C19?) and it seems to be working ok. i'm really surprised i didn't catch this earlier but it was kind of tricky since the noise was fading in and out.

[NZimmerman]

Last post mentions Q11. Working an Amp where Q10 and Q11 are hot enough as to not be touchable. Circuit board is discolored but not burnt. Listened with no issues for about 3 hours today but then got distortion and noise. More static then buzz. No adjustment possible with pots or switches. Blank Plug inserted into rear "effects" jack produces no change in output. I assume the problem is on the main amp. Can anyone comment as to the temp of Q10-Q11? Both have been resoldered as well as adjacent areas. Spending way too much time on this unit... Nothing new there though... Problem does come and go. Sounds great when it's great.. Thanks for any advice. -15 Reg was changed. +/- Reg caps replaced. 58Volt caps not changed but test good for value and leakage / ESR.

[Jazz P Bass]

Can you measure the static voltages around Q10 & 11 & compare them to the schematic?
The two transistors form part of the bias string.
Q10 & 1 should not get that hot, they are only rated for 1 watt/300V.
MPSW92 & MPSW42.

[NZimmerman]

I had Q9 pulled to check out of circuit. Noticed when I went to resolder it Part was a 2SC4793 so is Q14. Print shows thenm to be 2sc3298A. Anyway, I was all set up to read static voltages after resoldering the back rail parts and smoke came from more than one location. (sigh).. Looks like it is time to take it all back apart and get out the ohm meter. I'd prefer to run it on a bench where I can see whats up. Question: I believe it is reasonable to run just the Main Amp board with out connecting the Preamp and rear panel PCB. Opinion Please. Thanks!