Fender Rumble 100 bass amp - what causes DC shift on speaker ?

[chazpope]

Hi I obtained one of these in a dead state (cement resistors were dislodged) and basically got it to work by connecting the perished pcb lines with wires. So now it works - it can play really loud and all seems fine,

however ..


when played really loud (like you would at a gig) it tends to 'miss' notes - instead the speaker does a thump -moves in one direction and recovers in about a second, it then continues to play ok until the next peak. this seems to happen for repeated loud notes, or any attempt at slap playing that causes loud peaks. I am sure what happens is the DC state of the mid speaker point gets suddenly shifted - but I am not sure why. Could it be that the big caps 3300mf (or one of them) cannot cope with the load (they seem too small for such am amp anyway) or could it be that the 'limiter' circuit this amp has s playing up -

any ideas ?

[Jazz P Bass]

It sounds like the amp is "turning on" fully in one direction.
Which would be the output section of the amplifier.
I'd stop playing it.

[chazpope]

well yes it is the power amp, point is - how do I fix it ? if that's any indication speaker seems to be going in one way only

[Jazz P Bass]

Repairing the amp is going to require finding what is wrong.
You will need test equipment.
A volt meter at least.
Verifing Vdc power supplies is a good start.
Maybe one of the rails is collapsing.
It may be bad soldering.
Flip the board over & inspect the solder joints.
Sometimes there will be bent over leads that touch the next circuit trace.
These should be lifted up off the trace.

[chazpope]

well I thought I did all that before I posted. the problem happens only under load - at almost every peak, it is certainly not mechanically caused and also because it happens so fast it's hard to replicate for any length of time. Maybe I will try generating a constant signal with my computer to get it to stay there

[Jazz P Bass]

You did all what?
Did you monitor the + & - high voltage rails as the amp fails.
Volts dc should stay stiff.
Volts ac should be small (10's of mv's)
Did you resolder the power amp section.
I would.
If the high voltage rails do not budge at amp failure, move on to the preamp voltages + & - 15 Vdc.
Volts ac should be zero on these rails.
If this does not show any related symptoms, monitor the base voltage of the driver transistors.
If one of them goes falooey, that will certainly cause the speaker to move one way.
Please try to understand that there is not a flow chart to repair amps.
You have to find a symptom.
Then you can find the cause.

[52 Bill]

Is it possible to recreate the problem by mechanical means? Is it possible that there is still something loose on the pc board that is being moved when the amp is driven hard?

[Steve Conner]

Good point. Try bashing the amp while it's powered up, shake it, kick it, whatever. If that reproduces the fault, then there's another bad solder joint that you missed.

Another common trick is the chopstick test. Power it up with the circuit board exposed, and poke at the components with some sort of insulated stick. If some part has a bad solder joint, the amp will go crazy when you prod it.

Also try putting pressure on the circuit board itself to flex it. There may be a hairline crack in it. I recently fixed a drum machine like this, the previous owner did his best, but missed 3 cracked traces.

If that doesn't help, we're looking at some sort of weird electronic latchup.

[Enzo]

I'm with them, first MAKE SURE there is no mechanical sensitivity. Ball up yout fist and whack it, or use a rubber mallet. Pick up one corner of the amp a couple inches and rop it on the bench. If it ignores that, then it is electrical.

You note the output goes to DC but lloking at the speaker cone. Fair enoug. Now do it without a speaker and watch with a scope. Does it still do it, or only under load? If I had to guess, I;d say some semiconductor junction was breaking down under current.

[chazpope]

thanks guys. I am not a pro but I have built several solid state amps up to 200w, mostly 25-30 ago, and I have repaired probably another several amps that have been rejected as this one by shops as beyond repair.

I will re-do mechanical tests, and test the voltages under load (not sure yet how - I do not have a scope) - and report. I was hoping there may be an easy explanation that I am not aware of

Jazz p wouldn't you expect the voltages to be dropping with load anyway and even if one does more than the other it could be a consequence of the circuit behaving unsymetrically, who knows.

thanks - to be continued

[chazpope]

ok so - it's not mechanical, main +/_44 volts are fine, dipping at high volume to say 40 and when the crap happens - to 35.

next I suspected it may go in the crap state without speaker - and yes - it does it without speaker !

I can see that voltages on the bases of Q14 and Q15 stay low at normal play but any peaks send them wildly off by +-20 volts. This lasts a second and I can only see it with analogue meter. It's as if anticipates peaks and goes cuckoo before the speaker has made a sound. resoldered the whole pcb - no change. I cannnot quite comprehend the limiter circuit - I wonder if it can be disconnected for a test. run out of ideas for today and also it is midnight and I probably woke the neighbours already

whoever can help - the schematic is here under the 'Fenser rumble' (sic) thread

[g1]

Are you saying when the "crap happens" that the minus rail dips to 35 but the positive rail stays at 40? If so the DC appearing at the output is negative?

[chazpope]

no, both voltages behave the same - dip for a bit then recover. this I think is a consequence rather than a cause of the amp being destabilized and drawing more current for a bit

[g1]

OK, so both supply rails are doing the same thing. When the DC offset occurs, is it always the same direction (+ or -)? Or when the speaker was connected did it always stall in the in or out position? Or is it random (+ or - DC offset can both occur)?

[mozwell]

What happens to the voltage across D4 & D5 when this all happens.
If you temporarily disconnect D17 & D18 does this still happen
If you temporarily disconnect R89 does this still happen.
Is U7 in a socket, can you temporarily remove it, does the problem still happen.
Are the +/-16V rails constant, or do they dip when your problem occurs
Also check the preamp output U3 pin 7, does this go weird when your problem happens.

If you havent done it already, plug a lead between preamp out & pwoer amp in sockets, insert & remove the plug into each jack several times

It could be an issue with teh limiter (U7) or the soft start circuit for the constant current transistor Q3, faulty PTC RT2, etc