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

[chazpope]

Thanks guys for detailed instructions:

disturbance is always in the same (-) direction followed by the other direction - lasts a second or so - then goes to 0 with or without speaker

voltages D4 D5 stable, U7 not in socket, +-16V stable, U3 pin 7 stable

disturbance traced back to through Q7,8 Q21,20 to R89 where it goes from 0 to several volts, up to collector of Q22 where it goes from 16 to 10V, while the other side of R97 at U7pin5 stays stable at 16v

Went straight to disconnect R89 - problem is gone!

Now do you think that if I fail to fix the thing I could just play it without the bloody limiter

thanks

[Steve Conner]

Does the problem go away when U7 is removed from its socket?

If so, based on what you said, I'd suspect U7 is faulty and has massive control voltage feedthrough. This also fits Murphy's Law of electronics as the CA3080 is obsolete.

The circuit consists of a pair of Baker clamps that give smooth clipping behaviour by preventing the power amp transistors from saturating, and also provide "clipping present" outputs that activate a compressor based on the CA3080 (used as a voltage controlled amplifier)

Seems quite similar to Peavey's DDT, indeed the slow attack on the compressor is straight out of the Hartley Peavey white papers.

Edit: I think D17 and D18 are backwards.

[Jazz P Bass]

Peavey does have a CA3080 substitute available.
It is an ungainly "standup" module, but it works.

[chazpope]

Steve, no I meant 'not installed in socket' so I have not removed it. Signal flow wise does the limiter work left to right on the schematic or vice versa ? Asking this because I find pin 5 of U7 to be not shifting voltage while after R97 the collector of darlington Q22 is. Am I misreading the B arrows - do they mean they are all connected to earth or what?

[Steve Conner]

Well, I'm asking you to make another test. Put R89 (or whatever it was) back in circuit, then take the CA3080 out, and tell us if the problem is still present. If the problem goes away, then the CA3080 is dud.

The limiter works from right to left. The Baker clamp transistors are the detectors, and the CA3080 is the thing that modulates the amp's gain. The result is a feedback loop: "Am I clipping heavily? Then I'll turn my own volume down."

The CA3080 is a current controlled amplifier, so you won't see the voltage at pin 7 change (by more than a few 10s of millivolts) as it does its thing. Pin 7 is the base of a transistor, or the emitter or something. It might not seem to be doing anything, but the more current you pull out of it, the higher the gain of the 3080 will go.

[chazpope]

yes I will do that, might be in a day or two. Tonight I have a bass gig and maybe I'll play this baby and see what happens. Looks like a replacement CA3080 will be hard to find anyhow and changing it won't be fun - on this pcb the tiniest force makes the pads come off - I have not seen anything like this before it's as if they are glued with spit and held simply by the fact that wires are crudely bend behind holes. Around the cement resistors it had all fallen off and I had to put wires - but then my much posher Hotrod had same problem.

[J M Fahey]

Well, *worst* case , youŽll have a limiter-less Bass amp.
Essential in my book, but must acknowledge that there are still tons of such amps out there ... and people keeps using them.
Not the best, but you still have an usable amp.
Before, it was useless and dangerous, those DC transients might kill your speaker.
If you canŽt find the 3080, you might make a small board (perfboard is fine) with the Crate passive Opto Limiter and stash it somewhere inside your chassis.
Works like a charm.

[chazpope]

Steve, I took the 3080 out and reconnected R89 - it works OK - so like you say U7 must be toast. Thank you for your detailed explanation of how this works - it has saved me a lot of time and it's been educational as well. Still beats me how it did its nasty business to the main circuit - must have been through C25 which explains why it lasted just a bit.

Before I took the chip out I played it at a gig and it was OK, so I will give some time and see how it works out. Not sure if I will find the same chip and not sure whether to put it back - I am a guitarist and distortion is ok with me also I read somewhere that the amp is kind of lifeless because of the limitler.

Thanks everybody who helped - I would like to think I have a new amp now ! Going away for Xmas and fill think what to do with it after

Happy Xmas everyone!

[chazpope]

just a quick update that the thing has worked ok at several gigs, mostly played at full volume. I have already forgotten that its limiter has been exorcised.

It is not a great sounding amp, though it is louder than a Trace Commando 100W, probably just due to its bigger speaker.

[g1]

I'm seeing lots of ca3080's on ebay for a couple bucks each, free shipping.