Rickenbacker RB55 maybe

[mtlbasslad]

Good day all,

I put ? in the title because that's what the sticker on the back says - but I find no reference to this model on Rick site. However, there is an RB60 which seems similar...
Except that my power amp board is missing some parts on the schematic, IC Q13 for instance. By missing I mean no place to mount it...

Amp works OK with bass, but when plugging in extension cable from computer I get noise - hard to describe but a bit of crackling & whining. I'm calling it the sound your Rice Crispies makes when milk poured on accompanied by small child whining LOL.

Plugging in same cable to Joan's Fishman Loudbox no more noise, so not cable.

First suspect components? Bring on the ideas...

19334.pdf

19335.pdf

Thanks

[g1]

Do you hear any of that noise with bass at all?
If not, it is possible there is some kind of noise getting in from the computer that the Loudbox has better shielding against, or different ground scheme.

Is there a schematic available for the RB60 preamp?

[mtlbasslad]

Hello g1, here is the preamp -

19326.pdf

I'm curious about L1, never seen that in a power amp before... looks like a good antenna maybe?

[Mick Bailey]

L1 and the parallel resistor are part of the stability circuit to prevent oscillation.

[g1]

Any of those noises at all when not connected to computer?
Again I am wondering if it has poor shielding/filtering against digital type noise, where the Loudbox was designed when that kind of interference was more of an issue.

[The Dude]

FWIW: I often encounter a similar problem when using my computer at work as an audio source. In almost all cases, it's not a fault in the amp, but the computer radiating some garbage noise. I've learned to ignore it and it's never been a problem. Not saying this is the case here, but certainly could be. IMO, if you aren't going to be using the amp that way (with a PC as an input), I wouldn't sweat it.

[mtlbasslad]

Thanks for the info folks. I'll be putting it back together today - only problem I found was loose transformer mounting bolts...

Notice that the schematic says 'all caps 25v' but the layout specs 40v for the big filter caps - which is not much above the 36v rails, hmm.
Can someone enlighten me as to the purpose of R36 & R37? Bias & ?

Also note the 'parts not used' thing - by adding 2 more 4700uf caps the amp becomes a 90 watter? Really, with no other changes?

Info on this amp is pretty rare, but seems it was made 1986-89 in Santa Ana California - not so many home computers around then...
We will reserve it for what it was made for - bass.

[Enzo]

Note on schematic and layout states: all caps 25v unless marked otherwise.

Cap ratings are in "working volts", meaning 40WV caps are perfectly happy running at 40v. All the caps have surge ratings too, so your 40v cap might have a 50v surge rating, so for example when first turned on and before loading, the voltage on the cap may swing above the 40v for a bit, but settles back down. That is OK.

1k trimmer R37 is indeed the bias adjust. I don't see R36, point it out please. Oops, I found it. That trimmer is to center the output offset. Adjust it so zero volts DC is on the speaker lead.

Adding two filter caps doesn't increase the power. However if they use this board in another model with more powr, it might need more filtration. And likelier a heavier transformer.

[mtlbasslad]

Thanks Enzo, my education continues...

I will check & adjust R36, what about bias R37?

Another thing I noticed but forgot to mention - this amp has a 2 piece chassis, preamp up front & power amp at the rear - separated by air & wood, none of the connections shielded. Possible cause of strange noises? Would it be beneficial to line the cavity with some sort of shielding ? Would sheet aluminum work or would it have to be ferrous ?

Did you ever see one of these somewhat rare amps?

Hope the snow is not too deep in your neck of the woods!

Cheers, Lorne

[mtlbasslad]

Dc voltage at speaker lead is 0.007v - close enough to leave it alone?

[g1]

That should be close enough to zero.
For bias, what is the no signal DC voltage across each of the output transistor's emitter resistors (R27 & R29) ?

[mtlbasslad]

DC voltage across R27 & R29 is 0.001v... but I had to take the PA board off the heatsink for access - not sure if I removed a ground there...

[g1]

With .27 ohm emitter resistors, that works out to about 3mA per transistor. It's probably fine. If you had a scope you could check for crossover notch but I wouldn't worry about it.
If there is objectionable crossover distortion you will most likely notice it on the final decay of notes.

[mtlbasslad]

Ah, but I do have a Hantek dual trace Oscope... not the super deluxe gazillion mhz needed for digital work, but enough for audio. Why would I want to work on digital stuff? I still wear an analog watch LOL
Of course I'm still a little shaky about how to use the scope...

[Enzo]

If you never use the scope, you never learn to play it.