Shobud Amp SB15 (Solid State) volume fluctuates

[PRNDL]

On the bench is a Shobud SB-15 transistor amp highly prized and used every day by a steel guitar player.

The main problem was fluctuating volume, although he said it also gave shocks when he touched the chassis or used a volume pedal. Unfortunately, he had spliced in a 3-prong cord to the old 2-prong cable and clipped off the tip.

After properly installing a 3-prong cable with the ground on the chassis, the amp sounds fine. The transistors and 2 power supply electrolytics tested good (I assumed the PS caps were gone, but maybe not).

My question is whether the volume issue could be caused by bad grounding from the 2-prong cord.

(Also, a schematic would be nice if anyone has one).

Thanks!!

UPDATE
The owner states that the amp volume fluctuates (usually cuts in half) after about 30 minutes of playing. I only tested it for a few minutes.

The cap cans are two Mallory 4300µ 50VDC 65 surge.

The output transistors are a pair of RCA 40636, which seem to be quite hard to find.

He wanted the caps to be replaced just in case, because the amp is so old.

I'm wondering if lower voltage electrolytics last a lot longer than high voltage ones used in tube amps (and these are just fine).

[Enzo]

Dried up caps will tend to make the amps hummy - as in they don't filter as well. They could even congribute to amp instability. But I can't think of a way they'd make the volume hop up and down. And the third wire ground won't make things louder and softer either.

So the amp works fine for you? PLug a CD player into it and use it as a shop jukebox for the afternoon while you work on other stuff. You'll hear if it drops or not.

Have we explored the rest of the guy's rig? His cables. Does he use a volume pedal?

If the amp has effects loop return or power amp in jacks, check the shunt contacts on those.

If it will do it for you, then explore the signal path.

[PRNDL]

Thanks Enzo - you confirmed my ideas and added a few more things to consider.

It's a pretty simple amp - no effects loop. I'm fairly certain the grounding was the main issue - electric shocks from the chassis is not good.

The other suspects are the cables and volume pedal, but he didn't bring them. I'll ask that he does bring them when picking up the amp so we can test them.

The most distressing issue I've found is when an amp has to come back for more repairs, which is why I posted. Also, solid state amps are a bit different than tubes.

This amp has some surprising tone -- it has yellow Mallory caps, carbon comp resistors and a 15" JBL speaker. Mostly it sounds like transistor cheese, but there's something interesting in the high end and response that probably works well with steel guitar.

That and it's light weight.

[Enzo]

Steel guys are OK with solid state.

I often invite people to bring their rig along and set it up in the warehouse here. After all, a large percentage of problems in systems is the connections between parts.


I recently had a little Begringer powered mixer in for warranty service from a local store. Customer complains one power amp channel is out, no sound from speaker. Theres nothing wrong with this thing, NOTHING I can do to it makes the sound even remotely pause, let alone go missing. Send it back to customer, still not working. "Does this "Enzo" guy even TEST these things? Did he just hook a meter to it or something?" Word for word. Back it came - nothing wrong. I talk to the dealer and tell them they are welcome to set the whole thing up here to demo the problem, and did they test all their cords and speakers?

Customer claims he knows what he is doing, he is an electrical engineer, after all. Obviously Enzo doesn;t know what he is doing. SO the dealer contacts Behringer and gets an exchange for a new unit. Customer gets it and comes back the next day - "This one doesn;t work either. This one has a dead channel too." I told the sales rep to have them set it up for him. I never saw it again, but my inside guy told me they came over and set it up and sure enough one channel made no sound. Then the store people pointed out that one of the speaker cables was hte wrong type, and...here./.. connected a proper one and sound was restored.

I suggested that the least the dealer could do was infirm Behringer that the mixer they had returned was 100% functional - user error.


Long story to point out that customers often zero in on a piece of a system without considering other parts that could be involved.


Maybe this schematic will help, I bet the power amps are similar anyway.

40636 looks like RCA numbers, but I don;t see them in my RCA guide (1980), neither do I se the 40594/95. They do look up in my ECG cross for what that's worth. Are they TO3s? If so, I'd just replace them with MJ15003 or something and move on unless they are working.

Unlike tubes where you can lose one and the other tube carries on, in a solid state amp, if you lost one of hte output transistors, you'd either blow fuses, or you'd have severe distortion - half the waveform missing - not just a loss of volume. SO I suspect the PA is OK.

[gbono]

Sho Bud

If the schematic that is shown is close to the SB15 then I would look at the input/preamp section. JETs (God forbid MOSFETs) are very susceptible to ESD they usually get noiser over time but I have seen them degrade the input after ESD exposure and eventually fail.

I have a $30 Chinese infrared temperature gun (with laser pointer). I find this tool invaluable for scanning boards for hot spots (not as good as a real IR camera). You can check differential/relative changes in component temps quickly and fairly accurately.

I usually run a 1000Hz sine wave into a troublesome/intermittent amplifier at 1/8 power and let it "burn-in" for an hour with a scope on the preamp/PA outputs and a DMM on the supply rail.

[olddawg]

Scope channel 1 on the preamp signal. Scope channel 2 on the output signal. 1khz tone into the input. Unit on load bank. Turn up the input signal until the amp clips and back it up around 85% or there abouts. Let it cook. Use a heat gun on the preamp maybe even restrict cooling (but be careful). Check for corroded input jacks and ground points.

Enzo...I used to have a stack of printed warnings that went out with any amp with an output failure or no problem found that stated that I would not even look at their unit again unless thay brought in their speakers and wiring for inspection. I had played the game you described too many times, the only thing worse than hearing them say, "I'm an engineer" is, "my husband or dad's an engineer."

[Enzo]

I was thinking of dumb customer stories on the way to work this evening. I got the phone call yesterday.

Hi, is this the repair shop
yes
is my amp done?
which one is it.
It's a Fender
can you be a bit more specific?
It's black and has two speakers. I talked to you about it the other day.
OK then , how about your name then, what's your name?
Bob...
Hi Bob, um do you have a last name too?

[PRNDL]

The amp sounds great.

I've found some NOS 40636 transistors for about $5 each, but am wondering if it's really necessary to replace them when the amp sounds this good. It's the same issue with the capacitors.

[Enzo]

If the amp sounds good and you have cured the up and down [problem, then I wouldn;t swap out the transistors.