Fender BXR 200 Bass Head

[TomCarlos]

Hi Friends...

I have a Fender BXR 200 Bass Head that seems to have a couple problems (different problems). The main symptom is no output.

First, using a Bulb limiter, it stays sorta bright. I then removed that and am using my Kill A Watt meter, I checked for idle current- .70 amps.

Next, the plus and minus 57 vdc - those measure ok.

Then, the plus and minus 15 vdc supplies - they show plus and minus 10.5 volts. If I disconnect the cable that goes to the preamp board, I get back to the plus and minus 15 vdc. So it looks like there is an issue with the preamp board.

For now, I am thinking I can debug the power amp board with the preamp board disconnected?

Tom

[g1]

Tom, you will need to zip that file, it's not working.
And please verify, you are no longer using the bulb limiter?

[TomCarlos]

Correct... Bulb limiter removed. Not sure what happened... maybe some space debris on the preamp circuit board, but I now have it connected and seeing my Plus and Minus 15vdc.

I still have a very low output. So I will begin to trace the signal all the way through. Looks like the schematic calls for 4mVrms at the input. Let me see what I get coming out of the preamp. Will report back .

Here is the zip file of the schematic.

[TomCarlos]

Looks like the preamp is sending a signal to the Power Amp Board.

On the Power Amp board, TP1 and TP2 look ok.

TP3 is a tad low: -14.7 volts.
TP4 is a tad low: +14.3 volts

There is very little AC signal at TP5- only 100mVrms.

On the Output Transistors:

Q15 (2SC3281)
Collector: 57 vdc
Base: .6 vdc
Emitter: .1 vdc

Q17 (2SC3281)
Collector: 57 vdc
Base: .6 vdc
Emitter: .1 vdc

Q16 (2SA1382)
Collector: -57 vdc
Base: -.4 vdc
Emitter: .0 vdc

Q18 (2SA1382)
Collector: -57 vdc
Base: -.4 vdc
Emitter: .0 vdc

So it looks like the Output Transistors are not turned on? I will need to check everything feeding those transistors.

[Enzo]

Let's not get into a sea of transistor voltages, let's troubleshoot instead.

Yu have little or no signal at the output. You have signal at the preamp out? Is signal present at pin 4 of the 8-pin ribbon at the power amp end, TP1? Is it present at pin 7 of U1b TP2, upper right on powr amp drawing? for that matter did we try the patch cord across the FX loop jacks?

If signal at least gets to the board, but there is little at TP2, then try removing U2. it is the compressor IC. I guess first, does turning the compressor switch off make a difference?

See P2B, upper left? That sends the signal back to the preamp for the chorus, and it returns in the same connector. So sent out pin 1 and back through pin 3. Signal at pin 3? Pull P2B off and short pin 1 to 3. Output restored?

The amp could be stuck in mute. Middle left edge is Q1, and it could be shorted or stuck on. With the amp running, measure resistance from source to drain. Easiest way is to measure resistance of R6. If R6 measures some high resistance, good. If R6 measures something low like 100-200 ohms or something, then the transistor is bad or stuck on. Either way, if we remove Q1, then does output return?

otherwise, is there signal at R5? At R8? At the collectors of Q2 and Q3?

basically we want to get signal INTO the power amp, and then follow it left to right across the power amp to see where we lose it.

That is the whole point of the whole post - find out WHERE the signal goes away. Then we can figure out why it goes away there a lot easier than guessing.

[TomCarlos]

Hi Enzo...

Yes, there is signal at TP2.

I have a jumper across the Send Receive jacks.

I checked the compressor switch and circuit. I see a change in the output. So it looks like this is working.

I pulled the P2B jumper and shorted pins 1 and 3. No change.

With the amp running, I checked the resistance across R6 - 25 ohms. Looks very low. So I will pull Q1 and test.

I will also finish testing the items you mentioned above.

Thank you, Tom

[Enzo]

OK, stop at Q1. With only 25 ohms across the signal path there, no wonder nothing comes out the amp.

Now, is Q1 bad or is it just stuck on? before removing Q1, measure voltage at its gate. (Voltage at the righthand end of R7)

Remember, a JFET is normally ON (low resistance drain to source) until it is turned OFF by voltage at its gate. So with no voltage at the gate of Q1, we'd expect that low resistance. But if we put a negative voltage there, it turns the thing off (high resistance). I have no idea what the voltage might be, I'll guess about -20v.

Lower left, CR46, makes a negative supply, filtered by C34. Note the polarities, this is a negative supply. Through 1M R7 that charges timing cap C5. The 1M and the 4.7uf C5 takes a moment to charge up. Once it does, that turns off the gate of Q1. this brief delay is your power on mute. it takes a second before it lets signal into the power amp, so you don't get a loud turn-on thump. So then, now that we know how it works, do we have a negative supply at one end of R7? And at the other?

If you have no voltage on the gate, find out why. Is C5 shorted? (low resistance from the right end of R7 to ground) Or is Q1 shorted grounding the gate? If there is voltage at the gate, then Q1 doesn't work and should be replaced.

Since Q1 is a mute, it is not necessary for amp function, so during tests, you can run the amp without it.

[TomCarlos]

Once again Enzo, you got it. Dang, you are good.

There was 0 volts at the Gate. So I traced that voltage source all the way back to the F2 and F3 fuses and one of them was open. I guess you can still get a plus and minus 15 volts on the pos and neg regulators? I will test that again.

And you where CLOSE- I get -24vdc at the Gate.

The amp is working. So now, I will give it my customary 3 day test cycle before giving it back to my friend.

You saved me TONS of time!!!

Tom

[g1]

If you have a chance, pop F2 out. See if the Q1 gate voltage goes away, and see what you have for the +/- 15V. I think it may be there but weak, or it will fall off when doing any work.

[Enzo]

Look at the schematic, the fuses are served by a center tapped winding, one fuse per leg. Open ONE fuse and you still have half wave from the bridge. But the little mute supply comes off one of the fuses, not both. So lose that fuse, you lose the mute. Lose the other fuse instead and you still have the mute, but potentially some ripple might creep into your 15v supplies.

[TomCarlos]

Hi Greg...

Enzo replied and I confirmed by testing. If you remove F2, that removes voltage at the Q1 Gate. However, you will still see the +/- 15vdc on the pos and neg regulators. That is what I missed. I was seeing the voltage but did not realize only 1 side of the bridge was feeding it. And this explains why I was seeing less than 15 vdc on the op amps on the Power Amp board.

Besides fixin' the amp, this will make a very good lessons learned entry in my log book!

[Enzo]

Tom, the lesson I want you to take away is that troubleshooting is a systematic process, where we work to isolate the problem. We narrowed things down to Q1, then we decided whether it was a bad Q1 or a bad circuit, and poof, there you are.