Kustom 200 Blowing Fuses

[rf7]

This amp blows it's fuse, and I've narrowed it down to the power amp PC703 board, but I pulled the board and everything measures OK on it. The output devices check out also.

I originally found CR702 was open and R724 measured fine, but looked so toasted, that I replaced it.

What is the best way to proceed on this? I can't power it up with the board in circuit, as it draws too much current. Power supply puts out about +/- 37V dc.

Schematic here!

[R.G.]

1. Build a light bulb limiter and use it so you don't keep blowing fuses.
2. Install a shorting wire from base of Q704 to base of Q705, shorting all the biasing diodes. This will cause bad crossover distortion, but the amp should work and pass signals bigger than the crossover region.
3. When the amp otherwise works except for the crossover distortion, start reintroducing CR700, CR701, then CR702 in that order, checking for operation at each point.

It is likely that some other part in the biasing circuit (CR700-CR702) is having a problem, especially if CR702 was open. Opening the bias string turns the outputs all on simultaneously, and then Bad Things happen.

[Enzo]

Also, DO NOT connect a speaker or other load to the amp until you are sure it is not making DC on the output. An amp with DC on its output usually doesn;t draw much current without a load.

[rf7]

Thanks for the replies guys!

Even with the bias shorted, it draws too much current.

I brought it up without the output devices, and I'm getting -19Vdc on the output. I replaced Q707, because it measured weird with voltage applied. Now it measures better, but I've still got -19Vdc. I'm figuring Q705 and Q709 are the next likely candidates.

[R.G.]

I brought it up without the output devices, and I'm getting -19Vdc on the output. I replaced Q707, because it measured weird with voltage applied. Now it measures better, but I've still got -19Vdc. I'm figuring Q705 and Q709 are the next likely candidates.

If it's stable with no load and the output devices not there, let's cut to the chase. Measure the pin voltages on all the transistor and diode pins and post them here. Any DC offset problem will show up there.

[Enzo]

Let's not go on an easter egg hunt. Let's find the problem and repair it. Guessing, is it this one? and just replacing parts is not very efficient. It may not be a bad transistor at all, an open resistor can be every bit as deadly. Or a cracked trace.

Just replacing lots of parts is a good way to introduce more problems into the amp.

Just checking junction drops with your diode test on your meter will reveal many bad semiconductors. COnsider that outside of the ones in that incrediby complex low volts power supply, and absent the output devices, there are only 10 transistors in the amp. That takes a couple minutes to go down the row and check each.

You already shorted out the bias, good, now unsolder and lift CR703 and 704 to disable the two limiter circuits. They could be involved, so get rid of them for now.

You have -19v on the output, don;t assume that is because some negative side part is turned ON, it could also be some positive side part CAN'T turn on, or is open. It may well BE something on the V- side, but we can't assume that, not yet. The output stage is poised and balanced, so like a stretched rubber band, if one end is let go, the whole thing snaps the other way. SO a large offset could be something pulling it that way, or it could be the other side unable to pull it back.

[R.G.]

+1
Replacing components on suspicion without any evidence that they're the real problem is an expensive way to fix things, in all but one case.

I distinguish between three main techniques for repairing anything.
1. Based on the description of the problem, your experience with the circuits involved, and on a deep understanding of the circuit and components involved, form a theory of what few components could be bad, then test only those.
2. DC conditions test: measure the DC voltages on every pin of every active device, and then from an understanding of what devices can and can't do, and what the DC conditions tell you about the devices, pick out the few devices that could be causing the erroneous DC conditions.
3. Divide and conquer: Remove confusing, obscuring, and confounding sections of the circuit from the problem by temporarily disconnecting or inactivating them to remove them as contenders for the defect.

There is actually a fourth way that can be used only in special cases.

You started with approach #1. I've done this, and it works to some degree on some circuits. However, the stock Linn architecture solid state power amp is so interconnected inside that it is very, very difficult to make this work. The interconnectedness of SS power amps is what leads them to have a chain of destruction where a flaw in the output stage kills the output devices, this kills the drivers, and this kills the voltage amplifier, and so on. Resistors and caps can be overstressed and damaged along the way, and this can happen in milliseconds.

Enzo is suggesting changing to #3. SS power amps work fine without a load, so remove the load. I suggested shorting the bias string in the same vein, that prevents bias failures from causing issues. Enzo quite reasonably suggests removing the limiting circuitry, which may be making even a good transistor act bad. You even used this by removing the output devices. This approach WILL get you there, eventually.

My comment on reading all the DC voltages is based on a technique I use to get beginner circuit makers to be able to diagnose their effects circuits. If you know how the components act, and you know all the pin voltages, then when you find a component with wrong pin voltages, you know that there's something wrong right there. This is the reason service schematics have typical DC voltages noted on the schematic in the first place.

There are a few logic rules that go with this. You have to know what the voltages on a working/good bipolar, jfet, MOSFET, opamp, etc. look like for comparison. But measuring all the DC voltages is a deadly effective way to find things you didn't think of as a possible problem; like DC on the input to that amp and a shorted input cap, maybe.

The fourth method is economics based. Components used to be expensive and labor was cheap. So it made sense to spend labor (time) to find a bad component. This has reversed, so that components are relatively cheaper than labor. One bench hour is at least $50. I doubt the components in the power amp you're working on are over $15. If time is short and the components cheap and readily available, why troubleshoot? Remanufacture it. Replace all the semiconductors and electro caps, and quick-test all the resistors. Chances are, you'll fix it very quickly. Notice that if the problem is NOT in the thing you think it is, you'll have wasted the money, and that there are some situations where this can go very wrong.

I guess it's like the Kenny Rodgers song - you gotta know when to hold 'em, know when to fold 'em.

[rf7]

I went thru the board again, and I found a C to E short on Q705. That seems to be it. I should have caught that before, oh well. I put the old Q707 back in as it is OK.

I've heard that troubleshooting technique put as "Short the bias and pull the protection".

Enzo & R.G., thanks again for your assistance, it is much appreciated!