Amplifier Problem - Revised Schematic

Can't see what parts external to the circuit shown could cause any of your symptoms. The input and output are decoupled with capacitors. Furthermore, you confirm that the internal, DC side of the input and output are at the prescribed voltage. You have also confirmed that the supply voltage is OK. There is nothing else externally that could affect the circuit. I am assuming that the node at the lower left corner finds its way back to ground somewhere.

The question remains why the collector of Q205 is at

One of the marvelous things about stereo is that you've got two identical channels. So you have a good one to compare it to.

What is the base voltage of Q205 on the working side? And what is it on your broken side?

EADGBE wrote in news:7a5f62ce-00ec-4fac-84f3- snipped-for-privacy@e67g2000hsa.googlegroups.com:

You do NOT indicate the voltage on Q205's base, also Q204's collector.

If it is 11.9, then Q204 is not conducting and Q205 has no forward bias, will not conduct, and all other bad values follow from there.

Since the OTHER voltages around Q204 are correct, the collector of Q204 should be LOWER than the 11.9 rail by half a volt or so. It should thus be 11.4 on Q204 collector and Q205 base. If it isn't, then Q204 is probably bad, open on the base/collector junction.

bz wrote in news:Xns9A881ACC87A34WQAHBGMXSZHVspammote@130.39.198.139:

OR, the Q204 collector is not connected to to the 12 k resistor and the base of Q205.

You said you replaced Q204. You could have a bad transistor(new parts can be bad), a part soldered poorly, or a transistor soldered in wrong. Check for broken traces and make sure that the orientation is correct on Q204's replacement.

Make sure your transistors are oriented the right way round, i.e. B,C & E really are where you think they are. I remember once being stuck with a repair where the lead out of a replacement transistor was not the same as the original. Couldn't work out what the hell was going on for quite a while, but C and E were reversed. (don't just use a diode check for this)

Gareth.

As an example: BC184 and BC184L have different pinouts.

Gareth.

You need to get a scope or at least a signal tracer and see where the signal stops on the bad side. It's a stereo amp so it should be a no brainer to compare signals as you move through both sides. If all you have is a meter then you just don't have enough equipment to do repair work.

Centre base ('TO-18 style').

Base at one end.

I have revised the schematic further by adding the voltages at the base of Q205 and collector of Q204....

I guess I don't have enough equipment to fix this problem. I can't afford a 'scope right now. This will have to go on a shelf until I can deal with it.

You might want to look at pc scopes. Or ebay. If all you are doing is audio and possibly lower rf frequencies there are lots of decent used scopes on ebay.

"James" wrote in news:6eSdnTosdIIDSZDVnZ2dnUVZ snipped-for-privacy@earthlink.com:

You might download a program such as waveview or spectragram (demo versions of many programs are available), make yourself a resistor divider probe and look at the waveforms with your computer.

You don't need a scope for this one, just DC meter and brain power. For the output transistors, Q207 base should be about one Volt higher than Q208 base. If not, either Q206 is shorted or Q205 is not sourcing current. If I can believe the voltages you show on Q205, it probably has an open collector.

What's driving me crazy is that, for the sake of learning, I replaced Q205, Q206, and Q208, all with exact replacements that are all known to be good, and still got exactly the same voltage readings.

And I also tried replacing Q204.

Have you followed the recommended procedure for debugging direct-coupled circuits like this?

Step 1 - remove all transistors and check them for shorts and opens out of the circuit. An ohmeter usually suffices.

If I understand this correctly the nominally 5.8 V point on Q208 base is close to 0 V but the output node is OK at +6.4 V.

If that's right, Q208's VBE plus the drop across the lower 3.3 ohm output resistor is about 6.4 volts rather than about 0.6 V. That can't be so unless the lower 3.3 ohm resistor is open circuit, or it has 6.4 V across it and is boiling hot, or Q208 is open circuit B-E.

All points to Q205 collector being open apart from one thing. On the revised schematic, the output voltage at the junction of the two 3R3 resistors is measured as being correct. That doesn't add up at all. I could do with seeing this voltage checked.

I've put the circuit into Pspice and played around with various fault conditions to see what the volts do. Q205 is the only thing that seems to reproduce the symptoms.

Actually, I am measuring +7.3V at the junction of the two 3.3 ohm resistors between Q207 and Q208.

And +20 - +30 mV at the base of Q208 from a previous point if I am correct?

What's the voltage across the B-E of Q208 and what's the voltage across the lower 3.3 ohm resistor?

Where did you get this diagram. Is it completely from scratch ?

greg