Hartke 3500 No Sound, Not blowing fuses.

[pokey17247]

Hello. I have a Hartke 3500 bass amp that is getting signal up to the resistor that is in series with the electrolytic capacitor right before the differential input pair. The signal stops at the junction of the resistor and capacitor.
Please help!! Thanks in advance.

[MarkusBass]

So maybe this junction is shorted to the ground, possibly with shorted capacitor C301? Have you checked it?

Mark

[pokey17247]

Hi Mark. Lifted C301. No change. When I lift the + side of C302 to the diff.pair, I get signal at that junction again.

Anthony

[Enzo]

Just to verify your technique is working, have you checked for signal farther along, say the bases of the output transistors?

Does the relay on the power amp board click on?

Is there power in the circuit? What kind of DC is on the collectors and emitter of Q301?

I don't recall specifically, but some power amp modules like this one are not happy with the input cable unplugged. The input stages take their ground reference from the cable ground. So if you are feeding signal into the input connector from a generator, you might need to conect the input ground pin over to the rest of the power ground.

[pokey17247]

There isn't signal at the coil, well...just hum....however there is high DC. I replaced the relay, however that didn't solve it. Not certain if I can hear the relay come on or not. High voltage rails are supplying proper voltage. I'm testing for signal with guitar in the input and signal tracer.

[Enzo]

I wasn't expecting the relay to be bad, I wanted to know if it clicked on because that tells us whether the amp thinks it is ready or not. Can't hear it? Put your finger on it and feel the click, if it makes one. or monitor voltage ACROSS the relay coil. There should be voltage ON the coil all the time, it is connected to V+ through a resistor. The control IC pulls the bottom end of the coil down to drag current through it and turn it on, but the chip only does that if conditions are right.

You got the rows of 0.47 ohm 5w resistors for each output transistor. If you can get at them, is there DC on any of those? Or ther should be a coil of wire, and inductor made by coiling wire. Probably near the relay. The wire is enameled, but the ends should be exposed copper, is there DC on that? We are looking for DC voltage in the amp, preventing the relay from engaging.


Now if you meant the relay coil when you said coil, there is never signal there, any hum you hear is just power supply ripple. But the bare wire coil on the board is in fact in the output to the speaker, but inboard of the relay contacts. That SHOULD have no DC but all your signal.

You say high voltage wires are supplying proper voltage, OK, where? I will assume at the connector, but does it get to all the parts? Q301 is the input diffy pair transistor. If the V+ V- don;t get as far as it, then having them on the outputs is irrelevant.

[pokey17247]

Enzo,
I can't feel or hear the relay click on. Good advice to put fingers on it to see
Yes, there is DC voltage on the inductor.
Voltage is getting to Q301, about 78+ volts.

[Jazz P Bass]

Hartke HA3500 Schematic

If there is truly Vdc on the inductor, then your output stage is damaged.
What polarity is the voltage?
That may help, as it provides a clue .

[Enzo]

Or it is being driven that way.

[pokey17247]

Positive voltage on the inductor. About 43+ vdc.

[Enzo]

That is why your relay won;t click on, the protect IC is doing its job. Your amp is making DC, you'll need to find out why.

[pokey17247]

When you say "making DC", you mean there's DC where there shouldn't be? Or DC passing through a component?

[Enzo]

The output of your amp - that what goes to the speaker - SHOULD sit at zero volts until moved back and forth to be music. If circuits fail so as to upset that balance, then the result can be a DC voltage on the output. That will kill speakers, as their voice coil will move one direction and try to stay there. I said making DC as an informal way of saying DC appears at the output.

By output I mean right at the circuit itself. On the power amp is a relay that decides if the output goes to the actual speaker jack on the rear of the amp. So I don;t expect DC on that jack, just at the output circuitry. That is all those 0.47 ohm resistors and the bare wire inductor coil.

If an output transistor shorts, that connects one of the main power rails to the output line. But the whole point if a power amp is to control the current from those power rails to the output. SO a small component early in the circuit can make the rest of the circuit deflect voltage from where it should be.

[pokey17247]

So now I know there's DC at the junction of the inductor and the.47ohm resistors. If an output transistor was bad, wouldn't the fuse blow? Maybe one of the other transistors connected to the output is bad. I guess I should test them all.

[pokey17247]

Enzo..or to all who observe, I've found a faulty component. It's a 2SC4370. I can't seem to find a source for this part or a cross reference. In the schematic I believe it to be 2SC2238...also seems to be a tough find unless I use ebay.