Ampeg BA-110 Trouble shooting - help needed

[fuzzman72]

Hello

My Ampeg BA-110 started to produce a low pitched hum, not very loud, but annoying.
I've measured 450mV DC at the output without load and the input grounded through the return jack socket.
The DC offset seems pretty high to me.

I've also measured 0V (with the DMM set on mV) across R213 and R214 which are rispectively at the emitter and at the collector of the power transistors.
These transistors seems to be turned off and shouldn't be negative the voltage at Q206 emitter?

I've checked every components and connections in the power section without finding any problem.

Since I'm more into tubes and I'm not that good with transistors amps I need some help to end the trouble shooting.

[DrGonz78]

The output section on this amp looks like a quasi complementary type which has two npn transistors instead of a pnp/npn pair. As I recall the DC offset with a quasi complementary output can measure 350-450mv by design on a perfectly working amp. I remember running into similar DC offset on an amp repair years ago and that was something that I did not know at the time.

I would clean the headphone jack and see if that improved the hum issue.

[fuzzman72]

Thank you for your reply.

As I recall the DC offset with a quasi complementary output can measure 350-450mv by design on a perfectly working amp.

Now I now that the amp is correctly working, but what about 0V across R213 and R214?
I'm asking even just for educational purposes.

I've already cleaned the headphone jack with no results.

[DrGonz78]

Thank you for your reply.

Now I now that the amp is correctly working, but what about 0V across R213 and R214?
I'm asking even just for educational purposes.

What type of voltmeter are using? Since you know already to check the current (voltage) across those resistors it tells me you have probably checked this before on other amps. So I assume it's probably not your meter but it never hurts to ask what we are using to measure to make sure. Someone else here will have a better answer I am sure.

About the hum... is it a 60hz hum or 120hz hum? I would try to isolate exactly where in the circuit the hum is originating. The amp has line out, send, and return jacks. Is the hum present on the preamp or is it only on the output side?

[fuzzman72]

I'm using a digital multimeter and I'm sure it works fine.

The hum sounds more like 100Hz (I'm in Europe) and it comes from the output section.
The preamp is absolutely quiet.
If were the filter caps the culprit, the preamp should be noisy too, I suppose.

[DrGonz78]

I would measure for AC ripple at C18 and C21 just to be sure. The preamp section has another 100uf filtering stage on the power supply. If your meter has trouble measuring AC ripple effectively you can always put a 0.1uf 600v cap in series with your meter probe. Some meters get confused trying to read AC when DC is present and the cap in series will help by stripping off the DC voltage. Another thing to contemplate is that there could bad solder joint in the power supply area and they just need to be re-flowed.