Sunn Beta Lead - Distorted, low output

[stereomonostereo]

I picked up a Sunn Beta Lead that's acting up to where the output is distorted and fairly weak. It occurs on both channels, and also from the accessory line inputs. The service manual is a little difficult to read in spots, but most of the voltages i've checked out seem to be in the ballpark. I'm getting +42 and -42 on my rails. TP-5 looks to be saying 60 volts P-P, but i can't find any indicator of what the P-P indicates, and that leaves me stuck there.

I'm getting .1mV across R18 with no change when adjusting the bias pot.

[Enzo]

Is the bias transistor shorted?

[Jazz P Bass]

"TP-5 looks to be saying 60 volts P-P, but i can't find any indicator of what the P-P indicates"

The Test Points in the rectangles are signal voltages. (Volts ac)

PP indicates peak to peak of the voltage waveform.

[J M Fahey]

Agree and add: 60Vpp is another way of saying 21V RMS at the speaker out, meaning the amp can put out up to 21*21/4=110W RMS ... excellent for a 100W rated amplifier.

And why not state RMS instead and keep it simple?

Just guessing, but in general Multimeter AC scales can´t be trusted too much above 50/60 Hz, *some* up to 400Hz, so can´t be trusted for Audio measurements, while old style VTVM probes and all Scopes show Peak to Peak voltages only ... but can be trusted even at Mega Hertz frequencies, let alone any Audio one.

[Mick Bailey]

You need to get the correct static DC conditions, otherwise any other test is misleading. 0.1mv could be the output transistors just about conducting at idle, or it could just be some DC leakage reading. Given your bias pot isn't making any difference my thought is something in or around the bias circuit isn't working. Do you get -1.7v on the collector of Q14?

[OldTech]

Have run a sine wave strait to the power amp section ( bypass the pre amp) first and checked with a scope? If the power amp is OK work your way back. I use to fix these when they first came out many years ago and for the most part they are very reliable. Because of the age of the amp most of the coupling caps may be going. There are bunch of 2.2 mF and 2 mF caps in the audio change that will be way out of spec. Also confirm the + 15 and - 15 volts for the opamp power look at Q1 and Q11 on the power amp board they are the 15 volt regulators they can get really hot if the power supply caps are drying out and loading them down, also check R3 and R11 if they are cooked too..

[OldTech]

To follow up on the coupling caps in the pre-amp. As they go bad they leak DC to the next stage. This will cause distortion and start to change the bias on the switching FETs Q103-Q106 and DC offset the OP Amps too causing weird clipping and distortion. You will really need to use your scope to check for sure. Will take about an hour to replace the caps

[nsubulysses]

Why do you need a scope to check rather than DMM? Is it because there is DC on both sides of the cap anyway?

[Jazz P Bass]

A good digital DMM will be fine.

[stereomonostereo]

Thanks for all the replies. I suspected the bias transistor, as it really sounds like the output ICs aren't bias properly, but i'm getting -1.7vdc at the collector, -1.3 at the base and +1.2 at the emitter, which *seem* correct to me. My +15/-15 at Q1 and Q11 are there, and it's definitely happening in the power amp section. I get a clean, strong signal from the preamp.

[g1]

I'd figure you want more like +1.9V at Q13 emitter.
Q8 base want 1.2V and you have around .7V drop across E to B of Q7.

[stereomonostereo]

The max that the bias trim pot will adjust to is 1.2v on the emitter of Q13. The base of Q8 is 0.5v. So is that .7v drop correct for Q7? And if so, then that would point back to Q13?

[The Dude]

...... I suspected the bias transistor, as it really sounds like the output ICs aren't bias properly, but i'm getting -1.7vdc at the collector, -1.3 at the base and +1.2 at the emitter......

If your polarity indications are correct in that post, your bias transistor is open E-B. You have a 2.5v drop across the junction and it should max out at around .7V. Pull the transistor and check it.

[stereomonostereo]

I replaced the bias transistor Q13 and the voltage drop seems better now. I'm getting -1.9v at the collector, 0.8v at the base, and 1.4 at the emitter. The amp is still having the same problem, but i believe that part is at least where it should be

[g1]

What is your Q8 base voltage now?
How about across R21 and R50? (output transistor emitter resistors)