Eden wt 800 no sound 'R amp/out low'

[pontiacpete]

Thanks Jon. There’s 82v across R10 an 9. R10 is a lot hotter than R9 by about 40degrees, 140deg and 80deg. Q4 is even hotter at about 180deg F.
I was seeing a signal on the bases of Q3 and 4, and I was able to see it move with the volume pot. Now I see nothing. Not even a trace. I’m not really good with this new oscilloscope, it’s digital. I find it easy at times but now ... Both of my 465s are not working.

[pontiacpete]

Thanks Jon. There’s 82v across R10 an 9. R10 is a lot hotter than R9 by about 40degrees, 140deg and 80deg. Q4 is even hotter at about 180deg F.
I was seeing a signal on the bases of Q3 and 4, and I was able to see it move with the volume pot. Now I see nothing. Not even a trace. I’m not really good with this new oscilloscope, it’s digital. I find it easy at times but now ... Both of my 465s are not working.

[pontiacpete]

I can trace up to R4 but the signal now stops there. I can't understand why I had a signal on the bases of Q3/4, maybe I was reading it at Q4 all time.

[Jon Snell]

If R10 and R9 are getting warm, that means the transistors are drawing current and all I can suggest is checking the Vbe which should be around 0.6volts for a silicon junction. If it is, the emitters will be moving with the bases. If they don't, I would suspect the transistors 9 & 10.

[pontiacpete]

Vbe Q3 .57v
Q4 .57v

[Jon Snell]

That means there will be signal on the emitters, follow the signal until it stops, there will be your fault.
Q4 base signal?
Q4 collector signal?
Q8 collector signal?
Q11 base signal?
Q13 emitter signal?

Exactly the same for the positive side of transistors.

[nevetslab]

That means there will be signal on the emitters, follow the signal until it stops, there will be your fault.
Q4 base signal?
Q4 collector signal?
Q8 collector signal?
Q11 base signal?
Q13 emitter signal?

Exactly the same for the positive side of transistors.

I'd be looking at the DC voltages for the input stages Q3/Q5 & Q4/Q6, then the voltage gain stage xstrs Q7 & Q8. What is the voltage across collector resistors R8 & R11? They should be the same, though from your numbers, it's clear they are not. 82V across 100k is 820uA, so I'd expect around 2.21V across R8 & R11. As you're not getting that, what's the voltage across R19 & R22? They should be the same. I forget what the nominal current thru Q7 & Q8 is (and thru bias xstr Q9). With 2.21V across R8 or R11, that would put around 1.61V across the emitter resistors R19 & R22 for 16.1mA thru the voltage gain stage. With the imbalance of current thru both input stages and voltage gain stage, you've got one or more xstrs that aren't happy, and maybe resistor values that are no longer 'nominal'.

With bias being set for 7mV nominal across any of the output stage emitter resistors, the voltage between the bases of driver xstrs Q10 & Q11 would be about 2.0-2.1VDC, maybe less. Relative to the output buss, the voltage at each of the bases of Q10 & Q11 should be the same.

With your imbalance, I stop looking for signal until the amplifier stages are correct for nominal DC current/voltages. The circuit is fully symmetrical, so the voltages and current thru the stages will be nearly identical from top to bottom. Get those right, then your signal should be flowing right.

[pontiacpete]

Sorry for the delay. I was feeling a little too comfortable probing freehand for these voltages and zap, took out Q3,4,5,6 and 8. Replaced them all and R22.
Here are the voltages you asked for, across R8 there's 18v and across R11 there's 7.5V.
These voltages are probably a lot different from what they would have been if I hadn't clobbered all those components. But I hope we can carry on from here.
Thanks

[pontiacpete]

Also R19 there’s 16v
R22 there’s 4.5v

[nevetslab]

Also R19 there’s 16v
R22 there’s 4.5v

So, you have a ton of current flowing thru the Voltage Gain Stage...Q7, Q9 & Q8. 160mA thru Q7 & R19, dissipating 2.6W in R19, and thru Q8, 45mA, dissipating 200mW thru R22. I'd replace Q7 & Q8. With 18V across R8, there's 6.7mA thru Q3, while 7.5V across R11, there's 2.8mA flowing thru Q4.

Now, since there's an imbalance in current flowing thru Q7/Q9 & Q8, some of that current is flowing thru the Drivers Q10 & Q11. It could be a problem with the drivers, which are pulling too much current thru Q7 & Q8, and hence thru the resistors R8 & R11. You could try replacing Q7 & Q8 first, though you might see the same thing. And, R19 is questionable, as it's only a 1/2W resistor, with 2.6W being dissipated.

And, since the output xstrs are pulling current from the drivers, the problem could be all the way down there. You should also measure the voltage drop across each of the output stage emitter resistors R29 thru R32, to see if it's one or more of the output xstrs causing all this. Good hunting!

[pontiacpete]

Found I hadn’t replaced Q6 and 3 from my first (or second) fumble. Now it’s getting closer, Q8 has 4v across and Q11 has 1.8v. But they still aren't equal. R19has 3.6v and R22 has 1.2v.

[nevetslab]

Found I hadn’t replaced Q6 and 3 from my first (or second) fumble. Now it’s getting closer, Q8 has 4v across and Q11 has 1.8v. But they still aren't equal. R19has 3.6v and R22 has 1.2v.

Check the voltage drop across the two power resistors R24 & R25. They feed 6.8V 1W Zener Diodes that are common to the output buss, and feed the collectors of the driver xstrs Q10 & Q11. We need to find what's causing the imbalance in the Voltage Gain stage Q7, Q9 & Q8. The imbalance could just be that Q7 & Q9 are bad, but it may also be the drivers, or maybe one or more of the output xstrs that's pulling the excess current thru the voltage gain stage.

Also check the voltage drop across R24 & R25 6.8k 2W, between the power supply rails and the 1N4734 6.8V Zener diodes tied to the output buss. That junction of the resistors/zeners feed the collectors of the driver xstrs Q10 & Q11. Also check the voltage across the emitter resistors of the output xstrs R29-R34, to see if there's one or more power xstrs pulling an abnormal amount of current.

[pontiacpete]

R24 has 72.5v
R25 79.5v
R29-34 all have 0v across them

[nevetslab]

R24 has 72.5v
R25 79.5v
R29-34 all have 0v across them

It sounds like the lower zener diode D3 is shorted, or perhaps the electolytic cap C11 is shorted. DMM check should solve that. Lift one end of the zener to be sure. From the voltage across R24, that's about what you'd expect to see across R25. Correct that, then see what you have. You may still have issues with the drivers as well as the voltage gain stage xstrs Q7 & Q8.

Reviewing the circuit again under idle conditions, no signal.....if the supply voltages are +/-83VDC, then the voltage across emitter resistors in the front end R9, R10, R12 & R13 would be about 82.3V, which translates to 823uA flowing thru Q3 & Q4. That would put 2.22V across the collector resistors R8 & R11. Then, subtracting junction voltage of Q7 & Q8's Vbe, you have 1.62V across the emitter resistors of Q7 & Q8 for 16.2mA current thru Q7 & Q8. Looking at the emitter resistors of Q10 & Q11, which are across the Vbe junctions of the output xstrs, which would typically be around 0.55VDC, that would have about 5.5mA flowing thru R26 & R28.

This is around what I'd expect to see on a working module, more or less, unless I've overlooked things. On your working amp module, see what you measure on it.

I forgot about the DC Offset Adjust pot. Here, you have 1.4M (200k + 1M pot + 200k) straddling the emitters of the diff pairs, +/- 0.6V, or 1.2V across 1.4M. That's 857nA thru the network. The pot extreme positions would be 200k x 857nA, or +/- 0.171V being applied to both of the bases of Q5 & Q6 to zero the output DC voltage of the amp. You'd rarely have to adjust the pot to the extremes, but that's the voltage range across the offset adj pot. On the working amp module, it's more than enough to trim the DC output level to 0VDC. Adjustment of the trim DOES, of courses, alter the DC balance thru the stages, though it's a minor trim. You have a substantial imbalance, more than the trim can rebalance, though it will come into play once the faults have been found.

[pontiacpete]

D3 is not shorted measuring with the DMM nor is C11. I put a in a new C11 since I have one. But I don’t have the zener. Do you recommend getting one anyway?
FWIW R26 has .25v
And R28 has .53v