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Below is a single channel power amp circuit from the 1970s.

I was wondering if someone could help me understand this circuit from a modular point of view (I'm getting lost with what I presume is negative feedback circuitry)

power amp circuit

I'm trying to understand the circuit better, so that I can diagnose a problem with the bias that is affecting both of these circtuits (left and right channels). The emitter resistor voltage drop is 14mV in the specs, but adjusting VR201 will only give a max of 5mV on the device. Also, the schematic shows -1.3V after the bias diodes, but it's only reading -0.6V in practice. Because this is affecting both channels the same, I suspected the voltages, but these are now in spec and the problem remains.

Thank you!

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  • \$\begingroup\$ Check for the correct voltages across the bias diodes. \$\endgroup\$
    – user16324
    Commented Dec 6, 2014 at 15:28
  • \$\begingroup\$ So what is wrong with the amplifier and don't talk about bias currents? How did it "fail"? When was the last time it worked? Under what circumstances did it fail? \$\endgroup\$
    – Andy aka
    Commented Dec 6, 2014 at 16:05
  • \$\begingroup\$ @BrianDrummond Forward Voltages for the diodes read : D202(STV3H):1.61V D203(IS2473): 0.65V Which seem about right from the datasheets. \$\endgroup\$
    – UpTheCreek
    Commented Dec 6, 2014 at 18:43

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This is not definitive, but it may be due to component values drifting with age.

The network between TR203 and TR204 collectors is designed to drop approximately 4*Vbe plus twice the 14mv you expect, and is adjustable. Now if TR202 and TR203 are reasonably well balanced, both collectors will be around -1.3V (though TR2 may be a couple of volts different) dropping around 40V across R215, which should result in a current of about 6ma in each leg. This is easy to check, there should be 1 volt across each of R213 and R214.

If these are lower than 1V, check the value of R215. I've seen 40-year old resistors under similar stress increase value by about 50%, and that could account for the problem.

Anyway back to that network.

0.65V + 1.61V + (0.6V across VR201 from 6ma) is on the low side of 4*Vbe and I think that's where your problem lies. The STV3H should drop closer to 1.7V according to Audiokarma and that 0.1V is enough to make the difference.

Possible fixes:

  1. If R215 has increased, replace it.
  2. Replace the STV3H. I have no evidence that these "Stabistors" suffer long term drift but it wouldn't surprise me. Old style "metal rectifiers" certainly did. (Difficult to get, Audiokarma suggest a network - 2 Si diodes and a Schottky - that comes close)
  3. A modest increase in VR201 - maybe adding 22R or 47R in series with one of the diodes is easier.

The third of these looks like the simplest to me.

Incidentally this is all guessing the amp still basically works though probably with higher crossover distortion than originally designed. If it's actually dead, the above may not be relevant.

EDIT : the reduced current in each of R214/R213 is suspicious, and this reduced current probably accounts for the slightly low STV3H forward voltage.

Given R215 is still in spec, suggests TR202 collector voltage will be high, instead of reasonably close to -1.3V. In other words, TR202 is not conducting as hard as it should, probably due to reduced base current.

This in turn would point to too low voltage across R205 (less than across R204) from TR201 (right half) not seeing enough base current.

One suspicious component there : C205, 330uf, 6.3V. If that has gone leaky, it could create these symptoms (by diverting that base current to 0v), and old electrolytics are famous for developing high leakage current. So, if TR202 collector voltage is out of spec, this is my prime suspect.

EDIT 2 from new evidence.

Replacing the electrolytics seems to have increased (presumably : but check!) current in R213,R214 and thus increased the voltage at TR203 collector from 0.6 to 1.1V. Still lower than required : I presume VR201 adjustment is out of range.

We also have 0.93V across R204,R205 allowing something like 0.3V across R206, allowing about 5ma (R206=68R) or 3ma (R206=100R) divided between TR202,TR203.

Why 0.93V? Suggests 0.28ma in each leg of TR201. Is that consistent with 42V across emitter resistances R209,R210 (68k+5.6k)? 0.56ma so yes. Suggests we can eliminate theories like failure of TR201.

And we have the change in R206 value. At this stage I would try 68 ohms (or add 200R across the existing component) and re-measure.

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  • \$\begingroup\$ Fantastic info, thanks Brian! I'll digest this a little more and then take some readings. (Your guess in the final sentence is correct by the way :) \$\endgroup\$
    – UpTheCreek
    Commented Dec 6, 2014 at 20:16
  • \$\begingroup\$ (though TR2 may be a couple of volts different) Did you mean T203 here? \$\endgroup\$
    – UpTheCreek
    Commented Dec 7, 2014 at 11:20
  • \$\begingroup\$ Sorry I meant TR202 may be a couple of volts different from TR203, and that may vary with temperature. If TR203 isn't about -1.3V as shown, there will be DC on the amp's output (bad!) \$\endgroup\$
    – user16324
    Commented Dec 7, 2014 at 11:34
  • \$\begingroup\$ Thanks. So I'm reading only about 0.24V across both R213 and R214. I pulled R215 and it was a little high, but not by much : 6.94K - Do you think that's enough to cause the problem? \$\endgroup\$
    – UpTheCreek
    Commented Dec 7, 2014 at 14:43
  • \$\begingroup\$ No, I would regard R215 as in spec. \$\endgroup\$
    – user16324
    Commented Dec 7, 2014 at 15:18

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