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I've been trying to repair a Saunsui A-40 amplifier for a long time now, (when it was turned on, magic smoke started to flow).

I found most of the driver and power transistors all shorted, so i replaced them (had to use similar transistors because i couldn't find the originals)

I replaced every transistor in that schematic EXCEPT TR06, that one was not burned)

I checked every datasheet and the transistors i chose are suited for the job. (If it's necessary, i'll show the replacements i used).

The amplifier now turns on fine, but it has almost no output, and its completely distorted, only with volume below 3 you can hear really low an almost not distorted signal. I can't figure out whats causing the problem, every transistor seems OK.

I'll show next both channels schematic. (the full schematic is everywhere on the web)

Right Channel Measurements Tips Left Channel Component Values Transistors transistors

All electrolytic capacitors where replaced, and i made sure all resistors are not open circuit.

Voltages measure almost like the indication in the schematic (instead of +/-35V, i have +/-40V, because in my appartment i have 230Vac mains most of the time, all measurments are proportional to the ones in the schematic)

UPDATE:

Test conditions: 1kHz 1VPP sinewave to the input.

Voltage Across R78 = 0mV (this doesn't change with the trimmers)

Voltage @ TR10 collector = -2.5

Voltage Across C44 = 2V (meaning the Voltage @ TR12 is -0.5V when it should be 2.5V, is this correct?)

(i've tested with changing the trimmer to -1.2V @ TR10 collector, the TR12 collector is the same -0.5V)

Here is a screenshot of the osciloscope measuring

Channel 1 (yellow): Input

Channel 2 (blue): Collector of TR10 at a volume of 2 (almost OFF)

at 2

Here is a screenshot of the osciloscope measuring

Channel 1 (yellow): Input

Channel 2 (blue): Collector of TR10 at a volume of 5 (half way) (maxed to 10 its very similar to this one but of course, sharper square) at 5

So, first:

1- Can i solve the voltage @ TR12 collector problem by changing R58 and R69, or this could cause other problems? I doubt those resistors are causing the problem, what is?

2- Does the above problem have anything to do with the saturation of the signal right after TR10? ¿How can i solve that problem? (Note that this continues no matter what i do with the trim pot)

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    \$\begingroup\$ You should isolate the actual amplification stage and do some calculations, likely the biasing should be rather different for the transistors you used \$\endgroup\$ – PlasmaHH Nov 2 '16 at 15:28
  • \$\begingroup\$ Have you changed TR06 as well (long tailed pair) or did you leave that in circuit? \$\endgroup\$ – JIm Dearden Nov 2 '16 at 15:50
  • \$\begingroup\$ @JImDearden I'll edit and add that to the post. NO, thats the only transistor i didn't change because its working (or seems to be) fine. Besides it's very likely i won't find a direct replacement, and probably have to use two A1015 (loosing the nice heat and hfe compensation the 2sa798 has) \$\endgroup\$ – ndelucca Nov 2 '16 at 15:54
  • \$\begingroup\$ You still can find the 2SA798 on Ebay. \$\endgroup\$ – Decapod Nov 2 '16 at 15:56
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    \$\begingroup\$ Are both left & right channels similarly distorted? If one channel is OK, compare with the other non-working channel, with "mono" rather than stereo front-panel selection. A very careful visual inspection of all circuit-board connections is useful when magic smoke happens. \$\endgroup\$ – glen_geek Nov 2 '16 at 16:18
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I would start by making sure the thing is biased correctly. Look at the voltage at both bases of the differential pair. They should both be zero. If that is the case, then I'd look for the signal on R44. You should see a nice amplified SIN there. Next I would check for the points that say 1.2V. The bottom one on TR16 should be -1.2 not 1.2. It's hard to say more without component values. what is the value of R44?

Check to make sure substitute transistors have the same pin outs as the originals.

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  • \$\begingroup\$ R44 is 6.8k look at the other channel. \$\endgroup\$ – Decapod Nov 2 '16 at 16:01
  • \$\begingroup\$ @owg60 Thank you, i'll take a look at that when i get home in a few hours. YES the base of TR16 is -1.2v, should have noted that. Oh, and the values of the components are on the Right Channel schematic, R44 is 6.8k. \$\endgroup\$ – ndelucca Nov 2 '16 at 16:02
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    \$\begingroup\$ You may know this but TR12 is part of a circuit called a VBE multiplier. If you don't know, google it. It's job is to create a voltage source that just keeps all those emitter followers around 0A when the input is zero. The variable resistor VR02 adjusts this. When you changed the transistors you would have been extremely lucky if you didn't have to adjust this. if this is not done properly you will get distortion. \$\endgroup\$ – owg60 Nov 2 '16 at 16:20
  • \$\begingroup\$ Just as a side note to those running off to look at VBE multipliers now, TR12 is a VBE multiplier that isn't compensated for the Early effect. \$\endgroup\$ – jonk Nov 2 '16 at 16:37
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    \$\begingroup\$ @owg60 "every transistor seems OK" what the hell was i thinking.... Indeed both replacements i used for TR15 and 16 where reversed.... i guess i must have seen the datasheet wrong the first attempt, so basically i've been punching the head against the wall for a week because of my own comfort zone of not doubting myself..... Thank you you've made my day. If youll edit the Answer to this i'll mark it as Solved. Thanks again. \$\endgroup\$ – ndelucca Nov 3 '16 at 22:04

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