Voltages are normally with respect to ground.
If the output is at zero volts DC and the output stage is drawing current (volt drop over the 0.47 ohm emitter resistors) then that 99% checks all is OK dc wise. You can check things like the constant current sources and calculate the values expected but low gain is more typical of an AC fault.
Is the sound OK... just lower in volume. Not lacking bass at all ? Might be worth linking points 36 and 38 (inputs to power amps) with a wire to be absolutely certain each channel is receiving the same signal, just to be 100% sure it really is a power amp problem.
If the output is at zero volts DC and the output stage is drawing current (volt drop over the 0.47 ohm emitter resistors) then that 99% checks all is OK dc wise. You can check things like the constant current sources and calculate the values expected but low gain is more typical of an AC fault.
Is the sound OK... just lower in volume. Not lacking bass at all ? Might be worth linking points 36 and 38 (inputs to power amps) with a wire to be absolutely certain each channel is receiving the same signal, just to be 100% sure it really is a power amp problem.
Yes, both channels output stages are running around 100 mA measured on the .47R resistors. I tuned that as well as the outputs' offset to zero.
I have the signal connected directly to those points. There is an input (XP1, 5-pole DIN) on the back that goes directly to the power amp. And input signals are the same amplitude after C1 and C2.
I would say it is lacking a bit bass, but treble is low too. At 1 kHz, this channels unloaded output amplitude is around 1/3 of the other, on the scope. Otherwise undisturbed sinewave both loaded and unloaded.
I have the signal connected directly to those points. There is an input (XP1, 5-pole DIN) on the back that goes directly to the power amp. And input signals are the same amplitude after C1 and C2.
I would say it is lacking a bit bass, but treble is low too. At 1 kHz, this channels unloaded output amplitude is around 1/3 of the other, on the scope. Otherwise undisturbed sinewave both loaded and unloaded.
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So you have a 'scope 🙂
So don't overlook anything... check the input as measured on the junction of R5/C1 comparing for both channels.
Carefully while applying the sinewave (1khz) scope each end of the electroylitics in the signal path and confirm the same signal is on both ends. Lack of bass on the faulty channel points to a dried out cap. Not treble though. C5 the 120 pf worth checking it's not leaky and shunting input signal. Same for C11 (feedback network). Check resistors R48, R11 and R5 and R9 as these determine the gain. R48 and R11 would have to go low in value though to reduce gain... unlikely. C13 worth checking by bridging.
Have you scoped the output at the emitter resistors in case of problem from amp to sockets... don't overlook anything however unlikely.
So don't overlook anything... check the input as measured on the junction of R5/C1 comparing for both channels.
Carefully while applying the sinewave (1khz) scope each end of the electroylitics in the signal path and confirm the same signal is on both ends. Lack of bass on the faulty channel points to a dried out cap. Not treble though. C5 the 120 pf worth checking it's not leaky and shunting input signal. Same for C11 (feedback network). Check resistors R48, R11 and R5 and R9 as these determine the gain. R48 and R11 would have to go low in value though to reduce gain... unlikely. C13 worth checking by bridging.
Have you scoped the output at the emitter resistors in case of problem from amp to sockets... don't overlook anything however unlikely.
I only have a soundcard scope but that should be enough, except that it does not show DC offset.
Yes, input signals are similar at C1/R5 and C2/R6 junctions.
I have measured on the cases (collectors) of the output transistors and they are similar.
And the output waveform is not distorted neither with nor without speakers connected. Just too low amplitude.
In the weekend I will have time to take a look at it again... Will also try 100 Hz and 10 kHz to see how it behaves with those.
Yes, input signals are similar at C1/R5 and C2/R6 junctions.
I have measured on the cases (collectors) of the output transistors and they are similar.
And the output waveform is not distorted neither with nor without speakers connected. Just too low amplitude.
In the weekend I will have time to take a look at it again... Will also try 100 Hz and 10 kHz to see how it behaves with those.
Okay, stupid question time...
1. Have you checked the balance knob to make sure it is installed on the control correctly?
Meaning, if you move the control it's even on both sides?
Maybe the knob is off center.
2. Have you tried swapping the speaker inputs around to make sure it's not the speaker at fault?
Sorry if this has been answered already, I just jumped the gun and replied. 🙂
Scott
1. Have you checked the balance knob to make sure it is installed on the control correctly?
Meaning, if you move the control it's even on both sides?
Maybe the knob is off center.
2. Have you tried swapping the speaker inputs around to make sure it's not the speaker at fault?
Sorry if this has been answered already, I just jumped the gun and replied. 🙂
Scott
Gain is set by R48 and R7 + R1 parallel R2 on one channel, and R49 and R8 on the other. R2 is a trimmer to adjust the gains of the first channel, but shouldn't make this much difference. R8 and the R7 combination go through C4 and C3 100 uf caps, which may be dried up and causing the gain to go down. Try changing C3 or C4. (If you want to check there should be no signal on them if they are good). Unless I am misreading the circuit, and assuming the schematic is correct. Good luck with it.
The signal goes directly to the input capacitors. Scoped after them both channels still have the same amplitude.
Yes, and seen on a scope the output signal is lower on one channel both with and without speaker connected.
Hi -
here is link to schematic in djvu format.
http://www.rw6ase-dok1.narod.ru/arhiv/odissey_u010instr.djvu
and there info about odissei and other soviet amps (in russian, but with pictures 🙂 )
Ñòåðåîôîíè÷åñêèé óñèëèòåëü ''Îäèññåé Ó-010 ñòåðåî''.
oh, sorry - you have it. my mistake.
here is link to schematic in djvu format.
http://www.rw6ase-dok1.narod.ru/arhiv/odissey_u010instr.djvu
and there info about odissei and other soviet amps (in russian, but with pictures 🙂 )
Ñòåðåîôîíè÷åñêèé óñèëèòåëü ''Îäèññåé Ó-010 ñòåðåî''.
oh, sorry - you have it. my mistake.
Got it fixed. Replaced C4 and C14, then scoped and started wondering... wasn't it this channel that was weak? Then replaced C3 and C13 (equal on the other channel) and now both are equal.
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