Hi! I'm attempting to repair an old amp we have in the studio here and thought I'd post and see if anyone has any ideas as to what could be wrong. It is an old Hafler DH-500 power amplifier. It puts out VERY distorted signals...for instance, if I put in a sine wave it comes out sounding more like a sawtooth (and looking distorted on the scope as well) but if you put music in you basically just get a bunch of crackles. For now, I am testing out just one channel, but both channels are having similar problems.
The DC rails of +/- 90v seem fine, actually a little hot more like 95 each. Don't know if that would cause any issues.
I've attached the schematic for reference. I've traced the signal to the bases of Q3 and Q5 and it is clean up until that point. But after that it gets distorted from there on out. I've also noticed that on the emitter of Q1 and collector of Q2 (the Current sources, as the block diagram, which i will attach next, says) are distorted as well, but occasionally it actually looks like they are just half waveforms (when using a sine wave on input).
But after Q3 and Q5 everything is a mess and distorted including Q4 and Q6 as well as test points 'A' 'C' and 'D'. Does this point to dead Transisotrs at Q3 and Q5? That is definitely going to be the first thing I'm going to try to replace. Any other suggestions or obvious things I'm missing?
Another observation I've made is that, past the transistors I've mentioned, (and at the speaker I have hooked up) you get no sound for a wide range of voltages and then all of a sudden, at some certain threshold, the speaker will start playing or the waveform will show up on the scope...but prior to those transistors you have a fully variable waveform when you alter the input voltage.
I'm mostly doing this for the fun/learning experience of it all, so any help is hugely appreciated!
The DC rails of +/- 90v seem fine, actually a little hot more like 95 each. Don't know if that would cause any issues.
I've attached the schematic for reference. I've traced the signal to the bases of Q3 and Q5 and it is clean up until that point. But after that it gets distorted from there on out. I've also noticed that on the emitter of Q1 and collector of Q2 (the Current sources, as the block diagram, which i will attach next, says) are distorted as well, but occasionally it actually looks like they are just half waveforms (when using a sine wave on input).
But after Q3 and Q5 everything is a mess and distorted including Q4 and Q6 as well as test points 'A' 'C' and 'D'. Does this point to dead Transisotrs at Q3 and Q5? That is definitely going to be the first thing I'm going to try to replace. Any other suggestions or obvious things I'm missing?
Another observation I've made is that, past the transistors I've mentioned, (and at the speaker I have hooked up) you get no sound for a wide range of voltages and then all of a sudden, at some certain threshold, the speaker will start playing or the waveform will show up on the scope...but prior to those transistors you have a fully variable waveform when you alter the input voltage.
I'm mostly doing this for the fun/learning experience of it all, so any help is hugely appreciated!
