So the fault has gone but no sound, at, any input. All rails are up, correct voltages, outputs are all turned on, weird
One for the scope... trace the signal through although check the basics first such as offsets. Make sure all the voltages are correct on that opamp now.
the output from the rails is spot on so ill have to get checking the rest, must be something simple and generic to affect both channels the same time
so back to this then.I had to stop for a while and clean it up as i couldnt read anything on the board
so this is hard work.The references on the board dont match the drawings so ive had to spend time decifering, which i now have.
what i origianly posted QF01 is in fact QE 01and these are the op amps on the equaliser itself
where i am now looking is Q401 and 402.
One channel looks ok, but there is no sound at all out of even that channel
i also found 2 caps that had been removed(missing from the board)
what i origianly posted QF01 is in fact QE 01and these are the op amps on the equaliser itself
where i am now looking is Q401 and 402.
One channel looks ok, but there is no sound at all out of even that channel
i also found 2 caps that had been removed(missing from the board)
Its not easy when things don't match up but the signal should be traceable, at least if you break the amp down into blocks.
so that op amp must be faulty as i swapped it around and the voltages are ok on the other channel with that one fitted
The file won't open for some reason.
If you are looking at these then they are the very high gain phono stage and so only applicable to turntable inputs.
If you are looking at these then they are the very high gain phono stage and so only applicable to turntable inputs.
They set the gain (remember the opamp theory) and also provide the special RIAA response curve for vinyl. If the caps were not there the gain would be the total of the four series resistors added together (12k, 680, 150k and 5.6k) divided by the total of the two series 150 ohms to pin 7. So the basic gain is 561. So 1mv input gives 0.561 volts output but the caps shunt the resistors at AC and make the 'value' of the resistor seem less. How much less depends on the frequency.
https://en.wikipedia.org/wiki/RIAA_...boosted, and on playback, the opposite occurs.
https://en.wikipedia.org/wiki/RIAA_...boosted, and on playback, the opposite occurs.
Its something called 'external frequency compensation' and was quite common on older opamp. Lots of modern parts have the compensation already built in. Without it and the opamp might be unstable and oscillate.
well it is balanced now, i put 4 new ones in(can you test these in some way?)-but not fixed, so another fault to trace yet.
anyway i now have a common +25v on all of the pin no 1's of QE01/2 and Q401,Q402 instead of 0v, so something common to trace here
anyway i now have a common +25v on all of the pin no 1's of QE01/2 and Q401,Q402 instead of 0v, so something common to trace here
so current situation
Op amp voltages on Q 401 are almost correct apart from pin 7 as follows
1 0v
3 +24v
4 -25v
5 -24v
6 0v
7 -1.4v
8 +25v
but all the others, that is Q 401, QE 01 and QE 02 look like this
1 -24v
3 +24v
4 -25v
5 -25v
6 0v
7 -22v
8 +25v
All of the offsets are correct and output is 0.2mv on both channels
quintisential current ss correct as well on both
Op amp voltages on Q 401 are almost correct apart from pin 7 as follows
1 0v
3 +24v
4 -25v
5 -24v
6 0v
7 -1.4v
8 +25v
but all the others, that is Q 401, QE 01 and QE 02 look like this
1 -24v
3 +24v
4 -25v
5 -25v
6 0v
7 -22v
8 +25v
All of the offsets are correct and output is 0.2mv on both channels
quintisential current ss correct as well on both