Ok, well shoot. This amp has been working just fine since the day I bought it back in 91 except for the initial problem which I just didn't put the recording and listening in the same position. I guess I should have had it fixed years ago when my dad was still around, but just didn't. Well, I'll keep at it and be more careful poking around. Maybe I'll get it and maybe I won't. HAHA!! I'll keep you all informed.
Thanks
Thanks
Random thought, it is a bit early in the morning 🙂
Remove the IC and then link pins 1 and 5 on the board. That second chip is only a buffer.
Does it all work normally if you do that?
Remove the IC and then link pins 1 and 5 on the board. That second chip is only a buffer.
Does it all work normally if you do that?
R119 is in the phono section I think. You mean the 49.9 ohm?
It can stay because with the opamp physically removed the resistor is isolated.
The voltage on the link between pins 1 and 5 should be zero volts after the modification.
It can stay because with the opamp physically removed the resistor is isolated.
The voltage on the link between pins 1 and 5 should be zero volts after the modification.
Excellent. So that channel should all work and sound normal.
It is up to you how to proceed now. If you are leaving it like this then do the same mod to the other channel.
Also which opamp is it we are talking about? Is it the phono section or the output section that drives the output sockets?
It is up to you how to proceed now. If you are leaving it like this then do the same mod to the other channel.
Also which opamp is it we are talking about? Is it the phono section or the output section that drives the output sockets?
Hi folks - Many thanks to the contributors on this thread. One of my GFP-565's was behaving in a manner as described in the initial part of this thread regarding DC voltage when the recording/listening selectors were on the same source. DV voltage in the signal chain changed proportional to the volume knob's position. I replaced the tape monitor IC's IC401/IC402 (Adcom 7A's) with a pair of OPA2134's and all is good!
Excellent 👍
It always amazes me how these high end products can often have such seemingly basic flaws in their design. Oh well 🙂
It always amazes me how these high end products can often have such seemingly basic flaws in their design. Oh well 🙂
Well I'm back from vaca. I ordered the IC103 that I shorted out. It came while I was gone. So I replaced in all IC103, IC401/IC402 with a pair of OPA2134's and 1 - 49.9 ohm resistor that I smoked. I'm happy to report that all is great now. Got rid of the original problem that I was having when the recording/listening selectors were on the same source. It sounds fantastic.
I can't thank you all enough for helping me out with this and being so patient with me.
Hope you all have a great day
I can't thank you all enough for helping me out with this and being so patient with me.
Hope you all have a great day
Excellent! Glad it works and sounds great. Good job following through and letting us know the results.
Well I hate to say this but when I turned the system on today the right side volume stays on low but will increase as you turn it up. It just doesn't turn the sound completely off. The left side works as normal.
Did you replace the volume control?
The volume control should 100% remove all audio. As a test just apply a short to ground such as across the 10 meg.
The volume control should 100% remove all audio. As a test just apply a short to ground such as across the 10 meg.
The short to ground didn't stop the volume. Unless I didn't short it out right. Nope I didn't replace anything other than the opamps and resistor. I rechecked all my solders and voltage and as far as I can see it looks good. It's funny that I fix the main problem that I need to but created another.
I gotta say the sound is a bit better now. My tape deck is much fuller sounding, at least to my ears.
I gotta say the sound is a bit better now. My tape deck is much fuller sounding, at least to my ears.
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Wondering - Is this volume problem happening if you check through all possible positions of the recording and listening selectors (using different sources)?
All you are doing is shorting pin 3 to ground which should 100% remove all audio. Ideally this would be a perfect candidate for using a scope.
The other possibility is something is feeding back into the chain after the volume control or there is a high resistance ground somewhere (unlikely).
The sound is coming though the right side no matter what input I plug into. For example, I plug the tape deck into tape 1 input, turn the volume off the sound comes through but will increase turning the volume up. This is the same for every input. If I put the listening on something else, the sound goes away. Also turning the balance switch to the left will turn the right side off as normal. The left side works the way it's supposed to.
I think you will have to use some old fashioned fault finding techniques on this (a bit of wire to short things out 😉)
Can't remember... do you have a 'scope???
Look at the circuit:
You have three output terminals per channel. The one marked 'bypass' is the one to check first. If you apply a short across that 10meg resistor on pin 3 of the first opamp you should here zero audio coming through. I can't see any route where the other channel could break through. So try that first. Use the 'bypass output'.
The ground that the volume control and the resistor go to should have 0.000 ohms continuity to any other grounds. Can't see that being a problem but check.
Can't remember... do you have a 'scope???
Look at the circuit:
You have three output terminals per channel. The one marked 'bypass' is the one to check first. If you apply a short across that 10meg resistor on pin 3 of the first opamp you should here zero audio coming through. I can't see any route where the other channel could break through. So try that first. Use the 'bypass output'.
The ground that the volume control and the resistor go to should have 0.000 ohms continuity to any other grounds. Can't see that being a problem but check.