Hello everyone. I have a GFP 565 that when the recording and listening switches are put on the same input the woofers move forward and make a pop as I turn up the volume, the same as Gary A. Galo mentioned in his article in Stereophile. I have the service manual for this preamp, but I'm not really great at figuring out schematics. I am decent at soldering, tho and feel I'm capable of fixing this my self, but I'm in need of your help. Could someone tell me which op-amp to replace.
Kind regards
Kind regards
Have you got as far as making any measurements? which would be the first thing to do really. As well as checking around the opamps you should also check and confirm all supplies are correct.
Measure the voltage on pins 3 and 6 of IC202 and also pin 5 of IC204 and report the results.
Do the same for the other channel.
And check the supplies first.
Do the same for the other channel.
And check the supplies first.
Mooly, will do. I wish my dad was still alive. He was electrician by trade and sadly I didn't learn enough from him to trouble shoot for myself. Please excuse me if my questions are dumb.
Do you measure "...18V DC on both channels of the tape 1 outputs. Further investigation showed that the suspect dual op-amp had the same 18V on its inputs as well"??
This only needs a DC voltmeter (any!) and access to the Tape Out jacks. Anything over a tenth volt after a minute is wrong.
Ok, if I'm doing this right, I'm reading on the Tape 1 output 17.5. On the op-amps both channels pin 3 - 17.3, pin 6 - 34.9.
Thanks for all your help.
Thanks for all your help.
Something sounds amiss with the measurements there... -34.9 is way over the top of anything that should be seen. That is more like what you would see measuring rail to rail.
Put your meter negative lead to ground and keep it there. The metal chassis should be ground (zero volts) as will be the 'outer' of any RCA sockets on the rear panel.
Recheck those voltages on pin 3 and pin 6 of IC202.
Pins 2, 3 and 6 should be close to zero volts. You also must see +18v on pin 7 and you must see -18v on pin 4.
Put your meter negative lead to ground and keep it there. The metal chassis should be ground (zero volts) as will be the 'outer' of any RCA sockets on the rear panel.
Recheck those voltages on pin 3 and pin 6 of IC202.
Pins 2, 3 and 6 should be close to zero volts. You also must see +18v on pin 7 and you must see -18v on pin 4.
I did put the negative in the wrong place. So now putting it on the chassis Pins 2, 3, 6 are 0 volts. Pins 7 +17.4v and pin 4 -17.6v
Tape 1 output reads -16.8.
Tape 1 output reads -16.8.
So a definite problem with the tape 1 output.
You need to locate these depending which channel is at fault. There is an A and B, one for each channel.
The opamps should all follow the same theory. They need the - and + supply on the appropriate pins and the two inputs (pins 1 and 3 together with 5 and 6) should be at zero volts as should the output pins on 1 and 2.
See if you can locate one of those opamps with a problem in those voltages.
You need to locate these depending which channel is at fault. There is an A and B, one for each channel.
The opamps should all follow the same theory. They need the - and + supply on the appropriate pins and the two inputs (pins 1 and 3 together with 5 and 6) should be at zero volts as should the output pins on 1 and 2.
See if you can locate one of those opamps with a problem in those voltages.
Just to clarify the above first diagram. It is the opamp circuitry referred to as A and B, not those two big letters A and B on the relay coil at the left in that first diagram.
IC 401 pins 2,3,5,6,7 at 0v
pins 4 -17.6v pin 8 +17.5v
IC 402 pins 2,3 at -16.7
pins 4 at -17.6
pins 5,6,7 at -16.7v pins 8 +17.5v
Output tape 1 and 2 right side show -16.6v. The left side 0V
pins 4 -17.6v pin 8 +17.5v
IC 402 pins 2,3 at -16.7
pins 4 at -17.6
pins 5,6,7 at -16.7v pins 8 +17.5v
Output tape 1 and 2 right side show -16.6v. The left side 0V
A definite issue here.
The circuit diagram isn't the easiest to follow on a screen so you are going to have to do a bit of detective work. Lets work with pins 3 and 5 of the opamp which are the inputs.
We need to know whether the fault is the IC or whether the IC is being fed a valid but incorrect voltage from elsewhere.
Look at the circuit diagram and points marked 1 and 3 on those opamp subcircuits. Points 1 and 3 connect to opamp pins 3 and 5. We need to work back and see where those points go and that depends on how you have the switches set.
Look at the circuit at all times to follow this.
Did any of the other opamps have a high voltage on their output pin? Be aware that some of the opamps are 'duals' and some 'singles'. For example IC206 is a single. The output is on pin 6.
So very important. Do any of the other opamps have a voltage on their output pin?
A quick test for IC402 is simply to remove it. It is possible a faulty chip could be feeding a voltage back into its input.
I'm confused on that one. I've done my best to follow the circuit, but I really don't know what to look for. Sorry I'm doing my best to learn.
I've checked every opamp for high voltage and none give anything over 17.6v.
Here is what I've taken of opamp 402. Hope this helps.
I'll do this next week after relatives leave.
That's no problem 🙂
The voltages you have just posted all show problems. Look at the opamp here (IC402 is the other channel of course which is not shown on the diagram but it will be identical to this)
Pin 3 and pin 5 on the chip itself are the inputs. Lets just concentrate on those two pins. They both have to be at zero volts DC and your readings show they are nowhere near that. We need to know whether the voltage you measure on those pins is being fed into the chip from earlier in the circuit or whether that particular chip is actually at fault (internally faulty) and putting that voltage back onto the input line.
Removal of the chip would prove that.
Now look at the circuit as a whole in the manual and look at every opamp in there. The output pin is always the one at the tip of the triangle but depending on the type of opamp, the pin number can vary.
To cover all bases with this (and it only takes a minute to do) just carefully measure the output pin voltage on each opamp on the diagram. Remember some are 'duals' with two opamps in one package like IC401 and IC402 and some are 'singles' with one opamp in the package.
Note any that have a reading other than zero volts DC (they should ideally be almost zero but may have a few millivolts but no more).
There is no need to write them if they are correct. It is only any that are incorrect we are interested in. All the points circled should be zero volts DC.
THAT is fundamental. going back into the power supply section and getting readings off the voltage regulators 78M and 79M is a wise place to start. I went through that entire section replacing the TIP110/115 pair (there was about 1 volt difference there) and then the VR’s with SPARKOS lab’s units.
I didn’t measure outputs, just put it all back together on faith and am very happy with the results…
but referring to the OP’s issue, I’d start with upstream measurements first.
ADCOM IC 7A is a dual opamp chip, like OPA2134, OPA2604, or Burson, Sparkos or other substitute. Can be replaced for about $5-75 each depending on OP’s budget, tastes.
BTW, a question for the opamp gurus here.
Everything routes through IC's 205 and 206.
Big Sky Audio's (Mikerodrig27) recommendations for "updating" this preamp only mentioned replacing the gain control chips, IC 201/202, with OPA 134/604, etc.
Has anyone atually replaced 205/206 with similar opamps?
Everything routes through IC's 205 and 206.
Big Sky Audio's (Mikerodrig27) recommendations for "updating" this preamp only mentioned replacing the gain control chips, IC 201/202, with OPA 134/604, etc.
Has anyone atually replaced 205/206 with similar opamps?
Can't help with that specific question but be aware of fakes, particularly for devices like the OPA604 and dual OPA2604 as these have I believe been discontinued. Always always buy from official sources.
The circuit configuration of the Adcom suggests that opamps with low input bias current (so FET) are needed in some areas such the opamp the volume pot feeds into due to the direct coupling. Get that wrong and you could end up with noisy pots as you rotate them.
The circuit configuration of the Adcom suggests that opamps with low input bias current (so FET) are needed in some areas such the opamp the volume pot feeds into due to the direct coupling. Get that wrong and you could end up with noisy pots as you rotate them.