There are so many components in the preamp section of this amplifier that tracing the input signals is driving me crazy and I am seeking help from those more familiar with these amps. The right rear RCA input does not send signal to the right rear channel when used in 4ch mode, but the channel plays the signal if using 2ch mode and the RCAs are plugged in to the front RCAs. Many repairs were performed on the amp in the power supply, output and driver board sections but since all channels play clean audio, I assume the problem is in the preamp section/board. Any help?
Have you tried lifting and re-seating the preamp board?
Have you operated all pots and switches through their entire range while listening for audio to see if it every played through (even just for an instant)?
Have you operated all pots and switches through their entire range while listening for audio to see if it every played through (even just for an instant)?
Yes, I lifted and reseated, removed and reinstalled the preamp board multiple times. I cleaned all the switches and pots and tried every configuration but nothing allowed the right rear RCA input to produce sound in the right rear channel.
I started to trace the input signals from each RCA terminal but left and right go into different OP amps on different legs. I might be able to trace the right front signal as it enters the same leg on a corresponding OP amp behind the front RCAs but I was hoping someone could save me from that menial task...
I started to trace the input signals from each RCA terminal but left and right go into different OP amps on different legs. I might be able to trace the right front signal as it enters the same leg on a corresponding OP amp behind the front RCAs but I was hoping someone could save me from that menial task...
So, I decided to try all the switches and pots again and found something. I can get the right rear RCA input to play through the channel with the HPF switch on in either 12 or 24db, but nothing in the OFF position and the output is very low. However, the same HPF switch will only pass the left rear RCA input when the switch is in the OFF position but at a much higher level than the right side whenever it passes the signal.
That means that the switching is handled by the DG201 switching ICs. Are you familiar with them? If not, look up their datasheet.
How many of the DG201s are there on the preamp board?
How many of the DG201s are there on the preamp board?
There are 21 DG201BDYs on the preamp board. I have been trying to trace the right front and right rear signals at the same time since the circuits are similar. I can see the front right signal on the input and output of a DG201 behind the rear RCAs when the switch is in 2ch mode. I can also see it on the header pin for the board. The right rear RCA signal can only be found on Pins 1, 2 and 3 of NJM2068 and Pin 15 of the DG201 located next to it, but there is no signal on any other pin of that IC, on any other ICs on the preamp board, or any of the header pins. Does that mean the DG201 is defective?
Attachments
Which DG201 pins switch from 12v to ground when switching the 2ch/4ch switch?
There should be 4 of them. It will be the corner pins of the IC. Find all 4 pins.
There should be 4 of them. It will be the corner pins of the IC. Find all 4 pins.
I found the only DG201 that has voltage switching on the four corner pins when the 2ch/4ch switch is manipulated. In 2ch position, pins 8 and 16 have 13.7v on them and pins 1 and 9 have ground. In 4ch position, these voltages are reversed. I can see the audio from the right front input on Pin 2 and cutting in and out in pin 3 when the 2ch/4ch switch is moved. I can also see right front audio on pin 6, but no output on pin 7, if it is supposed to be there...
Attachments
I can see the audio from left rear on pin15, and the audio from right rear on pin 7, but it is very low compared to the other channel.
Switch set to 4ch.
Find the two inputs that are at 0v. Those are the two switch circuits that you're concerned with.
Plug the LR RCA in and confirm that you have signal on both terminals for that switch circuit.
Remove the LR and connect the RCA cable to the RR jack.Then for the other switch circuit that has 0v input, see if you have signal on both of the terminals for that switch circuit . Or do you only have signal on one terminal for that switch circuit .
Find the two inputs that are at 0v. Those are the two switch circuits that you're concerned with.
Plug the LR RCA in and confirm that you have signal on both terminals for that switch circuit.
Remove the LR and connect the RCA cable to the RR jack.Then for the other switch circuit that has 0v input, see if you have signal on both of the terminals for that switch circuit . Or do you only have signal on one terminal for that switch circuit .
Feeding a 100hz tone. Left rear goes in pin15 and comes out of pin14 clean in 4ch mode, and output of pin14 stops when switch is set to 2ch. The input from Right Rear on pin7 is not a clean sine wave like how the left channel is. The IC seems to be switching properly though as the signal passes out untouched on pin6 when in 4ch mode and pin6 goes dead when switched to 2ch mode. I suspect I have a problem upstream of this DG201.
Pic1 is the right signal
Pic2 is the left signal
Pic1 is the right signal
Pic2 is the left signal
Attachments
Last edited:
The excessive negative DC may indicate that there is a defective op-amp. Do you see any op-amps in the area with excessive DC on their output terminals or possibly an op-amp missing the positive regulated supply voltage. It happened in the area of the op-amps shown in the attached photo in some amps.
Attachments
I have found the opamp that feeds pin7. I get different readings on pins 1, 2 and 3. All opamps have regulated voltage. The other opamp for the right channel, and the matching two for the left channel all have identical signal on pins 1, 2 and 3. I don't have any NJM2068. Is there anything that can substitute it that I might have, or can I swap one from another location on this board?
What is the voltage on terminals 1, 2 and 3?
You could use virtually any op-amp for testing but I'd order the right op-amp.
You could use virtually any op-amp for testing but I'd order the right op-amp.
So, I removed the suspect opamp and the channel started to play in 4ch mode, just a little softer than the left rear. I remembered that I had a 500/1 here that needs parts so I took a NJM2068 from that and put it in. Now both channels play at the same level in 4ch mode.
The next issue with this rear input is that the left rear only plays when the HP switch is off. Not getting any output with the switch in 12 or 24db positions. How do I find that?
The next issue with this rear input is that the left rear only plays when the HP switch is off. Not getting any output with the switch in 12 or 24db positions. How do I find that?