Discussion in 'Marantz Audio' started by dbxdx5, Aug 8, 2017.
I'm going to re-check the solder joints and traces.
So the joints look fine, but I'm going to reflow them anyway. Also, I had replaced C703 on both boards with this Nichicon cap per Leestereo's restoration thread. This is a suitable replacement, correct?
Another problem I just discovered last night is a broken solder joint at the ground connection for the male RCA input at J701, left channel. This issue has been noted by others, including catrafter and RSlaug, and likely explains why my DC offset had been fluctuating wildly prior to replacing the trimmer on the that channel.
I was able to dial in the bias, but DC offset still isn't responding to either trimmer. Both channels hover in the .600V - .700V range. Could a bad speaker relay be at work here?
That should be about where it trips the protection relay to shut off. The only scenario I could imagine for the relay is if the coil is somehow shorted to the contacts but that's a long shot!
It's more likely the pot isn't soldered in correctly. Exactly where are you measuring the offset?
In an earlier post I noted how the old wire-wound trimmer had two additional outer legs that the Bourns was lacking. Well, I noticed a moment ago that the removal of one of these legs takes R710 out of the trimmer circuit.
Since the missing outer leg in question connects to the wiper in the old trimmer, I need to solder a jumper wire from the wiper through hole to the empty leg through hole.
Well, that didn't fix it.
@rBuckner - There's only way for the trimmer to be mounted, and I checked my soldering. It's all fine. As far as how I'm checking DC Offset, I'm following the steps outlined in the SM. I welcome any ideas for how to troubleshoot this.
I have NO 2325 here to look at so I am basing this off the service documentation, which can be risky.
I can't edit images at the moment so I am going to have to try and explain this.
On the new pot, Pin 1 goes in the hole on the pad with R711. Pin 3 goes in the hole in the pad that goes to R712.
I think this is what you have now. You then need to add a jumper wire from the hole in the pad with R710 to pin 2 of the pot.
In other words, jumper the pad circled in red in your picture to the center pin on the new pot.
Hope this helps.
Thanks Tom. So the jumper goes from Pin 2 to the pad with 711, not 710 like I thought?
The jumper goes from R710's pad to pin 2 of the pot. I have fixed my post. Sorry.
No worries. I had a feeling that was what you meant. I have the jumper in place and still no response from DC offset to the trimmer adjustment. I'll keep investigating. . . .
FWIW, here's what else I changed when I replaced the trimmers:
• C703 - 33uF tantalum swapped for a bipolar Nichicon UES1E330MPM
• R719 and R720 - 1.2k ohm/2 watt swapped with Panasonic ERG-3SJ122
• H718 and H719 - VD1122 512 swapped with 1N4148
You probably know this already, but, there should be a positive voltage (black meter lead to chassis) on the end of the pot that connects to R711 and a negative voltage on the end of the pot that connects to R712. Pin 2 of the DC offset pot should vary from the positive voltage to the negative voltage as you rotate it from end to end. 0VDC offset (at the output) should occur when pin 2 of the pot reads near zero volts.
Ok, so when I attach the black meter lead to chassis ground and the red lead to Pin 2 of the trimmer, I'm able to adjust the trimmer to about zero volts. However, when I then check DC offset at the speaker terminal - black lead still to chassis and red lead now to the red terminal - I get roughly 1.00 volt DC. So to sum up, I can adjust DC at the point of the trimmer and get it to zero, but not at the speaker terminal, where it hovers around 1 volt.
Bad speaker relay?
Possibly the bipolar cap I installed at C703 in place of the polar tantalum?
I doubt it's the relay.
I see no reason that C703 being a bi-polar would cause a problem unless it is shorted.
Meter set to read DC Volts, black lead to chassis, measure with the red meter lead:
Measure the offset on one end of L701 or on the pin spaced away from the others on J701. This is ahead of the speaker relay.
Does the voltage change when you adjust the trimmer? Can you set it to zero volts?
For J701, do you mean red lead to the center pin of the RCA? Actually, I see you mean J710.
Measuring J710 gives me around 1V and trimmer has no effect. At J808 and J809, I get a fluctuating value between .850V and 1V for both, and no effect from trimmer.
The wiper only goes to R710 NOT anyplace else, looks like it's connected to the trace under the trimmer.
I'm confused. Do you mean that Pin 2 shouldn't go straight through the hole in the board in front of it, but rather be bent to the side and be soldered directly to the pad that connects to R710? Why would that matter if I have a jumper going from the center hole in front of Pin 2 to the pad for R710?
Do you have pin 2 of the pot going anywhere else other than R710? If you have it going to the trace directly below the pot that trace is ground if your board is the same as in your picture. Did the original pot go thru that hole? The whole outer case of the old pot was the wiper contact correct? The original pot had 4 soldered contacts correct? One for each end of the fixed resistor and two, the case, for the wiper correct?
Ah, ok. So the pad under the trimmer is left empty, correct?
Yep, it's ground. It wasn't used originally was it?
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