Nakamichi Sr-4a receiver - weak left channel problem

[Naki]

Just acquired this pretty nice sr-4a receiver a couple weeks ago but I’m running into this problem with the left speaker channel having almost or no bass and just feels weak and empty.

Before everyone says to clean pots etc I have fully taken apart this and deoxed and lubed all pots from the inside. I’ve already switched speakers left to right, switched speaker wires and tried A and B channel but the Problem still only persists with the left channel.

Replaced JR2A-24DC relays for both A and B channels just incase they were acting up.
I Decided to try headphones and the problem still persists in the headphones. Really not sure where to go from here, I tested the resistance on both left and right channels and the left channel is definitely higher and out of the percentage of error. Digging deeper I believe it’s a small cap on the rear of the receiver.

I’ve ordered the cap and I really hope it fixes it but if not I’m just going to put it back in the box until I have to time to mess with it again.

any help or ideas would be appreciated

 

[northpaw]

Presuming that you have eliminated any potential problem with the pots and speaker relays (including their solders to the PCB, which not uncommonly develop cracked joints), that leaves the 2-pole (stereo) switches as the likely culprits, and possibly a cracked solder on one of the RCA inputs. The latter you can check by using a different input (or the tuner), and also by gently wiggling the RCA input cables where they enter the back panel, while music is playing through those inputs.

The switches are a more likely target, IMO. These include the Selector switch, the Speaker switches, and the Tone-defeat switch. The Selector switch knob is remote from the actual switch, located on the Main PCB. You can test if that switch is part of your problem by wiggling it while music is playing, and see if there is any sensitivity to it. If it does need attention, that style of switch is notoriously difficult to clean thoroughly without unsoldering it from the PCB, but cleaning can be attempted by placing the siphon from the cleaning spray directly at any openings you can get to, and doing repeated cycles of sprayings and operating the switch through its range of motion. If you do this, I recommend first using a residue-free cleaning spray, like CRC QD, to flush things out as much as possible, followed by a more sparing application of deoxit or similar. This prevents making a sticky mess of the PCB.

But before working on that switch, I would suggest evaluating the other switches. Before treating them, you can test each of them for sensitivity to your issue as outlined above, by operating them while music is playing; if one is the problem, you will hear it (possibly even temporarily fix it) as the switch is activated. From my experience with a couple of TA-2s, the Tone-defeat switch is a common troublemaker, but it can be any of the 2-pole switches in the signal path. I think the ones that are rarely activated by the user are the ones most likely to develop issues because they rarely have their contacts wiped.

The Tone-defeat switich is on the Tone Control PCB, and the Speaker switches on the Speaker Switch PCB (the SR-4A has lots of boards). If it is like the TA-2A, these are mounted inverted and low in the chassis, and not in a position that is uncovered by removing the bottom panel, but you will have to see, and find a way to get a siphon to the switch bodies. Given that you have tried listening through both Speaker A and Speaker B and the problem persists, and moreso that the headphone output has the same issue, it is almost ciertain that the problem does not originate at the Speaker switches as they are downstream from the headphone take-off point.

The other thing to do is to check the health of the subsidiary supply voltages. For example, the ±18V circuit is used to power the op amps in the Tone circuit, so it is possible that it plays a role if the Tone-defeat switch is not activated. On the sister Nak units I have worked on, the transistors on ±18V circuits run quite hot, and that, in combination with aging glue used to hold their pins in place during factory assembly, results in corrosion and compromised solder joints at the PCB.

 

[Naki]

Presuming that you have eliminated any potential problem with the pots and speaker relays (including their solders to the PCB, which not uncommonly develop cracked joints), that leaves the 2-pole (stereo) switches as the likely culprits, and possibly a cracked solder on one of the RCA inputs. The latter you can check by using a different input (or the tuner), and also by gently wiggling the RCA input cables where they enter the back panel, while music is playing through those inputs.

The switches are a more likely target, IMO. These include the Selector switch, the Speaker switches, and the Tone-defeat switch. The Selector switch knob is remote from the actual switch, located on the Main PCB. You can test if that switch is part of your problem by wiggling it while music is playing, and see if there is any sensitivity to it. If it does need attention, that style of switch is notoriously difficult to clean thoroughly without unsoldering it from the PCB, but cleaning can be attempted by placing the siphon from the cleaning spray directly at any openings you can get to, and doing repeated cycles of sprayings and operating the switch through its range of motion. If you do this, I recommend first using a residue-free cleaning spray, like CRC QD, to flush things out as much as possible, followed by a more sparing application of deoxit or similar. This prevents making a sticky mess of the PCB.

But before working on that switch, I would suggest evaluating the other switches. Before treating them, you can test each of them for sensitivity to your issue as outlined above, by operating them while music is playing; if one is the problem, you will hear it (possibly even temporarily fix it) as the switch is activated. From my experience with a couple of TA-2s, the Tone-defeat switch is a common troublemaker, but it can be any of the 2-pole switches in the signal path. I think the ones that are rarely activated by the user are the ones most likely to develop issues because they rarely have their contacts wiped.

The Tone-defeat switich is on the Tone Control PCB, and the Speaker switches on the Speaker Switch PCB (the SR-4A has lots of boards). If it is like the TA-2A, these are mounted inverted and low in the chassis, and not in a position that is uncovered by removing the bottom panel, but you will have to see, and find a way to get a siphon to the switch bodies. Given that you have tried listening through both Speaker A and Speaker B and the problem persists, and moreso that the headphone output has the same issue, it is almost ciertain that the problem does not originate at the Speaker switches as they are downstream from the headphone take-off point.

The other thing to do is to check the health of the subsidiary supply voltages. For example, the ±18V circuit is used to power the op amps in the Tone circuit, so it is possible that it plays a role if the Tone-defeat switch is not activated. On the sister Nak units I have worked on, the transistors on ±18V circuits run quite hot, and that, in combination with aging glue used to hold their pins in place during factory assembly, results in corrosion and compromised solder joints at the PCB.

Thank you for your reply I’ll check those switches I don’t think the selector switch is the problem but it could very well be the tone switch I’ll try it just in case. I’ll start checking voltages later tonight and report back.