RX-5100 Boombox Channel Volume/Tone Discrepancy

[cpt.shotwell]

I've been working on a pair of boomboxes and have run into this issue in both instances. I'll try to describe as concise as possible the issue in one to keep it simple and maybe I can replicate any possible solutions in the other.

Hitachi TRK-8080MKII
National (Panasonic) RX-5100 (japanese model) - this is the one I'll describe the issue in detail below...

Each channel has its own separate volume slider, and there's one general tone slider that affects both. I've sprayed out the REC/PB switch under the tape mech and worked it quite a bit. I've cleaned each volume slider with IPA and then flushed out with Deoxit. I've confirmed continuity on all wires heading from the board amp to the speakers.

The left channel is noticeably louder but also noisier, especially in the middle of the path of the volume slider. The right channel does not get as loud but is cleaner, less noisy, throughout the path of the fader. The tone control slider is only affecting the left channel and not the right. The left channel feels like it has the appropriate amount of volume, it's just noisy. The right channel feels soft overall and should definitely be getting a bit louder at the top.

This issue is present at the headphone output as well, not just the internal boombox speakers, of which there are 4 of (stereo, 2-way each channel). It applies to both the tape player and the radio. The sound is generally pretty good coming out of the speakers, aside from this issue. So it appears most functions are in good working order and I've cleaned and serviced as many areas as possible. I'm just stumped on this problem...

Any insight would be greatly appreciated. I'm well equipped to work on the unit and can follow some advice, just not sure where this problem could be originating from. Thanks

 

[caution]

You could swap the L/R sliders to rule out them out. If the inability to control the tone shifts to the other side, then you really do have a bad slider. Nevertheless, the issue lies in the preamp/mixer stages of the audio path, since all sources are affected. Recapping those (or hunting down the bad ones with an LCR meter) may help.

 

[caution]

Another thought crossed my mind - I noticed this unit has line-out jacks. You could try hooking up an audio source to the line-in jacks and see how the audio sounds on the line-out jacks. If it sounds okay, then you can rule out the preamp circuit, and focus on the mixing stage, and potentially the power amp stage.

 

[cpt.shotwell]

Another thought crossed my mind - I noticed this unit has line-out jacks. You could try hooking up an audio source to the line-in jacks and see how the audio sounds on the line-out jacks. If it sounds okay, then you can rule out the preamp circuit, and focus on the mixing stage, and potentially the power amp stage.

Alright I plugged in an iPhone to the line-ins and the line-outs to a DJ mixer and separate speakers. That audio sounded fine, balanced levels and same tone. So I think you're right, the preamp stage is alright. I'm preparing the strip the boombox down to each individual circuit board. are you familiar with the unit? trying to find a service manual, but I image the power amp area is around the heat sink and big capacitors, seen in the bottom of the first image. the second image is a smaller PCB that connects out to a switch on the tape mech. the third is the tone slider and two volume sliders

 

[caution]

I was looking at the service manual for the RX-5110. The main board and slider board are the same, but the 5110 doesn't seem to have that other board mounted to the back. I think that's the TPS (Tape Program Sensor) board. The 5110 doesn't have TPS.
Check it out here (PDF)

 

[hopey]

I'd be removing those pot sliders from the PCB and crack them open and clean them properly. You can't expect deoxit to work all the time. You would be suprised how much volume can be restored by doing it properly.

 

[cpt.shotwell]

I'd be removing those pot sliders from the PCB and crack them open and clean them properly. You can't expect deoxit to work all the time. You would be suprised how much volume can be restored by doing it properly.

thanks for the tip. yeah i might give those a good thorough cleaning before deciding to recap the entire board. not particularly fond/comfortable with doing like 30 caps on a PCB. my soldering equipment is basic, generally just for reconnecting broken wires.

 

[caution]

If it gives you any inspiration here's a guy that replaced 56 of them