Technics SA-301 Right Channel problems

[flinx]

I picked up the above unit from a thrift store for $20, cosmetically it is in good condition. Connected it to a set of 40w Panasonic speakers. It powered up and worked fine for a bit and sounded great. Then the right channel started to get scratchy and dropped out. After some trouble shooting I found part of the problem was a bad connection on the speaker end. I fixed that but the problem persisted. I verified it was not the speakers anymore by attaching the left channel speakers to the right channel as well as another set of speakers.

I opened the unit up and first thing I did was pop out the 2 large caps and test them, they are supposed to be 6800uf at 45v, but both of them measured around 5400uf and 5300uf. I swapped them with each other and then checked the circuit board for bad/cold solder joints. I did not see any obvious cracked solder joints, but ALL the soldering was grey not silver. I traced out the right channel and reheated/added solder/resoldered anything that did not look perfect.

There were some key components that had a large amount of flux built up around them and re-soldered those. I tested the unit and it seemed to be fine. I listened to it for a while then buttoned it up. Listened some more and mentioned to my wife that I may have fixed it and yes not 10 seconds later the right channel dropped out.

I turned the unit off and back on immediately and the right channel was back. I do notice that when the volume is turned up that the right channel power meter shows slightly less then the left.

I read on another forum on another site that the DC offset voltage should be less then 15mv for each channel. The left channel on mine measured about 13mv and the right measured around 17mv. Currently Both channels are working but if I increase the volume 1/3 to 1/2 way up sometimes the right channel drops out again. bouncing the power switch brings it back.

One other thing, this unit has marking that say it should be used with 16ohm speakers. mine are 8ohm.

Any Ideas?

[budm]

Re: Technics SA-301 Right Channel problems

Those two caps are for the filters for the Bi-polar power supply to supply the +/- for both power amps so that cannot be the cause of one channel drop out, this receiver uses direct-couple output circuits. 17mV offset is not a problem at all, the speaker protection relay will disconnect the speaker when it detects more than 2VDC at the speaker terminals.
Did you do any cleaning of the switches yet, especially the TAPE Monitor and the Muting switches? Just toggle the switches a bunch of times may clean out the oxide built up on the switch contacts. You should also look real close at the solder joints, especially on any transistors that are mounted on the heatsink, look for small ring around the solder joints, it will look like small cracks, use high magnifying glass.

[Th3_uN1Qu3]

Re: Technics SA-301 Right Channel problems

Did you do any cleaning of the switches yet, especially the TAPE Monitor and the Muting switches? Just toggle the switches a bunch of times may clean out the oxide built up on the switch contacts.

The switches can be a major pain in the ass on old equipment. Before you get into the electronics, toggle the switches like budm said, it may be just that. Some contact cleaner spray would be helpful, too.

[tom66]

Re: Technics SA-301 Right Channel problems

You are using 8 ohm speakers with a 16 ohm amp? Maybe it's just current limiting.

[abrsvc]

Re: Technics SA-301 Right Channel problems

If the channel is stopping with a volume increase, it is no likely to be the typical relay problem. A power sequence "fixing" the problem suggests that the spike is temporarily cleaning a connection or something like this. I would initially look for poor solder connections. Once that is completed, use freeze spray to see if a temp change can either cause or fix the probem.

Post your progress here.

Thanks,
Dan

[999999999]

Re: Technics SA-301 Right Channel problems

Since it starts out working then drops out presumably from heating, I vote for cracked solder joints on the output transistors like budm mentioned and would reflow those even if they look ok... using extra flux since these described gray-not-silver joints may be harder to solder well, likely some oxide buildup is present on the transistor leads.

If that alone isn't enough or doesn't help, I'd investigate replacing those after tracing the audio signal with a multimeter to see if it is still present right before those transistors and if it isn't, you just keep following the circuit backwards till you find the fault, or if it is present then replace the transistors.