| Chuck <ch@dejanews.net>: Oct 05 12:04PM -0500 >Is my thinking OK or did I miss something ? >I am a beginner in HiFi electronics but have basic skills in electricity >as an electrician for 25 years and do know how to solder.. Try pushing the tape monitor and other push button switches on the front panel a few times to see if the channel comes back. If it does, spray the switch with DeOxit 5 or equivalent. |
| dplatt@coop.radagast.org (Dave Platt): Oct 05 10:40AM -0700 In article <pp81ev$2tfg$1@adenine.netfront.net>, >3 End of preamp broken on left channel. >Most likely End transistors.. >Is my thinking OK or did I miss something ? Those are possibilities. Here's what I would suggest for further troubleshooting. [Turn the power off before changing any cable settings, of course.] First: the manual and schematic show that the preamp and amp are linked together via a set of jumpers on the back panel, going from "pre amp out" to "main amp in". First thing to do is remove and re-insert the jumpers, to see if you simply have a bad connection in the connectors there. If removing and re-inserting fixes the problem, then clean the connectors and jumpers with contact cleaner, reinsert, and you should be OK. Second thing to do is disconnect those jumpers, and replace them with a standard red/black RCA-connector interconnect cable. This time, swap the two channels - connect the left channel "pre out" to the right channel "amp in", and vise versa. If the problem remains in the left channel, then the fault lies in the amplifier section. If the problem moves over to the right channel, then the fault lies in the preamp section. If it's in the amp section: it might be the fuses (I'm not sure if these are among the ones you checked). It could be bad speaker on/off switches, but it seems unlikely that you've had the same failure in the A and B channel switches (but it is possible... the speaker-B switch might have failed long ago and you might not have noticed it until now). If the switches and fuses are all OK, and you're confident that the amp is getting a valid input signal, then you'd need to trace the signal through the amp section with a 'scope and DMM - see if your B1/B2 power supply voltages are OK, and see how far the signal gets through the left-channel amp before it vanishes. If it's a failure in the preamp section (if the dead channel "moves" when you switch the cabling between preamp and amp) then I think the big two suspects would be: - Bad, or dirty selector switches. In particular the "tape monitor" switches for Tape 1 and Tape 2 are both right in the signal path... if one of these has an open contact you'd lose a channel. It looks as if the "tone control defeat" switch is also a suspect, for the same reason. - An electrolytic interstage coupling capacitor which has failed in an "open" state. Here, also, signal tracing is your friend. Feed a known-good line level signal into the "tape 2 in" jacks, turn on "tape monitor 2", and see if you get a good signal... this is the last input "in the path". If that one works OK, try "tape 1 in" and the "tape monitor 1" switch setting. If that works OK, use "aux". With this approach you're working "backwards" from the preamp/amp interconnect, looking back along the circuitry a step at a time to see how far back you can go before the signal disappears. I've seen enough older receivers/preamps with dirty-switch problems that this is my lead suspect in cases like this. There's a reasonable chance that cleaning all the switches will fix your problem. Put a small squirt of DeOxIt D-5 into each switch, operate the switches a dozen times or so to break up any old grease and dirt and oxide and tobacco-smoke tar and etc., use another small squirt to flush out and lubricate the contacts. Put a small squirt into each RCA jack, then plug and unplug a cable a few times to break up the oxides and crud. Let dry for a few minutes, then test again. |
| dansabrservices@yahoo.com: Oct 05 11:30AM -0700 You can narrow this down to the AMP section by removing the shorting plugs (pre-amp out to AMP in metal plugs) and replacing them with an RCA cable. Let assume that the left channel is not working. Using a standard RCA to RCA cable, connect the left pre-amp out to the right AMP-in. If the right channel has sound as expected, then the pre-amp section is OK and the problem is with the amp section. With the power off using a meter, A/B the left and right channels. Measurements of the corresponding components should measure about the same. If you have access without the possibility of shorting components, use a DC meter and measure some voltage points. Again, the two channels should measure the same. With the symptom of slight sound in the output, I would suspect that a voltage is missing. You may have an open resistor that feeds the output section. |
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