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SX-838 No Output From Main Amp, Pre-Out is OK

bzenic

bzenic

Active Member
Here is my current situation: I don't have any output from main amp.
The tuner works fine and there is a good signal from Pre-Out RCA, I've tested that by connecting Pre-Out to another receiver AUX input. There is no any output on Phones jack as well.
The voltages on the protection board look good as well.
I have tried testing transistors on the main amp board without desoldering them by measuring diode fall between E-B and they seemed okay.
Testing the output transistors on the sink the same way was inconclusive.
Is there a reliable way to test the output transistors on the sink without desoldering?
Should I trace a signal from the signal generator?
 
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Jumpers installed? Yes, sounds like they are.
Try the finger test on the amps main signal input? Try to make it hum.
Corrosion on the jumpers?
Check the big filters for rail voltage.
The -45v regulated at the power supply pin 7. It powers the amps only.
Check all the fuses with your meter.
 
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Unfortunately I've eliminated al those causes.
I do have about 45 V on pin 7, the jumpers are in and definitely not corroded and on the top of everything the tuner/pre-amp works well.
But, no output on earphones and nothing on the speakers.
 
Can you run the amps with another pre or even your phone? I know but:
Something is not adding up. Both channels?
Check the -44v at the amps?
Check all the PS voltages?
 
I've tried connecting signal generator to AUX, nothing on the speakers.
I will check the voltage on the amp, that is the next step.
What pins are signal input to the amp, it looks like 11 and 15?
 
Connect the SG to the Main in.
3000hz
SG attenuation very low.
Volume at min first and gradually bring it up.
 
Does the relay click?

With the outputs, you can three-way test them for shorts while in-circuit. If they test at like 1.5k ohms or higher, that’s fine. Shorted is probably bad. Test with the receiver off.
 
Yes, the relay clicks and the tuner and pre-out work.
It's the power amp that does not produce any sound at all.
You mean to measure resistance on output transistors?
 
yes, a short is a short... ghosting can't compete.

Transistor tests can tell us if bad pretty definitively, but can't definitively say "good", just - "this ain't bad".
 
No apparent shorts on the output transistors. I've checked all the voltages on the main amp board according to the service manual, they are consistent, +42.9 V and -42.9 V, a little lower because I was using DBT. Next I should try to check if speaker switches work.
 
relay clicks? use your finger to inject hum into the amp-in jumpers.

before powering up, check the power amp, if it has 2sa726 (Q1 - Q4) or 2sc1451 (Q5 -Q6) replace them with ksa992 (matched pair) and ksc3503.

ALL are known troublemakers, the 2sc1451 is an amp-killer in that position.
 
If the relay closes a few seconds after power up, you can tentatively at least, dismiss shorted outputs. If touching the jumpers on the back of the unit doesn't give a hum, try touching the amplifier input coupling caps, or the differential input transistors themselves. Check that you actually have continuity from the amp board to the speaker terminals.
 
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This is the state of the receiver:
1. The relay clicks and tuning faceplate lights work
2. pre-out main-in jumpers are in
3. FM / preamp worked, confirmed by connecting pre-out to another receiver's AUX
4. The output transistors are not shorted
5. No output on "A" speakers when the switch is on "A"
6. The voltages on amp board seem okay
7. 2sc1451 and 2sa726 transistors on the amp board replaced
 
It's a connection issue. The fact that the relay closes tells us that the outputs are fine.
Speaker switch, relay, or both. Again, check your continuity from the amp board to the speaker terminals. Deoxit and work the speaker switches. Input coupling caps could be the culprit, but I kinda doubt it.
 
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I'm with Tarior on the leads from the relay out to the speakers. If the relay is new, Make real sure the output leads are on the correct pins (wouldn't be the 1st time someone has miswired the output side), connections out to the A & B switches (did you test the "B" spkr switch? I see no mention of you having done so). then from the swtiches to the spkr connections. +1 on cleaning the switches (the push button selector switches are the bane of the 838,939, & 1010.) and ensuring continuity.
 
When I touch the power amp input it produces a loud hum but it seems that the signal from the preamp is missing at the power amp input.
I don't understand how is that possible, is there a possibility that the control amp is the cause?
Or the source switch?
 
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Someone may have installed the wrong jumpers and busted out the RCA jacks?
You said you tried the pre outs?
 
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