No sound sx 838

Do not jumper the protection relay!!!

Relay clicking once after a several second delay means the amps have no faults, and the power supply is reasonably correct.

Being a new member, I HAVE to ask: are the preamp out - power amp in jumpers in place in the back, do you have a service manual and understand how to set the switches for sound? This unit DOES have extra counter-intuitive switches.
 
Yes the jumpers are in place and I do have a service manual. First the protection relay does not click when the unit is turned on.
 
Remove the jumpers.

First the protection relay does not click when the unit is turned on.
Then maybe a fault in the protection circuit or the protection circuit is doing its job.
The next step probably involves voltage measurements on a powered up unit. Are you comfortable with this? (lethal voltages inside)
Also do you have the tools (multimeter...)
 
Yes I'm comfortable with testing and I do have meter. I'm confused about when to use AC,DC or millivolt settings
 
Most everything on the boards are DC Voltages. Look at the voltages coming off the Power Supply(AWR-055). You have AC coming in from the fuses, then it gets folded,spindled mutilated, and RECTIFIED into DC. All outbound voltages are DC, EXCEPT PIN 22 which feeds Pin 12 on the Protection board (AWM-025) and The Dial pointer lamp, Speaker Lamps, and the Tuning Dial Lamps. This is 7.5V AC.

Does your meter Auto-Range or is it a manually selectable. If manually selectable, set it for 200VDC for most items. It should show you mV at that setting.

Check the offset and bias voltages. Adjust them as per the manual. If they adjust ok, you should get sound. Push in SPKR "A", Make sure all Toggle switches are UP (Tape Monitor especially), and turn selector to AM or FM. See if you at least get AM static or FM static on each. If it will pick up a station, good.

Cleaning the controls is the 1st step in any troubleshooting. Switches, and pots get dirty, and can disrupt the signal going thru them, even blocking signal. Cleaning them will allow the switches and pots to be ruled out in most cases. Get some DeOxit D5 or DNS-5 and Faderlube F-5. Read The idiots guide to DeOxit...Revisited.
 
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Pin 1: -39.6. 2: -12.9 3: -.303 MV . 4: gnd. 5: -39. 6: -39 . 7: ,-275 MV. 8: ,-.309 MV. 9: 35.2 . ,10: 34.9. 11; 34.9 . 12: 7.3 ac no I don't know where to check bias and offset
 
I was looking at that schematic, I don't have the 8ohm dummy resister is there some place i can order one by mail
Since the relay is not engaging the dummy load is not needed. More important to measure the dc offset ie, steps 2&3.
 
I tried steps 2 and 3 but I didn't get a millivolt reading I got a voltage reading of -41.5
Yep, that would do it (fry the speakers), so the protection circuit has elected not to engage the relay. Next step is ti determine which channel is faulty
then identfy the faulty parts.
 
Which measurement pin (5 or 20) gave the -41.5V?
Pin 5 indicates left channel fault, pin 20 points to right.

If both pins had high voltage the faulty channel will be the one
with the highest voltage, eg, -41.5 versus -41.3V.

If the left channel is faulty, suggest diode test transistors
Q7,9,11,13. Test Q8,10,12,14 for right channel, amp powered off/unplugged
 
OK I'll test them and get back with you
Diode testing of transistors can be a bit misleading, faulty components can sometimes pass. If they pass then
measure Vdc to ground/chassis for base then emitter then collector of the listed transistors. Please don't probe
between transistor legs, probe at connecting part eg a resistor, diode, cap...
 
I apologize if I seem a little ignorant....this is the first time I've tried anything like this. If this was a vehicle I could fix it no problem. Are all those you told me to test on the same board having a hard time locating. Your patience is appreciated
 
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The transistors should on the board mounted to the large heatsink, so it
won't be so easy to test. However pins are available for the output transistors
Q11,12,13,14

upload_2018-6-10_21-8-52.png
will look at other transistors
 
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