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SX-650: 3-4 s of Playback and 3-4 s Of Hum

bzenic

bzenic

Active Member
After I recapped the tone board and recapped and replaced all transistors on the vertical amp board (except the two on the heat sinks) I get only periodic playback of about 3-4 s and then the period of hum. This same sequence repeats on and on...you got the picture.
What would be the most probable culprit and what to try?
 
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Did you double check your work....no caps in backwards or transistors with different pin out that’s now incorrect?

Also, did you substitute any parts that weren’t on a previous vetted list?
 
Yes, I have checked everything. Crazy thing is, the playback sounds normal during the 3-4 s period but then gets replaced with weird hum.
I'm most suspicious about one 100 uF capacitor rated 35 V instead of 50 V as it should be by the spec. I ran out of 100 uF rated for higher voltage at one point and put this one in instead.
That is my candidate number one for the culprit.
 
Don't know if that would be the issue. Which capacitor is that? Someone here could probably figure out what the voltage is in that portion of the circuit.

When it cycles from music to hum does the protection relay click on/off? Does it happen with all input sources?
 
That is C34 on GWR-101 (power-amplifier-assembly 2).
The relay does not engage at all, this periodic behavior (playback...hum...playback...) is uninterrupted by the relay.
 
???

Under rated cap used is not good. That is suspect number one.

But, you say the relay is not engaging at all?!?!
How are you hearing playback/hum?

It may help to know..,
Did it work before you worked on it?
What transistors did you use as replacements?
 
Yes, the thing worked before my recapping and re-transistoring, damn.
No, the relay does not engage after the first time when the receiver is powered on.
That is why I think that somehow this underrated capacitor gets charged during playback and then leaks/shorts during hum period.
I used the replacement set bought on ebay.
 
Still mud.

Power on. Relay engages and you get clean sound. Through speakers. Yes/no?

Relay releases and you get hum!?! Through the speakers? Can’t happen.

Maybe relay does engage, stays engaged and you get music/hum/music/hum…?

Kit from eBay tells us nothing. There are some sellers there that appear to offer decent enough kits but to really tell what you did, a part by part replacement list is needed.

And, then, how well you did the work is a question. Even after 1000s of parts changes, I still get one wrong on occasion.

Get the right cap for the 100uf changed and see where things are.
 
blhagstrom said:
Still mud.

Power on. Relay engages and you get clean sound. Through speakers. Yes/no?

Relay releases and you get hum!?! Through the speakers? Can’t happen.
Click to expand...

No. The relay stays engaged all the time along.

blhagstrom said:
Maybe relay does engage, stays engaged and you get music/hum/music/hum…?

Kit from eBay tells us nothing. There are some sellers there that appear to offer decent enough kits but to really tell what you did, a part by part replacement list is needed.

And, then, how well you did the work is a question. Even after 1000s of parts changes, I still get one wrong on occasion.

Get the right cap for the 100uf changed and see where things are.
Click to expand...

Yes, that seems like the most logical option to try.
 
No luck. The capacitor was actually 100 uF, 50 V, so that was not the problem at all.
I refreshed all the soldering spots but still the same symptoms: period of playback of about 5 s and then some hum and noise for 4-5 s and then again playback...
At this point I have no idea where to look for the cause of this.
 
The same happens on FM and AUX when I connect the cd player.
And no, the relay is not kicking out the power. All this repeats periodically with a period of approximately 5 s...
 
watch the tone control (power) voltages on pin 10 (+21v) & pin 9 (-19v)

and also on the chips TA7136 pin 7 (+21v) and pin 4 (-19v)

referenced (black dmm probe) to a bare metal chassis ground.

That's the single point of failure common to both channels.

Those voltages start out at the PSU as +/- 33v regulated, then on the amplifier there are ~400 ohm resistors leading to off board pins.
these wires go to the phono amp area where there are zener diodes that regulate the +21 and -19v. that are sent to the tone control board.
there are caps and resistors that can be messed up somehow causing the power to cycle... they are scattered all over the inside of the sx-650.

that schematic's scans are a go%%$%# mess !!!
 
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There was a broken link on the pcb and that was the cause of all the crazy behavior.
The collector of Q10 was basically cut off from the rest of the circuit and hanging on its own.
Thank you, blhasgtrom and markthefixer for help
 
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