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Fixing and Debugging SX-780 with one STK-0050

bzenic

bzenic

Active Member
I have this situation:
Right now I have only one STK-0050 in, the other one is removed because it was bad.
When I play some FM station in STEREO the sound is very low volume and intermittent. If I slowly turn the tuning knob the loud noise waves occur and then settle to the previos low volume. When I switch to MONO the sound volume increases and it is better in quality.
I guess this happens because the other channel preamp (the one without STK) is also included.
I was checking a bunch of transistors using ATLAS semiconductor tester but so far could not find a bad one, I still have not tested all of them.
Could it be that a bad capacitor causes these symptoms?
Where should I look and how to proceed?
 
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I don't think its good to run the missing STK amp with out a feedback loop.
Use a 150 ohm (or close to) resistor between pins 1 and 3, and 0 and 8 on each missing module. This will simulate the STK and allow for voltage readings and balance checks. The protection relay should work using this procedure.
You should hear the one channel normally.
Try another source, like a CD player in the AUX.
The tuner sounds like its not working correctly.
 
I can certainly try adding 150 Ohm resistors in the missing STK slot, I have them.
The relay works fine, it clicks after 5 s and never disengages during playback.
Actually I tried CD player on AUX but the symptoms were similar:
very low volume and periodic noise in STEREO mode and louder, more consistent playback on MONO.
 
Mono is both channels combined.
One channel is not working.
In stereo with the balance to the good channel you should have the full channel.
 
Here's the update: I put the two 150 Ohm resistors on missing STK slot (pins 1-3, 8-0) to close the loop.
Using FM and AUX as source produced bad, distorted sound.
Then I swapped the STK-0050 and 2 resistors from one channel to another.
Now the sound was much better and consistent, I flipped source/tape switches back and forth and that affected sound by decreasing volume down a lot.
From all of this I can only conclude that one channel has some faulty component, likely in the amp driver section and the STK-0050 is probably okay.
I also cleaned source/tape and mono/stereo switches since it seems that they influenced sound.
 
Also remove and disassemble the selector switch and clean that. That will definitely affect sound quality.

Athanasios
 
Putting the 150ohm resistors in is only to be used for a test of the amp front end, it’s not meant for hooking up a load or a speaker to listen too.
You were to have measured and reported the voltages with them installed to determine if there is a problem before putting in a stk
 
This is the latest update:
On one channel I had original STK-0050, on the other channel I put one of the two STK-0050 replacements (without any previous adjustment of the VR on the replacement's circuit board).
With FM as a source it was playing ok. Phono seems to have some issue, producing nothing but a hum.
One weird issue I had: the balance did not work at all, both channels were playing no matter what side I was.
I also deoxited the balance pot.
The DBT filament was visibly lit while playing, that seemed suspicious to me.
It seems there is some weird coupling between the two channels.
 
Shouldn’t use the DBT when playing the receiver. The bulb will be bright with the volume above min.
If the DBT dims then it’s safe to plug in to the house line power.
You still have balance issues with the phono input and only a hum? But FM is good? Clarifying the issues.
 
No, balance doesn't work even when stereo FM playback is okay.
Both channels work at the same volume no matter at what angle the balance knob is turned.
 
Something is wrong with the balance pot.
Someone may of rigged the pot? Basically by-passed it. Visual inspection required.
The Phono EQ uses a op-amp for the gain, one in each channel. Check the voltages at the amp pins 4 and 8. (+-23v or close).
There are 100/25 filter caps (C301 & C302) at the PS for the +-23v supply. They could be bad. Or your missing the supply?
Check the voltage first and we'll go from there.
 
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I did a lot of refreshing of soldering points all over the amp board.
The balance knob started to work much better and I also got phono working on both channels.

Only one thing I need to make clear: is there something missing at the back of the balance pot?
It looks like a screw is missing at the back above the pot.

balancepot.jpg

balancepot.jpg

balancepot.jpg
 
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balancepot.jpg

This is the question: is that hole above the balance pot supposed to be empty?
 
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That should be a tubular rivet. It connects the center tap of the 1MΩ balance control to circuit ground. It is connected similarly to the loudness taps on each channels' volume controls.

SX-780VolumeBalancePots.jpg

If that connection is not reliable, that could contribute to a non-functioning balance control.
 
Just received a pair of STK-0050 ordered on ebay, I was just curious if they can be any good.
I measured the diode voltage between 1-3 and 8-0 pins.
1-3 : 1.22 V
8-0 : 0.77 V
surprise, surprise...
Wtf, are they using only one transistor for NPN component?
 
Some advice on the modules.
I never measured the pins for that check but I see what your getting at. I'd still try it.
Never run the modules without a heatsink attached. They get hot fast!
Make sure the metal back plate is contacting the heatsink. I had some that had the plastic mold cover protruding, creating a large air space between the plate and heatsink.
Use a good quality thermal paste.
I build all the STK modules I need using H. Brown and Ricks open source 0050 circuit. Its makes for a better finished product in sound and reliability.
 
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It's kinda like taking a Plain White '62 Belair Hardtop with a 283 2bbl and a Powerglide that's worn out, and swapping in a 409/425HP, close ratio Muncie 4spd, quiet mufflers and no badges. Still looks like Grandma's Sunday go to Church & weekly grocery getter, but wake it up and everyone pays attention at stoplights and the strip.

Putting in the modules is definately worth it.
 
Would that be an air-cooled powerglide?

Ironically, I just disposed of a '63 GM/Borg aluminum T10 close ratio (.223 first, IIRC) with a magnesium bell and two output yokes, in early September. It was also not Chinese, but some lesser unit would probably have worked to test a chassis.
 
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