Reviving dead SX-680

MyLastWord

Active Member
I bought this dead (no sound for both channels) SX-680 receiver for cheap to revive it.
The first order of business is to blow away all the dirt, then clean all the knobs, switches and the connections at the back with electronic contact cleaner.

Follow the service manual, I then replaced the transistors as follow.
- 2SA725s/2SA726s with KSA992
- 2SC1919s with KSC1845,
- 2SC1438 with KSC1845,
- 2SC1384 with KSC2690
- 2SD712 with KSC2073.

Next I replaced all electrolytic caps except the 2 big 8000uF/42V ( I don't have any handy).
One thing I learn during this exercise that replace the 220uF/63v with 390uF/63v (also don't have any 220uF/63v handy) will cause loud hums, so I put back the original 200uF/63V.

Searching for issues with SX-680 from this site, I replaced resistors R401 (33ohm) and R410 (100ohm) and replacing the Diodes D401/D402 10E2 with 1N4004 for good measure.

Once all of this is done. I start it up everything came up (except few light bulbs). Test few AM/FM channels all goods. Test Aux also good. All goods except 2 issues: 1) Only Left channels is working and 2) minor hums (audible when not play anything)
So I would think the right power pack STK-0039 would be dead. To test this, I swapped around the STK-0039.
Nothing changed, still minor hums, left channels still worked and no right channels. What!?!?
Measuring the voltage of the 2 STK-0039 , and they all seem within expected specs.

I also did try to replace the pair of 2SA798 with DIY combining 2 KSA992s and replacing the 2SK117 with 2SK30A nothing changed still so I ended up put them back.

At least, I and half way there (1channel to fix). I am running out of ideas, what to check and replace next and looking for help from collected wisdom/experiences here.
 
A 390 in place of a 220 shouldn't cause hums if it's in the power supply. Go through the preamp. Do you have a scope?
 
I think the 2SK117s are toasted. I checked the 2SK30A with one of the AVR transistor tester and it show the device as FET SGD. Check the 2SK117s they reported as GP NPN ECB.
I will try to put the 2SK30As in their place and KSA992 in the 2SA733.
 
C507, C508(?) - 4.7uf/25v, are the coupling caps for the line into the amps. Inject a signal there, maybe your finger? See if the amps are operating.
Its harder with the all in one board. Might have to solder a lead to the cap.
 
I have put 2 new STK-0039 modules in, now the left channel sound OK and the right channel sound weak. Swap modules around make no difference. Remove left module, the right channel still weak. Remove right module, the left channel still OK.
After poking and tracing back to Q505(L), and Q506(R) try to measure E-C-B vdc reading per schematic, both E-C-B voltages of Q505 and Q506 are comparable. However I expect the collector voltages (post 70 and 72) should be around 14v, but only get around 8v, while the bases are 2.6v the emitors are 1.9v. I checked the resistors around these transistor and the are within specs. I have tried replace these 2sc1919 (BCE pinout) with ksc1845 (ECB pinout) without any differences. Any idea to check why 8v instead of 14v or how to tackle this imbalance issue? could it be the problem with the balance or the volume pot?
This receiver has drove me nut with the imbalance issue.
 
Moving on to check voltage around the area of 2fets Q503, Q504. At one point I saw the voltage difference around D503 and D504. so I pull both out, as well the 4-100K resistor, R533, R534 and R537, R538. One of the resistor was measure around 110K instead. Put 4 new 100K resistors in and 2 1N4148 in for the D503 and D504. Finger cross, high hope... Plug back in.. and... NOPE.. no difference. I am pretty convince the problem with the Volume pot or the Balance pot. I have worked on this receiver for a while now, so time to give it up.:(:(
 
Check, with no input signal, the Vdc on pins 0 and 1 on the STKs, 0 should be ≈+1.4 Vdc. 1 should be ≈-1.4Vdc. Verify DC offset is within ±20 mVdc (no spec given in SM I could find). Measure Vdc from pins 3 to 8 on the STKs, they should be very close to the same comparing left to right channels (Bias). Use Deoxit and Faderlube on the tone control pots, they are in the negative feedback loop so flaws may have major impact.
 
The previous time I worked on this receiver, I soldered pin header on the board, for most of the transistors so they can be swapped in or out with original/replacement transistors. One of the original Q503 fets was dead (became a BJT) so I used 2SK30A as replacement for both. Since they are on pin header, it is easily to be swap around and I did without any different result. I have searched and read in this forum for much of info regard this receiver and its cousins (580 & 780). Cleaning the pots, buttons and switches was the first and many time I did. I did compare the voltages of the pins of the right module to the left, good working one and they are reasonable close (my DMM is not that accurate). I will try to that again and post the result here.
I did took out the volume and balance pot to clean and measure resistance behavior. The Balance resistance value seem to be function normal from side to side. The volume pot resistance value are slightly off when comparing from front to back row, but always in-sync when turn from Min to Max. At max, I think they are off by about 2K-3K, with the back is lower (The right side?). Not sure if this is the cause.
When I tested it in AM radio and thru head phone, if 5 being the middle of L-R, I have to turn to 9 (R) in order to have reasonable L-R balance, albeit low on volume.

From the block diagram of the SM, which would be the buffer amp Q502 or the pair Q505/Q506?
 
From the block diagram of the SM, which would be the buffer amp Q502 or the pair Q505/Q506?
Q505 and Q506 are the Buffer Amp block, basic common emitter voltage amplifiers; the FETs Q503 and Q504 are the Muting Gate block; and Q502 is the final driver transistor in the Muting Control block.
Is the imbalance present and the same in Mono mode as in Stereo mode?
 
Do you have a way to generate a 1kHz sine wave with level control as an input?
What DMM are you using? Can you post a picture so I can see the scale selection available, please?
 
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Thanks Merlynski, Both on mono (AM radio) and (Aux) Stereo. FM radio is another story for another day.. Sorry I don't have any signal generator or oscilloscope equipment.
 
Thanks Merlynski, Both on mono (AM radio) and (Aux) Stereo. FM radio is another story for another day.. Sorry I don't have any signal generator or oscilloscope equipment.
How about a smart phone with a tone generator app and an audio output, or a computer with an audio generator program?
 
Thanks Merlynski, Both on mono (AM radio) and (Aux) Stereo. FM radio is another story for another day.. Sorry I don't have any signal generator or oscilloscope equipment.
OK, that indicates to me the problem originates after the mono/stereo switch.
 
OK.. let me research on that smartphone/software... And can you give me an outline what to do with it then?
We will feed the same signal into the Aux inputs and the follow the levels through the amp with the DMM on a mVac range to see where it becomes unequal. May need to put a 0.1 to 1µF cap on the probe to isolate from dc.
 
I found Function Generator and Signal Generator android app. Going to install them and try it when I get home from work. the cap go between black and red (aligator clips with pin hooks attached) of the DMM right? I got 2 DMMs with 20mV and 40mV resolution though, not sure if the they are highly accurate..
 
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