bzenic
Active Member
Today, it was disaster. I was replacing caps and managed to solder C308 (100 uF) wrongly. Using DBT I turned the power on and immediately some smoke started to raise. I turned the power off and noticed that the wrapped resistor R318 parallel to Q20 (2.2 Ohm) was burned. I removed the resistor and noticed that it broke in half. What should be my next step after, of course, mounting the C308 correctly?
What else could have been burned or destroyed?
What else could have been burned or destroyed?
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Definitely Q20, probably Q22, small maybe Q24.
Check trace from Q20 emitter to C308 minus, all the soldered in jumpers probably kept current below damage threshold before resistor popped.
R318 2.2ohms 1/4w (flameproof) ( 660-CFP1/4CT52R2R2J ) did it's job....
Check trace from Q20 emitter to C308 minus, all the soldered in jumpers probably kept current below damage threshold before resistor popped.
R318 2.2ohms 1/4w (flameproof) ( 660-CFP1/4CT52R2R2J ) did it's job....
always check the bom first. yes they are designed as a on-purpose weak link.
"RD1/4PSF xxx J
Later manuals started using the exclamation point in a triangle as a signal:
And of course, things that had me thinking overnight - get reversed... (swapping chips between channels)
"RD1/4PSF xxx J
Later manuals started using the exclamation point in a triangle as a signal:
And of course, things that had me thinking overnight - get reversed... (swapping chips between channels)
No, not on the original modules.
Counterfeits? probably not.
Our DIY STK0050 ? No. The ebay listing proves it. 6 connections, 1,2,3, 8,9,0(10).
Attachments
bzenic
Active Member
Yesterday I replaced almost all el capos on the amplifier board.
Still getting noise on the right channel, it starts gradually with balance button turning from extreme left (left channel only) to the center, and then becoming worse turning all the way to the right channel.
The sound dramatically deteriorates with turning the volume up on the right channel.
I also retested the SX-780 AUX input with the signal taken from REC on the other receiver (SX-650).
Again, noisy right channel.
Thinking today of tracing signal with a small toy-oscilloscope, I may be able to detect the source of noise in FM STEREO audio signal.
Other than that I don't have too many ideas what to try.
Still getting noise on the right channel, it starts gradually with balance button turning from extreme left (left channel only) to the center, and then becoming worse turning all the way to the right channel.
The sound dramatically deteriorates with turning the volume up on the right channel.
I also retested the SX-780 AUX input with the signal taken from REC on the other receiver (SX-650).
Again, noisy right channel.
Thinking today of tracing signal with a small toy-oscilloscope, I may be able to detect the source of noise in FM STEREO audio signal.
Other than that I don't have too many ideas what to try.
I am explicitly asking ( seems unanswered to me) if these two IC's (Q3 & Q4) IN RED circles on the AUDIO board
have been swapped and tested to see if the problem swaps channels:
Next steps could be:
Swap power amp transistors between channels. one by one, retesting.
be SURE power is 0.0 before digging in EACH TIME!!!
Make two (about) 10 ohm jumper wires, put them across
d10 banded lead (15), d12 UN banded lead (16)
d7 banded lead, d9 UN banded lead
to drain off the residual charge.
have been swapped and tested to see if the problem swaps channels:
Next steps could be:
Swap power amp transistors between channels. one by one, retesting.
be SURE power is 0.0 before digging in EACH TIME!!!
Make two (about) 10 ohm jumper wires, put them across
d10 banded lead (15), d12 UN banded lead (16)
d7 banded lead, d9 UN banded lead
to drain off the residual charge.
bzenic
Active Member
It seems I made a breakthrough today, the main culprit seems to be the volume pot itself.
What was going on is that touching and turning the volume stem with fingers was somehow initiating the noise which would only go worse by turning volume up. I made a big improvement by simply removing old solder from volume pot contacts and putting fresh solder. Not to mention that I also replaced a lot of caps on the amp board.
Also it needs to be noted that this does not happen when the balance control is on extreme left.
Now it is much, much better but still not perfect on the right channel. I still get crackling noise on the right channel but now it occurs at a much higher volume of FM STEREO and everything altogether seems more controllable and cleaner.
@markthefixer
I think it is quite possible that you are right and there is still an issue with op amps. I will try to swap them again because I trust your expertise and I still did not find the root cause. I think that previous swap was possibly inconclusive because of the noise generated by volume pot so it is worth doing again.
What was going on is that touching and turning the volume stem with fingers was somehow initiating the noise which would only go worse by turning volume up. I made a big improvement by simply removing old solder from volume pot contacts and putting fresh solder. Not to mention that I also replaced a lot of caps on the amp board.
Also it needs to be noted that this does not happen when the balance control is on extreme left.
Now it is much, much better but still not perfect on the right channel. I still get crackling noise on the right channel but now it occurs at a much higher volume of FM STEREO and everything altogether seems more controllable and cleaner.
@markthefixer
I think it is quite possible that you are right and there is still an issue with op amps. I will try to swap them again because I trust your expertise and I still did not find the root cause. I think that previous swap was possibly inconclusive because of the noise generated by volume pot so it is worth doing again.
bzenic
Active Member
I swapped:
Q6 and Q5,
Q12 and Q11,
Q14 and Q13 where Q13 = 2SC1885 on the board was replaced with 2SC1573 ??.
I replaced them both with 2SC1845.
Q16 and Q15,
After all this I still get crackling on the right channel at higher volume on STEREO FM.
The next thing I will try is swapping 5 legged pairs.
Q6 and Q5,
Q12 and Q11,
Q14 and Q13 where Q13 = 2SC1885 on the board was replaced with 2SC1573 ??.
I replaced them both with 2SC1845.
Q16 and Q15,
After all this I still get crackling on the right channel at higher volume on STEREO FM.
The next thing I will try is swapping 5 legged pairs.
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bzenic
Active Member
Just to mention that the problem was always on the right channel no matter how I swapped the transistors.
Removed Q7 and Q8 and tested. Q7 was apparently bad with the gain on one side 334 (left 3 legs) and on the other around 20 (right 3 legs).
I replaced both transistors with a pair of KSA992FBU.
Things are much better now and finally no erratic behavior when volume is up.
I'm thinking of replacing Q9 and Q10 with pair of 2SC1845, their gain was fine but they also might be leaky although I could not establish that by with Atlas tester.
Removed Q7 and Q8 and tested. Q7 was apparently bad with the gain on one side 334 (left 3 legs) and on the other around 20 (right 3 legs).
I replaced both transistors with a pair of KSA992FBU.
Things are much better now and finally no erratic behavior when volume is up.
I'm thinking of replacing Q9 and Q10 with pair of 2SC1845, their gain was fine but they also might be leaky although I could not establish that by with Atlas tester.
bzenic
Active Member
@markthefixer and others, thanks a lot for your advice and time,
It's still a mystery to me how come only FM stereo playback was affected with this faulty driver transistors.
Some kind of interference and static still seems to find its way from the tuner and affects signal in the amp.
I can only ascribe that to the tuner imperfect adjustments and quite possibly some faulty parts.
It's still a mystery to me how come only FM stereo playback was affected with this faulty driver transistors.
Some kind of interference and static still seems to find its way from the tuner and affects signal in the amp.
I can only ascribe that to the tuner imperfect adjustments and quite possibly some faulty parts.
bzenic
Active Member
One more update from my end:
Although much improved I wasn't still happy with crackling on the right channel when I turn the volume up.
So I swapped STK form ebay (right channel) with original STK (left channel).
That completely reversed the situation: now I have left channel (ebay STK) causing terrible interruptions and the right channel (so far always bad no matter what pair of transistors I swapped) all of a sudden is perfect!
Turns out that all the time the fake chinese ebay STK was the main culprit.
Just to remind that I immediately measured asymmetric diode voltage drop on NPN and PNP side.
It is exactly that asymmetry that was causing all this craziness.
Never buy fake STK-0050 from ebay!
Although much improved I wasn't still happy with crackling on the right channel when I turn the volume up.
So I swapped STK form ebay (right channel) with original STK (left channel).
That completely reversed the situation: now I have left channel (ebay STK) causing terrible interruptions and the right channel (so far always bad no matter what pair of transistors I swapped) all of a sudden is perfect!
Turns out that all the time the fake chinese ebay STK was the main culprit.
Just to remind that I immediately measured asymmetric diode voltage drop on NPN and PNP side.
It is exactly that asymmetry that was causing all this craziness.
Never buy fake STK-0050 from ebay!