Have a Technics amp with left channel hum (only) and no audio on right. Audio is good from the preamp on both channels right up to the stablizer input (Test pins 18 & 19) See attached link for a schematic and my readings in red vs expected. Upload files feature isn't working at the moment and providing this google link seems to be the only way I can get a file up here.
Anyway - Getting very strange voltage readings around TR201/202 as well as the main amp IC201/202. TR201/202 are known for trouble based on some googling I've done already, including replacement solutions, but need help confirming, based on the voltage reading I've taken (noted in red), if one of both of them is actually the cause, or if one or both of the IC amps is blown, or something else is going on. Also did a cursory ESR meter check on the caps - all good unless I overlooked one by accident. Nothing else obviously burned etc.
https://drive.google.com/file/d/0B_Yuh1ijNVjmXzFQSmRoLXZKQkE/view?usp=sharing
Anyway - Getting very strange voltage readings around TR201/202 as well as the main amp IC201/202. TR201/202 are known for trouble based on some googling I've done already, including replacement solutions, but need help confirming, based on the voltage reading I've taken (noted in red), if one of both of them is actually the cause, or if one or both of the IC amps is blown, or something else is going on. Also did a cursory ESR meter check on the caps - all good unless I overlooked one by accident. Nothing else obviously burned etc.
https://drive.google.com/file/d/0B_Yuh1ijNVjmXzFQSmRoLXZKQkE/view?usp=sharing
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Steve - thanks for the reply. I can certainly pull that TR205 and put it on a tester.
As I want to learn the whats/whys - what specifically do you see in those voltages on the B C and/or E that indicates it might be open? What should be the expected values(s)?
It's sibling on the other channel, TR206, looks somewhat equally odd (but different), but you don't see any issues with that one? Especially the 18v base? (note - I may have written it down wrong, perhaps it was -18 - I have to double check)
As I want to learn the whats/whys - what specifically do you see in those voltages on the B C and/or E that indicates it might be open? What should be the expected values(s)?
It's sibling on the other channel, TR206, looks somewhat equally odd (but different), but you don't see any issues with that one? Especially the 18v base? (note - I may have written it down wrong, perhaps it was -18 - I have to double check)
Another 'why' question. You say 201/202 look like they are performing. Why is there 11.5v on b2 of 201 and 8v on b1 of 202 then the schematic shows they should be 0? Or am I reading the schematic incorrectly where it shows zeros by b1/b2 of 201? (they are sideways, which I thought was weird).
Consider the pair as balancing act. When the output goes positive, tr202 base goes positive, until the voltage reaches the breakdown voltage of the B-E junction. the transistor is "shut off", letting all the current supplied to the emitter go through 201, trying to make the output more negative, which doesn't happen because of 205 being open. If you changed 201/202, you would still get similar voltages. (However, They still may be bad or not up to spec. )
Pulled 205 - tested bad on a transistor tester just as you predicted. I still need to understand the why though - how did you know it was bad/open from those voltages readings? Specifically from the readings at it's B/C/E?
I need to order replacements. Do you recommend a specific site for identifying good matches and perhaps one that allows spec to be compared side by side? (these may be different sites)
I need to order replacements. Do you recommend a specific site for identifying good matches and perhaps one that allows spec to be compared side by side? (these may be different sites)
KSC2383 is readily available (mouser and other Fairchild distributors) and a pretty close match.
I do most of my substitutions based on a combination of datasheets, looking at how the transistor is used, and other posts as to suitability and availability. There are substitution lists around, and if I can find one, I use that as a starting point.
I do most of my substitutions based on a combination of datasheets, looking at how the transistor is used, and other posts as to suitability and availability. There are substitution lists around, and if I can find one, I use that as a starting point.
Steve, I was doing some research last night as well. 2SC2911 seem to be mentioned as a suitable replacement too. As I understand it, as long as most of the data value are equal to or greater they should work. I compared this to the 1855 and they seem to be a bit closer across the board, including one factor (HFE) which I think in layman's terms is effective gain. The spec hfe on the 1855 is 150, the 2911 is 100 and the HFE on the KSC is far lower at 60. In your opinion, so you think the 2911 would be a bit be better match (albiet in a diiferent package type)?
And maybe even more important - HOW COULD YOU TELL 205 WAS BAD FROM THOSE VOLTAGE READINGS?? I'm in a major learning mode and want to understand that. Please explain it to me. Thanks!
And maybe even more important - HOW COULD YOU TELL 205 WAS BAD FROM THOSE VOLTAGE READINGS?? I'm in a major learning mode and want to understand that. Please explain it to me. Thanks!
Hi Jimpkarl,
a transistor is bad if the voltage differential between Basis and Emitter is greater than about 0.7V. Effectively the PN (NP) junction is destroyed if the reading is greater than that of a normal PN junction like a diode is. Read some book on BJT.
This is a very easy way of telling.
a transistor is bad if the voltage differential between Basis and Emitter is greater than about 0.7V. Effectively the PN (NP) junction is destroyed if the reading is greater than that of a normal PN junction like a diode is. Read some book on BJT.
This is a very easy way of telling.
My bad - I didn't even look at the other channel. Tr202 is probably toast, as well as the output module. Was thrown off by the zero volts at the output, looks like multiple problems in the second channel. tr204 may not be good, either. tr206 is bad. Make a "dim bulb tester" - An ac line with an incandescent bulb in series. The bulb acts like a current limiter and will glow brightly if something is amiss, protecting other components. The output modules are no longer in production so finding a good replacement is a bit of a gamble. Good luck.
OK Replaced TR505/TR506 which were a 2SC1885 with new 2SC2911 and while things are "better" definitely not working. See attached pic for new measurements. Original problem was Hum on left channel, no audio on right. Now have audio on left, but heavily distorted, still no audio on right.
For the right channel where there is no audio output, I see two interesting things:
1) There is no audio at TR202 Base 2. Audio is present at both B1 and B2 on the left side TR201, but only B1 on the right side. There are lines in the schematic that go off to the left but that is just to a tone circuit which is switched out for testing, creating a straight loop back. Confirmed the switch is good.
2) Pins 9/10 of the IC202 chip (STK0039) have the same voltage. Disconnected that IC from circuit and seeing a short between those pics, so I assume that chip is blown. Have another on order but I not clear that is related to find #1 right above. ** Is there another part of the circuit I should look at for that missing audio at TR202 B2? **
For the left channel - now wondering was is the cause of the heavy distortion? Was the 2SC2911 an inappropriate replacement for a 2SC1885? Using a audio tester, audio seems clear at the base of TR201. Ho can I tell if the audio is being distorted by TR201, TR203, (the new)TR205 or the IC201?
https://drive.google.com/file/d/0B_Yuh1ijNVjmWEdxam0tTUwtNUE/view?usp=sharing
For the right channel where there is no audio output, I see two interesting things:
1) There is no audio at TR202 Base 2. Audio is present at both B1 and B2 on the left side TR201, but only B1 on the right side. There are lines in the schematic that go off to the left but that is just to a tone circuit which is switched out for testing, creating a straight loop back. Confirmed the switch is good.
2) Pins 9/10 of the IC202 chip (STK0039) have the same voltage. Disconnected that IC from circuit and seeing a short between those pics, so I assume that chip is blown. Have another on order but I not clear that is related to find #1 right above. ** Is there another part of the circuit I should look at for that missing audio at TR202 B2? **
For the left channel - now wondering was is the cause of the heavy distortion? Was the 2SC2911 an inappropriate replacement for a 2SC1885? Using a audio tester, audio seems clear at the base of TR201. Ho can I tell if the audio is being distorted by TR201, TR203, (the new)TR205 or the IC201?
https://drive.google.com/file/d/0B_Yuh1ijNVjmWEdxam0tTUwtNUE/view?usp=sharing
Unfortunately I don't have a scope.
I guess i have to start with replacing the rest of the transistor circuit. Maybe I'll put the new IC int he Left channel first when it comes - that would rule out the left channel IC being bad or not. If it's still distorted after swapping, then leaves TR 201 and TR203 (and possibly TR205 if my substitution was a bad choice - But I hear the distoration at that base so I suspect the problem must be further back at TR201 or TR203. From what I've read - TR201 is a PITA to replace - basically have to match some substitution pairs up.
Hi jimpkarl,
the thing with amplifiers is, they have a feedback loop. So any component in this loop can cause you grief. I had a look at the power supply circuit which is very basic, I am speculating that this unit has seen a substantial AC power glitch which may have caused the components to have failed due to over voltage for a split second.
Well, for now even if you have to replace TR201(TR202) it may not be as important to have a matched set. The amp will work reasonably ok with no matched transistors. Since there is feedback, it will find its balance, but it will be much better to have a matched set.
For troubleshooting, pretty much any transistor will do for now. Once you have it working again you can take care to match a set and install it.
the thing with amplifiers is, they have a feedback loop. So any component in this loop can cause you grief. I had a look at the power supply circuit which is very basic, I am speculating that this unit has seen a substantial AC power glitch which may have caused the components to have failed due to over voltage for a split second.
Well, for now even if you have to replace TR201(TR202) it may not be as important to have a matched set. The amp will work reasonably ok with no matched transistors. Since there is feedback, it will find its balance, but it will be much better to have a matched set.
For troubleshooting, pretty much any transistor will do for now. Once you have it working again you can take care to match a set and install it.
anytime I worked on any power amp... even with IC outputs... I checked every part in the circuit before powering it back on... any one bad leaky cap or diode or transistor wrong value resistor can put you right back where you started.... I always made a resistance check between one channel and the other channel... everything discharged.. from ground to each part... and anything that I was not sure of was replaced... how did I learn my lesson #1... I was working on a 4300 marantz and I replaced almost everything in the right channel $ 100 dollar in parts..... Plugged it in.... it blew up with smoke !!... OHHHH ALSO WHEN IT TURNED ON IT smoked MY 6X9 TEST SPEAKER. OHHH it was all ok till the relay clicked in... then poof !!!! second time I replaced all the same parts and one more small transistor under the heat sink that I never saw before.... checked everything 3 times...This time I used a 100 W bulb in the circuit so just maybe parts would not fly all over the room.... and it worked 100% .. after that time I checked everything 2 times and they always worked....
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