Weird, but ok...
Here is a run down of all transistors
IC 101
1 = 0.02
2 = 0.005
3 = 0.005
4 = -15.82
5 = -0.67
6 = -0.67
7 = 0.009
8 = 15.63
Not sure how I got more than 90v in previous measurement of Q105 and Q103. And I am not in the dim bulb.
Still makes no difference to me, all transistor seem to be working.
Here is a run down of all transistors
| Transistor | E | C | B |
| Q102 | -0.67 | 0 | -9.22 |
| Q103 | 1.97 | 87.7 | 2.56 |
| Q104 | 2.03 | 88.4 | 2.6 |
| Q105 | -2.05 | -88.2 | -2.6 |
| Q106 | -2.09 | -88.3 | -2.64 |
| Q151 | 0 | -9.23 | 0 |
IC 101
1 = 0.02
2 = 0.005
3 = 0.005
4 = -15.82
5 = -0.67
6 = -0.67
7 = 0.009
8 = 15.63
Not sure how I got more than 90v in previous measurement of Q105 and Q103. And I am not in the dim bulb.
Still makes no difference to me, all transistor seem to be working.
Other half of IC101 looks perfectly reasonable.
I'm increasingly suspicious of Q102, especially since I don't understand what it's supposed to do. Any idea?
I suspect it's responsible for the strange current that seems to be present at pin 5 of the opamp. As a test, if you short across R104 to ground, I predict output offset will improve dramatically. A further test is to remove the test short across R104, but now remove Q102. I think the problem channel will behave nearly identically to the other channel.
Then we figure out Q102 is all about; perhaps it's failed. I suspect its intent is to short the opamp's plus input during mono operation.
I'm increasingly suspicious of Q102, especially since I don't understand what it's supposed to do. Any idea?
I suspect it's responsible for the strange current that seems to be present at pin 5 of the opamp. As a test, if you short across R104 to ground, I predict output offset will improve dramatically. A further test is to remove the test short across R104, but now remove Q102. I think the problem channel will behave nearly identically to the other channel.
Then we figure out Q102 is all about; perhaps it's failed. I suspect its intent is to short the opamp's plus input during mono operation.
I shorted and grounded R104 and this triggers the safety circuit.
I removed Q102 and the DC is gone.
I did an additional test...
I switched to mono and Q102 presented 0, 0, 0.677vdc (ECB).
I am still unsure how Q102 and Q151 work together but it seems related to the mono/stereo.
So, researching...
I went back to my notes and I had replaced Q102 by KSC1845.
I swaped back the original transistor which is KSC2878 and everything looks fine now.
Comparing both transistor specs it looks like KSC1845 has Ic of 50mA while KSC2878 is 300mA.
Does it explain it? Perhaps a bad replacement choice?
The fact is when I swapped back the original KSC2878 it worked.
It may or may not be the Ic, but it worked.
I removed Q102 and the DC is gone.
I did an additional test...
I switched to mono and Q102 presented 0, 0, 0.677vdc (ECB).
I am still unsure how Q102 and Q151 work together but it seems related to the mono/stereo.
So, researching...
I went back to my notes and I had replaced Q102 by KSC1845.
I swaped back the original transistor which is KSC2878 and everything looks fine now.
Comparing both transistor specs it looks like KSC1845 has Ic of 50mA while KSC2878 is 300mA.
Does it explain it? Perhaps a bad replacement choice?
The fact is when I swapped back the original KSC2878 it worked.
It may or may not be the Ic, but it worked.
Q151 keeps Q102 turned off when in STEREO mode, switch to MONO Q151 turns on which turns on Q102 killing the RIGHT channel input. When Q151 is off there is a negative voltage on the base of Q102 keeping it off. When Q151 turns on it supplies a + voltage to the base of Q102 turning it on.
Glad we could help!
The KSC2878 is a very unusual (obsolete?) transistor in that it tolerates 25V of reverse base-emitter breakdown; apparently, its base is pulled to -15V by R115 when in Stereo mode. The device you substituted is far more typical and breakdown occurs probably around -7V, triggering that leakage current.
I'm baffled why shorting R104 would trigger shutdown, but my curiosity isn't worth putting your amp at risk.
Enjoy!
The KSC2878 is a very unusual (obsolete?) transistor in that it tolerates 25V of reverse base-emitter breakdown; apparently, its base is pulled to -15V by R115 when in Stereo mode. The device you substituted is far more typical and breakdown occurs probably around -7V, triggering that leakage current.
I'm baffled why shorting R104 would trigger shutdown, but my curiosity isn't worth putting your amp at risk.
Enjoy!
You may have this fixed and this may be a stupid question. Did you calibrate the amp after you recapture?
Also on these TFM models I would not monkey around and replace the speaker relays
I never had to replace an IC before only upgrading like the C1amp
Also on these TFM models I would not monkey around and replace the speaker relays
I never had to replace an IC before only upgrading like the C1amp
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There was a bad transistor: Q102.
I replaced the originals AJR39091 relays with correspondent modern version G2RL-1A4-EDC24A
And the NJM4565L (ICs in the input board) are still around.
After those replacements I was able to set the idle current and get the amp adjusted and working really well.
I replaced the originals AJR39091 relays with correspondent modern version G2RL-1A4-EDC24A
And the NJM4565L (ICs in the input board) are still around.
After those replacements I was able to set the idle current and get the amp adjusted and working really well.
thank you for the suggestion. I do use to bypass a few electrolytic caps with film caps often, this amp is direct coupled, so no ecaps on the audio path. there is a pair of caps in the input board and a switch that you can use to pull them into the circuit or not, but I replaced them with film caps.
I am still chasing down the noise on the right channel. Frequency is 1.1Mhz and level is about 200mV peak to peak.
I have replaced all transistors in the audio path but two, I ordered them today.
I replaced the FETs but it didnt fix the problem.