The Q109L part is a bit low on hFE, the range for BL graded A970's is 350-700. The current measured for 109L was a bit on the low side compared to the others too.
I'd go ahead and install new A992's in those locations. I don't think its especially important for Q108/109 to be well matched but look for some reasonably matched pairs in hFE & Vbe to install for each channel. If you have some well matched A992 pairs I'd save those in case they're needed for the current mirror in the input LTP.
I'd go ahead and install new A992's in those locations. I don't think its especially important for Q108/109 to be well matched but look for some reasonably matched pairs in hFE & Vbe to install for each channel. If you have some well matched A992 pairs I'd save those in case they're needed for the current mirror in the input LTP.
So hopefully you received your parts?
Once you get your parts and put the front end of this amp back together, the first thing I'd check after powering it up is for approximately +1V at the base of Q116 and -1V at the base of Q117. The bias potentiometer will affect this reading, you should see that voltage go up/down slightly when you adjust the bias pot.
After that it would be good to know the voltage drop across R103/104 which should be very close to 0.6V each if the LTP is working properly.
Also do you have approx 0V at the collector of Q106 now? This is probably going to vary by a few volts.
If all of that seems to be working okay then I'd install one each of the drivers and outputs. So probably Q116/117 & Q120/121. Adjust the bias to its minimum position and then power it up on the DBT.
Make sure you keep the signal input RCA shorted to ground during all of this testing and the front end of the circuit grounded to chassis as well.
If you get that far without any hiccups I'd say that you're in very good shape!
Once you get your parts and put the front end of this amp back together, the first thing I'd check after powering it up is for approximately +1V at the base of Q116 and -1V at the base of Q117. The bias potentiometer will affect this reading, you should see that voltage go up/down slightly when you adjust the bias pot.
After that it would be good to know the voltage drop across R103/104 which should be very close to 0.6V each if the LTP is working properly.
Also do you have approx 0V at the collector of Q106 now? This is probably going to vary by a few volts.
If all of that seems to be working okay then I'd install one each of the drivers and outputs. So probably Q116/117 & Q120/121. Adjust the bias to its minimum position and then power it up on the DBT.
Make sure you keep the signal input RCA shorted to ground during all of this testing and the front end of the circuit grounded to chassis as well.
If you get that far without any hiccups I'd say that you're in very good shape!
Yes, I received my parts yesterday and hope to start installing them today. Regarding transistor matching, I’ve read through your responses, and I just want to make sure I understand. Should I be matching the hFE in pairs of like-type transistors, e.g. q102 to q103 and q104 to q105? Does matching matter from left channel to right, or will bias adjustment make that negligible?
Yes you'll want to match in like-type pairs. Only the 992's will need to be matched and not for Q104/105. I would match the pairs for Q108/109 though and if it turns out Q102/103 needed to be replaced then you'd want well matched pairs for those too. The transistors across channels do not have to be matched to each other either.
What I normally do is pinch the transistor between my fingers for about 10 secs while I'm connecting the leads to the tester. Then I will test that transistor 3 - 4X back to back until the hFE & Vbe stabilize. Usually by the 2nd test its very stable. Then write down the values on the tape and go to the next transistor, pinch, connect, repeat...
Based on my experience with these On Semi A992 transistors the Vbe measurements will be within a few mV on all of them and hFE will vary a decent amount. If you can find some with identical vBE and very similar hFE then I'd call them a good match. If you bought 25 - 30 of them then you should be able to find several matches.
What I normally do is pinch the transistor between my fingers for about 10 secs while I'm connecting the leads to the tester. Then I will test that transistor 3 - 4X back to back until the hFE & Vbe stabilize. Usually by the 2nd test its very stable. Then write down the values on the tape and go to the next transistor, pinch, connect, repeat...
Based on my experience with these On Semi A992 transistors the Vbe measurements will be within a few mV on all of them and hFE will vary a decent amount. If you can find some with identical vBE and very similar hFE then I'd call them a good match. If you bought 25 - 30 of them then you should be able to find several matches.
I reinstalled Q101 and Q110. I replaced Q104 and Q105 with KSC1845. I replaced Q106 with KSC3503. I replaced Q107, Q108, and Q109 with KSA992 (the latter two matched) I replaced ZD11 with a new 10v zener, and installed the Canadian option ZD15/16.
q116-q119 are still removed.
I powered up the amp through the DBT, and I heard the main relay click, followed by the two on the power amp PCBs.
check for +1v at Q116 base
Q116L base: -73.3v
Q116R base: -72.9v
check for -1v at Q117 base
Q117L base: -74.9v
Q117L base: -74.4v
check voltage drop across R103 (close to 0.6v)
R103L: 1.316v
R103R: 1.277v
check voltage drop across R104 (close to 0.6v)
R104L: 1.324v
R104R: 1.324v
check for 0v at Q106 collector (might vary a few volts)
Q106L collector: 0.854v
Q106R collector: 0.799v
So, some things seem close, but the Q116/177 bases look very wrong still.
q116-q119 are still removed.
I powered up the amp through the DBT, and I heard the main relay click, followed by the two on the power amp PCBs.
check for +1v at Q116 base
Q116L base: -73.3v
Q116R base: -72.9v
check for -1v at Q117 base
Q117L base: -74.9v
Q117L base: -74.4v
check voltage drop across R103 (close to 0.6v)
R103L: 1.316v
R103R: 1.277v
check voltage drop across R104 (close to 0.6v)
R104L: 1.324v
R104R: 1.324v
check for 0v at Q106 collector (might vary a few volts)
Q106L collector: 0.854v
Q106R collector: 0.799v
So, some things seem close, but the Q116/177 bases look very wrong still.
Thats a lot more current flowing through R103/104, essentially double. That should not cause the issue of the high neg rail voltage on the base of Q116/117 but it could cause issues in other parts of the circuit. Mainly it'll greatly increase the dissipated power on Q110/111. However at least those voltages are balanced now which is good.
Whats your emitter voltage on Q106? I'm just wondering if this is where the whole 6.6mA is going or not.
Also whats your collector and emitter voltage on Q110 and the same on Q115.
I have a suspicion that Q115 is bad and leaking through.
Whats your emitter voltage on Q106? I'm just wondering if this is where the whole 6.6mA is going or not.
Also whats your collector and emitter voltage on Q110 and the same on Q115.
I have a suspicion that Q115 is bad and leaking through.
Shoot, I didn't see your 2nd message until I took apart the amp again. With Q115 out and in the transistor tester, I see:
Q115L:
BJT-PNP
hFE=113
iC=1.1mA
Vbe=598mV
Q115R:
BJT-PNP
hFE=74
Ic=.70mA
Vbe=594mV
I can measure Q106 again once I have it back together. Some of these parts are tricky to measure with my fat probes. I think next time I order from digikey I'll get some nice, sharp, thin Pomona probes.
Q115L:
BJT-PNP
hFE=113
iC=1.1mA
Vbe=598mV
Q115R:
BJT-PNP
hFE=74
Ic=.70mA
Vbe=594mV
I can measure Q106 again once I have it back together. Some of these parts are tricky to measure with my fat probes. I think next time I order from digikey I'll get some nice, sharp, thin Pomona probes.
If you could pull Q114 and test them as well that would be good.
So those Q115 measurements seem fine, the 115R's hFE is at the low end of what I'd expect to see though. The high neg rail voltages on all 3 pins of Q115 that you measured prior are very out of place so I'm still concerned about them. Something has failed here we just need to pin it down.
Did you happen to buy any of the KSA1381's? Those are the compliment for the C3503.
If so I have a test for you to try.
The pin out of the A1381 is opposite that of the Q115 SA1306. If you have one of the A1381's then insert in in the opposite direction back into one of the Q115 locations and see if your neg rail readings on all 3 pins of that transistor changes. You will not be able to leave this transistor installed for anything other than a test but it would at least test this Q115 transistor at full operating voltage.
If you did not buy any A1381's then what PNP transistors do you have on hand?
So those Q115 measurements seem fine, the 115R's hFE is at the low end of what I'd expect to see though. The high neg rail voltages on all 3 pins of Q115 that you measured prior are very out of place so I'm still concerned about them. Something has failed here we just need to pin it down.
Did you happen to buy any of the KSA1381's? Those are the compliment for the C3503.
If so I have a test for you to try.
The pin out of the A1381 is opposite that of the Q115 SA1306. If you have one of the A1381's then insert in in the opposite direction back into one of the Q115 locations and see if your neg rail readings on all 3 pins of that transistor changes. You will not be able to leave this transistor installed for anything other than a test but it would at least test this Q115 transistor at full operating voltage.
If you did not buy any A1381's then what PNP transistors do you have on hand?
One other thing I just thought of.
I know you replaced Q112 prior. Inspect it and make sure that you didn't mistakenly install it in the opposite direction. The TO-126 case of these transistors makes it very easy to install them backwards and not realize it.
You could be fighting a secondary problem that was introduced by accident along the way.
I know you replaced Q112 prior. Inspect it and make sure that you didn't mistakenly install it in the opposite direction. The TO-126 case of these transistors makes it very easy to install them backwards and not realize it.
You could be fighting a secondary problem that was introduced by accident along the way.
Q114L:
BJT-NPN
hFE=140
Ie=6.3mA
Vbe=618mV
Q114R:
BJT-NPN
hFE=127
Ie=6.2mA
Vbe=629mV
ICEs=1uA
I don't know why 114R gave an ICEs number but Q114L did not. I repeated the test and got the same-ish results.
I did not buy any KSA1381s, but could on the next go-round. For PNPs I only have what I have used on this amp: KSA992, and it's possible some of the old 2SA1492s I pulled are still good.
Good idea about the potential for Q112 installation issues. Looking at the data sheet, the side that should go on the flat side of the silkscreen is the side with 3 dots and the part label. Emitter, pin 1, should be on the side of the PCB nearest the output transistors, pin 2, collector, is in the middle, and pin 3, the base, should be on the side on the top of the PCB. I've just double checked and my install appears ok based on this.
BJT-NPN
hFE=140
Ie=6.3mA
Vbe=618mV
Q114R:
BJT-NPN
hFE=127
Ie=6.2mA
Vbe=629mV
ICEs=1uA
I don't know why 114R gave an ICEs number but Q114L did not. I repeated the test and got the same-ish results.
I did not buy any KSA1381s, but could on the next go-round. For PNPs I only have what I have used on this amp: KSA992, and it's possible some of the old 2SA1492s I pulled are still good.
Good idea about the potential for Q112 installation issues. Looking at the data sheet, the side that should go on the flat side of the silkscreen is the side with 3 dots and the part label. Emitter, pin 1, should be on the side of the PCB nearest the output transistors, pin 2, collector, is in the middle, and pin 3, the base, should be on the side on the top of the PCB. I've just double checked and my install appears ok based on this.
The one thing that stands out to me is the Ie for the NPN pairs is > 6mA where the PNP pairs are quite low. 115R is both low hFE and Ie. I thought they might have been low but wasn't sure until you tested the NPN's.
I have a couple of those devices and I was going to verify my own results but unfortunately my damn tester went kaput. I'm not sure what happened there, I've had it for close to two years using it a couple times a week with no issues. One day working the next nothing. I'll chalk it up to Made in China I guess. I ordered a replacement one up as I find it so handy I need another one.
The ICE will occasionally pop up its a leakage current and its not uncommon to see that in the single digit uA range. If it was 10 - 20uA I would be more concerned about it.
If Q117/119 are removed from the circuit and you do not measure a short from the neg rail to the base of Q117 then there's just no where that I see that the full neg rail on the base of Q117 could be coming from other than a failed Q115.
Q112 could be an issue if it failed again after you replaced it earlier but this is not probable to me and you've also verified that Q112 was oriented correctly too.
So if you're going to order something then I'd opt for a proper replacement transistor for Q114/115.
Toshiba no longer manufactures the 2SC4793/SA1837 pairs which are the popular successor to the long discontinued C3298/A1306 used for Q114/115 in your amp. However BDENT.COM has the C4793/A1837 transistors in stock and many folks on diyaudio.com have had good luck with BDENT providing genuine parts. I'd order some of those transistors up and try replacing Q114/115. I am betting the neg rail on the base of Q117 will be gone.
I have a couple of those devices and I was going to verify my own results but unfortunately my damn tester went kaput. I'm not sure what happened there, I've had it for close to two years using it a couple times a week with no issues. One day working the next nothing. I'll chalk it up to Made in China I guess. I ordered a replacement one up as I find it so handy I need another one.
The ICE will occasionally pop up its a leakage current and its not uncommon to see that in the single digit uA range. If it was 10 - 20uA I would be more concerned about it.
If Q117/119 are removed from the circuit and you do not measure a short from the neg rail to the base of Q117 then there's just no where that I see that the full neg rail on the base of Q117 could be coming from other than a failed Q115.
Q112 could be an issue if it failed again after you replaced it earlier but this is not probable to me and you've also verified that Q112 was oriented correctly too.
So if you're going to order something then I'd opt for a proper replacement transistor for Q114/115.
Toshiba no longer manufactures the 2SC4793/SA1837 pairs which are the popular successor to the long discontinued C3298/A1306 used for Q114/115 in your amp. However BDENT.COM has the C4793/A1837 transistors in stock and many folks on diyaudio.com have had good luck with BDENT providing genuine parts. I'd order some of those transistors up and try replacing Q114/115. I am betting the neg rail on the base of Q117 will be gone.
Yes, at one point I had a short on Q117R and Q119R from B to E. I traced it to another transistor and replaced that part. I don’t remember the left channel having a similar issue, but I could have forgotten by now.
Do you think I should order spares of the Q114/115 while I’m at it? They’re not super cheap, but it’s always more painful to order more later.
I’ll check for a short from the Q117 base to the negative rail tomorrow before I order just to be sure.
Do you think I should order spares of the Q114/115 while I’m at it? They’re not super cheap, but it’s always more painful to order more later.
I’ll check for a short from the Q117 base to the negative rail tomorrow before I order just to be sure.
A generally advised rule of thumb is to replace not only the stage that failed, but the prior stage too even if they measure good. The thinking behind this is that the stage prior to where the failure is at will have taken a heavy load, maybe even a brief overload, during the failure. Some folks will even recommend going two stages back on the replacement to be safe. You're just about there now. 
It probably wouldn't hurt to have a spare pair of the C4793/A1837's. They're getting rare and the only other option that's available is a clone that is made by the manufacturer UTC and sold through a UK distributor, so they'd take a while to show up in your mailbox.
Definitely make sure there's no short from the Neg rail to Q117 base.
It probably wouldn't hurt to have a spare pair of the C4793/A1837's. They're getting rare and the only other option that's available is a clone that is made by the manufacturer UTC and sold through a UK distributor, so they'd take a while to show up in your mailbox.
Definitely make sure there's no short from the Neg rail to Q117 base.
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