I received my C4793/A1837 transistors today. I thought I'd test them tonight before I install them sometime this weekend. I know I'm not necessarily transistor matching, but I wanted to see how they compared to the ones I pulled out.
2SC4793, NPN, replacement for 2SC3298: Q114
#1:
BJT-NPN
hFE=218
Ie=6.3mA
Vbe=614mV
#2:
BJT-NPN
hFE=189
Ie=6.3mA
Vbe=609mV
#3:
BJT-NPN
hFE=198
Ie=6.3mA
Vbe=607mV
2SA1837, PNP, replacement for 2SA1306: Q115
#1:
BJT-PNP
hFE=140
Ie=1.3mA
Vbe=570mV
#2:
BJT-PNP
hFE=153
Ie=1.5mA
Vbe=568mV
#3:
BJT-PNP
hFE=152
Ie=1.5mA
Vbe=569mV
So, definitely some higher numbers on the new parts, but not enough to make me feel this this was the problem. I'll wait until I hear from you to see if you have any other thoughts.
2SC4793, NPN, replacement for 2SC3298: Q114
#1:
BJT-NPN
hFE=218
Ie=6.3mA
Vbe=614mV
#2:
BJT-NPN
hFE=189
Ie=6.3mA
Vbe=609mV
#3:
BJT-NPN
hFE=198
Ie=6.3mA
Vbe=607mV
2SA1837, PNP, replacement for 2SA1306: Q115
#1:
BJT-PNP
hFE=140
Ie=1.3mA
Vbe=570mV
#2:
BJT-PNP
hFE=153
Ie=1.5mA
Vbe=568mV
#3:
BJT-PNP
hFE=152
Ie=1.5mA
Vbe=569mV
So, definitely some higher numbers on the new parts, but not enough to make me feel this this was the problem. I'll wait until I hear from you to see if you have any other thoughts.
The PNP's are the one's I was more worried about and they aren't massively different than the old ones. Better hFE and slightly better Ie. However the old ones would have passed the sniff test anyway. Its once you put the +/-78V to them in-circuit that I want to see what you measure.
I'd go ahead and install at least one set into a channel and see how it measures after changing them out.
I'd go ahead and install at least one set into a channel and see how it measures after changing them out.
Ok, so I replaced Q114 and Q115 in the left channel, hooked it up to the amp, leaving the right channel out. The DBT powered up and dimmed as normal, but the relays never clicked. I thought that maybe the limiter board needs both channels in place, so I drained the caps, installed Q114 and Q115 in the right channel, hooked it up to the amp, then powered on again. The DBT worked as normal again, but the relays still never clicked. Investigating further, I found that the 250v 0.5A fuse on the limiter board had blown.
Can I chalk this up to some issue with when I only had the left channel installed? I double checked the Q114 and Q115 were in the correct positions.
Can I chalk this up to some issue with when I only had the left channel installed? I double checked the Q114 and Q115 were in the correct positions.
Just looking at the schematic the 500mA fuse only feeds the limiter circuit & slow start relay coil and it sends voltage out to the "AC detection" input on the upc1237 protection chip. I would guess that the fuse was borderline and failed. It shouldn't be related to anything going on in the channel amps or due to Q114/115 even if you installed them improperly.
I have a bunch of fuses, but of course not that one. I”m going to see if I can find one+spares at the hardware store tomorrow.
Fuse replaced, the relays power up and click properly.
check for +1v at Q116 base
Q116L base: -73.4v
Q116R base: -72.9v
check for -1v at Q117 base
Q117L base: -74.7v
Q117L base: -74.1v
check for +1v at Q116 base
Q116L base: -73.4v
Q116R base: -72.9v
check for -1v at Q117 base
Q117L base: -74.7v
Q117L base: -74.1v
Well that sucks. I was hoping to see a change of some sort.
Check your readings on the 3 pins of Q112 & Q115 and see if they're still comparable to the picture of measurements you posted before.
Check your readings on the 3 pins of Q112 & Q115 and see if they're still comparable to the picture of measurements you posted before.
Q112L
B: -75.4v
C: -76.0v
E: -76.0v
Q112R
B: -75.0v
C: -75.8v
E: -75.9v
Q115L
B: -75.8v
C: -76.3v
E: -75.2v
Q115R
B: -75.5v
C: -76.1v
E: -75.1v
B: -75.4v
C: -76.0v
E: -76.0v
Q112R
B: -75.0v
C: -75.8v
E: -75.9v
Q115L
B: -75.8v
C: -76.3v
E: -75.2v
Q115R
B: -75.5v
C: -76.1v
E: -75.1v
Q112 seems okay.
What is the voltage on the base of both Q108/109?
Make sure that both R162 and R110 are still measuring good too.
What is the voltage on the base of both Q108/109?
Make sure that both R162 and R110 are still measuring good too.
R162L in-circuit: 18.24k
R162R in-circuit: 18.25k
R110L in-circuit: 1.093k
R110R in-circuit: 1.096k
Q108L base: 64.2v
Q109L base: 74.1v
Q108R base: 64.4v
Q109R base: 74.1v
The 108 bases look low, don't they?
R162R in-circuit: 18.25k
R110L in-circuit: 1.093k
R110R in-circuit: 1.096k
Q108L base: 64.2v
Q109L base: 74.1v
Q108R base: 64.4v
Q109R base: 74.1v
The 108 bases look low, don't they?
Sorry I'm helping my daughter with her room. We painted it last weekend and working on redecorating, moving furniture this week. I had forgotten how much work was involved with painting.
Yes those Q108's base readings are very low. Are you still showing a balanced voltage drop across R103/104? Those were fairly balanced after you got the front end LTP put back together. The voltages drops were much higher than expected but balanced.
When you check those also check the collector voltage on Q106 to make sure its still close to 0V like it should be.
Yes those Q108's base readings are very low. Are you still showing a balanced voltage drop across R103/104? Those were fairly balanced after you got the front end LTP put back together. The voltages drops were much higher than expected but balanced.
When you check those also check the collector voltage on Q106 to make sure its still close to 0V like it should be.
No worries at all. Today my wife and I planted a tree and did a bunch of other yard work, so I'm guessing we're probably both pretty wiped out.
R103L
rail side: 75.7v
transistor side: 74.4v
R103R
rail side: 75.4v
transistor side: 74.2v
R104L
rail side: 75.7v
transistor side: 74.3v
R104R
rail side: 75.4v
transistor side: 74.0v
Q106L collector: 0.41v
Q106R collector: 0.44v
R103L
rail side: 75.7v
transistor side: 74.4v
R103R
rail side: 75.4v
transistor side: 74.2v
R104L
rail side: 75.7v
transistor side: 74.3v
R104R
rail side: 75.4v
transistor side: 74.0v
Q106L collector: 0.41v
Q106R collector: 0.44v
Okay you'll need to pull Q102/103 and put them on the tester.
The voltage drops are similar on R103/104, so if its working properly the voltage drops on the other side of the Q102/103 devices should be similar as well but they're not. These should test as a close match. If they test good but have much different hFE & Vbe then I'd change them with some well matched A992's.
I probably should've had you remove and test this pair earlier when you found the other failed devices in the LTP.
The voltage drops are similar on R103/104, so if its working properly the voltage drops on the other side of the Q102/103 devices should be similar as well but they're not. These should test as a close match. If they test good but have much different hFE & Vbe then I'd change them with some well matched A992's.
I probably should've had you remove and test this pair earlier when you found the other failed devices in the LTP.
Ha Haa, Thanks!
Yes Tenelson will figure it out sooner or later. He's really the one persevering I'm just giving him my opinion as we go.
I'm surprised the thread has gone this long without any other contributors chiming in.
Yes Tenelson will figure it out sooner or later. He's really the one persevering I'm just giving him my opinion as we go.
I'm surprised the thread has gone this long without any other contributors chiming in.
Thanks guys! I'm confident we'll figure this out eventually, with Chamberman's expert help.
Q102L
BJT-PNP
hFE=413
Ic=3.8mA
Vbe=630mV
Q103L
BJT-PNP
hFE=364
Ic=3.3mA
Vbe=649mV
Q102R
BJT-PNP
hFE=461
Ic=4.2mA
Vbe=629mV
Q103R
BJT-PNP
hFE=366
Ic=3.4mA
Vbe=635mV
Do you think the hFE and Vbe are different enough to warrant replacement?
Q102L
BJT-PNP
hFE=413
Ic=3.8mA
Vbe=630mV
Q103L
BJT-PNP
hFE=364
Ic=3.3mA
Vbe=649mV
Q102R
BJT-PNP
hFE=461
Ic=4.2mA
Vbe=629mV
Q103R
BJT-PNP
hFE=366
Ic=3.4mA
Vbe=635mV
Do you think the hFE and Vbe are different enough to warrant replacement?
Yes change those out with the best matched pairs you can find. Its my understanding that Vbe match is most important for a current mirror pair. However you should be able to find some A992's with both matched Vbe and very similar hFE too.
One other thing to check BEFORE you power it up again. With it powered off measure DC volts across C104 or C105 whichever is easier to measure across. Or you could measure from R121 to the front end ground connection. I'm interested in whether or not the feedback caps are holding a significant charge.
If there is voltage remaining there it should slowly drop with the resistance of your meter on there. If there's voltage there then bleed that voltage off with a jumper to zero volts before you power up the amp again.
Its interesting that at one point several pages back you were able to get just about the right voltages at the base of Q116 & Q117. But then something happened and we're not able to get back there.
I also think its very interesting or maybe the word to use is suspicious that BOTH channels are failing and responding in the exact same manner.
If there is voltage remaining there it should slowly drop with the resistance of your meter on there. If there's voltage there then bleed that voltage off with a jumper to zero volts before you power up the amp again.
Its interesting that at one point several pages back you were able to get just about the right voltages at the base of Q116 & Q117. But then something happened and we're not able to get back there.
I also think its very interesting or maybe the word to use is suspicious that BOTH channels are failing and responding in the exact same manner.
With Q102/Q103 replaced,
R121L cap side to front end ground (chassis): -0.503v
R121R cap side to front end ground (chassis): -0.471v
I drained both channels to 0v and verified with the DMM. What were you thinking the issue is with those caps having some charge?
I agree that it's very interesting that I was getting the right voltages earlier. Normally I would think that something I changed caused the problem, like maybe I have a bad solder connection or a short. But in both channels? That seems unlikely.
Q116L base: -73.9v
Q116R base: -72.9v
Q117L base: -75.1v
Q117R base: -74.3v
R121L cap side to front end ground (chassis): -0.503v
R121R cap side to front end ground (chassis): -0.471v
I drained both channels to 0v and verified with the DMM. What were you thinking the issue is with those caps having some charge?
I agree that it's very interesting that I was getting the right voltages earlier. Normally I would think that something I changed caused the problem, like maybe I have a bad solder connection or a short. But in both channels? That seems unlikely.
Q116L base: -73.9v
Q116R base: -72.9v
Q117L base: -75.1v
Q117R base: -74.3v
I was thinking that if those feedback caps were holding a really high charge then maybe the latch up to the neg rail may be occuring. But I would think it would take a much higher voltage than you measured. I know a big start up thump can happen if these caps hold a charge, but the delay on relay for the speaker protection keeps this from being audible.
One part I just realized you haven't checked is Q111. The voltage drop across that part is very high. That's the reason I had you check the resistance of R162 earlier. I think you should pull Q111 and put it on the tester.
One part I just realized you haven't checked is Q111. The voltage drop across that part is very high. That's the reason I had you check the resistance of R162 earlier. I think you should pull Q111 and put it on the tester.