Hello, is there anybody available that can shed some light on this amplifier diagram. i have a red led light that is stuck on but the amplifier is working fine. dc offset is 6 and 8 mv and bias is set on both channels at 400 millivolts
It's likely transistor Q8, or less likely, Q7. The Q8's for each channel are highlighted in green, the Q7's are highlighted in magenta. In this photo, the Q7,8 on the left side of the board are for channel B, the pair on the right are for channel A. If the light is stuck on, it would either be a shorted Q8, or an open Q7. You can replace them with 2N5401.
Ok, i am waiting for some parts currently, i will be replacing the scr diodes on the pwr board.good to know you have some amps for sale, just in case i cant fix this one. I checked those transistor with my transistor tester dca-55 and they test good. i was thinking i might have a problem with the diff input pair.
If the amp is playing and DC offset is fine, then the diff pair is fine and likely everything else. It has to be something with those transistors or circuit if clipping is stuck on.
This all started when I decided to recap the boards. The 4.7uf 35 volt cap at the top left was also changed out but the cap connections were very close together. I am wondering if I may have made a mistake there. Also the pads where the transistors mount are very weak and it doesn't take much to lift the pads. .....I am going to take it apart again and go over the driver board with some high intensity magnification glasses........but also my dc offset is at about 24 millivolts on both channels.I was expecting it to be lower after the recap. I also replaced the led with one that I had,it's the same one I'm having the problem with. Maybe the led is a bad replacement . Could that cause a problem?.....my board has the mpsa29 for the diff pair. But it really seems hard to find a replacement.
Ok, so I traced the circuit down. The positive side of the led goes into the 3.3k resistor, we already know that resistor has been heat stressed although it reads 3.3k that is not the problem just a sign of something else? ...so then it goes to the collector of Q8 and the base of that transistor is connected to the collector of Q7...then it goes to R10 which is 10k ...from there it goes to Q2 ... all transistors including the 10k resistor check out good?
All transistor were removed and tested with a atlas dca-55 transistor tester . I also checked them with a dvom set to diode.
Wow, that's weird. Now I'm wondering what the voltage across R8 1.6k is. Does this amp play music? I know you said offset and bias were fine.
Yes, it actually sounds pretty good. Dc offset before I took it apart was -19mv exactly on both channels. Bias is set at 400 millivolts.
I would still check idle voltage across R8. I'm thinking it might be possible for Q2 to have a dead collector and still a good base-emitter junction to play and keep offset. Otherwise there would have to be something shorted in or on the board to get current to the LED. Or something open in the circuit to Q7
I did pull Q2 out of circuit and tested it with my atlas dca-55 does not show any open or shorted conditions
Q7 keeps Q8 off. So voltage across R8 should shows there is voltage to turn Q7 on. Or check the base-emitter voltage of Q7.
Ok, I took the amplifier apart again . I am going to replace the pn2907 transistors under the heat sinks. 1 of the legs of the transistor broke off. I don't think it has anything to do with the light staying on but I will be changing out the mpsa29...Q1 and Q2 diff pairs. And maybe Q8 and Q7 .......I will be installing to92 sockets for the diff pair that way I can plug in different transistors until I reach 0 offset or pretty close. I will report back when I reassemble , and thanks a bunch for your help. Frank
Hello, good afternoon. I recieved the mpsa29 transistors today after a long wait. I didnt know how to match a darlington transistor so i picked 4 out of a batch of 100.. I ended up with 4mv on the left and 15mv on the right,.and the fricken clip light is off, im so happy about that....Now if i only knew how to match a darlington transistor i could probably get the right channel down closer...Frank
That's good. Did you test the originals? I'm guessing Q2 was only functioning as a base-emitter junction. I match mine with a transistor tester. Otherwise you would have to build a test circuit to calculate gain. If all you have is a multimeter, you could match the base-emitter juction voltage with the diode checker function.
Q2 was giving me all kinds of funky readings, I do not understand alot of the readings but there was obviously something not right. I just got this, it has a curve tracer in it but i dont know how to use it yet
