Hi All! I need some help with my GFA-555 II. I have an Adcom GFA-555 II with a dead right channel. Stuart Easson sent me some original output transistors. Thanks Stuart! The old input board is crusty and a lot of lifted traces, so I got a new input board and 2 matching differential pair from Chris Hoppe. everything on the new input board is all new except the switch as recommended by Chris. I also replace the driver transistors 2SD1047/B817 with NJW3281/NJW1302. I replaced Q201/Q251 with KSC3503 and Q301/Q351 with KSA1381. the only 2 transistors that I didn't replace are the Q257 & Q307 bias compensation transistors. Now after putting things together. DC offset are about 1mv on both channels, voltage on the LT1006 pin 6 are about 4.5v on both channel. Bias on right channel is adjustable except the left channel. The left channel bias is just showing 0 and it's not moving at all. After power on for about an hour, the heat sink on the left side is still cool but the right side is warmer. I checked the Q257 & Q307,they are not shorted. I don't know what else to check ?. Thanks in advanced!
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I notice on the mark 11 the bias on both channels is routed via connectors are they making good contact ?
Duncan makes a good point. Those JST board headers and connectors sometimes corrode. You can test the bias comp transistor in-situ, just do a diode check from the bottom of the board.
Also you should see something like +/-1.2V on the DRV+ and DRV- terminals.
Make sure you have the right value components in the bias circuit. Fortunately you have one good channel, so start poking around with the meter and see where you find a difference.
Also you should see something like +/-1.2V on the DRV+ and DRV- terminals.
Make sure you have the right value components in the bias circuit. Fortunately you have one good channel, so start poking around with the meter and see where you find a difference.
the connectors and headers are brand new. I'll take a look at them again. stay tuned! Thanks Duncan and Chris.
its a brand new connectors and headers,
0v bias could be coursed by the solder joints on the copper side,
R117 681r, if its ends one side open then Q307 will turn on.
suggest to check continuity,
1) check continuity form collector of Q107 to collector of Q307,
2) check continuity from joints of R117 and R119 to base of Q307,
3) check continuity from collector of Q108 to emitter of Q307,
4) check R117, R118, R119 2k-B for their conditions.
5) check Q307, if C. B. shorted or defective.
6) check the pads condition.
0v bias could be coursed by the solder joints on the copper side,
R117 681r, if its ends one side open then Q307 will turn on.
suggest to check continuity,
1) check continuity form collector of Q107 to collector of Q307,
2) check continuity from joints of R117 and R119 to base of Q307,
3) check continuity from collector of Q108 to emitter of Q307,
4) check R117, R118, R119 2k-B for their conditions.
5) check Q307, if C. B. shorted or defective.
6) check the pads condition.
I just checked all these suggestions.
1. Ok
2. Ok
3. Ok
4. All are new and tested ok
5. Diode test ,Black to B- red to C= 1.140
black to C- red to B= 0.569
6. pads of output transistors look ok.
I don't know if I should replace the Q307? all the other components are new and tested ok before I installed them.
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They checked out ok.
I have a bit higher at around 1.7v the right side has about 1.6v
So far, everything is matching with the right side and tested ok.
Not sure about that reading on Q307, but it's in circuit. Can you unplug the bias cable and then try on diode check, B-E and B-C? Should be about 0.6V on each.
Yahoooooo! I finally found my problem. It's the thermal sensor wire. I had it in reverse. Thanks everyone for helping! Thanks Chris, for your help and a great board.
You're welcome, but are you sure about that? :^D The thermal breaker is just a switch and has no polarity. Maybe you fixed a bad solder joint, or fixed a solder-bridge in the process of reversing it?
Thank you,
I've learned something here,
I was too focus to a point and neglected the surrounding.
thermal switch S101, cuts off the bias to Q108 when heat is detected over 85 degree and protect the amp.
I've learned something here,
I was too focus to a point and neglected the surrounding.
thermal switch S101, cuts off the bias to Q108 when heat is detected over 85 degree and protect the amp.
That was what i thought that the switch has no polarity. but it could be that I touched up some solder joints again. but the area that I touched up, I checked the continuity was fine. So, I don't know for sure. But the only thing that I did for sure was changing the wire around. I was frustrated with this amp, it looks very simple but yet it drove me nuts,until I don't know what to do anymore. But I'm glad that the problem is resolved.