Guru
Joined: 12/08/2011
Location: AustraliaPosts: 357
| Posted: 06:27am 16 Jan 2023 |
Hi All
The Jukebox has two of these boards (Left & Right channel) and with one of them working I have been comparing signal levels.
The top of the diagram is +30 and the bottom is -30
The only description of the board are these 4 lines:
Q4 is part of the positive clamp circuit.
Output device Q1 draws current through Resistor R2. (Should that be R1 ??)
Q4 drops the base of output device Q1 to below R2, limiting current to a safe value.
Q3 acts on the negative signal component in the same manner as positive clamp Q4.
If I leave the Fuse F1 in (-31) the Q1 will overheat if I let it.
So with the F1 open I have put a 1khz signal into both boards at 1v p2p.
That is what was noted at left terminal 3.
There is no current flowing through R25 and R20 is at 28vdc.
However the 1V p2p drops to a dirty looking 0.3 v p2p at emitter of Q6 and the base of Q7 left side. I had not replaced C17 yet, but C15 is ceramic disc so probably ok.
I was simply going to replace all transistors.
I have already replaced all the electro caps and not touched other caps which are ceramic disc or mylar.
However after finding the Q7 on line (2N2919) at $60 decided to go for the Q5 & Q6 at $0.30 from Jaycar and replace C17.
Will follow your suggestion and test again in the morning.
Edited 2023-01-16 16:30 by Dinosaur