Had the amp working briefly long time back and then it was put in hibernation.
Now installed a new spare PT and powered up. Followed the sections "Checkout" and "Setting the Bias" step by step to get the amp started. 3 issues I am experiencing:
1. The 5842 voltage is not stable on one channel. I set it to 175v, but it keeps on jumping up and the tube red plates within a few seconds. The other channel is rock steady.
2. The heatsink for U2 U3 gets very hot.
3. With a 300-0-300V, 200mA rated transformer the B+ I am getting is <300V which seems low to me. The choke value is 1H, 40 Ohms, and the 300B tubes are biased at 65mA.
Where do you think my problem lies?
Now installed a new spare PT and powered up. Followed the sections "Checkout" and "Setting the Bias" step by step to get the amp started. 3 issues I am experiencing:
1. The 5842 voltage is not stable on one channel. I set it to 175v, but it keeps on jumping up and the tube red plates within a few seconds. The other channel is rock steady.
2. The heatsink for U2 U3 gets very hot.
3. With a 300-0-300V, 200mA rated transformer the B+ I am getting is <300V which seems low to me. The choke value is 1H, 40 Ohms, and the 300B tubes are biased at 65mA.
Where do you think my problem lies?
Sounds like the 10M45 has failed. It fails to a dead short, and red plates the 5842.
The B+ is too low, but these two problems may be related. Fix the red plate issue, and that may fix the B+.
One of the 10M45 pins is used as a via to move B+ to the opposite side of the board, and if not soldered on both sides, crazy negative voltages appear in wierd places.
Can't remember which 10M45 has that via, but be aware of this as you are making the repair. I had this happen when I replaced a 10M45. Fortunately George recognized the problem right away.
The B+ is too low, but these two problems may be related. Fix the red plate issue, and that may fix the B+.
One of the 10M45 pins is used as a via to move B+ to the opposite side of the board, and if not soldered on both sides, crazy negative voltages appear in wierd places.
Can't remember which 10M45 has that via, but be aware of this as you are making the repair. I had this happen when I replaced a 10M45. Fortunately George recognized the problem right away.
CCS replaced
Hi,
Replaced the CCS and voilà, the voltage on the 5842 on that channel is holding rock steady, and no more red-plating.
And the B+ is up too - holding at around 320v after biasing the tubes to ~63-64mA.
However, I am not yet totally trouble free...
I tried biasing the 300B tubes, and noticed that the 300B bias is not steady on one channel - btw this is the other channel and not the channel in which the CCS was replaced.
Might it be a case of a coupling cap issue (dry solder, bad connection, bad cap)? Or should I look at something else first?
Hi,
Replaced the CCS and voilà, the voltage on the 5842 on that channel is holding rock steady, and no more red-plating.
And the B+ is up too - holding at around 320v after biasing the tubes to ~63-64mA.
However, I am not yet totally trouble free...
I tried biasing the 300B tubes, and noticed that the 300B bias is not steady on one channel - btw this is the other channel and not the channel in which the CCS was replaced.
Might it be a case of a coupling cap issue (dry solder, bad connection, bad cap)? Or should I look at something else first?
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Yes, IIRC swapped the tubes to rule out a faulty power tube.
The jump is around >20mA. When setting the bias, the process usually goes fine till around 55-60mA and then becomes 0 or starts jumping (highest I have seen is >100mA). Finally got it steady (~60mA) and played music, then saw it fluctuating between 60-80+mA. The other channel is rock steady when no input, and when music is playing goes from 63.3-63.5mA, so very stable.
I have one more MOSFET in hand...
Any new substitute parts been identified?
The jump is around >20mA. When setting the bias, the process usually goes fine till around 55-60mA and then becomes 0 or starts jumping (highest I have seen is >100mA). Finally got it steady (~60mA) and played music, then saw it fluctuating between 60-80+mA. The other channel is rock steady when no input, and when music is playing goes from 63.3-63.5mA, so very stable.
I have one more MOSFET in hand...
Any new substitute parts been identified?
This makes me suspect the track in the pot, or the wiper arm, is bad. One way of testing this is to remove the pot, and then use jumper leads to switch the orientation of the pot, so that the wiper is sitting on a different part of the track than it would have been when it was installed.
Solid state parts are usually all or nothing, works right or doesn't work at all.
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