Had this amp serviced twice and still having issues. First time I had a red-plate tube and the amp was shutting down. Was told there were a few bad solder joints and I needed a new set of tubes. Next I Had some hum on one channel, and the port for getting the bias reading with a meter was not giving any voltage readings. Brought it back. pretty sure there was a loose molex plug for that feature- but I’m not sure what else he did
To clarify- the hum that I’m describing is coming out of the speaker. No idea of the frequency, but I’m guessing it’s 60hz
Anyway-If you follow the manual on setting bias for this amp, you turn the gain pots all the way down, then turn up the voltage to 500mv (2 x 6550/channel) for each tube
When you first start to turn up one of the pots, there is some audible hum, which I’m assuming is a balance issue between the 2 push-pull tubes, until you turn up the other pot to 500mv as well.
This is fine for the right channel.
Now on the left channel, the one with the “hum” issue, one of the gain pots will not zero out. In fact, when I turn it all the way down, it doesn’t go below 300mv..
So that pot I can adjust to 500mv
But what happens it when I try to adjust the other tube pot to raise the voltage, the hum gets louder and louder beyond 250mv.
So basically, with pots all the way down it’s
L( 0v, 300mv) R( 0v, 0v)
With gains set, and least amount of noise it’s
L(250mv, 500mv) R (500mv, 500mv)
Could this be from a bad potentiometer?
Thanks
To clarify- the hum that I’m describing is coming out of the speaker. No idea of the frequency, but I’m guessing it’s 60hz
Anyway-If you follow the manual on setting bias for this amp, you turn the gain pots all the way down, then turn up the voltage to 500mv (2 x 6550/channel) for each tube
When you first start to turn up one of the pots, there is some audible hum, which I’m assuming is a balance issue between the 2 push-pull tubes, until you turn up the other pot to 500mv as well.
This is fine for the right channel.
Now on the left channel, the one with the “hum” issue, one of the gain pots will not zero out. In fact, when I turn it all the way down, it doesn’t go below 300mv..
So that pot I can adjust to 500mv
But what happens it when I try to adjust the other tube pot to raise the voltage, the hum gets louder and louder beyond 250mv.
So basically, with pots all the way down it’s
L( 0v, 300mv) R( 0v, 0v)
With gains set, and least amount of noise it’s
L(250mv, 500mv) R (500mv, 500mv)
Could this be from a bad potentiometer?
Thanks
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The 502 and the 502B(rc) no longer have much in common.
You will have to open and take out the test equipment.
The first generations are known to fry their pcb, I don't know about the current one, start with a visual inspection.
You will have to open and take out the test equipment.
The first generations are known to fry their pcb, I don't know about the current one, start with a visual inspection.
Yeah that’s a big difference lol. I haven’t found a schematic for this amp specifically-yet.
Ok - one other check… I swapped the tubes to the opposite channels. Noticed that the issue followed the tube, where I cannot get the bias reading for this tube below 180mv. All the rest can be set to near 0v.
Bad tube?
Ok - one other check… I swapped the tubes to the opposite channels. Noticed that the issue followed the tube, where I cannot get the bias reading for this tube below 180mv. All the rest can be set to near 0v.
Bad tube?