ST-70 Problems with left channel….

I have an ST-70 with the VTA board and SDS Capacitor/Power Supply board, upgraded PT and the old A-470 OT
The problem I having is that the left channel is playing at a lower level and sound muffled/distorted and seems to be lacking mid range (vocals )

Now before I started to try to find the problem, I had bought the capacitance upgrade and 4 military-spec Russian PIO output coupling capacitors that I haven’t yet installed, so thought I would do that first just incase it fixed the problem. I also tried switching the tubes around but no luck……

I’m kid of new to these tube amps so I would appreciate if someone can help me sort out this problem. I’m not sure if the problem is in the driver board or possibly an output transformer.

I was thinking of temporarily switching the left output of the driver board to the right output tubes. That would definitely narrow things down…..What do you guys think? Can that be done? Or are there some tests I should perform first?

I think you are talikng about switching the OP tubes from one side to the other. Good place to start.


Steve

Do both sets of output tubes bias OK? If not try measuring the output tube cathode resistors.

Westy56 wrote:I think you are talikng about switching the OP tubes from one side to the other. Good place to start.


Steve

No, I tried that already.... I was thinking to hook up the left output wires of the VTA board to the right channel of the output tubes....So that the left channel will be using the right output transformer and tubes... so if the problem is still there I know the problem is in the input stage and not the output stage.... Does this make any scene?

BrianLW wrote:Do both sets of output tubes bias OK? If not try measuring the output tube cathode resistors.

Yes the bias is good on both sides....

Having one channel that works gives you an advantage. You can measure voltages at each of the tube pins on the bad channel and compare to the good channel. I would do this and report back any large differences before moving wires around.

Also, with the amp powered down, high voltage completely discharged, and output tubes removed, measure the resistance of the output transformer windings. If there is a big difference between channels, there is a problem with the transformer. If there is not a big difference, the test is inconclusive as it would not show a shorted turn.

BrianLW wrote:Having one channel that works gives you an advantage. You can measure voltages at each of the tube pins on the bad channel and compare to the good channel. I would do this and report back any large differences before moving wires around.

Also, with the amp powered down, high voltage completely discharged, and output tubes removed, measure the resistance of the output transformer windings. If there is a big difference between channels, there is a problem with the transformer. If there is not a big difference, the test is inconclusive as it would not show a shorted turn.

Ok I'll do some testing later today to see what I find

if you have swapped the output tubes and the problem still remains on the same channel, it's has to be either an assembly error on the VTA PCB or more likely the output transformer on that side. Yes, you could route the PCB outputs to the tubes on the opposite side as a good test of whether it is the PCB or the output transformer.

ok I just got home and tested all the resistors and found R21 open not sure what part of the circuit that is but it close to the input tube. I'm not sure if it was replaced before because R21 should be 51K but it has 47k in it in both channels..... now I'm not sure if I should put 47k back in or put 51k? or will it make no difference?

R19 thru R22 should all be the same value, normally 51K but 47K works fine if that's what the other ones are,
if that was bad it would definitely not work correctly, so that could be an easy fix!

I replaced that risistor and it fixed the problem! Sounding sweet again! Thanks to everyone for the advise.........