Balance and bias voltage unstable

[rayma]

Old gear can have all sorts of odd problems due to dirt/smoke/corrosion/little use.
It's best to just completely clean it up before any testing.

 

[solderbrain]

I decided to put my 1kHz sine wave in to the aux, 150mV P-P and have followed it through to the Level control, the output of the level control seems to go to pin 2 of the 7247 tubes. I see the signal going into the wiper and can see c1 vary out but there is nothing coming out of the c2 level out. Will try and take it apart and clean, but unless I am simplifying it more than it should be, looks like that is the issue, at least for this one, haha.

 

[rayma]

The grid of the tube following the level control is a virtual ground, being an inverting stage.
Very little signal will normally be present there, similar to this:
https://www.electronics-tutorials.ws/opamp/opamp_2.html

 

[solderbrain]

Hi thanks for the reply but what I’m saying is the signal is not leaving one side of the level control. 2 channels in, only 1 channel coming out.

 

[rayma]

The plate output has no signal in one channel?

 

[solderbrain]

I had to look at my RCA book, pin two is the grid, so nothing going to the grid.

 

[rayma]

Are you saying one channel's grid has signal, and the other channel's grid does not?

 

[solderbrain]

Yes sir

 

[rayma]

Ok, pull the tube just after the level control and then recheck at the wiper/grid node,
with the control turned up half way.

If the control is at near minimum, some controls will mute in one channel before the other,
due to poor tracking.

 

[solderbrain]

Pulled it, put back, same scenario, about 118 mV ac into each side and then one side is about 300 mV out and the other side is 9mV out. That is one question, if I check my RCA inputs at the back where the signal comes in, I have 212 mV AC on each one. then I measure each one as it comes into the level control and I have like i said about 118mV AC, then the one output is almost double and more than each input at 300 mV AC!

Here are the measurements on the 7247s:

Channel 1
Pin 1, Plate 2: 101.9 vdc
Pin 2, Grid 2: 300 mvdc
Pin 3, Cath 2: 3.8 vdc
Pin 4, 5 (shorted), Heat 1,2: 13.3 vdc
Pin 6, Plate 1: 147.2 vdc
Pin 7: Grid 1: 0
Pin 8, Cathode 1: 1 vdc
Pin 9, Heat M: 13.3 vdc

Channel 2
Pin 1, Plate 2: 102.8 vdc
Pin 2, Grid 2: 9 mvdc
Pin 3, Cath 2: 3.7 vdc
Pin 4, 5 (shorted), Heat 1,2: 13.3 vdc
Pin 6, Plate 1: 149.2 vdc
Pin 7: Grid 1: 0
Pin 8, Cathode 1: 1 vdc
Pin 9, Heat M: 13.3 vdc

 

[solderbrain]

Quick update, I was very curious so I jumpered the pin 2 jumpers (switched them) and the problem was persisting, no change.

 

[solderbrain]

Also, forgot to mention, I swapped the 7247 tubes and the problem stayed with channel 2.

 

[rayma]

When you say "out" where are you measuring, at the level control wiper?
Or at the speaker output?

 

[solderbrain]

The level control

 

[rayma]

Did you pull the tube and measure at the wipers again, at several settings of the control?

 

[solderbrain]

Let me try that now to be sure.

 

[solderbrain]

Okay with tubes (7247s) removed, it is nice and consistent, I have the input to the level control about 84 mV AC on both sides and then out at full level approximately the same.

 

[rayma]

At the max level is not hard for balance.
Without the tube, compare the levels at a pot setting of halfway, and also at a very low setting.

 

[solderbrain]

okay here we go:

1/4 volume level
C2: 0.9mV
C1: 0.8mV

1/2 level
C2: 16.9mV
C1: 21.4mV

3/4 level
C2: 52.1mV
C1: 56.7mV

Full level
C2: 84.0
C1: 84.2

 

[rayma]

Ok, the channel outputs are about 10% off. That's around 1dB tracking error. Not defective, just an old cheap control.

Now repeat with the tube in place. But measure only at the plates (HV present), not at the grids.
The grid is a virtual ground in this circuit, so it is not meaningful data.