Vox 1143 question

[Garydean]

I'm working on a 1967 Vox V1143 Beatle. The brilliant channel has about 5% output. All other channels are loud and clear. I took some measurements and found the bias to be off a bit (12V). I adjusted the bias pot on the preamp chassis to where I was getting about 15V (not 19V) on the preamp transistors. There's also around -6vdc on the chassis that contains the power supply and the output. The voltage reading on that chassis changes inversely when I adjust the bias. I'm feeling there's still a component issue someplace in the brilliant channel. Or possibly the mixer for that channel. Has anyone had a similar issue that they would like to share with me.

Thank you,
Gary

[52 Bill]

Originally Posted by Garydean

I took some measurements and found the bias to be off a bit (12V). I adjusted the bias pot on the preamp chassis to where I was getting about 15V (not 19V) on the preamp transistors. There's also around -6vdc on the chassis that contains the power supply and the output. The voltage reading on that chassis changes inversely when I adjust the bias.

From what I remember, there usually are two adjustment pots on the pre-amp chassis. One is for the tremolo and the other is for the limiter circuit. What does the control that you are adjusting hook up to? What are you reading -6vdc on the power amp chassis in reference to? Both of the chassis' should be ground connected.

Originally Posted by Garydean

I'm working on a 1967 Vox V1143 Beatle. The brilliant channel has about 5% output. All other channels are loud and clear.

Very common problem on all of the Thomas organ amps are bad electrolytics in the signal path.

Beyond that, watch out for the reverb switch and associated wiring. If it is dirty it can kill the signal from either the bright or normal pre-amp.

[Garydean]

Thanks Bill,
I didn't have the signal jumper plugged into the preamp board when I took the readings earlier. It's now connected and -6vdc went away.

Still a faint signal out of the bril channel. I tried scoping the signal thru the bril channel and my scope decided to quit on me. I'll investigate the reverb area. I tested all the other electrolytics and they measure good.

The control I was adjusting seems to be the limit level control. What should that be set on?

thanks!!

[52 Bill]

Originally Posted by Garydean

The control I was adjusting seems to be the limit level control. What should that be set on?

I'll have to look it up. You feed a signal into the amp and you adjust for something like onset of clipping. When I find the schematic and procedure, I'll scan it for you.

[Garydean]

Hey Bill,
I replaced one of the non polar electrolytics, C120, and it's fixed. The casing was cracked but It looked fine until I removed it. Thanks for the help!!

G

[52 Bill]

Originally Posted by Garydean

I replaced one of the non polar electrolytics, C120, and it's fixed.

Glad to hear you got it working.

I have uploaded at Ampix a scan of the factory set up procedure for the Limiter and the Tremolo Modulator pots. It's in my folder.