Oscillations

[Swampy]

Hi,

Have a Jennings Univox J7 with unwanted oscillation in audio spectrum traced to plate of V5. DC voltages are off also.

To date have tube swapped, replaced R38, killed oscillation by lifting R32 , R37 C12, or C13.

I can visualize how lifting these components breaks the feedback loop...what I don't understand is why this circuit has lost stability and why plate voltage is so high?? R34, 35 and 36 measure ok with multimeter

Matt

[Enzo]

Plate voltage is so high for the same reason the cathode voltage is so low - the right triode is not conducting as much. Hard to make predictions without part values.

What is that circuit? It looks like it is intended to be an oscillator, or maybe a flip-flop. The two triodes are cross connected plate to grid.

What effect does the switch have on things?

DO the DC voltages even out if the caps are all lifted? You measured the plate and cathode resistors but neglected to measure the four grid related resistors.

[Jazz P Bass]

Wow.
A Univox J7.
Wasn't that used as the lead instrument on 'Telstar' by the Tornadoes? (circa 1962)
Post the complete schematic if you have it.

[km6xz]

i am not familiar with the unit but it looks like the second tube is an Astable Multivibrator so one side should NOT be conducting until it is flipped by an external pulse. It could be used as a divider in a synth. So the unbalanced values are probably correct. What is the input source C9a connected to. That should be a pulse or tone generator of some kind and the output should be a square wave of half the frequency

[philbo]

I'd say get the DC bias levels fixed first, then chase the oscillation if it's still there.

It's a little hard to tell without the component values (or even tube numbers) on the schematic, but you might check R57 and R58 to see if either have changed value. They set the grid bias for the rightmost triode. The grid of that triode is too negative, driving the tube toward cutoff (conducting less than it should). This is why the plate voltage is too high. If the tube were conducting more, the plate voltage would decrease.

[Enzo]

I am inclined to accept Stan's explanation. I saw that the thing looked like a flip flop, but didn't think it through to realize that meant it would toggle to one side or the other. One side turns on, that winds up turning the other side off.

[philbo]

Yeah... pretty hard to tell what's going on with that little snippet of schematic. I'm inclined to agree with you & Stan. I was assuming the black numbers were correct bias voltages, but who knows? They could have been written on the schematic by somebody troubleshooting a broken one.

[Swampy]

Yah, an ancient and rare beast...bit of a challenge to work on, turret strips are under tubes mounted horizontally.

So oscillations cease with C12 + C13 lifted. R37 + 38 (not 57+58) measure good @ 470 K

With V5 pulled

1. 169 V
2. 48 V
3. 0 V
4. 0 V
5. 0 V
6. 268 V
7. 29 V
8. 0 V
9. 6.5 VAC

and in

1. 37 V
2. 23.7 V
3. 23.2 V
4. 0 V
5. 0 V
6. 262 V
7. 6.2 V
8. 23.2 V
9. 6.5 VAC

[Enzo]

How do YOU think the circuit ought to work? I see a keyboard as wanting to oscillate, otherwise, how will it make notes? We kinda need to know what we are trying to fix.

With V5 removed, it seems to me the voltages ought to even out. The resistors from the parts list seem to be 100k to ground from each grid, (not 470k), 470k from grid to opposing plate, then 47k plate resistors. Schematic says 250v B+. So I see two voltage dividers. R34-R37-R38, and the identical R35-R32-R33. From the values, 47k-470k-100k (617k total), and 250v, I expect 100k/617kx250v or 40v at each grid, and 570k/617kx250v or 230v at each plate. Thus your pins 2 and 6 look close, while pins 1 and 7 look low. What did R34 R35 measure? If i figure the same circuit with R34 as 470 instead of 47k, my numbers come back low, like yours. So whatever B+ is, I expect about 1/6 of it at the grid pins.

[hernanbaldi]

Hi, i´m working iin one of these, are the schematics available for dowload anywere ? Thanks !

[Swampy]

Hi, i´m working iin one of these, are the schematics available for dowload anywere ? Thanks !

univox schematic 1.pdf

univox schematic 2.pdf

[hernanbaldi]

Thanks Swampy !! I´m unable to download or view the attachment, i will try from another computer later. Thanks Again !!

[oc disorder]

Swampy.. (as per JP Bass) you need to zip 'em up and repost as the forum has a fault with files of the same size that have been posted before.
Then Hernanbaldi - you and everyone else will be able to view them.

[Pavel Fursov]

Can't download them too; reupload, anyone?

[knutknut]

Hi swampy. The divider stage produces the sub bass at half frequency. The voltage on pins 3 and 8 should be around 40v, regardless of which side is conducting. I would suspect a leaky capacitor across R36.