Bush VHF 64. unwanted oscillations.

[ian rose]

I am looking for help regarding high frequency oscillations which are provoked with the operation of the volume, bass and treble controls. volume control is part of the output transformer secondary feedback network to the triode AF amplifier section of the EABC80 [V5]. Bass and treble controls are interposed between the anode of V5 and the control grid of final stage V7 [EL84]. When the volume control is at its lowest level the HF oscillation is replaced with a 'hum'.

The fault is also apparent with V4 [EF89] removed and also apparent on the 'gram' position.

During the operation of the controls the HT voltage at the anode of triode section of V5 rises from the design voltage of 70 to 105 volts, indicating, perhaps, that the fault is allowing the V5 triode section to produce the oscillation?
I have replaced all paper capacitors. Checked and replaced as necessary all
resistors and checked all ceramics. Also made a substitution for V5. Checked all earth to chassis connections.
If any member has experience and has access to the circuit schematic and can spare a few minutes to have a look and share thoughts with me I would be most grateful as I would like to clear the bench for the next job!

Regards to all
Ian Rose

[ms660]

Have the feedback connections on the output transformer been reversed by mistake?

Lawrence.

[Peter.N.]

Is there a small capacitor across the primary of the output transformer?

Peter

[ronbryan]

Have the HT smoothing electrolytics dried out? - C61 (20uF) and possibly C62 (40uF) on the Bush service sheet.

Just try tacking something similar across C61 and see if it makes any difference. Likewise with C62.

Ron

[jmedley]

i note that you have replaced the waxies but this model is also full of the small
Hunts caps (mostly brown colour) that are even more problem than waxies
Get rid of them first
Good luck
Jim

[Edward Huggins]

Is the triode section of the EABC80 dodgy?

[ms660]

Forgot to ask, is this a recent fault since working on the receiver or is it an as found original fault?

Lawrence.

[Tractionist]

Originally Posted by jmedley

i note that you have replaced the waxies but this model is also full of the small
Hunts caps (mostly brown colour) that are even more problem than waxies
Get rid of them first

Seconded !!

[ian rose]

Thanks all for your replies.
Yes, there is a .01 cap across the primary of the op trans. in series with a 10k resistor. from which junction is taken a connection to the electrostatic tweeter.
Prior to my initial posting I had replaced all capacitors inc. electrolytics except the silvered micas in the RF section.
EABC80 had been changed.
After initial restoration the set worked perfectly and was 'on soak' for several hours before the fault appeared. It is useable but the hum and high frequency oscillation appears with certain settings of the tone and volume controls.
Feedback connections on the op trans.have not been moved.
Thanks all

[Boater Sam]

EL84, Bet?

[ian rose]

Thanks Sam,
Yep, EL84 has been substituted and voltages thereon are ok.

[ronbryan]

Philips Philettas, such as the BD244U have a 330k grid stopper connected very close to the UL41 control grid, so that the resistor and valve grid capacitance attenuate feedback that would cause parasitic oscillations. In the absence of better suggestions, you could try a grid stopper.

Ron

[Boater Sam]

Grid stopper is a good move but it should work without it, it did once, so there must be a reason

[ian rose]

Taking a day or so away from the shack. will report back soon re grid stopper possibility and further head banging. Any further suggestions gratefully rxd.
regards
Ian

[jmedley]

When I restored my VHF64 I did make a mistake which is very easy to do with so many caps all very close to one another. Triple check your work, many faults are man made.
This model has 2 types of audio output transformer, the change was related to cancelling hum. Please see attached pictures, check your manual against the transformer type
I have 3 types of manual if you need them. Manufacturers, Trader and Broadcaster
Good luck
Jim

[ian rose]

Hello all.
A slapped wrist for me, I fear.
On the umpteenth session of examining my wiring I discover that R26 was connected wrongly! Not surprising that oscillations occurred!
Bush VHF 64 now singing beautifully!
Thank you so much to all for your interest and suggestions. although the problem was man made by me your comments will, I am sure, be usefully remembered for future projects.
Good luck with your own projects.
Ian Rose

[Tractionist]

Easily done ..... especially in a VHF 64/94 ...... but these are fantastic receivers - enjoy the fruits of your labour [after you've have the skip-load of dud caps removed from your road] !!!

[ian rose]

I agree. A lovely Rx with plenty of audio available. fortunately the cabinet is in lovely condition and yes plenty of removed capacitors and one speaker for the bin.
regards
Ian

[Tractionist]

Ah ..... a detached speaker cone had me head scratching for a while - I was on the verge of scoping the thing up when [fortunately] I spotted it! The FM performance is superb .... the Editor of 'Valve & Vinyl' is on record as stating that this was the best monaural Hi Fi that he had ever heard!!