Bush VHF 41 Output Transformer Phase Wiring

[alanworland]

I have been working on the above set, recapping etc and am about to replace the output transformer.
Am I being over cautious in thinking about the wiring with regard to phase?
The transformer is not marked in any way and primary and secondary windings can only be identified by resistance.
Is it important?

Alan

[Tractorfan]

Hi,
The output transformer forms part of the negative feedback circuit and, as such, must be phased correctly. I built a valve amplifier a while ago and got it wrong, thus creating an very effective oscillator! It caused great distress to the output valve with sparks and a bright glow within, and I had to unplug PDQ! After reversing the primary connections, all was well once more. Resistance checks will only show which is primary (quite a high resistance) and which is secondary (low resistance), or if a winding is open circuit. It won't indicate phase.
I'm not sure of the correct way to phase an unknown transformer. Perhaps run the set up slowly on a Variac and look out for any potential fireworks?
Cheers, Pete.

[crackle]

Hi Alan
Are you replacing it with one from another Bush VHF41?
As mentioned the transformer secondary does have a connection for the negative feedback. But I would think it may be possible to work with one that doesn't, by altering the resistor value in the neg feedback.
Perhaps someone can advise.

Mike

[Herald1360]

Only if it has negative feedback connections. Which it does, and odd ones too.

See attached snippet:

P is primary and identified by its resistance but its phase is unknown.
S(1) is whole secondary (or possibly just the bit from S2 to C57)
S(1) is the section connected to the loudspeaker.

So.... getting secondary connections right is relatively easy but primary is a 50/50 bet. Or in the real world, your first attempt will be wrong and the set will squawk or at least sound awful at which point turn off quickly and reverse the primary connections

Edit: After reading the postings put up whilst typing, if the TX isn't the right one, it could get complicated as suggested. It might be better to leave the feedback off, as the winding resistances suggest that the feedback voltage may be considerably higher than the speaker output voltage. You might get away with connecting C57 to the output valve anode (the phasing will be correct, but the level may not).

[alanworland]

Thanks, its the original transformer going back.
Its my own fault I usually photograph such items, but it was disconnected before I got round to it! Having said that the wiring (quite sound) was so dirty I am not sure it would have helped.
I can run it up slowly on a variac ready for a quick turn off!

Alan

[Herald1360]

Originally Posted by Herald1360

P is primary and identified by its resistance but its phase is unknown.
S(1) is whole secondary (or possibly just the bit from S2 to C57)
S(1) is the section connected to the loudspeaker.

Just noticed the typo.

Red section should read:

S(2) is the section connected to the loudspeaker.

[dseymo1]

Usual advice in this situation is to try it and see.
If the result is oscillation, reverse the phase.
I suppose with careful use of a dual-trace scope (or crafty Lissajous with a single trace) you could work it out.

[ukcol]

This would be my approach.

Referring to the snippet in post #4 from Chris.

Temporarily connected a switch between the junction of C57-R31 and chassis. Close the switch and turn the set on. Adjust the volume for a normal listening level and the tone control for maximum treble cut.

Momentarily open the switch and listen to the change in audio. If you have the phase right there will be a drop in volume level. If you have the phase wrong there will be an increase in volume and/or instability. Using a switch allows you to remove the feedback quickly if anything dramatic happens.

[alanworland]

Thanks for help, needless to say I wired it the 'wrong' way first!
Not without its problems though, am stations received, FM station can only be picked up with the variac supplying about 70% of mains?
Chassis upsidedown next for some voltage investigation, more posts to come I reckon.

Alan