Bestone model TRF - R.I.P :(

[egerton]

I've been listening to my newly restored Bestone TRF for over a week now and it's been working fine, but today it suddenly died mid song, it just cut out and became silent. I took a few measurements and soon worked out the output transformer primary has gone open circuit. Such a pity. I've removed it and measured the secondary resistance as around 0.5 ohm and the speaker speech coil as 5 ohm, I think someone commented the primary should be around 5K going by the circuit. Can I obtain a suitable replacement easily enough or should I consider a repair service? Never had to replace one before...

Thanks. Paul

schematic: https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu

[martin.m]

Hi Paul.
I would just use one from any scrap radio chassis that has a single ended valve output stage. If originality is important, there are forum members who can rewind vintage transformers.

Regards
Martin

[Herald1360]

The resistance of the primary of a suitable transformer won't be 5K- that's about what the impedance it will present to the anode of the output valve will be. This is speaker impedance times the square of the transformer turns ratio. The actual primary resistance measured at dc will be around 300-600 ohms or so.

As others have said, any gash output transformer of a similar physical size will work- if the match isn't right, you'll just lose a bit of output or gain a bit of distortion. Neither is likely to be very noticeable. Even a 240V:6V small mains transformer will work, though not so well!

[egerton]

Thanks Martin & Chris for your replies. I have an old Ecko A23 & Philips G85U that are single ended but the o/p trans. in both have a tap and power is fed to it and split between anode and supply rail, so not sure I could press one of those trans. in to service on the Bestone?

Paul

['LIVEWIRE?']

I'm not familiar with the Bestone TRF radio, but, AFAIK, you could still use the o/p transformer from either the Ekco or Philips radios. That tap is simply a hum reduction feature, so it can either be ignored and HT= connected between the 'top' end of the winding and the o/p valve anode in the usual way or the Tx could be wired as in the Ekco or Philips radios to make use of the Hum reduction feature. I once used an o/p tranny without the hum cancelling winding to replace a faulty one in a Bush radio which did have the feature. The radio worked fine with the replacement transformer!

[Boater Sam]

A tapped transformer will work, just use the section with about the correct 300 to 650 ohm resistance.

Bear in mind that the original failed for a reason, probably grid current in the output valve due to a leaky coupling capacitor to grid 1. Worth checking else it may do it again.

[egerton]

That tap is simply a hum reduction feature, so it can either be ignored and HT= connected between the 'top' end of the winding and the o/p valve anode in the usual way or the Tx could be wired as in the Ekco or Philips radios to make use of the Hum reduction feature. LIVEWIRE

Thanks for the suggestions for the trans. The Bestone has a choke so I was thinking I could remove this if I used the tapped trans. and wired it in correctly - it would basically be the same circuit. I will measure the resistance of the o/p transformers to see if they match the choke resistance in the Bestone.

A tapped transformer will work, just use the section with about the correct 300 to 650 ohm resistance.

Bear in mind that the original failed for a reason, probably grid current in the output valve due to a leaky coupling capacitor to grid 1. Worth checking else it may do it again.
__________________
Boater Sam.

Thanks Sam. I have already replaced 'that' cap. (C5 in my schematic) plus C9 and all electrolytics so I'm confident the caps. are ok. I noticed from the start that there was a quite a lot of random 'scratching like' noises that I thought may be a component breaking down that I couldn't locate. I guess it was the transformer on the way out. It is clearly over 80 years old!

[ms660]

I will measure the resistance of the o/p transformers to see if they match the choke resistance in the Bestone.

The resistance of the hum reduction section (the section with the fewest turns) on the proposed replacement transformer will be nothing like the resistance of the existing HT choke.....It will be much lower. To reduce hum, a hum reduction output transformer in effect has to be balanced, it would have a resistor in series with the HT output feed to the rest of the receiver, I would stick with convention if using a hum cancelling type, connect the tap to HT, and the bottom end to output valve anode.

Lawrence.

[egerton]

The resistance of the hum reduction section (the section with the fewest turns) on the proposed replacement transformer will be nothing like the resistance of the existing HT choke.....It will be much lower.

I was wondering about that. I was thinking about how a change like this may affect the HT supply voltage. This is an American set adapted by Bestone for the UK market and I replaced the line cord dropper with an auto transformer. Just about managed to mount it on to the midget chassis making a integrated solution. I will be careful to maintain the HT voltage if using a replacement trans. it was only around 100V DC on the smoothing cap.

[ms660]

Here's a bit from one of my reference books on the hum cancelling type of output transformer, note the resistor on the primaries output leg:

https://www.vintage-radio.net/forum/...9&d=1495094806

Lawrence.

[egerton]

That's kind of you Lawrence, thank you very much I'll read with interest. Is this sometimes called a 'humbucker'? I have heard this term for guitar pickups and various other things. Just wondered....

Paul

[paulsherwin]

Yes, it's sometimes called a humbucking coil or winding. You can just ignore it if you don't need it.

[ms660]

Originally Posted by egerton

That's kind of you Lawrence, thank you very much I'll read with interest. Is this sometimes called a 'humbucker'? I have heard this term for guitar pickups and various other things. Just wondered...

Some folks call any coil to do with hum a humbucker.... Usually the term humbucker is for guitar pickups or a coil that's in series with the speech coil in a field coil loudspeaker.

Lawrence.

[egerton]

I have "borrowed" the o/p transformer from my Philips G85U and put it in the Bestone ignoring the humbucker connection now that I'm educated about it. I'm pleased to report the set is now working very well again. I had to modify the bracket a little as it's mounted on the speaker directly whereas in the G85U it was on the chassis. The G85U was working so it's now back in my to do list. I'll keep an eye out for another transformer. Many thanks to all contributors to another successful repair.
Paul

[AC/HL]

To quote Mark Twain, "The reports of my death are greatly exaggerated"