UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 4th Jul 2019, 12:22 pm   #1
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Unknown Home Built TRF radio

First posted on August 2011 under the title: Radio Identification Required

Link to original posting:

https://www.vintage-radio.net/forum/...ad.php?t=73641


I have now got round to starting with refurbishing this radio.
In the first posting there were some thoughts that this was a commercially built radio, however I am now certain it is a home made job.
The build quality is very poor. The soldering is probably the worst I have ever seen, the wire routing leaves a lot to be desired.

I have tried to find some information on this but came up blank. It was most likely to have been a magazine project such as the PW magazine.

So far I have done just what was essential to test it.
First the AC supply was, as with many early radios, Neutral switched. I consider that to be very dangerous practice, so it was changed to Live switched.

The transformer on top of the chassis was next. I originally thought this to be an auto-transformer, it is not. It is a 6.3V heater transformer that was wired incorrectly, with one side of the primary connected to Live, whilst the other was connected to ground via the 6.3V winding. That was corrected.

3 old paper capacitors and one electrolytic were removed and replacements tacked in.
The set was then tested, it worked on both wavebands.

My intentions are now to draw out the circuit and layout including the wiring, then to gut the chassis totally so that it can be fully cleaned and probably painted for protection. All the wiring needs replacing as it is calcified and breaking up. The set will then be fully rebuilt.

I now have a couple or so questions:

The heater transformer supplies the 4 valves including the rectifier, with one side of the winding grounded. Would it be better to have a transformer with 2 winding's so that the rectifier could have its own heater supply that was not grounded?

The rectifier Anodes are connected together directly to AC Live, but I think they should have a resistor in each anode of at least 100R. Is that correct?

The AC On/Off is a single pole switch mounted on the rear of the wave change switch. Is it possible to obtain a double pole switch to replace it? I have my doubts about that possibility.

However there could be an alternative. There is a potentiometer (appears to be 20K) that seems to be acting as just a volume control, that is all it is actually doing. However the centre tag that would normally be the wiper is riveted to the metal casing, that is grounded, so either one end or the other of the pot is moved to, or nearer to, ground, but the track is dodgy.

This could be replaced by a standard pot with the wiper grounded, and maybe with a 2 pole switch. That would of course mean replacing the wave change switch with a 2 way device, easily obtained.
Does that sound viable?
What is this pot meant to be doing? Reactance?

I have added a picture of the chassis underside showing the switch and the pot.

Regards,
Dave
Attached Thumbnails
Click image for larger version

Name:	PICT0007.jpg
Views:	609
Size:	110.0 KB
ID:	186172  
davidgem1406 is offline  
Old 4th Jul 2019, 12:40 pm   #2
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Unknown Home Built TRF radio

If the pot has its three connections used (centre one grounded) it could be that it alters the bias (gain) for the RF amp while at the same time shunting the antenna circuit, an arrangement like that was often used for TRF receivers of that vintage, if that's the case the pot might be antilog, it might be worth checking its law if you want to replace it.

Lawrence.

Last edited by ms660; 4th Jul 2019 at 12:52 pm. Reason: Addition
ms660 is offline  
Old 4th Jul 2019, 2:08 pm   #3
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
Default Re: Unknown Home Built TRF radio

That looks suspiciously like a 'Premier' or 'Barton' kit radio. These were prolific in the 50's and 60's and used whatever valves were the cheapest and available at the time. There were strange combinations used and even a small VHF transmitting valve could be found in the output stage of some of them. Some were superhets, others were TRF but all using the same basic chassis and cabinet. The only circuits available were the ones supplied with the kit although I have a superhet version of one and the circuit is available on this website (under Barton). The TRF's were quite basic so it shouldn't be difficult to reverse engineer the wiring and commit it to paper.

What valves are used in the set? Mine also has a heater transformer that had neutral switched. I changed it but my set was actually very well built and only required some basic work to bring it back to working order. It benefitted hugely from a proper alignment though....I guess the original constructor just peaked it up on known signals.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 5th Jul 2019, 9:38 am   #4
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Unknown Home Built TRF radio

Valves are listed in the original thread.

The heater transformer would be happy to see a 6V6 rather than the unusual 349A.

Don't understand why moving mains switch function elsewhere would require the wavechange switch to be replaced. Just ignore its redundant OFF position or even modify the redundant switch to act as a limit stop!
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 5th Jul 2019, 10:41 am   #5
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Hi Lawrence,
Yes, it seems to be doing something like that. The centre is grounded, one end to the aerial coil, the other to HT via 100K and 220R grounded. Seems rather strange to me!
I cant check if it is Log or Lin as the connection between wiper and track jumps all over the place. I will try to look inside when I remove it.


Hello Sideband,
I am in progress with drawing the circuit at the moment, so far I have done the HT supply and audio output.
The valve line-up: Vi 6K7G, V2 6SJ7, V3 349A (6V6G), V4 6X5G.
Almost no alignment available on this one, just 2 capacitors, 1 to each section of the tuning capacitor.


Hello Herald 1360,
Just to remove the redundant Off position, when the switch is removed it may be possible to alter the stop position for the rotor. Nothing decided for certain as yet.

Regards,
Dave.
davidgem1406 is offline  
Old 5th Jul 2019, 11:03 am   #6
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Unknown Home Built TRF radio

Quote:
Originally Posted by davidgem1406 View Post
Hi Lawrence,
Yes, it seems to be doing something like that. The centre is grounded, one end to the aerial coil, the other to HT via 100K and 220R grounded. Seems rather strange to me!
It might be similar to the arrangement in this:

https://vintage-radio.net/forum/atta...0&d=1144184530

R1 being the 100k and R2 being the 220 Ohm?

Lawrence.
ms660 is offline  
Old 5th Jul 2019, 11:38 am   #7
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Correction to my earlier posting #5. The 220R I said was going to ground, does not, It goes to the cathode of V1


Lawrence, that really looks very much like what I have drawn so far, especially with the correction I just made, that part is identical.
Nice one, thanks.

Regards,
Dave
davidgem1406 is offline  
Old 5th Jul 2019, 1:48 pm   #8
Hartley118
Nonode
 
Hartley118's Avatar
 
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
Default Re: Unknown Home Built TRF radio

Quote:
Originally Posted by ms660 View Post
It might be similar to the arrangement in this:

https://vintage-radio.net/forum/atta...0&d=1144184530

R1 being the 100k and R2 being the 220 Ohm?
In my experience, this TRF circuit works a whole lot better than it deserves to. In the example which I own, the volume pot controls some positive feedback around the first stage. That increases the sensitivity in much the same way that reaction works in more conventional TRF circuits.

However, I can’t for the life of me see how that volume pot with its associated resistors and decoupling capacitors can actually introduce positive feedback in that first stage. Maybe it merely acts as a combined gain controller/aerial attenuator and relies on stray coupling for the positive feedback.

Martin
__________________
BVWS Member
Hartley118 is online now  
Old 5th Jul 2019, 5:10 pm   #9
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
Default Re: Unknown Home Built TRF radio

Yes it does usually depend on stray feedback [often the grid- and anode-coils of the RF stage are unscreened....] to provide a whiff of regeneration. In some examples you may find a 'gimmick' capacitor - made from a couple of short lengths of insulated wire twisted together - between the anode of the RF stage and the grid-coil.

The wiring of the mains-switch: remember that back when these radios were designed/built it was most likely they'd be run from either a bayonet-adapter into a light socket, or from something like a 5A 2-pin socket. Which, being reversible, meant that whichever side the on/off switch was fitted there was a 50% chance of it being in what we'd now consider the 'wrong' wire.
G6Tanuki is offline  
Old 5th Jul 2019, 6:06 pm   #10
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 901
Default Re: Unknown Home Built TRF radio

I've a Hallicrafters S72 with a BFO (likely for CW originally, but works for SSB). There is no product detector or mixer at the final IF. It's purely stray pickup between the oscillator and the IF!

EIGHT battery valves!

If that TRF uses less than 20W, you could run it off a shaver outlet mounted in a surface mount cooker switch case as a case. Cheapest type of isolation transformer for 115V / 230V. I usually fit a 1nF 2K XY rated ceramic cap between the secondary centre tap and the mains earth. I put an extra dummy hole for the 5A plug "earth" pin as I have put 5A three pin plugs on all the "live chassis" or dodgy looking metal panel battery/mains valve sets to avoid them being plugged into regular mains. They sell new 5A 3pin plugs but with part insulated L & N pins. Apparently still legal on new builds in Ireland for lighting. The German Shuko sockets still exist here too.
I adapt the shutter / plate of the shaver transformers so the twin Blade USA plugs can only fit in the 115V socket. Only some of the USA sets will run off < 20W due to 50mA series heater off dropper. The S72 takes a staggering 100mA as it has two series chains of 50mA battery valves. A couple have 115V filament rectifiers on mains.
Mike. Watterson is offline  
Old 11th Jul 2019, 10:41 pm   #11
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

I have now done the drawings for the circuit, chassis underside and topside.
I now have to complete them and add all the small components and wiring.

The circuit turned out to be identical to the Premier AC TRF that Lawrence supplied the link to.

Also I have to decide what can be done about fitting a double pole switch for the AC on/off.

The chassis is now going to be stripped right down.

With the aerial and oscillator coils on my unit, both have a length of insulated wire soldered to one of the terminals, then wound as a single turn between 2 of the coils. I guess that is for some coupling?
When I remove them from the chassis I will put a picture of one here.

Regards,
Dave.
davidgem1406 is offline  
Old 11th Jul 2019, 10:50 pm   #12
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Hi Mike,
Thanks for the insight into that way of doing things but in reality it is not something I would do. For AC/DC chassis I always fit a DP switch, a new 2 core cable, and a 13A plug correctly connected.

I don't know the overall consumption of this set as I have not made any measurements for that, but the HT alone is 15.9mA. Seems a bit low to me, but the 2 front end valves only have 90V HT.

Dave.
davidgem1406 is offline  
Old 17th Jul 2019, 4:36 pm   #13
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

I have now found that the gain pot is faulty with O/C tracks. This is some special type of pot that uses two wire-wound tracks, and I guess, pretty impossible to replace now.
Im sure a standard pot is not going to cut any ice for this.

I have drawn the equivalent circuit of the pot and attached it here so that it will be easier to understand how it works, easier than a written description anyway.

Any thoughts and ideas welcome.

regards,
Dave.
Attached Files
File Type: pdf Gain Pot Equivalent Circuit.pdf (5.0 KB, 186 views)
davidgem1406 is offline  
Old 17th Jul 2019, 4:59 pm   #14
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default Re: Unknown Home Built TRF radio

Couldn't a "stereo" dual gang carbon pot be wired to do the same thing?

I've only seen linear WW pots, I don't know if other laws would be practical.
Does the section acting as a screen feed dissipate enough to justify being wire-wound?
__________________
--
Graham.
G3ZVT
Graham G3ZVT is offline  
Old 17th Jul 2019, 6:37 pm   #15
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Unknown Home Built TRF radio

Other laws are possible for WW pots, but involve tapered formers and/or differing wire thickness or resistivity along the track. Not cheap! A simple two slope approximation to log law isn't too hard to implelment, though.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 18th Jul 2019, 11:40 am   #16
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Hello Graham,

I had not thought of that, it could be wired to work in the same way. I think tone controls are linear, not certain on that though.
As for the value it seems to be 20K, but does have a military number of 10C/18*8, the stared number could be either an 0, an Q, or another 8.
I found a whole list of these 10C/ numbers a short while ago for mica capacitors but now I cant remember where.

I have no idea as to what the screen current would be, but the total HT current was 56mA.

The only valve using full HT is the output, the 2 front end valves use a 90V HT.


Hi Herald 1360,

I don't know if it is a log pot that we need, as it is for front end gain, I guess it would be linear.

Dave.
davidgem1406 is offline  
Old 18th Jul 2019, 11:46 am   #17
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Unknown Home Built TRF radio

Are you sure the pot connects directly to the screen grid?

Lawrence.
ms660 is offline  
Old 18th Jul 2019, 9:07 pm   #18
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Hi Lawrence,

Yes, I have the circuit for the Premier AC TRF, that's identical and the pot is connected to G2 by a 100K resistor.

I have added the circuit for the front end taken from my drawing.

Dave.
Attached Files
File Type: pdf Front end.pdf (9.1 KB, 146 views)
davidgem1406 is offline  
Old 18th Jul 2019, 9:12 pm   #19
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Unknown Home Built TRF radio

I did think it was the same as the schematic I linked to, but the reason I asked was that your pot drawing in Post#13 seems to suggest different, so just trying to clarify.

Lawrence.
ms660 is offline  
Old 19th Jul 2019, 3:18 pm   #20
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Unknown Home Built TRF radio

Hi Lawrence,

Yes, I see what you mean, I did not show the resistor.
The drawing was more to show how the pot functioned rather than the external connections.

Dave.
davidgem1406 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:45 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.