Marconi TF1246 - UK Vintage Radio Repair and Restoration Discussion Forum

ed731pdh · 26th Feb 2011, 5:20 pm

Part 2 of what to do on a rainy saturday afternoon.

A Marconi TF1246 RF signal generator, 5 valves, rugged case, big friendly dials and switches. Sort of operational when obtained except the A frequency setting was giving no output. Add to difficulties a major strip down due to a screening can the size of a baking tray and there was my wet play session booked in.

The case cleaned up really well, a few nicks and bangs here and there but nothing more than what I'd expect for a device made in 1959.

Getting the lid off, four valves, passives and a transformer sit on a rear chassis inside the lid whilst the actual oscillator sits within the screening can secured by 8 screws. The chassis cleaned up nicely, no major issues and all passives in good order, this military spec kit is built to last when compared to the commercial gear. Getting the can off, switching is done by having the coils for each range mounted on a indented twelve position carousel switch, the whole lot rotates through 360 degrees on a central shaft. Contact from the carousel to the oscillator valve is done via silver plated contacts, quite hefty ones on first impression. Removal of the carousel and cleaning of each of the silver contacts was first order followed by a full internal checkover which revealed nothing of obvious fault. Reassembly and testing using a scope found A bank still dead even though the silver contacts were operating correctly. A full inspection of the coils on A bank found a hairline break in a coil to trimmer connection, re-soldering restored full operation.

Output quality is excellent with no distortion of the signal. I have a second one of these units which is totally dead and an associated Q meter to restore at a later date.

chroma04 · 26th Feb 2011, 7:57 pm

Certainly looks to be a sturdy piece of kit. It should last for another fifty-odd years.

Very nicely done.

Regards,

Adrian.

glowinganode · 26th Feb 2011, 9:46 pm

Quote:

Originally Posted by ed731pdh

I have a second one of these units which is totally dead and an associated Q meter to restore at a later date.

Well done on the repair, I assume it's for use with the Q meter as it doesn't appear to have an output attenuator.
Is it a TF1245? I have one of these that I'd like to get going, do you have any service data? I did try tracing out the deceptively simple circuit, but couldn't make out if some of the components are caps or crystal rectifiers.
I'm also after a TF 1246 to go with it, so I'd be glad if you'd consider me if you decide to part with your spare one.
Rob.

ed731pdh · 27th Feb 2011, 12:03 pm

Strangely enough, the Q meter is a TF1245 model, not in too bad a shape but needs some TLC. The attached osc, I suspect, needs new valves but is in very good condition otherwise. I managed to obtain a service manual for it off the net, no problems.

One thing to watch with the TF1246s, as mentioned the coil wire on one bank had disconnected itself from a solder tag. What Marconi had done was sleeved the wire with a black PVC type material. The sleeve goes brittle, snaps, and takes the internal wire with it. Easy fix, slide off the sleeve on the affected wire and resolder without.

MichaelR · 27th Feb 2011, 12:42 pm

Paul,

The Qmeter works with the TF1246 as its freq source. There is also a TF1427 . I have a comlete sytem here that I use for Inductor Q measurements. If you look at the legend on the RF output on the front panel it specifies it as an output connection for the TF1245 Q meter.

These are first class Q meters

Mike

glowinganode · 27th Feb 2011, 11:38 pm

Thankyou gentlemen, sorry to hijack your thread.
I'd really like to get my TF1245 Q-meter working, either with the matching generator or a standard bench sig-gen. Do you have any service datafor the TF1245?
Roughly what is the output amplitude of the TF1246 generator?
Many thanks, Rob.

MichaelR · 28th Feb 2011, 2:41 pm

Rob and Paul,

I have a set of working instructions not service data, the marconi test forum maybe able to help you there.

It mentions that for using any low frequency sig gen you need 22V 0.8W signal and you have to use the supplied cable for the RF connection. Cases must be tied together to stop the opportunity of a tuned circuit developing between the 2 units.

My unit has tie bars to the Q meter and whatever source I am using ( 2146 or 2147)

It looks not straightfoward to use another sig gen as a source for the purposes of getting reliable and accurate Q measurements.

Mike

ed731pdh · 28th Feb 2011, 10:05 pm

Copied a couple of key pages form the TF1246 part of the manual.

ed731pdh · 28th Feb 2011, 10:13 pm

Glowinganode, last odd pages to get you on your way.

ed731pdh · 28th Feb 2011, 10:14 pm

I would also like to mention that I tried to upload the manual but the file limit kicked in.

Oh well, nice try while it lasted.

glowinganode · 28th Feb 2011, 10:15 pm

Thanks both, that's a great help. At least I have a ball park figure to play with.
Rob.

glowinganode · 18th Oct 2011, 9:32 pm

Paul has very kindly passed on his spare unit to me.
This is the totally dead one Paul mentions in his opening post.
I checked the power supply first, replacing the moulded and electrolytic caps.
All the resistors were within 20%, the original fuses were fitted and no signs of distress.
Applying power the HT soon rose and stabilised just over 300V, the trimmer adjusts the output from around 250 - 330V, and all seems in order.
Any idea what the nominal HT voltage should be?
Unfortunately the 5763 in the RF stage is down to air, so this is as far as I have got.
I may use the one from the power supply (strange choice?), and modify the p/s to use a 6BW6 instead, of which I have plenty.
Thanks again Paul.
Regards,
Rob.

glowinganode · 19th Oct 2011, 12:09 pm

I tested the good 5763 valve, it came up around 75%, so decided it was worth a try.
The modification to the power supply was easily done, and see no reason why it can't be left.
The oscillator works well on most ranges, giving around 40V pk-pk emf.
It seems some of the lower frequency ranges (wound with finer wire) have suffered the same fate as Pauls, ie. corrosion caused by the plastic sleeving.
This is a fault that owners of TF2002's know only too well.
I'll have to get my glasses on and the incy-wincy soldering iron out for this, wish me luck...
Rob.

ed731pdh · 19th Oct 2011, 7:26 pm

For cleaning the rotary connection pins I used Goddards Silver Dip and Q tips. Bit of elbow grease needed but worth the effort for long term reliability.

glowinganode · 21st Oct 2011, 4:26 pm

Well I've had a go, I managed to get a couple of ranges working. One range is still low on output, which I can only put down to a short circuit turn.
No joy on the lowest frequency range, which is wound with very fine wire, it must be o/c somewhere.
It's wave wound with silk covered enamelled copper wire, so it's bound to be a tricky job.
Really all the coil packs on the turret could do with reworking, the pastic sleeving replacing with silicone rubber.
Looking at the circuit diagram a bit more closely, it seems the regulated supply is only feeding the screen grid of the oscillator valve, via a 50k pot. Lord knows why they used a 5763 as a pass valve, when the load can only be a few mA's. Maybe it was a common unit, from a different piece of kit.
Rob.

ed731pdh · 21st Oct 2011, 6:55 pm

The outputs on mine tend to be low of certain settings. I suspect that this is nothing more than a design issue rather than anything else. With the o/c coil, have a look where the flylead enters the main winding, it has a habit of going there. I unwound a couple of turns to get a decent connection with no detrimental effect to the output.

26th Feb 2011, 5:20 pm	#1
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Marconi TF1246 Part 2 of what to do on a rainy saturday afternoon. A Marconi TF1246 RF signal generator, 5 valves, rugged case, big friendly dials and switches. Sort of operational when obtained except the A frequency setting was giving no output. Add to difficulties a major strip down due to a screening can the size of a baking tray and there was my wet play session booked in. The case cleaned up really well, a few nicks and bangs here and there but nothing more than what I'd expect for a device made in 1959. Getting the lid off, four valves, passives and a transformer sit on a rear chassis inside the lid whilst the actual oscillator sits within the screening can secured by 8 screws. The chassis cleaned up nicely, no major issues and all passives in good order, this military spec kit is built to last when compared to the commercial gear. Getting the can off, switching is done by having the coils for each range mounted on a indented twelve position carousel switch, the whole lot rotates through 360 degrees on a central shaft. Contact from the carousel to the oscillator valve is done via silver plated contacts, quite hefty ones on first impression. Removal of the carousel and cleaning of each of the silver contacts was first order followed by a full internal checkover which revealed nothing of obvious fault. Reassembly and testing using a scope found A bank still dead even though the silver contacts were operating correctly. A full inspection of the coils on A bank found a hairline break in a coil to trimmer connection, re-soldering restored full operation. Output quality is excellent with no distortion of the signal. I have a second one of these units which is totally dead and an associated Q meter to restore at a later date. Attached Thumbnails

26th Feb 2011, 7:57 pm	#2
chroma04 Pentode Join Date: Jul 2008 Location: Doncaster, South Yorkshire, UK. Posts: 116	Re: Marconi TF1246 Certainly looks to be a sturdy piece of kit. It should last for another fifty-odd years. Very nicely done. Regards, Adrian. __________________ Chroma 04

27th Feb 2011, 12:03 pm	#4
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 Strangely enough, the Q meter is a TF1245 model, not in too bad a shape but needs some TLC. The attached osc, I suspect, needs new valves but is in very good condition otherwise. I managed to obtain a service manual for it off the net, no problems. One thing to watch with the TF1246s, as mentioned the coil wire on one bank had disconnected itself from a solder tag. What Marconi had done was sleeved the wire with a black PVC type material. The sleeve goes brittle, snaps, and takes the internal wire with it. Easy fix, slide off the sleeve on the affected wire and resolder without. Attached Thumbnails

27th Feb 2011, 12:42 pm	#5
MichaelR Retired Dormant Member Join Date: Mar 2006 Location: Sheffield, South Yorkshire, UK. Posts: 1,587	Re: Marconi TF1246 Paul, The Qmeter works with the TF1246 as its freq source. There is also a TF1427 . I have a comlete sytem here that I use for Inductor Q measurements. If you look at the legend on the RF output on the front panel it specifies it as an output connection for the TF1245 Q meter. These are first class Q meters Mike

27th Feb 2011, 11:38 pm	#6
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Marconi TF1246 Thankyou gentlemen, sorry to hijack your thread. I'd really like to get my TF1245 Q-meter working, either with the matching generator or a standard bench sig-gen. Do you have any service datafor the TF1245? Roughly what is the output amplitude of the TF1246 generator? Many thanks, Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

28th Feb 2011, 2:41 pm	#7
MichaelR Retired Dormant Member Join Date: Mar 2006 Location: Sheffield, South Yorkshire, UK. Posts: 1,587	Re: Marconi TF1246 Rob and Paul, I have a set of working instructions not service data, the marconi test forum maybe able to help you there. It mentions that for using any low frequency sig gen you need 22V 0.8W signal and you have to use the supplied cable for the RF connection. Cases must be tied together to stop the opportunity of a tuned circuit developing between the 2 units. My unit has tie bars to the Q meter and whatever source I am using ( 2146 or 2147) It looks not straightfoward to use another sig gen as a source for the purposes of getting reliable and accurate Q measurements. Mike

28th Feb 2011, 10:05 pm	#8
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 Copied a couple of key pages form the TF1246 part of the manual. Attached Thumbnails

28th Feb 2011, 10:13 pm	#9
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 Glowinganode, last odd pages to get you on your way. Attached Thumbnails

28th Feb 2011, 10:14 pm	#10
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 I would also like to mention that I tried to upload the manual but the file limit kicked in. Oh well, nice try while it lasted.

28th Feb 2011, 10:15 pm	#11
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Marconi TF1246 Thanks both, that's a great help. At least I have a ball park figure to play with. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

18th Oct 2011, 9:32 pm	#12
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Marconi TF1246 Paul has very kindly passed on his spare unit to me. This is the totally dead one Paul mentions in his opening post. I checked the power supply first, replacing the moulded and electrolytic caps. All the resistors were within 20%, the original fuses were fitted and no signs of distress. Applying power the HT soon rose and stabilised just over 300V, the trimmer adjusts the output from around 250 - 330V, and all seems in order. Any idea what the nominal HT voltage should be? Unfortunately the 5763 in the RF stage is down to air, so this is as far as I have got. I may use the one from the power supply (strange choice?), and modify the p/s to use a 6BW6 instead, of which I have plenty. Thanks again Paul. Regards, Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

19th Oct 2011, 12:09 pm	#13
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Marconi TF1246 I tested the good 5763 valve, it came up around 75%, so decided it was worth a try. The modification to the power supply was easily done, and see no reason why it can't be left. The oscillator works well on most ranges, giving around 40V pk-pk emf. It seems some of the lower frequency ranges (wound with finer wire) have suffered the same fate as Pauls, ie. corrosion caused by the plastic sleeving. This is a fault that owners of TF2002's know only too well. I'll have to get my glasses on and the incy-wincy soldering iron out for this, wish me luck... Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing. Last edited by glowinganode; 19th Oct 2011 at 12:10 pm. Reason: grammar

19th Oct 2011, 7:26 pm	#14
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 For cleaning the rotary connection pins I used Goddards Silver Dip and Q tips. Bit of elbow grease needed but worth the effort for long term reliability.

21st Oct 2011, 4:26 pm	#15
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Marconi TF1246 Well I've had a go, I managed to get a couple of ranges working. One range is still low on output, which I can only put down to a short circuit turn. No joy on the lowest frequency range, which is wound with very fine wire, it must be o/c somewhere. It's wave wound with silk covered enamelled copper wire, so it's bound to be a tricky job. Really all the coil packs on the turret could do with reworking, the pastic sleeving replacing with silicone rubber. Looking at the circuit diagram a bit more closely, it seems the regulated supply is only feeding the screen grid of the oscillator valve, via a 50k pot. Lord knows why they used a 5763 as a pass valve, when the load can only be a few mA's. Maybe it was a common unit, from a different piece of kit. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

21st Oct 2011, 6:55 pm	#16
ed731pdh Retired Dormant Member Join Date: Jan 2011 Location: Pulborough, West Sussex, UK. Posts: 114	Re: Marconi TF1246 The outputs on mine tend to be low of certain settings. I suspect that this is nothing more than a design issue rather than anything else. With the o/c coil, have a look where the flylead enters the main winding, it has a habit of going there. I unwound a couple of turns to get a decent connection with no detrimental effect to the output.