Barlow Wadley IR 800C Receiver - UK Vintage Radio Repair and Restoration Discussion Forum

robkay15 · 8th Jan 2012, 11:36 am

Hi I'm new around here and was wondering if someone could cast their eye over this schematic.
The AM on the radio works great but when switched to SSB the gain drops and I can't detect any oscillation from the BFO TR18. The voltages all seem correct on the BFO circuit when it is switched in, limited a bit at the moment as just moved house so bench and test gear are not yet fitted up.
Replaced C79 which was o/c with a 3.3uF, switch seems to be all working. Can't see the wood for the trees

Any thoughts appreciated.
Regards
Rob

robkay15 · 8th Jan 2012, 12:11 pm

This is the XCR 30 MK 11.....same schematic...

Rob

frsimen · 8th Jan 2012, 5:24 pm

Hello Rob,

I thought it looked like an XCR30, but the model number on the circuit threw me!

From your description, it's quite likely just the BFO not oscillating. There won't be much output from the product detector without the carrier from the BFO. As the voltages are correct, the resistors probably aren't at fault, so there's not too much else to consider.

Check that the collector of VT18 is at close to 0v. If it isn't, check the continuity of the BFO tuned circuit, T4. Assuming no problems there, something loading the secondary of T4 could stop the oscillator. A faulty product detector diode D9 or D10 would be a good candidate, as would a short in C68.

If still no joy, one of the capacitors in the area could be faulty - C91, C94 or C95. I imagine that C91 and C95 are fairly critical value wise, but you could bridge C94 with something around the same value as a test.

Once you've checked those, you are only left with Q18 itself or T4 in the BFO circuit.

I'm afraid this looks to be a list of all the BFO components, but at least it's a fairly simple circuit.

Paula

robkay15 · 8th Jan 2012, 9:16 pm

Hi Paula....
Thanks for your thoughts, spent a couple of hours wading through this today. I've changed 5 or 6 resistors around the if circuit that had drifted up in value, then removed most of the components from the bfo and checked or changed them, scoped the output still nothing.....the only items in the bfo that have not actually been changed are the transistor(checks o.k. out of circuit with a fluke m/meter) and t4. Dug the scope and balanced it on a box just to make sure there was no output, perhaps its worth getting a few spare transistors and substituting, t4 seems to read o.k. its not o/c but i'm wondering if it has some corrosion, shorted turns or the tuning capacitor inside the can could be faulty

Its a Toko coil, just wondering if it would be possible to get a replacement, once i've broken it(when i try this next week

) to get it open. Did lift one of the diodes to see if anything was loading the oscillator, but appears not, but will have another check.
Regards
Rob

frsimen · 9th Jan 2012, 8:06 am

Hello Rob,

That doesn't sound very encouraging. I would suggest you exhaust all other possibilities before doing anything to T4, as it will be very difficult to find a replacement for that. You could be right about the corrosion, though. One or two corroded strands can stop an oscillator, as I discovered earlier this year.

If you have a signal generator, you could try injecting a signal into the base of TR18, sweeping it around 455kHz to confirm that T4 is tuned to the correct frequency and that TR18 is amplifying. That won't help much with broken strands, but it may give some clues.

Paula

robkay15 · 15th Jan 2012, 12:22 pm

Come to the conclusion it must be the coil, haven't taken it out of the board yet....not sure if i'll be able to get one of these......is it possible to rewind them?
Thanks
Rob

BobGreen · 24th Jan 2012, 12:00 am

I suppose this RX was the iconic use of the fabled 'Wadley Loop' tuning arrangement as adopted in the Racal RA17 but Racal may have had an interest in Barlow Communications at that time. It was in apartheid times and all a bit shady. Most famous of the Wadley Loop incarnations was the 'Frog-7"; a very good 'grey box' by Yaesu (FRG-7) from the land of the copyists and improvers.

Bob

JohnBradford · 17th Oct 2012, 6:19 pm

Hi Rob,

I expect you will have sorted this by now, but I had one with the same fault; the problem was that the BFO coil can wasn't grounded due to a dry joint - not the first time I've found this on a BW, the standard of soldering is sometimes not great.

cheers, John.

robkay15 · 1st Nov 2012, 7:03 pm

John just noticed your reply, I shelved the set for a while as other things came along, I will dig it out again and do some more checks around the BFO coil.....
Many thanks for your thoughts.
Rob

8th Jan 2012, 12:11 pm	#2
robkay15 Retired Dormant Member Join Date: Jan 2012 Location: Stoke on Trent Posts: 10	Re: Barlow Wadley IR 800C Receiver This is the XCR 30 MK 11.....same schematic... Rob

8th Jan 2012, 5:24 pm	#3
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Barlow Wadley IR 800C Receiver Hello Rob, I thought it looked like an XCR30, but the model number on the circuit threw me! From your description, it's quite likely just the BFO not oscillating. There won't be much output from the product detector without the carrier from the BFO. As the voltages are correct, the resistors probably aren't at fault, so there's not too much else to consider. Check that the collector of VT18 is at close to 0v. If it isn't, check the continuity of the BFO tuned circuit, T4. Assuming no problems there, something loading the secondary of T4 could stop the oscillator. A faulty product detector diode D9 or D10 would be a good candidate, as would a short in C68. If still no joy, one of the capacitors in the area could be faulty - C91, C94 or C95. I imagine that C91 and C95 are fairly critical value wise, but you could bridge C94 with something around the same value as a test. Once you've checked those, you are only left with Q18 itself or T4 in the BFO circuit. I'm afraid this looks to be a list of all the BFO components, but at least it's a fairly simple circuit. Paula

8th Jan 2012, 9:16 pm	#4
robkay15 Retired Dormant Member Join Date: Jan 2012 Location: Stoke on Trent Posts: 10	Re: Barlow Wadley IR 800C Receiver Hi Paula.... Thanks for your thoughts, spent a couple of hours wading through this today. I've changed 5 or 6 resistors around the if circuit that had drifted up in value, then removed most of the components from the bfo and checked or changed them, scoped the output still nothing.....the only items in the bfo that have not actually been changed are the transistor(checks o.k. out of circuit with a fluke m/meter) and t4. Dug the scope and balanced it on a box just to make sure there was no output, perhaps its worth getting a few spare transistors and substituting, t4 seems to read o.k. its not o/c but i'm wondering if it has some corrosion, shorted turns or the tuning capacitor inside the can could be faulty Its a Toko coil, just wondering if it would be possible to get a replacement, once i've broken it(when i try this next week) to get it open. Did lift one of the diodes to see if anything was loading the oscillator, but appears not, but will have another check. Regards Rob

9th Jan 2012, 8:06 am	#5
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Barlow Wadley IR 800C Receiver Hello Rob, That doesn't sound very encouraging. I would suggest you exhaust all other possibilities before doing anything to T4, as it will be very difficult to find a replacement for that. You could be right about the corrosion, though. One or two corroded strands can stop an oscillator, as I discovered earlier this year. If you have a signal generator, you could try injecting a signal into the base of TR18, sweeping it around 455kHz to confirm that T4 is tuned to the correct frequency and that TR18 is amplifying. That won't help much with broken strands, but it may give some clues. Paula

15th Jan 2012, 12:22 pm	#6
robkay15 Retired Dormant Member Join Date: Jan 2012 Location: Stoke on Trent Posts: 10	Re: Barlow Wadley IR 800C Receiver Come to the conclusion it must be the coil, haven't taken it out of the board yet....not sure if i'll be able to get one of these......is it possible to rewind them? Thanks Rob

24th Jan 2012, 12:00 am	#7
BobGreen Retired Dormant Member Join Date: Jan 2011 Location: Egham, Surrey, UK. Posts: 219	Re: Barlow Wadley IR 800C Receiver I suppose this RX was the iconic use of the fabled 'Wadley Loop' tuning arrangement as adopted in the Racal RA17 but Racal may have had an interest in Barlow Communications at that time. It was in apartheid times and all a bit shady. Most famous of the Wadley Loop incarnations was the 'Frog-7"; a very good 'grey box' by Yaesu (FRG-7) from the land of the copyists and improvers. Bob

17th Oct 2012, 6:19 pm	#8
JohnBradford Retired Dormant Member Join Date: Oct 2012 Location: Oxford, Oxon, UK. Posts: 9	Re: Barlow Wadley IR 800C Receiver Hi Rob, I expect you will have sorted this by now, but I had one with the same fault; the problem was that the BFO coil can wasn't grounded due to a dry joint - not the first time I've found this on a BW, the standard of soldering is sometimes not great. cheers, John.

1st Nov 2012, 7:03 pm	#9
robkay15 Retired Dormant Member Join Date: Jan 2012 Location: Stoke on Trent Posts: 10	Re: Barlow Wadley IR 800C Receiver John just noticed your reply, I shelved the set for a while as other things came along, I will dig it out again and do some more checks around the BFO coil..... Many thanks for your thoughts. Rob