Baffling BFO - UK Vintage Radio Repair and Restoration Discussion Forum

cendoubleu · 14th Jan 2009, 12:17 am

This evening I decided to tackle the inop BFO on my AR88. I checked the plate voltage and OK at about 60v. Looking at the circuit I see there are several capacitors (C82 - C88) and resistors (R24,28 and 29). Out of these, the only one I can find is C84 (part of one of the bathtubs) and another capacitor coming from the BFO coil, which has no markings on it. As for the rest and the resistors, either I'm going blind (possible!), they are well hidden or this receiver has had some serious mods done to it. (It is actually an R1156A, the RAF version of the '88). Could anyone possibly point me in the right direction?

Charlie

Station X · 14th Jan 2009, 9:07 am

I think some of the components are inside the BFO can. The BFO injection level of an AR88 is very low. Designed for CW, not SSB,

The differences in the RAF version are minor. Toggle off/on switch, additional headphone jack and a muting facility wired to the back panel.

G4XWDJim · 14th Jan 2009, 10:12 am

It's a long time since I measured the BFO volts in an AR88 but I have a feeling that 60v is a bit high. I recall I think about 40 ish when it's oscillating. Have you tried measuring the anode pin volts with the valve pulled out just in case it's not taking current and the measured volts are being set by leaky capacitors.

Jim

Skywave · 14th Jan 2009, 11:01 am

Before going down the dismantling / inspection route, it will be worthwhile checking that the BFO is on the right frequency. It might have been mal-adjusted and off freq.

Al / Skywave

XTC · 14th Jan 2009, 11:49 am

How about the valve? 6J5s seem to go on forever but this one might be down or have corroded/dirty pins depending on how the set has been kept. If the set works on AM, the local oscillator valve should be OK in the BFO so try a swap before taking things to bits.

Pete.

G4XWDJim · 14th Jan 2009, 12:31 pm

You can check if the 6J5 is oscillating by shorting the grid to deck whilst measuring the anode volts which should then drop a little.

Jim

cendoubleu · 14th Jan 2009, 2:22 pm

Thanks for all the suggestions folks. I shall do the LO/BFO valve swop first to see if that makes a difference. I know there is a small mod to make the BFO more effective by increasing the coupling, but this just doesn't work. As far as the 60v on the anode, I assumed that this was because I was measuring with a DMM. All the voltages on the other valves I have measured (AF,PA etc) have all been about 20v higher than stated in the manual. I shall look at finding a couple of replacement 6J5's if tests show it may be going soft.
Thanks once again for all the help and interest.

cendoubleu · 15th Jan 2009, 1:15 am

Found the cause of the problem. The 2nd IF valve (6SG7) was in place of the BFO valve and vis-versa. Clearly 6SG7's don't make good oscillators!. Once sorted, the sensitivity issues I had notcied were improved and the BFO is now working...well good enough for CW but very hard to tune SSB. I will try the mod which improves the coupling to see if this improves things. So now I have an RX which works well. I shall continue to change out the bathtub caps as time permits and will at some time remove the front panel and re-paint etc but for now I have a working RX for which I am very happy.
Thanks once again for the advice and help, I couldn't have got this far without it.

Charlie

XTC · 15th Jan 2009, 12:17 pm

Charlie,

Good news.

Possibly you feel a bit daft - looking at circuit diagrams and all the rest and then finding the valves were swapped about.

Look on the bright side.....

You could have spotted the valves were swapped after you'd dismantled the BFO can, or worse, started messing with the IF alignment, then you'd have felt much more daft, and angry with yourself. And even more daft and furious if you'd broken something in the process.

Especially with complicated things like AR88s, it's very easy to fix on the idea that a problem must have an involved solution, and overlook the bleedin' obvious.

I remember seeing a cartoon years ago in which half a dozen boffins in heavy framed specs and lab coats had taken a huge computer to bits. One of them was pointing to a plug lying on the floor by a socket.

"I think I've found the problem chaps".

We all do it.

Pete.

cendoubleu · 15th Jan 2009, 1:15 pm

yes I did feel a trifle daft Pete but at least as you say it wasn't after doing major dismantling. I don't know at which point they were swapped but I have a feeling that it was probably me about 18 months ago when I first started looking at the set. Mind you, if I ever get round to homebrewing a valve receiver I shall remember that 6J5's make good oscillators and the other type doesn't! although that probably depends on the cct topology.
Hopefully this weekend I shall be able to sit down and have an extended listening test and check out how good it is. Just a shame we're at a low in the sunspot-cycle.

Skywave · 15th Jan 2009, 1:38 pm

Quote:

Originally Posted by cendoubleu

. . I shall remember that 6J5's make good oscillators and the other type doesn't! Although that probably depends on the cct topology. . .

Your second sentence is correct: either can be configured as an oscillator. It's all a question of circuit design.

Al / Skywave.

14th Jan 2009, 12:17 am	#1
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Baffling BFO This evening I decided to tackle the inop BFO on my AR88. I checked the plate voltage and OK at about 60v. Looking at the circuit I see there are several capacitors (C82 - C88) and resistors (R24,28 and 29). Out of these, the only one I can find is C84 (part of one of the bathtubs) and another capacitor coming from the BFO coil, which has no markings on it. As for the rest and the resistors, either I'm going blind (possible!), they are well hidden or this receiver has had some serious mods done to it. (It is actually an R1156A, the RAF version of the '88). Could anyone possibly point me in the right direction? Charlie

14th Jan 2009, 9:07 am	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Baffling BFO I think some of the components are inside the BFO can. The BFO injection level of an AR88 is very low. Designed for CW, not SSB, The differences in the RAF version are minor. Toggle off/on switch, additional headphone jack and a muting facility wired to the back panel. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

14th Jan 2009, 10:12 am	#3
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: Baffling BFO It's a long time since I measured the BFO volts in an AR88 but I have a feeling that 60v is a bit high. I recall I think about 40 ish when it's oscillating. Have you tried measuring the anode pin volts with the valve pulled out just in case it's not taking current and the measured volts are being set by leaky capacitors. Jim

14th Jan 2009, 11:01 am	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Baffling BFO Before going down the dismantling / inspection route, it will be worthwhile checking that the BFO is on the right frequency. It might have been mal-adjusted and off freq. Al / Skywave

14th Jan 2009, 11:49 am	#5
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: Baffling BFO How about the valve? 6J5s seem to go on forever but this one might be down or have corroded/dirty pins depending on how the set has been kept. If the set works on AM, the local oscillator valve should be OK in the BFO so try a swap before taking things to bits. Pete.

14th Jan 2009, 12:31 pm	#6
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: Baffling BFO You can check if the 6J5 is oscillating by shorting the grid to deck whilst measuring the anode volts which should then drop a little. Jim

14th Jan 2009, 2:22 pm	#7
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: Baffling BFO Thanks for all the suggestions folks. I shall do the LO/BFO valve swop first to see if that makes a difference. I know there is a small mod to make the BFO more effective by increasing the coupling, but this just doesn't work. As far as the 60v on the anode, I assumed that this was because I was measuring with a DMM. All the voltages on the other valves I have measured (AF,PA etc) have all been about 20v higher than stated in the manual. I shall look at finding a couple of replacement 6J5's if tests show it may be going soft. Thanks once again for all the help and interest.

15th Jan 2009, 1:15 am	#8
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: Baffling BFO Found the cause of the problem. The 2nd IF valve (6SG7) was in place of the BFO valve and vis-versa. Clearly 6SG7's don't make good oscillators!. Once sorted, the sensitivity issues I had notcied were improved and the BFO is now working...well good enough for CW but very hard to tune SSB. I will try the mod which improves the coupling to see if this improves things. So now I have an RX which works well. I shall continue to change out the bathtub caps as time permits and will at some time remove the front panel and re-paint etc but for now I have a working RX for which I am very happy. Thanks once again for the advice and help, I couldn't have got this far without it. Charlie

15th Jan 2009, 12:17 pm	#9
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: Baffling BFO Charlie, Good news. Possibly you feel a bit daft - looking at circuit diagrams and all the rest and then finding the valves were swapped about. Look on the bright side..... You could have spotted the valves were swapped after you'd dismantled the BFO can, or worse, started messing with the IF alignment, then you'd have felt much more daft, and angry with yourself. And even more daft and furious if you'd broken something in the process. Especially with complicated things like AR88s, it's very easy to fix on the idea that a problem must have an involved solution, and overlook the bleedin' obvious. I remember seeing a cartoon years ago in which half a dozen boffins in heavy framed specs and lab coats had taken a huge computer to bits. One of them was pointing to a plug lying on the floor by a socket. "I think I've found the problem chaps". We all do it. Pete.

15th Jan 2009, 1:15 pm	#10
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: Baffling BFO yes I did feel a trifle daft Pete but at least as you say it wasn't after doing major dismantling. I don't know at which point they were swapped but I have a feeling that it was probably me about 18 months ago when I first started looking at the set. Mind you, if I ever get round to homebrewing a valve receiver I shall remember that 6J5's make good oscillators and the other type doesn't! although that probably depends on the cct topology. Hopefully this weekend I shall be able to sit down and have an extended listening test and check out how good it is. Just a shame we're at a low in the sunspot-cycle.