Best germanium diode for FM detector - UK Vintage Radio Repair and Restoration Discussion Forum

PaulR · 6th Sep 2011, 8:09 pm

Can anyone advise me of the best germanium diode to replace those in the FM detector of an otherwise valve set? There are several listed on ebay with IN34A seeming the most popular.

Thanks

Paul

neon indicator · 6th Sep 2011, 10:47 pm

I've used OA47 and recently got a lot of 1N60 which are similar.

At a glance the 1N34 looks slightly "better" than a 1N60, But I have not looked at RF characteristics.

It starts with 1 (one) not I (eye)

I can't see any reason why a 1N34 would not be fine. Or 1N60.
The OA47 is very low PIV, only 10V
1N34 is 60V
1N60 is 50V

The datasheet for 1N34 says for TV-IF (that's 36MHz to 45MHz), FM Radio and AM RF use. So it will work.

There may be a better one. But I don't know of it. It should be good enough.

John_BS · 7th Sep 2011, 9:50 am

Paul: Are you replacing valve diodes, or germanium? In either case, can you give the types?

John

neon indicator · 7th Sep 2011, 11:54 am

Replacing valve circuit detector diodes can be a bit more complex than replacing germanium. The 1N34 will replace OA90 and probably OA91 (though it is 110V PIC approx).

PaulR · 9th Sep 2011, 2:19 pm

Thanks all. Sorry not to have replied earlier, but I have been away.

It is to replace the germanium diodes in a Clarke and Smith 88/12. The FM tends to drift a bit and I wondered whether replacing the diodes might help. I don't know which ones are there at present as I cannot find any markings on them.

Paul

kalee20 · 9th Sep 2011, 2:43 pm

If tuning is drifting, then replacing the discriminator diodes won't help. Unless, it has AFC derived from the discriminatior - and even then changing the diodes is only going to make a miniscule difference.

More likely, one of the low-value capacitors in the FM front end has been replaced at some time with one of a different temperature coefficient. The down side is that I'm not familiar with this unit to suggest which one! But I'd forget the diodes, unless somebody else knows something I don't...

neon indicator · 9th Sep 2011, 5:20 pm

Tuning drifting is a problem at the Front end. Valve heats up components at local oscillator.

It's unlikely changing diodes will help. Probably swapping some capacitors for modern NPO types at the VHF L.O. is what is needed. Common problem with valve sets. Some don't have AFC or not enough AFC for warmup period.

PaulR · 13th Sep 2011, 1:35 pm

Thanks all. The actual tuning drift isn't too bad after the set has warmed up. I wondered about the diodes as the FM alignment was also out and having adjusted it, it now seems to be going out again. If it had an EABC80 I would be thinking about replacing it!

Paul

ENGLISH VICTOR · 17th Sep 2011, 1:21 pm

What do you mean by F.M. alignment? R.f, I.F. or discriminator alignment.
Victor.

PaulR · 17th Sep 2011, 1:37 pm

I meant discriminator alignment. I got it pretty good when I bought the set but it seems to be drifting off again as sibllants are becoming more intrusive again.

Paul

kalee20 · 17th Sep 2011, 7:08 pm

If sibilants are becoming intrusive, can they be corrected by twiddling the tuning knob again (without losing signal strength)? If so, then it is, as per earlier posts, oscillator drift in the front end. But if not then you could have discriminator alignment drifting off - not likely, but possible (such as a loose core in the discriminator transformer).

PaulR · 17th Sep 2011, 7:23 pm

There are two issues.

Firstly the tuning drifts a little as the set warms up - this happens with other valve FM sets I have with no AFC and I can live with it.

Secondly sibilants were intrusive when I first tried the set and I adjusted the discriminator and managed to reduce this. I haven't got a sweep generator but a successful method was suggested to me. I now feet that it drifting again and I wondered whether the germanium diodes were to blame, maybe going out of balance somehow. It isn't very bad but I thought it might be worth trying replacements to see whether they help. The core seemed OK when I adjusted it and I doubt that it will have moved again.

Thanks

Paul

neon indicator · 18th Sep 2011, 1:19 am

Very unlikely to be the diodes.

Sideband · 18th Sep 2011, 8:58 am

OA91's were, I think, used for discriminators. It's probably not too critical but they should be fairly well balanced. You should also check the de-emphasis components. If there is a problem with those, there could be excessive sibilance. What set are you working on?

SB

Mike Phelan · 18th Sep 2011, 9:37 am

Hairdryer and freezer needed, Paul?

ENGLISH VICTOR · 18th Sep 2011, 3:20 pm

Excessive sibilants are not usualy the sign of the discriminator being out of alignment. Misalignment is more likely to manifest itself as distortion.
As Sideband has suggested take a careful look at the pre-emphasis network; there are two components usually a resistor and a capacitor. It would need the resistor to go low in value or the capacitor to go low. A leaky capacitor can produce insufficient de-emphasis but will also produce low level audio. In valve tuners the values are frequently 47K and 1000pF for 50uS for European and British standards and 1600pf and 47K for the American 75uS standard.
I have known the 1000pF to go very low in the past when ceramicons were employed.
If the forward and reverse resistances of the diodes are within 10% I would forget about them. The hair dryer sounds like a good idea.
Victor.

Amraduk · 18th Sep 2011, 4:07 pm

Hello Victor,

Quote:

Originally Posted by ENGLISH VICTOR

As Sideband has suggested take a careful look at the pre-emphasis network;...

Pre-emphasis takes place at the transmitting end, it is de-emphasis that is carried out in the receiver!

Regards,

Dave.

johnlees · 18th Sep 2011, 9:31 pm

We had one of those Clarke and Smith receivers at the school where I taught. It was forever going off tune and I frequently had to go out in the middle of assembly to retune it. It was a pain in the neck.

DangerMan · 19th Sep 2011, 12:15 am

The Mullard recommended diode (when all types were available) was OA79, but they may be hard to get now.
If the set was better after realignment when first restored, maybe it has continued to dry out and that's why it has drifted?
However, if it drifts off tune every time it warms up, rather than a cumulative deterioration, then that's a different problem, and these aren't likely to fix it... as others have said.

Pete

PaulR · 19th Sep 2011, 3:30 pm

Thanks all

I didn't replace any components when I first got it as all the electrolytics including the one in the discriminator reformed well and the others are Mullard mustard types. It seems that it might be a problem in the de-emphasis circuit and I will have a look there. I am not worried by the slight tuning drift as it isn't too bad.

Paul

6th Sep 2011, 8:09 pm	#1
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Best germanium diode for FM detector Can anyone advise me of the best germanium diode to replace those in the FM detector of an otherwise valve set? There are several listed on ebay with IN34A seeming the most popular. Thanks Paul

6th Sep 2011, 10:47 pm	#2
neon indicator Retired Dormant Member Join Date: Jun 2010 Location: Co. Limerick, Ireland. Posts: 1,183	Re: Best germanium diode for FM detector I've used OA47 and recently got a lot of 1N60 which are similar. At a glance the 1N34 looks slightly "better" than a 1N60, But I have not looked at RF characteristics. It starts with 1 (one) not I (eye) I can't see any reason why a 1N34 would not be fine. Or 1N60. The OA47 is very low PIV, only 10V 1N34 is 60V 1N60 is 50V The datasheet for 1N34 says for TV-IF (that's 36MHz to 45MHz), FM Radio and AM RF use. So it will work. There may be a better one. But I don't know of it. It should be good enough.

7th Sep 2011, 9:50 am	#3
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,779	Re: Best germanium diode for FM detector Paul: Are you replacing valve diodes, or germanium? In either case, can you give the types? John

7th Sep 2011, 11:54 am	#4
neon indicator Retired Dormant Member Join Date: Jun 2010 Location: Co. Limerick, Ireland. Posts: 1,183	Re: Best germanium diode for FM detector Replacing valve circuit detector diodes can be a bit more complex than replacing germanium. The 1N34 will replace OA90 and probably OA91 (though it is 110V PIC approx).

9th Sep 2011, 2:19 pm	#5
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Best germanium diode for FM detector Thanks all. Sorry not to have replied earlier, but I have been away. It is to replace the germanium diodes in a Clarke and Smith 88/12. The FM tends to drift a bit and I wondered whether replacing the diodes might help. I don't know which ones are there at present as I cannot find any markings on them. Paul

9th Sep 2011, 2:43 pm	#6
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,077	Re: Best germanium diode for FM detector If tuning is drifting, then replacing the discriminator diodes won't help. Unless, it has AFC derived from the discriminatior - and even then changing the diodes is only going to make a miniscule difference. More likely, one of the low-value capacitors in the FM front end has been replaced at some time with one of a different temperature coefficient. The down side is that I'm not familiar with this unit to suggest which one! But I'd forget the diodes, unless somebody else knows something I don't...

9th Sep 2011, 5:20 pm	#7
neon indicator Retired Dormant Member Join Date: Jun 2010 Location: Co. Limerick, Ireland. Posts: 1,183	Re: Best germanium diode for FM detector Tuning drifting is a problem at the Front end. Valve heats up components at local oscillator. It's unlikely changing diodes will help. Probably swapping some capacitors for modern NPO types at the VHF L.O. is what is needed. Common problem with valve sets. Some don't have AFC or not enough AFC for warmup period.

13th Sep 2011, 1:35 pm	#8
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Best germanium diode for FM detector Thanks all. The actual tuning drift isn't too bad after the set has warmed up. I wondered about the diodes as the FM alignment was also out and having adjusted it, it now seems to be going out again. If it had an EABC80 I would be thinking about replacing it! Paul

17th Sep 2011, 1:21 pm	#9
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Best germanium diode for FM detector What do you mean by F.M. alignment? R.f, I.F. or discriminator alignment. Victor.

17th Sep 2011, 1:37 pm	#10
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Best germanium diode for FM detector I meant discriminator alignment. I got it pretty good when I bought the set but it seems to be drifting off again as sibllants are becoming more intrusive again. Paul

17th Sep 2011, 7:08 pm	#11
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,077	Re: Best germanium diode for FM detector If sibilants are becoming intrusive, can they be corrected by twiddling the tuning knob again (without losing signal strength)? If so, then it is, as per earlier posts, oscillator drift in the front end. But if not then you could have discriminator alignment drifting off - not likely, but possible (such as a loose core in the discriminator transformer).

17th Sep 2011, 7:23 pm	#12
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Best germanium diode for FM detector There are two issues. Firstly the tuning drifts a little as the set warms up - this happens with other valve FM sets I have with no AFC and I can live with it. Secondly sibilants were intrusive when I first tried the set and I adjusted the discriminator and managed to reduce this. I haven't got a sweep generator but a successful method was suggested to me. I now feet that it drifting again and I wondered whether the germanium diodes were to blame, maybe going out of balance somehow. It isn't very bad but I thought it might be worth trying replacements to see whether they help. The core seemed OK when I adjusted it and I doubt that it will have moved again. Thanks Paul

18th Sep 2011, 1:19 am	#13
neon indicator Retired Dormant Member Join Date: Jun 2010 Location: Co. Limerick, Ireland. Posts: 1,183	Re: Best germanium diode for FM detector Very unlikely to be the diodes.

18th Sep 2011, 8:58 am	#14
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: Best germanium diode for FM detector OA91's were, I think, used for discriminators. It's probably not too critical but they should be fairly well balanced. You should also check the de-emphasis components. If there is a problem with those, there could be excessive sibilance. What set are you working on? SB __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

18th Sep 2011, 9:37 am	#15
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Best germanium diode for FM detector Hairdryer and freezer needed, Paul? __________________ Mike.

18th Sep 2011, 3:20 pm	#16
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Best germanium diode for FM detector Excessive sibilants are not usualy the sign of the discriminator being out of alignment. Misalignment is more likely to manifest itself as distortion. As Sideband has suggested take a careful look at the pre-emphasis network; there are two components usually a resistor and a capacitor. It would need the resistor to go low in value or the capacitor to go low. A leaky capacitor can produce insufficient de-emphasis but will also produce low level audio. In valve tuners the values are frequently 47K and 1000pF for 50uS for European and British standards and 1600pf and 47K for the American 75uS standard. I have known the 1000pF to go very low in the past when ceramicons were employed. If the forward and reverse resistances of the diodes are within 10% I would forget about them. The hair dryer sounds like a good idea. Victor.

18th Sep 2011, 9:31 pm	#18
johnlees Hexode Join Date: Oct 2004 Location: Cheltenham, Gloucestershire, UK. Posts: 283	Re: Best germanium diode for FM detector We had one of those Clarke and Smith receivers at the school where I taught. It was forever going off tune and I frequently had to go out in the middle of assembly to retune it. It was a pain in the neck. __________________ Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)

19th Sep 2011, 12:15 am	#19
DangerMan Heptode Join Date: Jul 2009 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 719	Re: Best germanium diode for FM detector The Mullard recommended diode (when all types were available) was OA79, but they may be hard to get now. If the set was better after realignment when first restored, maybe it has continued to dry out and that's why it has drifted? However, if it drifts off tune every time it warms up, rather than a cumulative deterioration, then that's a different problem, and these aren't likely to fix it... as others have said. Pete

19th Sep 2011, 3:30 pm	#20
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Best germanium diode for FM detector Thanks all I didn't replace any components when I first got it as all the electrolytics including the one in the discriminator reformed well and the others are Mullard mustard types. It seems that it might be a problem in the de-emphasis circuit and I will have a look there. I am not worried by the slight tuning drift as it isn't too bad. Paul