Baird / Radio Rentals M232 - FM terrible! - UK Vintage Radio Repair and Restoration Discussion Forum

Nickthedentist · 28th Dec 2007, 11:35 pm

Hello everyone,

You may remember I rescued this set from being dumped a few weeks ago, see here: https://www.vintage-radio.net/forum/...ad.php?t=20730

To recap, it's a Radio Rentals set, equivalent to the Baird M232 which appears on Paul's second CD of circuit diagrams. It's a basic but solidly-built AC/DC model with LW, MW and VHF.

I've just re-capped it (all waxies and electrolytics), and it works really well on MW and LW. Loud, sensitive and generally pleasant to listen to.

However, on switching to FM, things are very different

. When a station is tuned in, there's a very loud 50/100Hz "squarewave" hum which completely obliterates the programme. De-tuning the set slightly makes things better, but the hum is still at least as loud as the programme. There is complete silence between stations.

What's going on here? The FM front end uses a UF80 followed by a UCC85. Wiggling/tapping the valves makes no difference to the symptoms. Sadly, I haven't got spare valves to substitute.

Looking forward to your bright ideas,
Nick.

Sideband · 28th Dec 2007, 11:55 pm

Hmmm...! This seems to be some form of modulation hum. It could be caused by a heater/cathode leak/short in one of the front-end valves. I've not got the circuit of this set. You mention a UF80 and UCC85 in the tuner? So the UF80 is the RF amp so how do they use the UCC85?

Probably one of these causing the problem. If there are any heater decouplers, it will be worth checking them as well.

Can't help with bottles I'm afraid. Plenty of the 'E' series but none of the 'U' types!

Rich.

Nickthedentist · 29th Dec 2007, 1:19 am

Thanks, Richard, I've ordered a UF80 and a UCC85. Will let you all know the outcome.

Nick.

Mike Phelan · 29th Dec 2007, 9:48 am

Nick
If it is a H-K leak, can you prove it by disconnecting the heater line at the "hot" end with the set running? Should last long enough to determine if the hum goes before the signal does.
Does sound like a H-K leak in one of the front-end bottles or an earth off somewhere.

Nickthedentist · 29th Dec 2007, 4:29 pm

Quote:

Originally Posted by Mike Phelan

If it is a H-K leak, can you prove it by disconnecting the heater line at the "hot" end with the set running?

Thanks Mike. I tried disconnecting the heaters as suggested... the hum lessened (but did not go) leaving a very "thin"-sounding programme. Not sure what to think, so I'll wait until the two new valves arrive.

Cheers,
Nick.

ronbryan · 29th Dec 2007, 7:46 pm

Nick

I had this "hum on FM only" problem on a Philetta I repaired for Howard. It was a faulty UABC80 - remember on this valve the FM discriminator diodes are separate from the AM detector diode.

Ron

Nickthedentist · 29th Dec 2007, 7:56 pm

Thanks Ron, that gives me hope that a new valve will do the trick.

Although a very plain (ugly?) radio, it gives a very good account of itself on AM (the ferrite rod must help) and I'm gradually warming to it. Getting FM working too will no doubt help me like it even more

.

Nick.

Sideband · 29th Dec 2007, 8:11 pm

Another good point Ron. I've had a few hum problems with these valves but also affecting AM.

Nick. Besides the items already mentioned check also the capacitor across the rectifier.
Rich.

Steve_P · 29th Dec 2007, 9:23 pm

What I'd do is clean the wavechange switch first, then switch to FM, disconnect the FM feed from the UCH81 and set a signal generator to 10.7 Mhz. If there's no 50Hz mains hum then, then the IF's are probably OK. If there is, change the UABC80 and look at the FM discriminator caps. And of course the caps on the reccy.

If it is not there, and the valves don't cure it, check the small caps on the heater chain.

Cheers,

Steve P.

Mike Phelan · 30th Dec 2007, 9:48 am

Post #5 does suggest that there is an H-K leak or lack of heater decoupling, but the hum still remaining sans heater supply is looking more like the UABC80 or something around the ratio detector; especially as the hum increases when it is tuned in.
We'll watch!

Nickthedentist · 4th Jan 2008, 6:46 pm

Quote:

Originally Posted by Mike Phelan

If it is a H-K leak, can you prove it by disconnecting the heater line at the "hot" end with the set running?

Quote:

Originally Posted by Nickthedentist

I tried disconnecting the heaters as suggested... the hum lessened (but did not go) leaving a very "thin"-sounding programme. Not sure what to think, so I'll wait until the two new valves arrive.

I suspect the radio was not quite properly tuned-in. Accurate tuning was impossible with the very loud hum that was originally present. Tuning on FM seems really quite critical on this model.

Quote:

Originally Posted by Richard_Newman

It could be caused by a heater/cathode leak/short in one of the front-end valves.

Thanks to all of you who suggested this, and especially to Richard, who took the time to write me a detailed circuit description of the FM front end (apparently it's quite an unusual circuit configuaration).

>>> A new UCC85 (from Colomor - NOT cheap, but very efficient and friendly) has restored hum-free FM reception<<<

I will hang onto the old one and do some resistance and Megger checks in it if I get time.

Nick.

Nickthedentist · 17th Jan 2008, 2:31 pm

Hello everyone,

This set has now been in daily use for a week or so, mainly on FM. Sound quality is extremely good and it's been completely reliable.

However, tuning on FM is quite "critical", with only the slightest nudge to the tuning knob making all the difference between perfect reception, a lot of sibilance and distortion, or complete interstation silence.

This is in complete contrast to MW and LW, where half a turn either way makes little difference.

But the biggest gripe istuning drift on FM. Five minutes after switching on, the programme is lost completely, and slight re-tuning (towards the 88 MHz end of the scale) is required to bring it back. The same thing happens a second and third time. After about 20 minutes, it's spot-on, and will play away indefinitely without intervention.

Is this normal for one of these sets? If it is, it's a shame, as it does seriously reduce its usefulness for occasional listening.

Nick.

paulsherwin · 17th Jan 2008, 2:40 pm

I don't know this set, but FM tuning is often quite critical with valve FM sets, and tuning drift during warmup is quite common. *All* my valve FM sets drift a little, though not as badly as you describe.

Another UCC85 may help, but then again it might not

Paul

howard · 17th Jan 2008, 5:10 pm

Hello Nick,

I have 8 (plus 2 more in theatre) 1950/1960s valve VHF sets, Bush VHF61 & VHF62, PYE Continental VHF2D, Philips Philettas BD244U & B2D04U, Philips B3G63A, Philips Reverbeo B7X14A and a Philips B5X04A, and like Paul's sets one or two tend to drift on VHF very slightly in the first few minutes of warm up but nowhere near as bad as your Baird so I fear it has a fault.

Howard

Nickthedentist · 17th Jan 2008, 5:17 pm

Do yours have AFC, Howard?

howard · 17th Jan 2008, 5:34 pm

Hello Nick,

None of them as far as I am aware have AFC and none of them drift right off station like your Baird. On switch on I notice that the station tuned in from the previous session might be a touch out but once tuned in after a couple of minutes usually stays put but might require minor retuning after a few minutes. The later Philips plano sets are better in this respect.

Howard

howard · 17th Jan 2008, 7:03 pm

Hello again Nick,

Just to qualify what I posted earlier, I've just powered up two of my sets and run them for 30 minutes to test for drift.

Firstly my 1954 Philips BD244U AC/DC AM/FM set which like your Baird has U* series valves. On switch on, no drift found on Radio 4 from previous session and no drift whatsoever after 30 minutes either !

Second my Bush VHF80 (I forgot this one and my VHF81 above and also my Stella ST160A, so I've got 13 VHF valve sets) and on switch on it was slightly off tune from the local station on VHF from yesterday so after a couple of minutes I tuned it in properly and after 30 minutes it was still spot on.

There's no doubt Philips sets are best for FM stability during warm up and neither of my Philips plano sets seem to drift either. My Bush sets tend to drift slightly but never off station.

Howard

Nickthedentist · 17th Jan 2008, 7:09 pm

Thanks for that comprehensive test, Howard

Very interesting, but does make me feel that I've got a fault.

Any other ideas other than a substandard valve, anyone?

Nick.

Steve_P · 17th Jan 2008, 7:17 pm

First, is the IF okay. The UABC80 and the FM discriminator. Caps in the IF cans? Resistors on the Discriminator. Also, in the FM can, components in there?

The IF should be 10.7 Mhz. Check it. It sounds as though it's a bit harder to tune than it should be.

Cheers,

Steve P.

radiogammon · 17th Jan 2008, 7:22 pm

Hello Nick'

I have just checked out my RR 232 on FM and the tuning drifts very slightly during the first few minutes (not enough to go off station) and then settles down, requiring no further adjustment. The tuning on yours seems very sharp - the pointer on mine needs moving several mm to take it off station. Sounds like some adjustment of the FM tuning coils is required, but I am no expert on this and perhaps someone with more experience of this would like to comment.

Nice sounding set, though, in spite of the relatively small speaker compared to the large cabinet.

28th Dec 2007, 11:35 pm	#1
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Baird / Radio Rentals M232 - FM terrible! Hello everyone, You may remember I rescued this set from being dumped a few weeks ago, see here: https://www.vintage-radio.net/forum/...ad.php?t=20730 To recap, it's a Radio Rentals set, equivalent to the Baird M232 which appears on Paul's second CD of circuit diagrams. It's a basic but solidly-built AC/DC model with LW, MW and VHF. I've just re-capped it (all waxies and electrolytics), and it works really well on MW and LW. Loud, sensitive and generally pleasant to listen to. However, on switching to FM, things are very different . When a station is tuned in, there's a very loud 50/100Hz "squarewave" hum which completely obliterates the programme. De-tuning the set slightly makes things better, but the hum is still at least as loud as the programme. There is complete silence between stations. What's going on here? The FM front end uses a UF80 followed by a UCC85. Wiggling/tapping the valves makes no difference to the symptoms. Sadly, I haven't got spare valves to substitute. Looking forward to your bright ideas, Nick. Last edited by Darren-UK; 13th Jun 2008 at 12:49 am. Reason: Reset font to default.

28th Dec 2007, 11:55 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Baird / Radio Rentals M232 - FM terrible! Hmmm...! This seems to be some form of modulation hum. It could be caused by a heater/cathode leak/short in one of the front-end valves. I've not got the circuit of this set. You mention a UF80 and UCC85 in the tuner? So the UF80 is the RF amp so how do they use the UCC85? Probably one of these causing the problem. If there are any heater decouplers, it will be worth checking them as well. Can't help with bottles I'm afraid. Plenty of the 'E' series but none of the 'U' types! Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

29th Dec 2007, 1:19 am	#3
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Baird / Radio Rentals M232 - FM terrible! Thanks, Richard, I've ordered a UF80 and a UCC85. Will let you all know the outcome. Nick.

29th Dec 2007, 9:48 am	#4
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Baird / Radio Rentals M232 - FM terrible! Nick If it is a H-K leak, can you prove it by disconnecting the heater line at the "hot" end with the set running? Should last long enough to determine if the hum goes before the signal does. Does sound like a H-K leak in one of the front-end bottles or an earth off somewhere. __________________ Mike. Last edited by Mike Phelan; 30th Dec 2007 at 9:43 am. Reason: Typo

29th Dec 2007, 7:46 pm	#6
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Baird / Radio Rentals M232 - FM terrible! Nick I had this "hum on FM only" problem on a Philetta I repaired for Howard. It was a faulty UABC80 - remember on this valve the FM discriminator diodes are separate from the AM detector diode. Ron

29th Dec 2007, 7:56 pm	#7
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Baird / Radio Rentals M232 - FM terrible! Thanks Ron, that gives me hope that a new valve will do the trick. Although a very plain (ugly?) radio, it gives a very good account of itself on AM (the ferrite rod must help) and I'm gradually warming to it. Getting FM working too will no doubt help me like it even more . Nick.

29th Dec 2007, 8:11 pm	#8
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Baird / Radio Rentals M232 - FM terrible! Another good point Ron. I've had a few hum problems with these valves but also affecting AM. Nick. Besides the items already mentioned check also the capacitor across the rectifier. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 29th Dec 2007 at 8:30 pm. Reason: Added more info.

29th Dec 2007, 9:23 pm	#9
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Baird / Radio Rentals M232 - FM terrible! What I'd do is clean the wavechange switch first, then switch to FM, disconnect the FM feed from the UCH81 and set a signal generator to 10.7 Mhz. If there's no 50Hz mains hum then, then the IF's are probably OK. If there is, change the UABC80 and look at the FM discriminator caps. And of course the caps on the reccy. If it is not there, and the valves don't cure it, check the small caps on the heater chain. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

30th Dec 2007, 9:48 am	#10
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Baird / Radio Rentals M232 - FM terrible! Post #5 does suggest that there is an H-K leak or lack of heater decoupling, but the hum still remaining sans heater supply is looking more like the UABC80 or something around the ratio detector; especially as the hum increases when it is tuned in. We'll watch! __________________ Mike.

17th Jan 2008, 2:31 pm	#12
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Baird / Radio Rentals M232 - FM terrible! Hello everyone, This set has now been in daily use for a week or so, mainly on FM. Sound quality is extremely good and it's been completely reliable. However, tuning on FM is quite "critical", with only the slightest nudge to the tuning knob making all the difference between perfect reception, a lot of sibilance and distortion, or complete interstation silence. This is in complete contrast to MW and LW, where half a turn either way makes little difference. But the biggest gripe istuning drift on FM. Five minutes after switching on, the programme is lost completely, and slight re-tuning (towards the 88 MHz end of the scale) is required to bring it back. The same thing happens a second and third time. After about 20 minutes, it's spot-on, and will play away indefinitely without intervention. Is this normal for one of these sets? If it is, it's a shame, as it does seriously reduce its usefulness for occasional listening. Nick.

17th Jan 2008, 2:40 pm	#13
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Baird / Radio Rentals M232 - FM terrible! I don't know this set, but FM tuning is often quite critical with valve FM sets, and tuning drift during warmup is quite common. All my valve FM sets drift a little, though not as badly as you describe. Another UCC85 may help, but then again it might not Paul

17th Jan 2008, 5:10 pm	#14
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Baird / Radio Rentals M232 - FM terrible! Hello Nick, I have 8 (plus 2 more in theatre) 1950/1960s valve VHF sets, Bush VHF61 & VHF62, PYE Continental VHF2D, Philips Philettas BD244U & B2D04U, Philips B3G63A, Philips Reverbeo B7X14A and a Philips B5X04A, and like Paul's sets one or two tend to drift on VHF very slightly in the first few minutes of warm up but nowhere near as bad as your Baird so I fear it has a fault. Howard Last edited by howard; 17th Jan 2008 at 5:16 pm.

17th Jan 2008, 5:17 pm	#15
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Baird / Radio Rentals M232 - FM terrible! Do yours have AFC, Howard?

17th Jan 2008, 5:34 pm	#16
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Baird / Radio Rentals M232 - FM terrible! Hello Nick, None of them as far as I am aware have AFC and none of them drift right off station like your Baird. On switch on I notice that the station tuned in from the previous session might be a touch out but once tuned in after a couple of minutes usually stays put but might require minor retuning after a few minutes. The later Philips plano sets are better in this respect. Howard

17th Jan 2008, 7:03 pm	#17
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Baird / Radio Rentals M232 - FM terrible! Hello again Nick, Just to qualify what I posted earlier, I've just powered up two of my sets and run them for 30 minutes to test for drift. Firstly my 1954 Philips BD244U AC/DC AM/FM set which like your Baird has U* series valves. On switch on, no drift found on Radio 4 from previous session and no drift whatsoever after 30 minutes either ! Second my Bush VHF80 (I forgot this one and my VHF81 above and also my Stella ST160A, so I've got 13 VHF valve sets) and on switch on it was slightly off tune from the local station on VHF from yesterday so after a couple of minutes I tuned it in properly and after 30 minutes it was still spot on. There's no doubt Philips sets are best for FM stability during warm up and neither of my Philips plano sets seem to drift either. My Bush sets tend to drift slightly but never off station. Howard

17th Jan 2008, 7:09 pm	#18
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,865	Re: Baird / Radio Rentals M232 - FM terrible! Thanks for that comprehensive test, Howard Very interesting, but does make me feel that I've got a fault. Any other ideas other than a substandard valve, anyone? Nick.

17th Jan 2008, 7:17 pm	#19
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Baird / Radio Rentals M232 - FM terrible! First, is the IF okay. The UABC80 and the FM discriminator. Caps in the IF cans? Resistors on the Discriminator. Also, in the FM can, components in there? The IF should be 10.7 Mhz. Check it. It sounds as though it's a bit harder to tune than it should be. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

17th Jan 2008, 7:22 pm	#20
radiogammon Retired Dormant Member Join Date: Dec 2002 Location: County Down, Northern Ireland, UK. Posts: 275	Re: Baird / Radio Rentals M232 - FM terrible! Hello Nick' I have just checked out my RR 232 on FM and the tuning drifts very slightly during the first few minutes (not enough to go off station) and then settles down, requiring no further adjustment. The tuning on yours seems very sharp - the pointer on mine needs moving several mm to take it off station. Sounds like some adjustment of the FM tuning coils is required, but I am no expert on this and perhaps someone with more experience of this would like to comment. Nice sounding set, though, in spite of the relatively small speaker compared to the large cabinet.