BC348 - some repair advice - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 17th Aug 2006, 9:56 pm

I am working on a BC348 I have had for many years and have built an integrated PSU and done the heater rewire. First thing I found was one of the paper capacitors was a dead short and it evidently isn't the only one as it draws 40mA with no valves.

Anyone had any experience with these receivers? Is it worth finding the faulty ones or are the paper caps so unreliable I would be better replacing the lot? Not an easy job on this set....Peter

jim_beacon · 17th Aug 2006, 10:53 pm

Peter,

generally the caps are reliable. Remember that, as with the R1155, chassis is NOT ht -VE on these sets, and beware of the crystal filter - they are susceptable to damp, and can become very lossy, otherwise they are an excellent set.

My experiences are here:
http://www.g1jbg.co.uk/mil3.htm
and here:
http://www.g1jbg.co.uk/mil3rep.htm
one day, I'll get round to finishing it! I do have a few spares if you're stuck.

Jim.

PJL · 17th Aug 2006, 11:04 pm

Found 2 more that are dead shorts. These are the 18 x 0.01uF/500V paper block capacitors ...guess what I'll be doing for the next few evenings.

PJL · 17th Aug 2006, 11:27 pm

Hi Jim, mine was also modified but for an external PSU which I don't have. The heater rewire was pretty poor so I have redone it and built an internal PSU. The dynamotor was very inefficient so dissipation should be OK.

The 911 regulator was missing and from the spec. is overrun so I have put in zeners. These capacitors are a nuisance but as yours is working I will try just replacing the failed ones.

Paintwork is poor and the front panel and name plate are missing along with most of the front panel screws which appear to be an odd US size. Inside is clean and otherwise unmodified. I plan to rewire the dial lamp to work from 6.3V but it won't be as bright as original.

Aub · 18th Aug 2006, 8:35 am

Hi

I had one of these sets for a couple of years in the seventies. At least, i think it was the 348, but I believe that there was a 342 and I was never sure which model mine was.

Mine has a separate power supply, which I assumed was standard - I mean it looked the same as the set and had a proper plug and socket arrangement.

As I recall, it tuned 1.5 to 18Mhz and I used it mainly as a tuneable IF for a 2 metre converter back in my G8 days. ( 2 to 4 Mhz IF ). Altogether it was a good set and I never had any electrical problems with it, although if I cranked the tuning knob round fast it would make a horrible noise, as if the mechanism needed greasing or something.

Hope you get it working well.

Regards

Aub

JHGibson · 18th Aug 2006, 4:48 pm

I rebuilt my BC-348 recently and found that all the paper caps had a few megohms leakage. There are only a few places where this leakage becomes critical, such as the coupling cap to the control grid of the output tube, and the decoupling caps on the AGC line. However once I had the chassis upside down on the bench and replaced those caps, I could not stop myself and replaced all the rest. Now I don't have to worry about future cap failures.

On 28 volts, the dial lamp circuit consists of two #44 lamps (6.3 V @ 0.25 A) in series, also in series with a 68 ohm fixed resistor and the 200 ohm dimmer pot. For 6.3 V I rewired the lamp sockets in parallel and subsituted # 47 lamps (6.3 V @ 0.15 A) to avoid overloading the dimmer control, and shorted out the 68 ohm resistor.

The 915 kc crystal is quite lossy - expect at least 6 db loss in the crystal position. It is also important to align the IFs at the crystal's frequency which might not be quite 915 kc. If the crystal is seriously lossy, it can often be reactivated by dismantling the holder and washing the blank with hot water and detergent and then rinsing it well.

The 991 neon regulates (poorly) at about 60 V.
John.

PJL · 1st Sep 2006, 5:01 pm

Mine has flat black Micamold capacitors for the x18 paper capacitors. Of these I have found 4 that are either a dead short or hopelessly leaky. The remaining ones are just very leaky but as they are only used in decoupling positions I wonder if they were always rubbish.

After two trips to Maplin with the correct part numbers, first returning with 10Amp shottky diodes and then a 6V and not 33V zener, I am going to remember my glasses next time.

PJL · 1st Sep 2006, 8:38 pm

A bit unstable at high gain, whistling on AVC and needs alot of switch cleaner too...but it all works!

FrankB · 11th Sep 2006, 11:45 pm

I have a number of the BC348/BC312 series receivers.
Just shotgun all the caps including any mica types.
Otherwise it seems one after the other will fail.

radiodu24 · 22nd Feb 2008, 1:17 pm

hello from france

the voltage regulator RCA991 is missing on my BC348 and I do not find a piece of spare .
can I replace it with a zener and how?
.
merci

Sean Williams · 22nd Feb 2008, 5:24 pm

I MIGHT have a neon stabiliser here that will do the job

A Zener would work, but to be completely honest the set isnt that stable with the neon stabiliser

Will come back to you later

Bon Chance!

Sean

Sean Williams · 22nd Feb 2008, 6:08 pm

Yes, I have found Neons type NE48 - these are for the BC348.

Send me a PM

Cheers
Sean

Oldtimer74 · 14th Apr 2008, 3:17 pm

I had a BC348 as my first receiver when I got licenced in 1952. I did a complete rebuild and incorporated some modifications - in those days we were more interested in getting performance rather than keeping things original. If you want to improve the performance and also incorporate the 21 Mc/s amateur band I can give you the references to the articles which appeared in the RSGB Bulletin and SWM in the 1950s.
Pat

XTC · 14th Apr 2008, 10:39 pm

I have a BC348R. Some of the Micamold caps had been replaced, a couple of others had failed S/C taking other parts with them. The best course was to replace the lot, I found a handful of flat green ITT 400V caps with long leads which don't look too bad. I'd go along with Frank B's advice, apart from the micas.

The metal canned caps add a lot to the character of the set. The ones in this set were just hopeless and some had already been replaced, so I took them out and replaced them with new caps on a tagboard. The old ones are set aside in a box against the day when I find another BC348 to restore properly.

The pot at the end of the tuning shaft was missing, but the bracket was still there. It was a 3.5K? pot. I replaced that with a 4.7 with a 10K across it, I think. I checked out various possibilities of pot and resistors and I think that was it.

The wiring is cotton covered and seemed good so I left it alone.

When the set was up and aligned, it birdied like crazy. After some experimenting, it turned out to be down to the wiring to the mains switch on the front panel - no I didn't fit it, it was inherited. A length of braided sleeving and a screen for the mains switch sorted that out.

The 991 neon works intermittently but the set is amazingly good on SSB.

I recall the power supply and biasing of the O/P valves are a bit strange, in that the resistance of the choke is used as the bias resistor. I think there was already a separate bias resistor installed and I replaced it and the cap and and took the choke out of circuit - the already installed choke input PSU was more than adequate. I left the O/P transformer alone and use the set with an external solid state amp replacement for headphones.

An impressive set.

Pete.

FrankB · 18th Apr 2008, 12:31 am

I would still suggest replacing the Mica Mold caps.

They are starting to leak/short out in most of the sets I see here.

At my job, we now replace them as a matter of course to prevent callbacks.

In the one set I rebuilt that I didn't replace them all in, came back with several bad after a few days of solid use, even after a week of in-shop burn in.

17th Aug 2006, 9:56 pm	#1
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	BC348 - some repair advice I am working on a BC348 I have had for many years and have built an integrated PSU and done the heater rewire. First thing I found was one of the paper capacitors was a dead short and it evidently isn't the only one as it draws 40mA with no valves. Anyone had any experience with these receivers? Is it worth finding the faulty ones or are the paper caps so unreliable I would be better replacing the lot? Not an easy job on this set....Peter

17th Aug 2006, 10:53 pm	#2
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: BC348 - some repair advice Peter, generally the caps are reliable. Remember that, as with the R1155, chassis is NOT ht -VE on these sets, and beware of the crystal filter - they are susceptable to damp, and can become very lossy, otherwise they are an excellent set. My experiences are here: http://www.g1jbg.co.uk/mil3.htm and here: http://www.g1jbg.co.uk/mil3rep.htm one day, I'll get round to finishing it! I do have a few spares if you're stuck. Jim.

17th Aug 2006, 11:04 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BC348 - some repair advice Found 2 more that are dead shorts. These are the 18 x 0.01uF/500V paper block capacitors ...guess what I'll be doing for the next few evenings.

18th Aug 2006, 8:35 am	#5
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: BC348 - some repair advice Hi I had one of these sets for a couple of years in the seventies. At least, i think it was the 348, but I believe that there was a 342 and I was never sure which model mine was. Mine has a separate power supply, which I assumed was standard - I mean it looked the same as the set and had a proper plug and socket arrangement. As I recall, it tuned 1.5 to 18Mhz and I used it mainly as a tuneable IF for a 2 metre converter back in my G8 days. ( 2 to 4 Mhz IF ). Altogether it was a good set and I never had any electrical problems with it, although if I cranked the tuning knob round fast it would make a horrible noise, as if the mechanism needed greasing or something. Hope you get it working well. Regards Aub

18th Aug 2006, 4:48 pm	#6
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: BC348 - some repair advice I rebuilt my BC-348 recently and found that all the paper caps had a few megohms leakage. There are only a few places where this leakage becomes critical, such as the coupling cap to the control grid of the output tube, and the decoupling caps on the AGC line. However once I had the chassis upside down on the bench and replaced those caps, I could not stop myself and replaced all the rest. Now I don't have to worry about future cap failures. On 28 volts, the dial lamp circuit consists of two #44 lamps (6.3 V @ 0.25 A) in series, also in series with a 68 ohm fixed resistor and the 200 ohm dimmer pot. For 6.3 V I rewired the lamp sockets in parallel and subsituted # 47 lamps (6.3 V @ 0.15 A) to avoid overloading the dimmer control, and shorted out the 68 ohm resistor. The 915 kc crystal is quite lossy - expect at least 6 db loss in the crystal position. It is also important to align the IFs at the crystal's frequency which might not be quite 915 kc. If the crystal is seriously lossy, it can often be reactivated by dismantling the holder and washing the blank with hot water and detergent and then rinsing it well. The 991 neon regulates (poorly) at about 60 V. John. Last edited by JHGibson; 18th Aug 2006 at 4:52 pm. Reason: spelling!

17th Aug 2006, 11:27 pm	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BC348 - some repair advice Hi Jim, mine was also modified but for an external PSU which I don't have. The heater rewire was pretty poor so I have redone it and built an internal PSU. The dynamotor was very inefficient so dissipation should be OK. The 911 regulator was missing and from the spec. is overrun so I have put in zeners. These capacitors are a nuisance but as yours is working I will try just replacing the failed ones. Paintwork is poor and the front panel and name plate are missing along with most of the front panel screws which appear to be an odd US size. Inside is clean and otherwise unmodified. I plan to rewire the dial lamp to work from 6.3V but it won't be as bright as original.

1st Sep 2006, 5:01 pm	#7
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BC348 - some repair advice Mine has flat black Micamold capacitors for the x18 paper capacitors. Of these I have found 4 that are either a dead short or hopelessly leaky. The remaining ones are just very leaky but as they are only used in decoupling positions I wonder if they were always rubbish. After two trips to Maplin with the correct part numbers, first returning with 10Amp shottky diodes and then a 6V and not 33V zener, I am going to remember my glasses next time.

1st Sep 2006, 8:38 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BC348 - some repair advice A bit unstable at high gain, whistling on AVC and needs alot of switch cleaner too...but it all works!

11th Sep 2006, 11:45 pm	#9
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: BC348 - some repair advice I have a number of the BC348/BC312 series receivers. Just shotgun all the caps including any mica types. Otherwise it seems one after the other will fail.

22nd Feb 2008, 1:17 pm	#10
radiodu24 Retired Dormant Member Join Date: Jul 2007 Location: FRANCE near Bordeaux Posts: 2	Re: BC348 - some repair advice hello from france the voltage regulator RCA991 is missing on my BC348 and I do not find a piece of spare . can I replace it with a zener and how? . merci

22nd Feb 2008, 5:24 pm	#11
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: BC348 - some repair advice I MIGHT have a neon stabiliser here that will do the job A Zener would work, but to be completely honest the set isnt that stable with the neon stabiliser Will come back to you later Bon Chance! Sean __________________ Engineers make things work and have spare bits when finished

22nd Feb 2008, 6:08 pm	#12
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: BC348 - some repair advice Yes, I have found Neons type NE48 - these are for the BC348. Send me a PM Cheers Sean __________________ Engineers make things work and have spare bits when finished

14th Apr 2008, 3:17 pm	#13
Oldtimer74 Rest in Peace Join Date: Apr 2008 Location: Worcester, Worcestershire, UK. Posts: 330	Re: BC348 - some repair advice I had a BC348 as my first receiver when I got licenced in 1952. I did a complete rebuild and incorporated some modifications - in those days we were more interested in getting performance rather than keeping things original. If you want to improve the performance and also incorporate the 21 Mc/s amateur band I can give you the references to the articles which appeared in the RSGB Bulletin and SWM in the 1950s. Pat

14th Apr 2008, 10:39 pm	#14
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: BC348 - some repair advice I have a BC348R. Some of the Micamold caps had been replaced, a couple of others had failed S/C taking other parts with them. The best course was to replace the lot, I found a handful of flat green ITT 400V caps with long leads which don't look too bad. I'd go along with Frank B's advice, apart from the micas. The metal canned caps add a lot to the character of the set. The ones in this set were just hopeless and some had already been replaced, so I took them out and replaced them with new caps on a tagboard. The old ones are set aside in a box against the day when I find another BC348 to restore properly. The pot at the end of the tuning shaft was missing, but the bracket was still there. It was a 3.5K? pot. I replaced that with a 4.7 with a 10K across it, I think. I checked out various possibilities of pot and resistors and I think that was it. The wiring is cotton covered and seemed good so I left it alone. When the set was up and aligned, it birdied like crazy. After some experimenting, it turned out to be down to the wiring to the mains switch on the front panel - no I didn't fit it, it was inherited. A length of braided sleeving and a screen for the mains switch sorted that out. The 991 neon works intermittently but the set is amazingly good on SSB. I recall the power supply and biasing of the O/P valves are a bit strange, in that the resistance of the choke is used as the bias resistor. I think there was already a separate bias resistor installed and I replaced it and the cap and and took the choke out of circuit - the already installed choke input PSU was more than adequate. I left the O/P transformer alone and use the set with an external solid state amp replacement for headphones. An impressive set. Pete.

18th Apr 2008, 12:31 am	#15
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: BC348 - some repair advice I would still suggest replacing the Mica Mold caps. They are starting to leak/short out in most of the sets I see here. At my job, we now replace them as a matter of course to prevent callbacks. In the one set I rebuilt that I didn't replace them all in, came back with several bad after a few days of solid use, even after a week of in-shop burn in.