Cossor CR4212A - UK Vintage Radio Repair and Restoration Discussion Forum

Pilot Mariner · 8th Sep 2009, 9:38 pm

Hello - I have 2 Cossor CR4212A sets that I am working on in parallel, both exhibit the same problem - when switched to the Bass+ setting, the bass notes tend to distort. This starts to happen from fairly low volume settings and makes the set virtually unlistenable at medium volumes. The usual checks have drawn a blank.
Anyone any ideas - thanks

Darren-UK · 8th Sep 2009, 10:57 pm

Does the problem occur on all stations on all wavebands? Is it worse on ILR MW stations?

I'm wondering if the radios do actually have a fault or if it's down simply to compression. Try the temporary disconnection of any tone corrector caps and experiment by bridging with caps of varying values until you find a happy compromise (which is all it'd be).

Pilot Mariner · 8th Sep 2009, 11:12 pm

Hi Darren - it does seem to be on all wavebands but alot more prominent on VHF.

Darren-UK · 8th Sep 2009, 11:25 pm

Presumably you've checked for dirty switchgear and any nasty old caps that need expulsion from the relevant part of the circuit? Is all in order at the output valve and the power supply?

Mikebay · 9th Sep 2009, 9:32 am

An old favorite for bass distortion in valved sets used to be the cathode bypass resistor in the output stage, this would fail or "lose" capacity, allowing the bias point to change with the power demand of the output valve. A possibility?
Mike

howard · 9th Sep 2009, 1:10 pm

Hello,

I've got one of these in mint condition, and it's a really nice radio. They're clones of the Philips B4X23A and originated in Belgium as the Aristona SA4212.
If you've got spare EL84 and EABC80 valves try swapping them. Change the 1300 volt black pitch capacitor on the output transformer as well.

The two loudspeakers in these small Philips plano sets are small and tend to boom a bit if the volume is turned up, especially on VHF.

Howard

Pilot Mariner · 9th Sep 2009, 1:56 pm

Hi Folks thanks for the replies

Darren - All switchgear is OK, HT approx 250V. I have checked the "That" cap and it is OK being the better quality type, mmm but there is a steady small +ve voltage (approx 150mV) on the EL84 grid. If I disconnect the cap or short the other end to ground, it is still there which further rules the coupling cap out. Could this be something to do with it?
I haven't checked the tone associated components yet, maybe the bass is being boosted too much?

I have tried other EL84s all the same.This is in both sets.

Mike - Cathode bypass resistor is OK, but I have replaced some other out of tolerance resistors - no change.

Howard - Yes my 2 are also in mint condition and I have another one with a scruffy cabinet and some missing parts that I have used for spares.
I have tried other EL84s and EABC80s- no change.
I haven't change the black pitch capacitor yet but it is worth a try.
I have fed the output to a larger speaker with the same results.

Mark

Darren-UK · 9th Sep 2009, 2:26 pm

Yes, try changing the black cap. If, as Howard says, it's on the o/p transformer then it'll likely be a tone corrector. Initially just disconnect it and see what happens; the radios will probably screech at you but the idea is to see if the distortion vanishes.

Opinions will differ but I like to get rid of those black Philips caps almost as a matter of routine.

Pilot Mariner · 9th Sep 2009, 10:53 pm

Hi - I have changed the black pitch capacitors. This has had no effect on the "fault".
Maybe this is how they are supposed to be, but to my ears it sounds terrible.

Steve_P · 9th Sep 2009, 10:58 pm

The Cossor manual for this is available up top there. Worth having.

Anyway, C47, C57, C44. Try a new rectifier valve. I hope we haven't got any shorted turns on the O/P transformer.

Cheers,

Steve P.

Mikebay · 10th Sep 2009, 7:59 am

Mark,

I should have said the cathode bypass capacitor, not resistor. Old age and all that!
Mike

8th Sep 2009, 9:38 pm	#1
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 439	Cossor CR4212A Hello - I have 2 Cossor CR4212A sets that I am working on in parallel, both exhibit the same problem - when switched to the Bass+ setting, the bass notes tend to distort. This starts to happen from fairly low volume settings and makes the set virtually unlistenable at medium volumes. The usual checks have drawn a blank. Anyone any ideas - thanks

8th Sep 2009, 10:57 pm	#2
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Cossor CR4212A Does the problem occur on all stations on all wavebands? Is it worse on ILR MW stations? I'm wondering if the radios do actually have a fault or if it's down simply to compression. Try the temporary disconnection of any tone corrector caps and experiment by bridging with caps of varying values until you find a happy compromise (which is all it'd be).

8th Sep 2009, 11:12 pm	#3
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 439	Re: Cossor CR4212A Hi Darren - it does seem to be on all wavebands but alot more prominent on VHF.

8th Sep 2009, 11:25 pm	#4
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Cossor CR4212A Presumably you've checked for dirty switchgear and any nasty old caps that need expulsion from the relevant part of the circuit? Is all in order at the output valve and the power supply?

9th Sep 2009, 9:32 am	#5
Mikebay Rest in Peace Join Date: Nov 2006 Location: Leominster, Herefordshire Posts: 174	Re: Cossor CR4212A An old favorite for bass distortion in valved sets used to be the cathode bypass resistor in the output stage, this would fail or "lose" capacity, allowing the bias point to change with the power demand of the output valve. A possibility? Mike

9th Sep 2009, 1:10 pm	#6
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Cossor CR4212A Hello, I've got one of these in mint condition, and it's a really nice radio. They're clones of the Philips B4X23A and originated in Belgium as the Aristona SA4212. If you've got spare EL84 and EABC80 valves try swapping them. Change the 1300 volt black pitch capacitor on the output transformer as well. The two loudspeakers in these small Philips plano sets are small and tend to boom a bit if the volume is turned up, especially on VHF. Howard

9th Sep 2009, 1:56 pm	#7
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 439	Re: Cossor CR4212A Hi Folks thanks for the replies Darren - All switchgear is OK, HT approx 250V. I have checked the "That" cap and it is OK being the better quality type, mmm but there is a steady small +ve voltage (approx 150mV) on the EL84 grid. If I disconnect the cap or short the other end to ground, it is still there which further rules the coupling cap out. Could this be something to do with it? I haven't checked the tone associated components yet, maybe the bass is being boosted too much? I have tried other EL84s all the same.This is in both sets. Mike - Cathode bypass resistor is OK, but I have replaced some other out of tolerance resistors - no change. Howard - Yes my 2 are also in mint condition and I have another one with a scruffy cabinet and some missing parts that I have used for spares. I have tried other EL84s and EABC80s- no change. I haven't change the black pitch capacitor yet but it is worth a try. I have fed the output to a larger speaker with the same results. Mark

9th Sep 2009, 2:26 pm	#8
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Cossor CR4212A Yes, try changing the black cap. If, as Howard says, it's on the o/p transformer then it'll likely be a tone corrector. Initially just disconnect it and see what happens; the radios will probably screech at you but the idea is to see if the distortion vanishes. Opinions will differ but I like to get rid of those black Philips caps almost as a matter of routine.

9th Sep 2009, 10:53 pm	#9
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 439	Re: Cossor CR4212A Hi - I have changed the black pitch capacitors. This has had no effect on the "fault". Maybe this is how they are supposed to be, but to my ears it sounds terrible.

9th Sep 2009, 10:58 pm	#10
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Cossor CR4212A The Cossor manual for this is available up top there. Worth having. Anyway, C47, C57, C44. Try a new rectifier valve. I hope we haven't got any shorted turns on the O/P transformer. 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...?

10th Sep 2009, 7:59 am	#11
Mikebay Rest in Peace Join Date: Nov 2006 Location: Leominster, Herefordshire Posts: 174	Re: Cossor CR4212A Mark, I should have said the cathode bypass capacitor, not resistor. Old age and all that! Mike