Cossor Radio 523 low volume help!? - UK Vintage Radio Repair and Restoration Discussion Forum

williamjbh · 26th Nov 2018, 2:25 pm

Hi, new to the forums here and looking for some help please!

I have rescued an Ekco Radiogram (TRG252./I) and discovered for myself the problem with carbon residue in selector switches and that if you use contact cleaner you then have to wash that off with isoprop! I'm really happy to have got it all working again - the radio is superb with a very small and basic dipole aerial and just waiting for some new rubber mounting bushes for the record player motor and a new mains transformer (one side only producing 195Vac the other side of the tap the correct 250Vac!) before it can be completed - I'm now working on a Cossor 523 Melody Master.

This poor old radio had suffered a catastrophic failure having been switched on for the first time for many years by the previous owner without having done any checks (or re-conditioned the main rectifier reservoir capacitors) - The rectifier was overloaded and melted down shorting the anodes and cathodes together and to the 6.3V heater supply. Amazingly the transformer is fine!

I also found that the old laminate valve bases for the rectifier and output BBQ5 were carbonised and arcing - these have been replaced, the rectifier also and all the other valves tested. The series resistor for 100Hz filtering (2K 2W) was obviously damaged, measured at 2.4K and replaced with a 3Wmetal film 2K resistor. The valves were then all removed and the rectifier heater supplied separately so I could then wind up the Mains input to the transformer slowly with a variac to increase the HT on the 32+32uF filter capacitor gradually to recondition it - and it is now fine, with all the valves fitted and 240Vac in, I'm getting 290Vdc with 3.2Vrms ripple on the first 32uF cap and after the 2K resistor 228Vdc with 72mV ripple.

The radio works - on all bands - and on VHF I can receive Radio 2, and 3 and less so Radio 4 - the problem is the output and hence volume is very low, as in I can hear the radio channel I'm tuned to (appears to be clear with no distortion) but the volume control is on maximum - VHF being the worst. Somewhere I'm losing gain, either something is leaking or the signal is being blocked or the gain is being drastically reduced.

I have been steadily testing/replacing the capacitors in the circuit but not yet got to the source of my problem - any clues would now be greatly welcomed!

The best improvement so far was achieved by replacing the 3nF cap across the secondary of the first FM aerial transformer and I have noticed that the signal appears to be stronger with only one side of the dipole connected (the arm pointing North!)

Allegedly the magic eye was working fine when the receiver was first switched on - before it went bang - but now there's not enough signal getting through for the twin diode differentiator to generate enough voltage to operate the magic eye (or presumably the DAGC circuit). I have tested the MR1 'delay diode' and it appeared to be open circuit and replaced it with a 1N4007 diode - this had no noticeable effect (is that a suitable diode to use?)

One thing that did make the volume jump up was when I probed the anode (pin 6) of the heptode (IF amplifier) 6AJ8/ECH81 with a DMM connected to chassis to measure the anode volts (which were a little high but ok). I am truly puzzled!

For now I am continuing to work slowly though the capacitors - testing and replacing as I go but any pointer would be greatly appreciated - I'd dearly love to get this radio working as well as the Ekco receiver!

I look forward to hearing from you!

Will

G8HQP Dave · 26th Nov 2018, 2:36 pm

Measure voltages and compare with the service sheet. Don't replace capacitors unless you have reason to believe that they are faulty or are likely to become faulty. This is especially true in the RF part of the circuit. In the VHF tuner part don't even move capacitors.

williamjbh · 26th Nov 2018, 3:36 pm

Hi Dave; completely agree re caps (especially the wax covered pF range devices in the VHF!) I had to replace the 3nF cap (C2) it was dead - and virtually no signal at all before it was replaced - no idea how it got damaged - especially if the set was supposedly working before). the AM volume AF signals are also low - so do not believe that the problem lies with the receiver parts but the later gain stages?
So far I have only probed out the voltages on V1, V2 and V5 (RF amp 6AQ8/ECC85, IF amp 6AJ8/ECH81 and o/p pentode 6BQ5/EL84) and all looked sensible. The -ve voltage from the differentiator was very small, as I mentioned the signal strength/output is low! -0.5V ish and I would expect more like -10V but I'll do a proper survey against the service sheet to see if that gives me any more clues..

Boater Sam · 26th Nov 2018, 6:12 pm

Establish if the audio stages are working correctly, gram input?

Then if so it has to be the mixer or IF stages. Has the "twiddler" been visiting?

Can you inject a signal and work back from the output stage to the aerial

williamjbh · 28th Nov 2018, 1:39 am

Hi Sam,

Pretty sure the output stages triode + pentode are working got a healthy hum when probing it but I'll certainly check!

I have now measured all the voltages as per the service sheet and all are pretty much where they're supposed to be; the anodes of the FM RF i/p amp (ECC85/6AQ8) were a bit high suggesting the valve might be a little tired, tried a known good valve in its place and the voltages were a lot closer to the mark, and it hasn't made an appreciable difference...

The main clue for me is the lack of -ve dc Volts being generated by the FM differentiator/detector the EABC80/6AK8, there's just not enough signal reaching it.

As for the twiddler having paid a visit

I certainly I haven't and I'm pretty certain no-one else has either, all the adjusters look undisturbed,...

I'll get the sig gen out and test the o/p stags (just to be sure) and then return to tracing through and testing/replacing components, obviously, any more help suggestions would be appreciated; having come so far I don't want to give up on this set!

Kindest regards

Will

Silicon · 28th Nov 2018, 2:59 pm

Have you replaced the 5uF capacitor in the discriminator circuit? Is the positive terminal connected to chassis? It is C41 on the Trader sheet.

There may be a bad connection on the valve socket of V2 (ECH81) or possibly a bad component nearby.
Try tapping the components in that area with a plastic rod.

G8HQP Dave · 28th Nov 2018, 5:54 pm

You might have low IF gain (probably EF85 or could be EF89). A weak valve would affect AM and FM.

PJL · 28th Nov 2018, 9:32 pm

Not sure the 1N4007 is suitable in an AGC circuit as the forward voltage will be higher and the reverse leakage current will be an issue.

26th Nov 2018, 2:25 pm	#1
williamjbh Diode Join Date: Nov 2018 Location: Wimborne, Dorset, UK. Posts: 3	Cossor Radio 523 low volume help!? Hi, new to the forums here and looking for some help please! I have rescued an Ekco Radiogram (TRG252./I) and discovered for myself the problem with carbon residue in selector switches and that if you use contact cleaner you then have to wash that off with isoprop! I'm really happy to have got it all working again - the radio is superb with a very small and basic dipole aerial and just waiting for some new rubber mounting bushes for the record player motor and a new mains transformer (one side only producing 195Vac the other side of the tap the correct 250Vac!) before it can be completed - I'm now working on a Cossor 523 Melody Master. This poor old radio had suffered a catastrophic failure having been switched on for the first time for many years by the previous owner without having done any checks (or re-conditioned the main rectifier reservoir capacitors) - The rectifier was overloaded and melted down shorting the anodes and cathodes together and to the 6.3V heater supply. Amazingly the transformer is fine! I also found that the old laminate valve bases for the rectifier and output BBQ5 were carbonised and arcing - these have been replaced, the rectifier also and all the other valves tested. The series resistor for 100Hz filtering (2K 2W) was obviously damaged, measured at 2.4K and replaced with a 3Wmetal film 2K resistor. The valves were then all removed and the rectifier heater supplied separately so I could then wind up the Mains input to the transformer slowly with a variac to increase the HT on the 32+32uF filter capacitor gradually to recondition it - and it is now fine, with all the valves fitted and 240Vac in, I'm getting 290Vdc with 3.2Vrms ripple on the first 32uF cap and after the 2K resistor 228Vdc with 72mV ripple. The radio works - on all bands - and on VHF I can receive Radio 2, and 3 and less so Radio 4 - the problem is the output and hence volume is very low, as in I can hear the radio channel I'm tuned to (appears to be clear with no distortion) but the volume control is on maximum - VHF being the worst. Somewhere I'm losing gain, either something is leaking or the signal is being blocked or the gain is being drastically reduced. I have been steadily testing/replacing the capacitors in the circuit but not yet got to the source of my problem - any clues would now be greatly welcomed! The best improvement so far was achieved by replacing the 3nF cap across the secondary of the first FM aerial transformer and I have noticed that the signal appears to be stronger with only one side of the dipole connected (the arm pointing North!) Allegedly the magic eye was working fine when the receiver was first switched on - before it went bang - but now there's not enough signal getting through for the twin diode differentiator to generate enough voltage to operate the magic eye (or presumably the DAGC circuit). I have tested the MR1 'delay diode' and it appeared to be open circuit and replaced it with a 1N4007 diode - this had no noticeable effect (is that a suitable diode to use?) One thing that did make the volume jump up was when I probed the anode (pin 6) of the heptode (IF amplifier) 6AJ8/ECH81 with a DMM connected to chassis to measure the anode volts (which were a little high but ok). I am truly puzzled! For now I am continuing to work slowly though the capacitors - testing and replacing as I go but any pointer would be greatly appreciated - I'd dearly love to get this radio working as well as the Ekco receiver! I look forward to hearing from you! Will

26th Nov 2018, 2:36 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Cossor Radio 523 low volume help!? Measure voltages and compare with the service sheet. Don't replace capacitors unless you have reason to believe that they are faulty or are likely to become faulty. This is especially true in the RF part of the circuit. In the VHF tuner part don't even move capacitors.

26th Nov 2018, 3:36 pm	#3
williamjbh Diode Join Date: Nov 2018 Location: Wimborne, Dorset, UK. Posts: 3	Re: Cossor Radio 523 low volume help!? Hi Dave; completely agree re caps (especially the wax covered pF range devices in the VHF!) I had to replace the 3nF cap (C2) it was dead - and virtually no signal at all before it was replaced - no idea how it got damaged - especially if the set was supposedly working before). the AM volume AF signals are also low - so do not believe that the problem lies with the receiver parts but the later gain stages? So far I have only probed out the voltages on V1, V2 and V5 (RF amp 6AQ8/ECC85, IF amp 6AJ8/ECH81 and o/p pentode 6BQ5/EL84) and all looked sensible. The -ve voltage from the differentiator was very small, as I mentioned the signal strength/output is low! -0.5V ish and I would expect more like -10V but I'll do a proper survey against the service sheet to see if that gives me any more clues..

26th Nov 2018, 6:12 pm	#4
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Cossor Radio 523 low volume help!? Establish if the audio stages are working correctly, gram input? Then if so it has to be the mixer or IF stages. Has the "twiddler" been visiting? Can you inject a signal and work back from the output stage to the aerial

28th Nov 2018, 1:39 am	#5
williamjbh Diode Join Date: Nov 2018 Location: Wimborne, Dorset, UK. Posts: 3	Re: Cossor Radio 523 low volume help!? Hi Sam, Pretty sure the output stages triode + pentode are working got a healthy hum when probing it but I'll certainly check! I have now measured all the voltages as per the service sheet and all are pretty much where they're supposed to be; the anodes of the FM RF i/p amp (ECC85/6AQ8) were a bit high suggesting the valve might be a little tired, tried a known good valve in its place and the voltages were a lot closer to the mark, and it hasn't made an appreciable difference... The main clue for me is the lack of -ve dc Volts being generated by the FM differentiator/detector the EABC80/6AK8, there's just not enough signal reaching it. As for the twiddler having paid a visit I certainly I haven't and I'm pretty certain no-one else has either, all the adjusters look undisturbed,... I'll get the sig gen out and test the o/p stags (just to be sure) and then return to tracing through and testing/replacing components, obviously, any more help suggestions would be appreciated; having come so far I don't want to give up on this set! Kindest regards Will

28th Nov 2018, 2:59 pm	#6
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,169	Re: Cossor Radio 523 low volume help!? Have you replaced the 5uF capacitor in the discriminator circuit? Is the positive terminal connected to chassis? It is C41 on the Trader sheet. There may be a bad connection on the valve socket of V2 (ECH81) or possibly a bad component nearby. Try tapping the components in that area with a plastic rod.

28th Nov 2018, 5:54 pm	#7
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Cossor Radio 523 low volume help!? You might have low IF gain (probably EF85 or could be EF89). A weak valve would affect AM and FM.

28th Nov 2018, 9:32 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Cossor Radio 523 low volume help!? Not sure the 1N4007 is suitable in an AGC circuit as the forward voltage will be higher and the reverse leakage current will be an issue.