Cossor501AC (Actually a 494AC) - raised from the grave?

[John G8MWF]

I recently purchased, at a moderate price, my first Cossor 501AC which is a departure from the DAC90A that I have done most of my restoration.

It is NOT in original condition, it is rusty and has had a LOT of work done on it over the years.

So I powered it up via the serial lamp and eventually the audio amp was working with low volume but no stations. This just turned out to be dirty pins on V3 but the audio is still low and distorted at 'high volume' on strong stations.
There is quite a lag when the volume control is turned up and is almost like a bad AGC line fault.

So, THAT capacitor and C7 the AGC decoupling capacitor were change with some improvement but nothing radical.

Voltage measurements, no serial lamp in place, on the 32uF = 202Volts and 16uF = 154Volts and the Anode of V4 = 187Volts Cathode 10.6Volts. All way too low.

As I have never worked on a Cossor 501 before and I don't have any spares or want to spend any more than I have to on this project - my question is - is the fault likely to be with the rectifier valve V5 (7Y4) or are there some other conditions that could be causing this fault?

Any help would be appreciated!!

[Boater Sam]

Best guess is the reservoir cap being bad.
7Y4 is a robust rectifier but if it is below par 2 silicon diodes say 1N4007 and 100 Ohm series resistor is cheapest fix.

[PJL]

It's worth checking the electrolytic is not getting warm and the mains transformer is not hot. Also check the voltage on the rectifier anodes which should be around 250V AC as it could be a mains transformer fault. If these are OK then I would go with your diagnosis.

[Steve_P]

I hope the voltage adjustment has not been taken down at some stage. It was a common bodge!

Those Valves are not the best. But I would have a look at R17 - This is between the Output Transformer and the Grid of the 7C5 (May be a 6V6GT) / C26 (The big can),

Another thing to try is pulling out the the valves in turn and see if one makes a lot of difference.

I would also look at the Tone capacitors - both go to chassis from the Tone Control and the Capacitor on the anode of the output valve,

As for the lack of stations - Check the voltages round the 7S7.

Cheers,

Steve P.

[John G8MWF]

It turns out that half of the HT transformer winding is open circuit - not the easiest fault to cure!

Thank you all for your help and advice - perhaps I can do the same one day.


Kind regards


John

[Steve_P]

You should be able to get something in there that would do. Any AC Only set with 5 valves would do here.

Get to the next radio meet and see what people have!

Cheers,

Steve P.

[TrevorG3VLF]

You can reconnect the good transformer winding to feed a bridge rectifier with the addition of two diodes, still using the valve. The good HT winding would conduct current in both directions and have double the dissipation but the overall transformer dissipation would not change. The chance of transformer saturation would be eliminated.

The good winding would be connected between the rectifier anodes with no connection to chassis. A diode would be connected from each anode to chassis.

[John G8MWF]

Just a thought . . . . what if I put a bridge rectifier in with a 1K in series on the 'good' half of the winding?

[Cobaltblue]

I did have a couple of spare transformers from this chassis not sure where they are at the moment

I will have a look at the weekend when I am at home.

Cheers

Mike T

[John G8MWF]

Originally Posted by TrevorG3VLF

You can reconnect the good transformer winding to feed a bridge rectifier with the addition of two diodes, still using the valve. The good HT winding would conduct current in both directions and have double the dissipation but the overall transformer dissipation would not change. The chance of transformer saturation would be eliminated.

The good winding would be connected between the rectifier anodes with no connection to chassis. A diode would be connected from each anode to chassis.

Hello Trevor. Yes I like your idea and I am going to try a temporary fix later today when I get the diodes in.
The Anode current is around 30mA an I also intend to place a 1K resistor in series to allow for approx. 30 volts increase by using silicon diodes.
Then I will see what happens and ,if it works, if the resistor is needed or not.

MikeT - thank you.

[Boater Sam]

You are quite unlucky to have a failed mains transformer. I have had dozens of these and they have all been OK.
Could I suggest that the audio coupling cap may well have caused the transformer to fail?
It would be worth checking the output transformer too before you get too involved.
Sam.

[TrevorG3VLF]

Hello Trevor. Yes I like your idea and I am going to try a temporary fix later today when I get the diodes in.
The Anode current is around 30mA an I also intend to place a 1K resistor in series to allow for approx. 30 volts increase by using silicon diodes.
Then I will see what happens and ,if it works, if the resistor is needed or not.

MikeT - thank you.

You will not need a resistor, the HT current goes through the valve as before. The opposite winding end is referenced to chassis through a diode (drop about 1V) instead of directly to chassis.

One end of the good winding is already connected to a valve anode. You will have to disconnect the chassis connection and connect it to the other anode.
Simples.

Trevor

[John G8MWF]

Originally Posted by Boater Sam

You are quite unlucky to have a failed mains transformer. I have had dozens of these and they have all been OK.
Could I suggest that the audio coupling cap may well have caused the transformer to fail?
It would be worth checking the output transformer too before you get too involved.
Sam.

Hello Sam,
The mains transformer is definitely o/c one side of the HT. It also looks like one side originally connected to the o/c winding of the 7Y4 is suspect.

There is about 202 volts RMS coming out of the surviving HT winding, this is now being fed into a 1N4007 bridge rectifier network, yes the centre tap was remove from ground, with the 7Y4 removed and the DC only measures 238 Volts!

The set still exhibits the same problems as before only slightly louder.

So, I know that the mains HT transformer is damaged possibly U/S, I suspect that . . . . both the 7Y4 and the 7C5 have had it and they may all relate to the current state of the damage.

The ripple across the 32uF is around 3Volts on a DVM but NO hum is apparent so I think that may still be good - although the cap is dated September 1949 and is probably not the original.
There has been a LOT of soldering all over the chassis and this may be why it was put on eBay !!!!!!

The only good point so far is that there has been no smoke - so far and the dial lamp now works!

I may well just keep this one for the case and few spares, if any, and wait until a more viable set comes along.


Cheers


John

Addendum - I just noticed a label on the back of the cardboard cover stating that this is a 494 AC and not a 501 AC as stated by the seller!

[Boater Sam]

I thought you had some audio, yet now you say 7C5 output valve is duff?
With 238v HT it should be able to work.
494 is not massively different from the 501.

[John G8MWF]

Hello Sam,

I DO have audio which is quite low level and distorts at high/full volume. There is also a strange time lag for the volume to increase and it goes quieter for a while and then gradually increase's.
I thought this was an AGC problem but that measures ok.
The output transformer resistance looks nominally ok too.

Therefore . . . until I get my now very dirty mitts on a known good 7C5 and replace it I suspect that the output stage valve is the culprit!

[Boater Sam]

That's guesswork not deduction then?

There are other reasons for distortion, the time lag does have me beaten though. Doubt its the valve, I have never had that before.
The 7C6 is the first triode AF amp valve and detector/agc diodes, are there any problems around that?

[ms660]

Is V3's grid return resistor ok?

Lawrence.

[electrogram]

Also the coupling cap from volume control to v3 grid

[John G8MWF]

All of the normal suspects and THAT (capacitor C3) was the first to be changed out along with the AGC decoupling cap.
As I previously mentioned the chassis is in a sorry state and dirty valve pins may also be contributing to the problem.

The output from the 7C6 detector remains good to the volume control.
So it's mostly guess work and some deduction - it will be a very quick test once I get my hands on a 7C5!

[mark_in_manc]

Scruffy Cossor 500s (wooden cabinet) have very little value, especially when the plastic 'gold' trim (in which the set differs from yours) is missing or goes all bendy, as they do. You might well pick one up for nearly nothing, and the innards are often (but not always) similar to your radio.