Cossor 500 conundrum

[PaulR]

I have resurrected a Cossor 500 from the loft to use in the kitchen. I replaced the usual capacitors and any out of spec resistors a while ago. The big let down on these sets is, I think, the valve holders which are not in keeping with the overall quality of the rest of the set and I replaced two of them originally. Another has crumbling contacts and so a replacement is on order.

Anyway, the set works but with quite a lot of hiss on LW. I wondered about the alignment but before I started I checked the anode voltages which are as follows:

V1 194, 74
V2 194
V3 71
V4 199

These are all somewhat down except V3 where it is well up. The voltage drop across R11 is 124. This led me to think that V3 was on its way out but when I test it on my valve tester the mutual conductance is over what the book says it should be and the diodes test ok.

The rectifier is new but I never replaced the smoothing caps.

Should my next course of action be to replace V3 anyway and should I also replace the smoothing caps? I am wary of buying a "used but tests ok" 7C6 as mine actually tests ok.

Thanks

Paul

[Cobaltblue]

Hi Paul

If you measured the anode of V3 with a modern digi meter you can expect it to read high as the readings in the manual are taken with a 20K Ohm/Volt meter.

As for the long wave these sets are usually a bit deaf on the Long waves without using an external Aerial.

The internal Aerial is untuned and not very effective.


Good luck with the set

Cheers

Mike T

[PaulR]

Thanks Mike

I appreciate what you are saying with regard to the voltage on V3, but the others are actually down. The Trader sheet shows:

V1 208 96
V2 204
V3 46
V4 268

V1 and 2 are fairly close but V3 is 50% high and V4 is nearly 70v low.

In view of what you say about LW it may be working as well as it would anyway, but if there is something I can do to get it a bit nearer the intended spec I would like to give it a go.

Paul

[Cobaltblue]

Hi Paul the HT is certainly a bit low.

You need to make sure C18 has no leakage and unless it's new I would replace it on sight.

If excess current through V4 is not the cause then I would check C22 for low capacitance and ESR and if they are OK your rectifier may be on the way out.

A quick test for C18 being the cause would be to short out R12 and see if the anode of V4 rises.

Cheers

Mike T

[PaulR]

Thanks Mike

I changed C18 when I first got the set (off a skip). It is looking like the smoother needs doing then as the rectifier is brand new.

Paul

[PaulR]

A close look at the smoothing caps shows that I must have changed them when I first did the set and made such a good job of the re stuffing that it cannot easily be seen!

I checked the grid of the output valve to find it has +14 volts on it and the cathode has +8. With the valve removed there is nothing on the grid. Does this show that the valve is faulty?

[PaulR]

Sorry - please ignore my previous posting. I misread the voltage as it was on an unfamiliar meter set on the auto range. It was actually on the 200mV range so the voltage was very small.

Paul

[Station X]

So what was the cathode voltage then?

[Cobaltblue]

Was the Cathode voltage 8?

Sound a bit low for a 6V6 or a 7C5 which I would expect to be at least 12 volts.

Worth checking the Cathode resistor is 270 Ohms. if your output valve is a 6V6 or a 7C5.

The voltages given on the trader sheet are for the 67PT, the bias resistor for that valve is 180 Ohms.

When I get back to Poole I can check the Manufacturers information.

Cheers

Mike T

[PaulR]

The cathode is at 8.5v and R16 reads 262 ohms (it has loctal valves)giving a cathode current of around 32mA which is near enough what the Trader sheet shows.

I wonder whether it is actually OK despite the odd valve readings. Could it be that the replacement smoothing caps I used have a higher ESR than the originals?

Paul

Edit

Posted at the same time as Mikes post above. R16 should be 270 per the Trade sheet. I have found another sheet where the readings were done off a set with loctal valves and the cathode reads 9V on that

[Cobaltblue]

I have just checked the values given in the 494 Sheet which is virtually identical chassis wise to a 500/1 with Loctal valves and the HT is still too low.

It could be the caps you could croc clip an additional 8 or 16uF smoothing cap across C22 temporarily and see if the volts rise substantially in which case there may be a problem with C22.

Certainly it doesn't look as if the output valve is dragging the HT down.

Cheers

Mike T

[PaulR]

Thanks for the help, all. I have now replaced the V1 valve socket and it is working well. I think that a bit of re-alignment might help but I need to do a some of work on my signal generator first.

I haven't done anything about the low HT as the set is fine with lots of volume so it doesn't seem worth disturbing it further. I think the problem may well be with the replacement smoothing caps, but I will leave them.

I can now start on a nice Murphy set I got a while ago.

Paul