Cossor 520 with almost no audio

[epwg123]

Hi,

I've picked up a Cossor 520 with no sound from any of the three wavebands, all it emits is a slight mains hum.

I've cleaned out 60 odd years of dust and used contact cleaner on all the swithes and valve sockets. That's as far as I've gone with maintenence as this is my first foray into the world of valves.

Out of curiosity I plugged in a bluetooth to audio converter into the phono input and a lovely clear sound came out of the speaker from my phone. Doing this I found that the volume and tone knobs both work.

Does anyone have an idea what needs to be fixed to bringing it back to full life?

Thanks

[Station X]

Hello and welcome to the forums.

A couple of simple questions. Are the correct valves fitted in the correct positions? Have you connected an aerial? A couple of metres of wire will do for testing.

[epwg123]

I’m assuming that the valves are in their correct positions as the internals looked untouched. The valve between the two metal capacitors is the only one not glowing. I’ve tried the internal loop and about a metre long length of wire and get nothing but the hum.

[Station X]

You must assume nothing. Someone may have tried to fix the radio in the past, swopped the valves round, given up and stuck the set in the attic.

If the valve is not glowing (some valves need to be viewed in the dark to check they're glowing) this is a sure sign of a fault, but not necessarily with the valve.

Do you have a test meter to carry out further testing?

[Nuvistor]

The valves use a B8A valve base, some of theses type of sockets have a spring next to the glass pip on the valve. Remove this spring clip before removing the valve or you risk breaking the glass.

[epwg123]

Yes, I do have a meter

I’ve been using the service sheet for the 500 as a guide- https://www.************************...-maker-500.pdf

Hoping just a simple component has died of old age

[Station X]

The correct service sheet is available here:-

https://www.service-data.com/section...0-melody-maker

The first step should be to check the heater continuity of the valve that does not light up. All the valves in this set have their heaters connected to pins 1 and 8 which are located on either side of the pip or spigot. With your meter on ohms range touch the meter probes together so you can see what a short-circuit and open circuit look like on the meter. Now apply the meter probes to pins 1 and 8 of the valve. If the heater is OK you will see a low resistance. The valve must be removed from the set to carry out this test.

Report back here with the result.

[crackle]

Only the manufacturers service information is specific for this radio and has most of the details you will need. The RER covers 500 501 502 & 520, the R&TVS lacks some details.
None of the service sheets "up top" give any details of component layout.
There are Trader service sheets available for the 500 and 501 which give the above and below chassis details and layout.
I would recommend the Trader sheets for the 500 if you have not already got them.

Depending on where you are, you may need a long wire aerial, as long as you can make it, 20 feet or so.
Mike

[cathoderay57]

I think I have most of the spare valves for this radio and so if you find a duff one in the set, give me a shout. I can supply tested valves. Generally they are available on eBay and elsewhere but not always tested.

Cheers, Jerry

[Ed_Dinning]

Hi, if you suspect it is that valve you can bring it over and I will test it for you.
Note that although the audio is working it will pay to change certain capacitors to avoid damage to other components. That done and with the set working you can either feed it with Bluetooth or use as a radio.

Ed

[epwg123]

The value between pins 1 and 8 is approx 130 ohms, its a 62VP valve.

As a total valve innocent, does this mean there is life I it and the fault lies elsewhere?

PS. An update, on switching on the radio and turning off the light the valve is actually glowing.

So.... does this mean the gremlin lies elsewhere?

[TrevorG3VLF]

The valve could be OK.
Measure the DC voltages on the valves and compare with the published data. It will not work if there is no current.

It is a superhet and requires an oscillator. Check V1 triode grid for a negative voltage.

[stitch1]

Back to basics ... have you cleaned the waveband switch and valve pin/holders?


John

[G8UWM-MildMartin]

The earlier 494 and 500/501 versions of that chassis were notorious for the SW section of the primary of the oscillator coil, wound with resistance wire, going O/C.
I don't know whether it was sorted by the time the 520 was released, but probably worth a quick continuity/resistance check anyway, or even just a quick solder of a bit of wire across it while trying the set on MW.

[FERNSEH]

Link to a repair of a Cossor 520:

https://www.vintage-radio.net/forum/...d.php?t=141208

DFWB.

[epwg123]

Over the next few days I'll be going through the radio testing each component and replacing as necessary. The waxy tubes called capacitors are first on the list.

As a side note I played back Orson Well's War of the Worlds through it this morning and the hums, clicks and hiss on the mp3 file felt right at home coming out of the speaker

[Station X]

Originally Posted by epwg123

Over the next few days I'll be going through the radio testing each component and replacing as necessary. The waxy tubes called capacitors are first on the list.

Compulsive Component Changing (CCC) is not the way to repair a radio. It's quite likely that you'll introduce new faults or wiring errors. You can't easily test capacitors with a DMM as they need to be tested at the working voltage.

I strongly suggest taking voltage readings on the valve holders and comparing the results with the figures in the service sheet. If you find a reading which looks wrong, there are many members her who will advise you what to do next.

[epwg123]

I shall try that and report back

[Station X]

Given that the Power Supply and Audio Frequency (AF) stages are obviously working I suggest checking V2 and then V1.

Sleeve all but the tip of your probe to reduce the risk of creating a short-circuit to an adjacent wire or tag.

[ms660]

Ensure that the audio output valve's control grid coupling capacitor is in good order, usually one of the first things that should be replaced (C25 on the BRTR service data sheet)

Lawrence.