Cossor melody maker 501AC

[Guineafowl]

Hi all,

Problems: No stations, although touching the gram socket gives a hum. All valve voltages are down by about 20-50%. Removing V4 (AF output) lets them recover, but even stepping up the voltage for a brief period doesn’t bring it to life. Removing V3 doesn’t have this effect.

I’ve replaced all the wax and electrolytic caps. Resistors all fine.

Injecting a 1.5 MHz signal with audio tone, by clipping sig gen lead to insulation of V1 mixer grid lead will produce a tone from the speaker, when the dial is tuned correctly. There can’t be too much wrong if this works, can there?

Another problem I have is the Trader sheet (1133) doesn’t quite match the set. Mine has Cossor valves with grid caps, but the sheet doesn’t show these.

Any advice much appreciated.
A

[ronbryan]

There is a note on the second page of Trader Sheet 1133 indicating other valve ranges fitted to this set, including the loctal types and mentioning minor circuit differences.

The Cossor service data shows loctal valves 7S7 7B7 7C6 7C5 7Y4 and is available from the service data on this site (top right of this page).

If the HT is low, it could be caused by the rectifier being low emission, assuming that you have already checked the reservoir and smoothing electrolytic capacitors.

Ron

[Guineafowl]

Thanks, Ron.

I saw that bit about the valves, but my set seems to have some hybridisation of the different circuit values. I’ll leave that alone for now.

Yes, I thought rectifier, but should I not see some reception, even slight? And shouldn’t boosting the voltage bring it to life?

A

[ronbryan]

These sets are not very good/sensitive in my opinion. What sort of external aerial are you using?

You have to start somewhere and getting the HT correct would help.

Forum member Sideband has had a problem with the capacitors tuning the IF transformers - there may still be a thread on the Forum concerning that issue. Low IF gain could contribute to your problem - you could check the IF alignment at 470kHz to see if all cores tune. This is the thread: https://www.vintage-radio.net/forum/...ad.php?t=39392

Ron

[Guineafowl]

I put a long wire from the A socket out of the window.

Pulled the rectifier and replaced it with 2 1N4007 and an inrush limiter of 180 ohm.

Voltages back up, and some vague stations come in, but terrible motorboating and noise.

What would your next move be? I may look to the IF transformers next. Thanks for the link!

[Boater Sam]

What is it like on the gram setting?
Establish where the instability is starting from, audio stage or IF?
You can pull valves on this set whilst it is running.

[Guineafowl]

A 400 Hz tone through the gram plays nicely through the speaker.

One of the IF cores doesn’t seem to tune, so will be stripped down.

Cheers

[cathoderay57]

Anybody near you with a valve tester to check out v1 & v2? Sounds like the AF stages are OK and in my experience the signal diodes in the DDT (v3) rarely fail. Agree IFT caps are worth looking at for causing low gain but the motorboating is usually caused by bad HT decoupling often on g2. Sure you haven't missed a cap? Not just the waxes, the small brown Hunts jobbers are notorious for leakage (if there are any).

[Boater Sam]

You had low HT, are the reservoir and smoothing caps up to snuff?
HT line decoupling after the Audio output screen grid is always a suspect.

V1 and V2 share a g2 decoupling cap, 0.1uf C19 on the makers sheet. This is a cost saving measure, it would of been better to put a resistor in the HT line after V2 g2 and another cap after the resistor but it obviously has worked as built.
Ensure that C19 is a good cap.

[kalee20]

As Ronbryan says, this radio is not particularly sensitive (however, it sounds quite good with a strong signal).

I'd suspect the IF transformers. Maybe the parallel capacitors, or maybe the tuning slugs have fallen off the adjustment screws, so they're wildly off-tune. That would explain the appalling sensitivity, with your modulated signal.

[Cobaltblue]

There seems to be issues on this chassis with low Q on the RF coil.

You can't miss it it's that great big thing sticking up on the chassis left hand side. (From the rear)

So far I have not got to the bottom of the reasons for this.

The same basic circuit was used on several sets starting with the 494.

It's true that this set is not particularly sensitive with the internal aerial which was really designed only for local radio reception.

If you get desperate for parts let me know

Cheers

Mike T

[Guineafowl]

Originally Posted by cathoderay57

Anybody near you with a valve tester to check out v1 & v2? Sounds like the AF stages are OK and in my experience the signal diodes in the DDT (v3) rarely fail. Agree IFT caps are worth looking at for causing low gain but the motorboating is usually caused by bad HT decoupling often on g2. Sure you haven't missed a cap? Not just the waxes, the small brown Hunts jobbers are notorious for leakage (if there are any).

No valve testers nearby, sadly. There are lots of squarish brown caps of low values about. I’ve got a Fluke 87V and a Megger IR tester (50-1000V) - I wonder if these would be suitable to test them (capacitance/leakage)?

Originally Posted by Boater Sam

You had low HT, are the reservoir and smoothing caps up to snuff?
HT line decoupling after the Audio output screen grid is always a suspect.

V1 and V2 share a g2 decoupling cap, 0.1uf C19 on the makers sheet. This is a cost saving measure, it would of been better to put a resistor in the HT line after V2 g2 and another cap after the resistor but it obviously has worked as built.
Ensure that C19 is a good cap.

You got me there - electrolytics were replaced with some salvaged from scrap modern boards. (My main hobby is solid-state electronics repair, but I got sucked into restoring a Regentone radiogram and found I rather liked these glowing things). I’ll put some proper new ones in.

Originally Posted by kalee20

As Ronbryan says, this radio is not particularly sensitive (however, it sounds quite good with a strong signal).

I'd suspect the IF transformers. Maybe the parallel capacitors, or maybe the tuning slugs have fallen off the adjustment screws, so they're wildly off-tune. That would explain the appalling sensitivity, with your modulated signal.

IFs will be stripped.

Originally Posted by Cobaltblue

There seems to be issues on this chassis with low Q on the RF coil.

You can't miss it it's that great big thing sticking up on the chassis left hand side. (From the rear)

So far I have not got to the bottom of the reasons for this.

The same basic circuit was used on several sets starting with the 494.

It's true that this set is not particularly sensitive with the internal aerial which was really designed only for local radio reception.

If you get desperate for parts let me know

I may take you up on that - cheers. The tuning cap gang gives a nasty burst of static on all stations at a particular point in the dial, and I may need a rectifier, but I’ve learnt to tackle one thing at a time on these things... I already salvaged the IF amplifier with some kitchen foil.

Well, plenty to think about, but I’m off Bass fishing in Devon for a few days. Will update when I get back. All replies and advice much appreciated

[cathoderay57]

You mentioned "my set seems to have some hybridisation of the different circuit values.". Not sure what you mean by this, or what valves you have got fitted. As far as the rectifier is concerned I have got three used 7Y4 if it is the Loctal/B8G type, and a used EZ35=6X5G if it is the octal. Please let me know which it is and I can PM you with valve tester results and a price. I might have some of the other valves as well and so will send you details for them while I'm at it. A digital voltmeter on the resistance range is no good for testing capacitor leakage. You would need to be careful not to get a lethal shock, but if you disconnect the capacitor from the circuit (taking careful note where it came from beforehand) and then connect one lead to the radio HT supply and the other to the +ve meter probe with the -ve probe on the chassis and the voltmeter on at least a 300v DC voltage range you will see if the cap is blocking dc or not. Obviously don't try this for any capacitors marked with a working voltage less than say 250v. To resolve the shorting tuning cap vanes first unplug from the mains. Then, you should be able to find the shorting spot while using the ohm meter on the continuity range (hopefully with beeper) between the rotor vane (chassis) and stator vane. Note however that the aerial side will have to have the stator wire removed otherwise there is a path to chassis via the ae coils and the ae shunt R1. For the oscillator side just switch to MW or LW which puts isolating caps into circuit. Then it's a matter of carefully bending them with a screwdriver so that they do not short. One last thing be careful inside the IFTs; the coil wires are very fine and easily broken. Take the opportunity to first do a continuity check on them, as one could already be broken, which could be cause of the fault! Cheers, Jerry

[Guineafowl]

Back now!

Thanks for this comprehensive answer, Jerry. I can’t find a recitfier tube for sale - would you be willing to sell me a Cossor 6X5G? If you have the others I might consider a job lot, if only for substituting in for diagnosis.

OM10, OM6, OM4, 6V6GT, 6X5G are the ones fitted. The OM6 has been kitchen-foiled, which isn’t ideal.

I haven’t had a chance to strip the IF tranny that wouldn’t tune, nor test the mica caps, but will update soon.

My Megger unit will apply 50-1000V across the cap under test (250V range probably best) and measure the DC resistance, so might be of use.

Cheers
A

[cathoderay57]

Welcome back. I'll test some valves and send you a PM. I have got all of them but not Cossor except maybe a Cossor 6V6G: EZ35=6X5G; ECH35 (OM10); EF39 (OM6); EBC33 (OM4). Not sure about tests using the Megger - if it has a digital display the results might be misleading. I use an old analogue Avo 8 Mk7 on the resistance range and you can see the cap charge up. OK, it's only testing at 15v but unless I get a leakage resistance of more than 10M I will usually replace, and always replace audio couplers if there is any visible leakage at all. I never bother to test or replace cathode decouplers. Mica caps are usually OK but not the fat "Micamold" devices that I recently encountered on an AR88. Cheers, Jerry

[Guineafowl]

That would be great.

The Megger is an old analogue BM10 - it certainly shows up the leaky wax caps (kohm range readings) versus their polyprop replacements (Gohm range readings).

Cheers
A

[Heatercathodeshort]

Not much help but I have two of these with a miserable performance. Low gain with noisy reception. I put it down to low Q coils. They are sitting under the workbench at the moment. Good luck with yours. John.

[mark pirate]

Not much help but I have two of these with a miserable performance.

Snap! One was working & in excellent condition, after replacing the usual capacitors, there was no improvement in performance whatsoever!
I also have the wood cased version, after replacement of the smoothing caps, the set is now working, but I am loathed to replace the caps on such a lacklustre set.
Not one of Cossors best efforts, easily outperformed by a Bush DAC90.....

Mark

[Guineafowl]

AH! I’m also fiddling with a Bush DAC90A at the moment. Very low voltages on at all points so a new rectifier is on the way. There may be a thread about this soon...

[Sideband]

The Cossor 501 is not particularly sensitive. I sold mine on in the end. The DAC90A will far out-perform it