Cossor Melody Maker 494 A.C. Valve radio

[stuboy57]

Looking to restore this Cossor Melody Maker 494 A.C. Valve radio, would this make an ideal first valve radio restore?
I haven't plugged it in at all as I am aware of the risks, ( the original owner said "the radio starts up but no sound just hissing") so no not going to try it....plus I guess a good starting point would be to replace all the "waxy" capacitors?.....now I have downloaded the service data, trader sheet etc,
Where is the best place to buy the replacement capacitors and how do I go about the values and voltages? i.e 0.01 MFD modern day equivilant?
Would someone be able to give me a list?
Also the tuning dial pointer cord has broken, where do you get replacement cord?
I have removed the chassis from the bakerlite case and have given it a bit of a clean
any help would be much apprieciated......be gently though as I am quite new to this forum but willing to learn from all you experts out there

[Boater Sam]

Before you get too absorbed, read my Thread "Failure, Cossor RIP, Oh Dear!" on this forum.

https://www.vintage-radio.net/forum/...d.php?t=137282

Do NOT twiddle the brass IF transformer adjusters either on top or underneath.

Turn the chassis from right way up to upside down whilst listening carefully, if you hear any click, one or more of the cores have broken off the brass screws and you have a huge problem.
Sam.

[Andrewausfa]

Ideal set to start with I'd say.

Capacitors wise, you'll want and you may as well get 10 of each but in brackets what you actually need: 100nF (3). 4.7nF (3) 22nF (1) all at 630v. One of the 4.7nF is on the output transformer check that as it may be a 1000v component. You want axial polyester film or just search for MKT1813.

Electrolytics: 32uF (1) 16uf (1) these are in the big can and rated at 450 volts. Then there is a seperate 25uf (modern 22uF) rated at 25v.

Loads of place to get these all from, RS, Farnells etc. Handy conversion chart for uF-mFd-nF here http://www.justradios.com/uFnFpF.html

I bought a large shank of strong beading string which rubbed with a little wax seems to work for dial cord.

Andrew

[telstar]

Originally Posted by stuboy57

any help would be much appreciated......be gently though as I am quite new to this forum but willing to learn from all you experts out there

Hi. I have one of these radios too, but I have some work to do on mine as well. The tuning cord can be sorted by using fishing line. Radios that employ thicker cord, I use the sort that builders use for plumb lines (made by Draper).
On my set the core that the various aerial coils have been wound on has been broken at its base. I have some replacement aerial cores, but it is getting the circuit re-tuned or finding a way of repairing the card core at the base. The cabinet has also a crack in one side.
The voltage adapter on the mains transformer requires some attention on these radios (uses what looks like a battery snap for a PP9 battery), these have poor connections so replace with a wire directly to the appropriate mains selector. I also wired the "live" mains wire to the single pole switch.
There are variants in the valves used in these radios, and resistor & capacitor value modifications for each type of valve line-up used (so I discovered when I was asked to restore one many years ago). When I finished the restoration, it worked really well indeed!
By all means replace the wax capacitors (I have an insulation tester that soon discovers how low some of these capacitors are in resistance). Look out for resistors that exceed their tolerance value. You will be surprised in what you will find in variation.
Regards
Mike

[David G4EBT]

Originally Posted by stuboy57

..... I have downloaded the service data, trader sheet etc,

It's quite a decent set - it has a large speaker, a tone control and three wavebands. It has a rudimentary 'frame aerial' wound on the back panel, which will pull in a few stations on MW/LW but would benefit from an external aerial, especially on short waves. It has a double-wound mains transformer so the chassis is isolated from the mains input, and hence, is safer to work on than sets with a 'live chassis' - the Bush DAC90A for example.

This is one of those cases where the Maker's sheet in some respects gives less detail than do the Trader Sheet and ERT sheet.

The Maker's data gives no component layouts for above and beneath the chassis, nor a dial string diagram. The Trader Sheet and ERT Sheet each give that detail, and it's worth having all three sets of data. However, if you quote any component numbers on the forum, do make it clear which data you are referring to, because they differ. For example, the Maker's sheet calls the audio stage cathode by-pass capacitor 'C29' whereas the Trader Sheet and ERT sheet refer to it as 'C21'. The output stage audio coupling capacitor (a 'must change' item), is called 'C26' on the maker's sheet, C18 on the Trader Sheet, and C19 on the ERT sheet. And so on it goes.

When restoring any valve set, which can take many hours over many weeks, I'd recommend making a simple cradle to hold the chassis upside down to work in as it minimises the chance of damaging something - a valve for example - if you try to balance the chassis on bits of wood or whatever. A cradle makes it easier to work on, and safer to carry out live testing.

I've attached some pics of a Melody Maker 501, similar in appearance to your 494, which I restored some time ago. The first pic shows it mounted on a simple cradle made from 4 pieces of flat steel and a scrap piece of plywood. Second and third pics are before & after underside views of the chassis. Note that the original 2-core mains flex was restrained only by having a knot tied in it, which I replaced with a 3-core flex and cable restraint. If the flex on your set is original, it will be the same, so I'd recommend changing it.Third and fourth show the original unstrained 2-core flex and the replacement 3-core and rear chassis apron cable restraint.

Plenty of room to work, which makes restoration a bit easier.

Hope that helps a bit.

Good luck with your restoration should you go ahead with it.

[stuboy57]

Originally Posted by Andrewausfa

Capacitors wise, you'll want and you may as well get 10 of each but in brackets what you actually need: 100nF (3). 4.7nF (3) 22nF (1) all at 630v. One of the 4.7nF is on the output transformer check that as it may be a 1000v component. You want axial polyester film or just search for MKT1813.

Electrolytics: 32uF (1) 16uf (1) these are in the big can and rated at 450 volts. Then there is a seperate 25uf (modern 22uF) rated at 25v.

Thanks for all your comments: I was going to replace the 6 "waxy" capacitors 1st which on the trader steet are C1 and C16 which are 0.005MFD 500V, C10,C17, C18 which are 0.1 MFD 500V and C11 which I think is 0.1 MFD 500V, think I have got all that right can someone confirm if correct, this is my 1st valve restore.
I guess you cannot get 500v capacitors so 630v is the next highest
also do they need to be fitted a certain way + - etc
I know these might be silly questions but really excited about getting it restored, no rush as and when a little at a time

[ms660]

630 volt capacitors will be fine, no problem, the voltage rating is the maximum voltage they are designed to reliably operate at, not the voltage they have to operate at.

C17 on the Trader sheet is 0.02 so use a 0.022uF.

Non of the capacitors you have mentioned in post #6 are polarized ie; there's no + or - connection.

Lawrence.

[Boater Sam]

Many of the old paper wax capacitors have a band on one end. That is the outer foil and would have been connected to the low potential connection in the circuit. This is thought by many to reduce hum levels.

Most modern foil capacitors are not so marked, but you can find the outer foil ends if using wound foil capacitors. This is done by connecting one end to a 'scope and holding the capacitor in the fingers, the 50Hz so picked up is of greater amplitude when the outer foil end is connected.
But as has been said, modern parts unless electrolytics are not deliberately polarised.
Sam.

[kalee20]

Yes, it's a good set to start with! Chassis comes out easily, all one-piece, it's well spaced out, dial drive is simple. These Cossors are disappointing in sensitivity so need a decent aerial, but sound quality is good.

Really, of all the waxies, the only one to change immediately is the one coupling to the output valve, as Dave G4EBT says. The rest may be below par, but are unlikely to stop the radio working. So get replacements in, but don't change them yet. It's easy to introduce an extra problem which never existed!

If it has been switched on recently and didn't blow up, then the HT electrolytics are likely to be OK. You can feel the can for the first few minutes when you switch on yourself, but if it stays cold then leave alone. They either go leaky (and get hot) or dry out (you get loads of hum).

Look forward to progress!

[ukcol]

SAFETY. You can read in various threads here that there are safety issues when working on valve radios. But in addition there is a particular point to watch with regard to your set.



There will be voltages on the mains tapping connections on top of the mains transformer. If the plug has been wired so that the on/off switch is in the live conductor there will be about 40 volts (with respect to earth) on the 200-215 volt tapping when the set is switched on. You can feel 40 volts but it is relatively harmless. If the plug is wired so that the on/off switch is in the neutral conductor there will be full mains voltage on the 240 - 255 volt tapping. This voltage will be present when the set is plugged in regardless of whether or not the set is switched off. THIS PRESENTS A SIGNIFICANT DANGER.

Be aware of this as it is easy to put your hand somewhere silly if you are distracted. Ideally you should make up an insulated cover for the top of the transformer so you cannot touch the tappings.

[David G4EBT]

Regarding which is the outer foil of non-polarised capacitors, if you look at the second picture in post #5 above - the unrestored chassis of my Cossor '501 Melody Maker' - you'll see several waxy paper caps with the outer foil marked with a band at one end. As Sam said, the outer foil should be grounded to prevent hum being induced. I've attached a pic below of the caps that I removed from that set, which shows the bands more clearly. (Incidentally, should anyone wonder why I didn't replace the 25uF 25V cathode bypass cap in the pic of the restored chassis, I did. Modern ones look so tiny and silly that I stuffed the old can).

Though the outer foil isn't marked on modern non-polarised caps, it's quick and easy to check them, either with a scope, or in the absence of a scope, using the input of an amplifier. If you hold the cap between your fingers without touching the wires at either end, your body acts as an antenna and will pick up 50Hz hum. If the cap is then connected to either a scope or the input of an amplifier, hum will be noticed with one end grounded, but not when the other end is. If no hum is observed, the end which is grounded is the outer foil, which should be grounded in the set.

When I buy a batch of caps, I check them all and mark the outer foil with a felt tip pen so when I come to use them, I ground that end. It only takes minutes to check a batch. I wrote it up as a thread a couple of years ago here:

http://www.vintage-radio.net/forum/s...+of+capacitors

The difference is quite marked, as can be seen on the scope traces below. The first trace shows the cap connected to the scope with the outer foil grounded and the cap held between my fingers - as can be seen, the trace is a flat line. On the second trace, the outer foil (as discovered on test) was not grounded, so the cap picked up 50Hz hum induced from my fingers. Confusingly, that particular cap had a black line at each end!

Many may not be aware of this phenomenon or bother to give it a second thought and may even consider it to be borderline OCD. I didn't bother until I became aware of it, and many times it won't cause a problem, but then if it does, we could be on a wild goose chase, changing smoothing/reservoir caps, output valves etc, trying to reduce or eliminate hum.

Hope that's of interest.

[stuboy57]

quite pleased with myself with re stringing the tuning dial.....used some beading string from Hobbycraft.....took about an hour or so but as I've never done one before I was well pleased....also will the square type capacitors need replacing? thanks for all the comments, really helpful advice

[Station X]

Originally Posted by stuboy57

also will the square type capacitors need replacing?

Not unless they're actually faulty. These type of capacitors are very reliable.

[Boater Sam]

Flat thin rectangular waxed caps are mica dielectric, very seldom faulty. As they are used in the tuned circuits and IF amps, changing them will upset the alignment badly.

[stuboy57]

Originally Posted by Andrewausfa

Ideal set to start with I'd say.

Capacitors wise, you'll want and you may as well get 10 of each but in brackets what you actually need: 100nF (3). 4.7nF (3) 22nF (1) all at 630v. One of the 4.7nF is on the output transformer check that as it may be a 1000v component. You want axial polyester film or just search for MKT1813.

Quick question are the 47nf capacitors used in place of the 0.005mfd capacitors? that's capacitors C1,C16,C20 on the trader service sheet as I cannot find any 0.005 mfd capacitors
Also need to wire or plug the speaker in as is was not attached,does one go to the chassis and the other in the EXT LS on back of set?
Carefully finding my way, thanks for the help so far

[ms660]

For 0.005mfd use 4.7nF (aka 4n7) not 47nF.

Lawrence.

[Boater Sam]

In the centre of the chassis at the back there are 2 sockets (one above the other) for the internal speaker and left of these are 2 more for the external extra extension speaker.

They go to the same place really.

[stuboy57]

Started replacing the capacitors today, started with the 3 off 0.1 mfd ones C10, C11, C18 on the trader sheet, thought I would plug the radio in just to see what was happening, so plugged it in via my lamp limiter, the 100 watt bulb was dimly lit, all I have at the moment after replacing the 3 capacitors is a hum which increases with the volume knob, all the valves vary in hotness, but the big can is cold and the hum reduces when I touch it.....is this progress in the right direction?, I waiting for the other capacitors to arrive so I can replace them as well (ordered the wrong ones 1st time round), also where can I get the small replacement plugs that go on the speaker and aerial sockets? another question what else can I check using my multimeter?
sorry for the silly questions but it's all new to me and I'm really enjoying it...thanks in advance...all help appriciated

[Boater Sam]

Originally Posted by stuboy57

but the big can is cold and the hum reduces when I touch it.....

That sounds to me like the chassis is not earthed, it can be as it has an isolating mains transformer and it will be a whole lot safer if it is.

[stuboy57]

Yes I used the original corded flex and plug (13 amp fuse), after checking, but yes I am going to replace the flex and earth the unit as David G4EBT has in his Cossor, where can I get a cable restraint?