Pye 15A capacitor question

[IvorBlister]

I am about to undertake my first restoration of a valve radio. In the past I have built a number of valve amplifiers and radios but this is my first restoration so am looking for any advice and also if someone could identify 'that cap' for me. I intend to replace all the capacitors but would like to know which that one is. I have downloaded both the Trader and Manufacturers sheets.

May I also add my thanks to Kev (Oldelectronics) for his valuable help and advice so far.

Any help with this radio greatly appreciated, thank you.

Nick

[Nuvistor]

Its the grid (g1) coupling capacitor to the output valve(s) from the previous stage. The previous stage usually has a high voltage on it, any leak in that capacitor will drive the output valve(s) g1 positive and overload the valve, its cathode components, output transformer and power supply potentially causing expensive damage.

[ms660]

I don't think there is one as we would normally think of it according to a schematic I'm looking at (EBL31).

But that's not to say that the g1 coupling capacitor shouldn't be replaced.

Lawrence.

[Nuvistor]

Hi Lawrence,
Thanks, I don’t have the circuit to hand but my description hopefully will be useful to Nick if he comes across one in the future.
Regards

[Station X]

C20, Trader Sheet, is the closest you'll get to it.

[IvorBlister]

Thanks very much to you all for the informative answers. Hopefully the restoration will go well.
Nick

[ms660]

I would say that capacitor wise C22 is the biggest threat to g1 of the output valve, see if you can figure out why, it'll be good training.

Lawrence.

[IvorBlister]

Thanks for the challenge Lawrence, I only wish my technical knowledge was sufficient to be able to answer the question.

One thing I do find confusing is the Trader sheet and manufacturers sheet number some of the components on the circuit diagrams differently. I have been using both sheets as some diagrams are better on one than the other.
Nick

[ms660]

Yes, Trader and manufactures usually give different circuit references but hopefully the same component values, errors can occur with some of them.

With the Trader for this receiver, it's best to print out both parts of the schematic and join them up with a bit of clear tape use a pencil or marker to highlight any part of the circuit you are tracing or replacing.

C22 going short would upset the grid bias on the output valve which would be unwelcome.

Lawrence.

[ukcol]

Snip from the Trader sheet to illustrate Lawrence's question.

HINT: What order of voltage may be on the plates of C20 and C22?

EDIT: Crossed with Lawrence's latest post.

[IvorBlister]

Thanks Lawrence for the explanation. I will have to give some thought to this to gain a better understanding. Also I had printed off the sheets and joined them together, makes things clearer. I have also printed out enlarged versions of the components layout diagrams and highlighted the components to change. Have also taken numerous photographs of the actual layout of the radio as it is at the moment.
Nick

[Neil Purling]

I hope that you have the EBL31 o/p valve & it is healthy as they are not cheap. Not in PX4 territory thank goodness, but I hope it & the o/p transformer haven't been roasted. That is what would happen if C22 was excessively leaky. Maybe the power transformer & rectifier too

[paulsherwin]

This radio doesn't really have a 'that capacitor' because it is a short superhet without an audio preamp stage. In a full superhet 'that cap' would go from the anode of the preamp to the grid of the output valve. However C22 will certainly cause some fireworks if it fails short circuit.

EBL31s can be a bit pricey, but there's always the possibility of using the old octal base to build an adaptor for something like an EL84 (or even an ECL80) and some solid state diodes.

[IvorBlister]

Thanks again guys. I managed to obtain three of these radios for a total of £8.99. Two look quite rough but are complete, the third looks in quite good condition for its age. The wiring is point to point (?) and does not use tag strips etc. From first look some of the capacitors seem to be behind other components or rotary switches, so I am expecting problems soldering the replacements in position. Perhaps not the best first set to attempt a restoration on? Fortunately I have three sets of valves, although I don't know the condition of them. Checking the dc resistance of the transformers and speaker all are within the values on the circuit diagram.
Nick

[paulsherwin]

They are very solidly built sets and not difficult to work on.

It's not a good idea for beginners to remove old components by desoldering them. There is likely to be a strong mechanical joint even with the solder removed, so it's very hard work and the final results don't look good without good soldering skills. You can also cause damage.

Use a technique called 'pigtailing', which means cutting the wires close to the old component and soldering the new one to the old wires. With care this can make a very neat repair and is exactly what a repairman would have done in the 50s and 60s. Even experienced restorers replace caps in this way in run of the mill sets. A later restorer can always revert to full component removal if needed.

[IvorBlister]

Thanks Paul. Useful tip and I guess it reduces the risk of big blobs of solder shorting out adjacent tags too by inexperienced solderers.
Nick

[paulsherwin]

It's not so much that. The soldered connections in these things are extremely substantal and you need a lot of heat and persistence to melt the solder, especially if the component is soldered to the chassis itself. You then need to unwrap the wire with needle nosed pliers and sidecutters. It takes ages and the risks of heat or mechanical damage are very high, especially if you've never done it before.

You could try doing a bit of complete component removal on a scrap chassis - good practice and it doesn't matter if something gets wrecked.

[Neil Purling]

Member G4EBT had an article in the BVWS journal about the EBL31 several years ago.
The European ct8 side-contact EBL1 has a larger diameter top cap. As converted to International Octal the valve is around 2 inches taller. That is not an issue in the Pye 15A, but you would not get the back on an Ekco A22. We had examples of EBL1/31 and converted EBL1 side-by-side. The solution was an old base containing a pair of diodes and a B9a base to take an EL84, which are common. There was even the g1 connection on the side so nothing had to be altered in the set.

The Ekco uses exactly the same valves at the Pye, including that odd 4 volt rectifier.
I do not remember what the Trader said the HT ought to be. My photo files showed that I used the same transformer as now resides in my Mullard 3-3, so it was therefore 300V.

The image shows that preset tone control and some not so nice waxies before eviction.
That grey cased Dubilier component is probably a replacement. Despite the different case the guts are the same & they make good firecrackers. One gave me such a fright both feet left the ground!
It was not in that Pye, but shows they too aren't to be trusted one bit.

[60 oldjohn]

I agree with Paul re Pig Tailing the joint, I would add that after cleaning the original wire with fine emery cloth (not wire wool) I would treat the wire with a flux suitable for electronics, any one who has not used extra flux will be surprised at how it helps the solder flow on old wires.


John.

[G6Tanuki]

Originally Posted by 60 oldjohn

I agree with Paul re Pig Tailing the joint,.

Agreed too: the usual trick was to snip the leads off the 'dead' component as close to the dead part as possible, leaving a couple of free ends.

Then we took an offcut of tinned wire from some other component and wound it round something like the shank of a 1/8-inch drill or a small screwdriver so you ended up with a coil of 4 or 5 turns.

Tin the free ends of your old component still attached to the radio; put the 'coils' over them then put the suitably trimmed ends of the replacement component into the coils.

A few seconds with some trusty Ersin 60/40 and the job's done.