Pye B16T - Pre Restoration Advice Sought.

[DoctorWho]

Further to my Wanted ad here:

https://www.vintage-radio.net/forum/...ad.php?t=25810

I am posting here for some pre-restoration advice for the B16T tabletop set,

Firstly, if I borrowed an MW22-17 from another set, is this a direct plug in replacement that I could use to test the set, or would I need to alter connections and change the heater voltage using a transformer? This is something I am looking in to until a CRT becomes available, so as to allow me to start work on the set.

I shan't know any more until I look inside the set and get the chassis out, so I'll update this thread as questions arise, but any advice on this initial question would be greatly appreciated, as, basically, if I can borrow an MW22-17 and directly plug it in to the B16T that's great, but if I need to make any changes, such as heater voltages or base connections, then best to find out about this first.

Thanks in advance for any advice.

Peter.

[Framer Dave]

Helo Peter,

The MW22-7, MW22-14 and MW14C all have a B8B base with the same pin-out. The 22-7 has a 600mA heater, the others 300mA, this need not worry you as the B16T uses a mains transformer. The 22-14 and 22-14C both have max. 9kV EHT against the 22-7's 7kV, but I have a 22-14 in my B16T and it has a beautiful bright picture on 6.5kV, so no problems there.

The MW22-16, 22-17 and 22-18 have a B12A base. You would have to find a B12A holder and make up an adaptor, perhaps using the base from your dud tube.

Dave

[DoctorWho]

Thanks Dave, that's useful to know, I be able to use the same heater supply from the transformer for the MW22-17?

Could anyone give me the base connections for both CRT's so that I can see what would need to go where?

Thank you.

Peter.

[Framer Dave]

B8B connections: clockwise from spigot, 1 - h, 2 - g, 3 - k, 4 - a1, 5,6,7 - i.c., 8 - h.

B12A connections: 1 - h, 2 - g, 6,7 - i.c., 10 - a1, 11 - k, 12 - h, other pins blank (probably absent).

All the tubes quoted have a 6.3V heater.

Dave

[DoctorWho]

Thanks for this information Dave, I'm just trying to work out what goes where from the original connector to the temporary one.

Does this look right:

Original Base to Temporary Base:

Pin 1 to Pin 1
Pin 2 to Pin 2
Pin 3 to Pin 11
Pin 4 to Pin 10
Pin 7 to Pin 7
Pin 8 to Pin 12

If they are both 6.3 volt heaters then, presumably, the set's own heater supply will be alright?

Sorry to sound dim, but I want to make sure I do this right before attempting anything.

Many thanks,

Peter.

[Framer Dave]

Peter,

You're nearly 100% right, but your table should look like this:

Original Base to Temporary Base:

Pin 1 to Pin 1 (heater)
Pin 2 to Pin 2 (grid)
Pin 3 to Pin 11 (cathode)
Pin 4 to Pin 10 (1st anode)
Pin 8 to Pin 12 (heater)

Don't make any connections to the pins marked i.c., these are designated as internal connections by the manufacturers and must not be connected to the external circuit.

The set's own 6.3V supply will be fine for the replacement tube.

Dave

[DoctorWho]

Thanks Dave, that's exactly what I needed to know.

All the best,

Peter.

[DoctorWho]

Well it now seems like time to start on this set with the borrowed CRT.

I can replace the other condensers in the set, but is it safe to use the original EHT smoothing condenser?

More details to follow when I have a chance to get the chassis out of the cabinet.

[Danny]

Hi no dont use the origanal eht condenser,s especialy if thay are visconol,s these have been mentiond before so dont want to dwell here. Danny

[DoctorWho]

They are indeed Visconol's, so thanks for the warning. It may have been discussed before, but I've missed that totally, so am very grateful for the warning.

Can you use the set with the original disconnected, or can you get replacements anywhere easily?

Thanks for the advice,

Peter.

[beery]

Hi Peter,
the original EHT capacitor in the B16T was a cylindrical metal can with a bakelite top. This was is usually very reliable.
Visconal capacitors are Bakelite all over (with a small metal rim at the base) and they usually fail. If you have a visconal cap in your set I would disconect it before you try anything because if it fails it can take the EHT transformer with it.


Before you do anything with this set check the condition of the heater wires that go from the power unit to the valves on the rest of the set. These are two heavy rubber wires (red and black) which usually crumble. You will need to replace these before you even consider powering the thing up. I think 32A wire was used here.
If you want to check your EHT transformer then remove the HT and EHT rectifiers, make sure the top connection of the EHT rectifier is not able to touch anything. Apply power to the transformer and leave it on for 5 minutes whilst observing it to check that it does not smoke or sizzle or buzz loudly. Switch it off and see how warm the laminations are.


One way of testing the cap is to buy a high voltage resistor of say 100K and put it in series with the capacitor. The resistor will probably fry if there is any significant leakage.

When I worked on my HMV 1804 I used a new 0.033u 8KV cap across the output of the EHT rectifier, a high voltage resistor, and then one of the original EHT caps connected to that.
I then used my EHT meter to compare the voltage readings across each cap. I left it running for a few minutes and then measured again.

All of this was done with caution as it is very dangerous if you don't know what you are doing.

I'll PM you about where to get EHT caps.

Cheers
Andy

[DoctorWho]

Thanks Andy, that's very useful to know. I'll investigate further.

All the best,

Peter.

[FERNSEH]

Originally Posted by beery

Hi Peter,
the original EHT capacitor in the B16T was a cylindrical metal can with a bakelite top. This was is usually very reliable.
Visconal capacitors are Bakelite all over (with a small metal rim at the base) and they usually fail. If you have a visconal cap in your set I would disconect it before you try anything because if it fails it can take the EHT transformer with it.

The EHT capacitor will be OK. If you wish to replace it, consider Ask Jan First
(Frag Jan Zuerst)
For test purposes it is a good idea to insert a 1/4watt 10Kohm resistor in series with the HVR2 valve anode. It will serve as a fuse. Be very careful and make sure that there no risk of coming into contact with this extra component. The Murphy V114 uses a 60mA fuse in this circuit position.

My B16 receivers are fitted with the MW22/7 CRT. I found that inserting a 100Kohm resistor at the tube anode cap stopped internal CRT flashover.

DFWB.

[FERNSEH]

Ask Jan First: http://www.die-wuestens.de/eindex.htm

or: www.FJZ-AJF.de

FJZ has a 0.1mfd 10KV capacitor supplied by FTCap Germany.

DFWB.

[DoctorWho]

Thanks for the input David. I did actually approach Ask Jan First, but he's out of stock at the moment, is there anywhere else you can get these high voltage condensers, the 0.1uf @ 10kv seem particularly hard to obtain.

Thanks for the tip about the resistors, what exactly is the internal CRT flashover? I shall be testing the set initially with an MV22-17, so will this be required?

Thanks again,

Peter.

[Framer Dave]

Fernseh's suggestion of a 100K resistor in series with the EHT feed was incorporated by Pye from serial no. 05001 (R79 in Trader sheet 894). Later, R49 (220K) was inserted in the A1 feed to the tube from serial no. 05701 for the same reason.

Internal flashover in the tube is due to the EHT arcing to other parts of the electron gun. This may well not occur with an MW22-17, but it will do no harm to have the resistors fitted.

Dave

[DoctorWho]

Thanks for the further clarification Dave, this is very helpful information to know. I'm hoping to get started on this set in the next few days, so I'll follow this through and make the modifications if they have not already been made.

[beery]

Hi Peter,
just a few extra notes...

Before you think of powering up the whole set you will need to pay attention to the following...

Replace the smoothing caps. Don't add a higher value cap here, especially the 1st electrolytic after the rectifier. The UU8 looks amazing on paper, but it its normal operating limits are a bit optimistic, so it will object to any extra loading.

Replace the foam rubber around the lopty and the anode lead for the EL38 line output valve. I have plenty of new foam rubber which you can have for free.

Replace the waxies. In this set values such as 0.1uF etc. come in the form of Sprague oil caps. These come in metal cans that are sometimes wrapped up in masking tape. These caps often work, so leave them in for the moment.

The worst job is replacing all those 5nF wax caps in the RF section. Some are hidden under the copper screening cans underneath the chassis. Make sure that the new caps will not short against the screening cans when they are put back.

That's it really. Other things to note... The insulation on the wires of the frame oscillator transformer often fails. Break off the old rubber insulation, unsolder one lead at a time and slide on silicone sleeving.
The most anoying thing about this set are those nasty presets. Depending the the condition of the ones in your set you may have to think of ways to replace them
Also another point. Don't try to unscrew the RF cores fully as this will break the ferrites which ar molded on the end of the threaded brass rods.

Cheers
Andy

[DoctorWho]

Thanks for the pointers here Andy, very much appreciated! It's also useful to know about keeping to the same value condensers, I remember having problems with the UU8 in the Ekco TSC48, so I presume it's a similar situation.

Thanks also for the kind offer of some foam rubber, I don't have any and so would be very grateful of this.

Am I right to assume that the 5nf condensers can be replaced by modern 4700pf ones?

Thanks also for the warning about the RF cores, i don't think I'd be touching them anyway, but it's very useful to be aware of a potential problem like this.

Thanks again, I'll let you know how it goes.

All the best,

Peter.

[Heatercathodeshort]

Visconol EHT caps are always faulty, even NOS ones. I had three .1uf 7kv ones and fitting them in one of my D16T's resulted in them snapping S/C in a few moments even after attempting to reform them. They only blew the mains fuses in my case and even the HVR2 survived. They have a date code at the bottom of the data printing. The metal type mentioned by Andy are made by DUBILIER and I have never had one fail, very reliable condensers, a first rate company. The bad guys are on the left in the attached picture. The good guy is on the right. Those sliders are a real pain and are very difficult and time consuming to modify. A double stack of conventional pots is the only answer so that the spindles fit the 'slots'. They loved these horrors prior to 1950..... Good luck with it. John.