Westminster CTA5350

[ViragoRider]

I picked up a Westminster CTA5350 radio as I rather like the imposing look of it. My visual inspection has discovered some modifications that I would appreciate some advice on how I can put it back to factory spec. I have included some images for reference.

Firstly, the aerial socket isn't connected at all and there is a floating ground wire that presumably is no longer used because of the modifications to the original circuit see the first image where I have marked the wire and the disconnected aerial socket.

At some point a huge electrolytic has been added - second image. It is a 16-16-16 uf @ 350v and is attached to the chassis by a single self tapper using a vacant valve screw hole. I'll replace this with something more appropriate as I don't like the idea of high voltage being exposed like this.

The third and most troubling issue is the band selection switch. This should be a 4 position switch but the one in my set has just 2 positions. I have no idea yet how this has been wired as I'm struggling to see without disassembling the switch. Note from the two images that the switch is not properly fitted, the two holes for the assembly are not in use, only the main nut is holding it in place. I thought the switch might be a little seized so gave it a bit of force to see if it would move beyond the two positions but all that did was loosen off the nut holding the switch in place. I'm going to take the switch apart to see if i can make some sense out of it. At the moment, I'm flying blind although I do have a copy of the schematic.

I should add at this point that the set came fitted with a round pin 3 pin plug, I've not seen one of those for many a year, so it's safe to say the set hasn't been used for a very long time.

[Station X]

Do you have the circuit?

I have two of these sets and one of them had a rather tatty circuit diagram glued to the inside of the cabinet. Subsequently a forum member redrew it for me. I can email you a copy if you like. I could also take pictures for you, but it may take a day or two for me to get a round tuit.

[Station X]

Circuit attached. You may have to zoom it.

[ViragoRider]

Thank you for the schematic. I would appreciate some photos of the switch assembly to compare it to mine and also the wiring from the aerial socket. This would be a great help.

[Station X]

Today I finally found a "Round Tuit", so I opened up one of my sets and took some pictures. I actually took more pictures than I posted here.

The aerial socket is wired to a "Coil Pack" with the wave change switch inside it. Your set appears to be missing its "Coil Pack"?

If you are desirous of owning one of these sets I have two and would be prepared to let one go. Transport might be a problem though. I am extremely reluctanct to post or courier Bakelite sets.

[ViragoRider]

Hi Station X, Thank you for getting these photos online. It looks like my set is a bit of a lost cause as there's been too much alteration done to it. The missing Coil Pack makes it beyond reasonable repair. A Shelf clean it is then and I'll seek out a different set. Thank you also for your offer to part with one, but I'll decline on this occasion. I do think they are good looking sets though.

[Station X]

AF sections might still be working in which case you could feed audio to the PICK UP/GRAM socket or use Blue Tooth?

[ViragoRider]

Hi All,

I managed to find another Westminster CTA5350. The seller was only asking £10 for it so I couldn't refuse. This one also has all its innards and on first power up today, I managed to tune in Radio 4 on long wave so this one is worth resurrecting I think.

I haven't done anything to it yet and have a couple of questions before I go 'wading in'.

Although I can tune in one station, the set is very quiet, barely audible even on full volume. I suspect that will improve when I change leaky caps. I have the schematic that Station x (Graham) kindly sent me and I note C13 is a 0.002uf at 1000v. I only have 630v rated caps, Will these be okay to substitute or do I need that extra voltage handling for that cap? Similarly, C10 is rated at 750v, so the same question applies there. I note on the schematic that the electrolytics are not polarity marked, so I'm assuming the ones already fitted are in the correct orientation and will replicate the fitment. For C15 (25uf @ 25v) I have a 22uf @ 50v. Will that be okay to substitute or should I get something a little higher? I've heard that if no exact match is available, a higher value can be used but I wondered if the nearest lower value would suffice in this instance?

There aren't any voltages marked on the schematic, so I have nothing to reference in that respect. Is there a general "rule of thumb" for voltages in situations where voltage references aren't stated? I obviously have B+ and filament voltages and the radio is working in a fashion. I don't have a valve tester but I do have a spare set of valves from my first CTA5350 that I have swapped out to test them but there was no appreciable difference in the performance of the radio.

The first set I purchased had a chocolate brown dial glass and this set has a cream dial glass. They are identically marked so I wonder if they changed the colour from one iteration of the set to another. I can't decide which one I like the best.

[cathoderay57]

C13 will be fine if it is a 630v polyester. The required voltage rating of capacitors is usually determined by the HT voltage of the set. Measure the HT voltage across C14. Assuming C14 and C16 and the rectifier are in good condition it should be somewhere in the region of 250v dc, in which case a 400v rating for the remaining caps (replacing paper ones) should be fine. In sets where the smoothed HT exceeds 300v dc it's better to use 630v caps. For electrolytic HT smoothing capacitors then for a set such as this capacitors rated at 450v will be OK. In an AM set the negative connection of the electrolytics goes to the point at or closest to chassis potential (earth). In FM sets (not this one) the electrolytic across the FM ratio detector diode (frequently an EABC80) is the exception where the positive lead goes to chassis. Don't assume that the way you find the existing electrolytics connected is correct - a previous repairer could have made a mistake. C15 at 22uF 50v is fine and there will be no audible difference. The tolerance of old capacitors was often wide so the original might not have been exactly 25uF even when new. You need to plug an aerial wire into the aerial socket to get any sort of decent reception; it won't work without. Jerry

[Station X]

Please be aware that in the two sets I repaired the tone correction capacitor was not connected as shown on the diagram. IIRC it's connected anode to ground rather than across the output transformer or perhaps it was the other way round?

[ViragoRider]

I spent the weekend recapping the set and the volume on long wave is now considerably higher, my dim bulb doesn't glow at all now with a 100w bulb in place whereas there was a feint glow before the recap.

With an aerial connected and me coupling to it by holding the wire I get a feint signal on the stronger MW stations and also some popping on SW, but there is no appreciable audio output on these two bands. I have done the trick with a detuned AM radio and can hear the oscillator running on all bands so I'm going to check the coils on each circuit. I'll use the LW coils as a reference measurement. I realise this will just be a comparative measurement and I don't know what to expect at this stage but I have to start diagnosing the poor reception somewhere. I've cleaned the switch assembly and the tone and volume pots and they work well without any scratching.

I also don't have any alignment procedure for the set although I am a way off that stage at the moment. My tone correction cap was also connected anode to ground rather than across the output transformer.

[cathoderay57]

Voltage measurements (between valve electrodes and chassis) for the anodes, grid2 and cathodes of the valves would be useful. Although there is no service sheet available to provide a spec value, the measurements should give us a clue as to whether the voltages are in the right order. Jerry

[ViragoRider]

Here are the voltages I measured:
Valve Anode Volts Grid 2 Volts Cathode Volts
V1 - 7S7 AH 186 / AT 72 110 0.055
V2 - 7B7 184 108 2.67
V3 - 7C6 94 0.718 0
V4 - 7C5 266 0.014 8.8
V5 - 7Y4 279 / 279 a.c.

Looking at the resistors I note that R4 according to the schematic is listed as 56k but on my set, there is an 82k resistor there which I have measured at 98k - on the edge of the high tolerance. I noted from the photos that Graham (Station X) posted, there is also an 82k resister in that position. Could this have been a design revision?

I am getting healthy reception on SW now - turns out I just needed to wait for evening time. MW is the only band that is troublesome now. The voltage measurements were taken with a Fluke 77 with the set switched to the MW position.

[cathoderay57]

Thanks for posting the voltages. V1 cathode voltage seems very low; V1 triode and heptode and V2 all share a common cathode resistor and so the current through R3 should be somewhere between 10-20mA giving a cathode voltage of 1.5-3.0v. It would be towards the low end if you measured voltages while tuned into a strong signal since the AGC would tend to turn down both V1 mixer section current and V2 anode current. Worth checking C2 since if it has gone near short-circuit that won't help; maybe also V1 and V2 have lost some emission. V3 cathode should be at zero volts because it's connected directly to chassis. V3 anode voltage looks reasonable. Re-check grid 2 voltage of V4 (it cannot be 0.014v since it is connected directly to the smoothed HT line and you are getting HT at V1, 2 & 3). V4 anode voltage looks reasonable but the valve is only passing 27mA through its cathode resistor indicating perhaps that the valve has only about 50% emission. Once you get the dc voltages sorted out it sounds like the mixer section needs an alignment tweak on MW. Don't mess with the IF tuning slugs or trimmers if it is OK on the other bands. Hope that helps. Cheers, Jerry

[cathoderay57]

I notice on second reading that you quote a cathode voltage on V2 of 2.67v which as you can see from my reasoning is about right. Thing is, V1 & V2 cathodes are connected together so you must have mis-measured the voltage at V1 cathode. Given that, the good news is that the voltages are all probably OK. J.

[Techman]

Originally Posted by ViragoRider

my dim bulb doesn't glow at all now with a 100w bulb in place whereas there was a feint glow before the recap.

If the lamp limiter is still in line (and I suspect it might be) the set won't perform properly and also the voltage readings will all be wrong. I've noticed this happening a number of times lately, so it needs to be explained to 'newcomers' that the 'dim bulb/lamp limiter' is only for initial safe testing purposes and not to be left in line when fault finding and when taking meaningful voltage and current readings.

[ViragoRider]

Rechecked the suspect measurements and the correct values are as follows:

V1 and V2 Cathode 2.46v
V4 Grid2 175v

R3 is reading 197ohms which will give a cathode current of 12.3mA, is it worth changing this to up the current a little?

I remeasured the cathode voltage on V4 and it is showing 8v today. R14 measures 380ohms giving a cathode current of 21mA - even lower than it was on the first reading.

I'm not using a dim bulb for these readings and suspect that minor differences could be down to line voltage fluctuations and the set not being fully warmed up. The set is also not tuned to any station and the tuning cap is fully open.

The resistors are all measuring on the high side of tolerance. R5 should be 47k 10% but is measuring 61k - Would changing this make any significant difference?

Thank you for the guidance - Pat

[cathoderay57]

Hi Pat. R3 is only just over 20% tolerance and replacing it won't make a lot of difference. I would replace R5 since the triode oscillator anode voltage is maybe a tad low and R5 is 23% high. It might increase the level of oscillator injection slightly but probably won't make a vast amount of difference. Worth checking R15. If it has gone high it will reduce HT supply to everything except V4 anode. If that is OK and V4 still draws a total of 21mA (including g2 current fed via R15) than it is probably low emission. That said, at 50%-odd emission or even lower, output pentodes can still function quite happily and provide years of useable output volume. Replacement valves for this set are not too difficult to find and not too pricey but the overall benefit is likely to be negligible. Cheers, Jerry

[ViragoRider]

Hi Jerry, I'll replace R5 and R15 is measuring 5.9k - 25% high so that has to go too. I don't have any of those in my stash so will have to put an order in. Thanks for the advice. Pat

[ViragoRider]

I had a spare V4 from the other chassis I've got (the one at the start of this thread that is missing its "coil pack" so I swapped it out and the Anode voltage now reads 184v - up from the 175v measured with the original V4 in place. Cathode voltage on this V4 is 8.4v