Gec BC5645 caps.(Mw/LW/FM) 1960 ish

[Pete_kaye]

I have got this one working but with poor volume and slight distortion .It is totally original and might be the original GEC valves.i measured the voltage to the grid of the output valve and it read 6V DC so I switched off.
There are 4 large wax caps as well as numerous smaller Hunt's types.i have removed the suspect cap but cannot read the value >it could be 0.01uF 100v or 400 or 1000v .One of the other similar looking ones are 1000V,connected to the tone control;would they all be the same?

[Tim]

Hi Peter.
I had one of these in about 2 years ago( and I still haven't got round to finishing the report........
ISTR I replaced all the waxies and Hunts moldseals, + low voltage electrolytics.
Low volume in my set was caused by lower than normal HT (U78 only 10% emission on one half!) and a duff N709/EL84. I only had 2.5 volts at the grid, so yours might be even more tired. ISTR also the tone control is c/w the volume, but not sure if it is in the output valve anode circuit OTTOMH. If so use 1000 volt caps, if not there is no need. In the latter case 400 volt types will be fine throughout the set. I used some PCB mounting types, as many of the Hunts connections weren't too far apart.
Not bad sounding set when done, but my example wasn't fabulous on VHF. I think the EF80's(Z719?) were at the limit of performance when the set was new! It will need a GOOD VHF aerial. Good luck
Tim

PS I have this one down as 1956

[Nickthedentist]

As I'm sure you know, it does absolutely no harm to err on the side of caution with this type of cap, and use something that might be rated for a higher voltage than the original. Having just one highish rating simplifies stock-keeping too.

This is in contrast to electrolytic caps, where it's theoretically best to use something whose voltage rating is the same or not much higher than originally fitted.

Nick.

[Sideband]

I've got one of these. 400v types are quite suitable except the tone corrector which is usually rated at 500 - 1000 volts...that's the one across the output transformer primary. Better to use a higher voltage type here.

Tim you're right about it struggling on VHF especially at the high end. I found if I tried to increase the high end, the oscillator just stopped.



Rich

[Steve_P]

Just looked at the service sheet.

THAT capacitor is 0.01uF. 630v are the ones I use, but 400v would be OK. C53 (Tone Control) is also 0.01uF. Most are 0.01 uF by the way.

Don't power it until THAT cap is changed. This is C54.

Trader Sheet 1336.

Cheers,

Steve P.

[Pete_kaye]

It was finished as far as I was prepared and time to reassemble-however when refitting I had a problem relocating chassis onto lugs so jiggled around until Ok.The valves were out at the time .
I tested just before fitting the back on and both Mw and Lw were absent while FM still OK.I took it out and thought I had possibly damaged the internal aerial which is a spider coil for Mw with LW in the centre.
MW worked if I pressed on the paxolin former in one spot...I took it out and measured the resistances of coils unsoldered in isolation .The LW coil as 29R but 15R on circuit diagram ,the resistor across it read 130K but 100k on colours so I replaced it.The mw coil was about 1 ohm(not specified) and not affected by finger pressure.I put it back together and no change at all except I noticed some weak MW stations on LW ONLY IF I touched the coil.I had a look at the switch and noticed some gunge yellow powder underneath and remembered I had sprayed the switch with proper cleaner just before reassembly.I then found out the hard way that there is high DC voltage on the top pins of the switch,fortunately with a neon tester.What now?

[Steve_P]

Ooops!! Check the wavechange thourougly and all the contacts. Spray liberally with switch cleaner and work the switch back and forth, then leave to dry. Then spray again.

Check all the wiring and make sure nothing has come off anywhere.

Cheers,

Steve P.

[Pete_kaye]

Some progress...I found a 0.01 400V capacitor that had become disconnected when I was replacing chassis .It was connected between Aerial and Earth connections .This got MW back but no joy on LW .The switch looks OK.
I was wondering about soldered connections to the LW Litz coil.I have checked resistance and it is still 29R.(T.sheet says 15) and have good blobs of solder on ends.I ahve looked at which components are switched in for LW and there aren't many but theyall seem OK.What do you suggest now?

[Steve_P]

Looking OK does not mean it is! Check it properly. Resolder and check everything and make sure the Oscillator is going on LW.

Another radio will tell you. 198 kHz plus 470 kHz should put you in the area. Transistor sets do have their uses. Or make sure the Grid of the Triode is Negative.

Cheers,

Steve P.

[Pete_kaye]

I tried the oscillator test and got a high pitched whistle in the transistor set around 660KHZ It had 3 peaks very close to each other.I also tested on MW and got a similar effect when valve set tuned to 1089Khz on ~640KHz(1089-470).What is this telling me?
I also found I can get LW 198 R4 weakly with the main LW coil removed plus Dublin (louder) and some MW stations IF I hold the MW coil tightly.All very mysterious.

[Steve_P]

If the switching's OK, change C21, C20, C17, R6 and R5.

I've got a nasty feeling the coil's been damaged. Check all the wiring to the aerial and then take the coils C6 and C7 out and check them thourougly. L6 should be about 0, L7 should be about 15 ohms.

Cheers,

Steve P.

[Pete_kaye]

Well the problem has gone away and I think it was a poor connection from the LW Litz coil as I took it out ,trimmed the ends ,cleaned and resoldered them back in and Lw was normal again.It is now back in the cabinet and working well.I didn't mention earlier that I improved Lw reception by fitting a 3 -ply mains lead and earthing the chassis.This got rid of background noise,particularly at night.
I am now looking for a FM range signal injector as it seems to be slightly down on volume and tuning is not precise as my other VHF set(Stella239).thanks for the advice

[Sideband]

Originally Posted by Pete_kaye

Some progress...I found a 0.01 400V capacitor that had become disconnected when I was replacing chassis .It was connected between Aerial and Earth connections .

Not sure how much difference this will make but according to the circuit (and the one I have just restored) this capacitor should be .005uF (nearest preferred is .0047uF). As it's bottom-end coupling for the frame aerials, it could affect sensitivity.

FM is slightly lower volume than AM. This does not necessarily indicate a fault. Tuning also seems a little more critical as well. I think it's just the way they are. Make sure that the 5uF capacitor in the ratio detector is in good condition. A 4.7uF will be OK as a replacement. If this capacitor is faulty, it will cause distortion and make tuning critical/impossible. Note POSITIVE to chassis with this!

Rich.