GEC 5842 alignment

[andrewn]

I'm restoring a GEC 5842. I've got the radio working well on FM but the AM bands seem to have either a fault or alignment issue. All the stations were at the wrong locations on the dial so I adjusted L11/12 to correct that. The odd thing is I can't receive BBC Somerset on 1566KHz/191 metres even though signal strength is very strong here. The last station heard is Absolute Radio on 1215Khz then silence above it. Generally there is very little radio noise between stations and only strong stations can be heard. There is a continuous hum in the speaker like when an audio amp has no input connected however it disappears at minimum volume.
I'm fairly new to repairing valve radios so not sure how sensitive the set should be compared to a transistor radio.

[cathoderay57]

Hi Andrew, I wonder if you have replaced any other parts in the radio yet; if so it would be useful to know what has already been done. Second question is: does the dial read true on LW i.e. Radio 4 on 1500m, or is the station accuracy out on both LW and MW? In the absence of any other info I would suspect the padder capacitor C29 although these are typically silver mica types and rarely fail. Also the oscillator coupler C28 and the HT RF bypass capacitor C26. All component numbers refer to Trader Sheet 1231. Cheers, Jerry

[cathoderay57]

Also worth checking the voltages on the frequency changer V3 X719 compared to the service sheet: Osc anode (pin 8) 30v, Mixer anode (pin 6) 200v, Mixer screen grid (pin 1) 45v. The symptoms of seemingly failing to oscillate at the HF end of MW could be a result of low oscillator anode voltage (it's very low to begin with!) so if it measures <30v DC check R16 (100k).

[Station X]

Is the pointer or W.H.Y correctly aligned at the end of the scale?

[Silicon]

If I was working on this radio I would replace capacitor C57 and either replace capacitors C48, C54, C58, C61, C62 or verify that they were working correctly. I am using the numbers on the Trader service sheet.

Faulty electrolytic capacitors will give incorrect voltages and noise on the power supplies.

That could affect the RF and audio stages.

Also check R11, R12 and C22.

[andrewn]

Thanks for your replies. So far I have replaced all electolytic and wax capacitors. I also replaced all the Hunts 0.01uf caps as many were crumbling apart. Its had a new DH77 valve due to o/c heater. All other components are original. I did have to replace the tuning drive cord so I'll double check my work.I will take some voltage measurements and report back.

[andrewn]

I've checked the pointer was positioned correctly and found I hadn't positioned it exactly correct so I altered that but still no better so I measured the voltages on V3 as suggested. Pin 1=47V, pin 6=218V, pin 8 varied as I turned the tuning from 25-30V. I next checked the 100K resistor and found it measured 124K so I replaced it and now measure 35V or so. The new resistor still didn't make any difference but in the end I managed to bring in BBC Somerset but adjusting the local osc quite a lot (5mm) and trimmers T1&T3. Stations are now correctly positioned at the HF end but become inaccurate towards the LF end. On LW, R4 is found at 1600m and is so weak the volume has to be at max. I will most likely use this radio on FM so I think I can live with the poor AM unless anyone has any other ideas?

[cathoderay57]

If you have a multimeter that measures capacitance then disconnect one end and check the padder capacitor C29. Should be 420pF. If it measures a markedly different value that could cause the reported tuning misalignment. You won't find an exact replacement but should be able to find a 400pF and 22pF or a 390pF and 33pF to substitute if need be. Cheers, Jerry

[Skywave]

Originally Posted by andrewn

On LW, R4 is found at 1600m and is so weak the volume has to be at max. I will most likely use this radio on FM so I think I can live with the poor AM, unless anyone has any other ideas?

And in that regard, you have a P.M.

Al.

[andrewn]

Originally Posted by cathoderay57

If you have a multimeter that measures capacitance then disconnect one end and check the padder capacitor C29. Should be 420pF. If it measures a markedly different value that could cause the reported tuning misalignment. You won't find an exact replacement but should be able to find a 400pF and 22pF or a 390pF and 33pF to substitute if need be. Cheers, Jerry

Hi Jerry, I've checked C29 and it measures correct. I decided to replace R11 & R12 (Trader sheet, R13 & R14 on mfr service sheet) as they had drifted high. That didn't affect the tuning but I think it has reduced the audio hum a bit. I'm reasonably pleased with MW but the LW reception is terrible with its built in frame aerial. Connecting a long wire to the ext aerial socket worsens it and the trimmer has no effect. Could you advise if the radio is intended to be used with an external aerial? I tried a 10m long wire.

Regards, Andrew.

[cathoderay57]

Hi Andrew, do you have any other radios capable of LW reception, and if so, what's the reception like on those? Here in Bristol R4 LW is very strong. If the GEC has an aerial socket then an external aerial should improve things but certainly not make reception worse. It sounds like there's a fault in the front end somewhere, maybe an open circuit on one of the LW coils or a bad trimmer. The postage stamp trimmers sometimes get split mica dielectric or corroded plates. I'll have another gander at the circuit. Cheers, Jerry

[Graham G3ZVT]

I've got a BC5842 too
Just the opposite of yours, LW/MW is fine, but no VHF reception. The tuner voltages look reasonable but I don't think the LO is oscillating. Tried swapping the two X77, no joy. Bought a pair of used valves, still no joy, perhaps all four are bad.
How is your EM80? mine was shot. I bought a Russian equivalent described as "dim", which it is. Does anyone know if the brightness can be improved, even at the cost of longevity?

[andrewn]

Hi Jerry, I've just tried a modern radio and to be honest R4 is not all that strong though certainly a lot better than the GEC. I thought there used to be several stations on LW but other than one weak foreign station that's it. I'm not too worried because I don't usually listen to R4 and if I did want to I can use FM. Perhaps I'll try and test the trimmer anyway. I notice reception is much improved if I put my hand near the frame aerial but the wiring looks fine. Regards, Andrew.

[andrewn]

Originally Posted by rambo1152

I've got a BC5842 too
Just the opposite of yours, LW/MW is fine, but no VHF reception. The tuner voltages look reasonable but I don't think the LO is oscillating. Tried swapping the two X77, no joy. Bought a pair of used valves, still no joy, perhaps all four are bad.
How is your EM80? mine was shot. I bought a Russian equivalent described as "dim", which it is. Does anyone know if the brightness can be improved, even at the cost of longevity?

The FM works well on mine but sometimes the tuner cam gets stuck. I oiled it but its not perfect. The EM80 seems a bit dim. The green lines only widens on a strong signal.

[cathoderay57]

Had another look at the circuit on Trader Sheet #1231. If the LW alignment is out of adjustment anyway, the change in capacitance caused by connecting an external aerial wire could well make things worse. There is no oscillator trimmer adjustment on LW so if the LW osc alignment is still out then it is possible that you introduced the LW error when you previously adjusted L11. I recommend that you test the values of C17 (100pF) and C27 (460pF). If those are both OK then follow the alignment procedure as shown in the Trader Sheet, starting on MW first. If you don't have a signal generator then start with R5 Live on MW (693kHz or 433m on your scale). Re-adjust L11 until it appears at the right point of the scale (N.B. don't confuse with R5 Live also on 909kHz or 330m). Then find a broadcast station near 1.5MHz (I think you say you can get R Somerset 1566kHz/191m) and adjust C25 for max output. Then tweak C19 while "rocking the tuning" which means move the tuning one way and adjust C19. If sig strength improves continue in that direction until no further improvement occurs. If sig strength gets worse, turn the dial the opposite way and adjust C19. Re-check R5 Live on 433m. If out you are now supposed to be able to adjust the inner turn on coil L9. Then switch to LW and tune to R4 and adjust C18 for max signal, while rocking the gang as before. If that doesn't yield better results I would begin to suspect either somebody has previously tweaked the IF alignment or one of the fixed trimmer caps in the AM IF cans has drifted out. I don't recommend you fiddle with that unless you have a decent signal generator. Cheers, Jerry