GEC 4940 Radio

[sobell1980]

Thanks, Les. Post #35.
Great pictures, shall get on with that and make myself a bottom cover . Really enjoyed repairing this set. Shall enjoy using it even more. Thanks for your time to reply Les.
Kind regards.
DAVE.

[Phil G4SPZ]

Originally Posted by Herald1360

The TX primary looks like an inductor... if the speaker falls off the secondary, the primary then looks just like an inductor and any fast di/dt caused by impulsive signals can produce much bigger voltages. One reason for the parallel C is to limit these by storing energy and turning the whole arrangement into a damped tuned circuit.

Ah, that makes sense, thanks Chris. So the C is effectively there to protect the transformer and valve in the event of an open-circuit on the speaker side. I hadn't thought about it in that way before.

[MajorWest]

The theory behind rectifiers can be a bit confusing and there is something else I ought to throw in here:

As has been clarified already, it's the A.C. that's fed directly to the rectifier anodes so this is why the voltage would be tested for A.C. What's confusing is we normally test anodes for D.C. voltage but these other valves are further down the circuit connected to HT and here we're discussing rectifiers.

This may help anyone who ever wonders about this:

(1) When an AC voltage is fed to the anode of the rectifier valve only the positive cycles can pass because it's a diode.

(2) As with any other valve, current (electrons) still flow from cathode to anode but the actual voltage at the anode will cause the current to jiggle due to the alternating cycles. Also, A.C. can be imposed upon D.C.

(3)The cathode is the source of the D.C. ripple as it enters the circuit. This ripple is smoothed by chokes and the reservoir capacitor.

(4) Amazingly you can get negative voltage from the lower amplitude of the ripple which is how simple AVC is tapped into.

(5) More amazingly the D.C. ripple can be stepped back up to A.C. via capacitors.

It has been known to have back to front rectifiers where only a negative A.C. cycle can pass through the cathode to emerge as a pulse at the anode but that confuse me too and fortunately it's rare.

[Phil G4SPZ]

Can you please give some examples of points (4) and (5) where these techniques are used in domestic radio circuits? Thanks,

Phil

[MajorWest]

I think I refer more so to detectors where part of the signal voltage would be dumped and that the said voltage would be negative. It was usually tapped off and used as simple AVC.

If an alternating voltage is directed to the anode of a valve, we all agree I think only the positive half cycles will pass. As you know this principle can be used with anode bend detectors as well. But at the cathode it will be a ripple which is why the H.T. smoother is designed (as well as chokes). I'm saying the ripple itself still contains the essence of a sine wave so the lower amplitude part of it ought to be more negative (just attenuated).

Where it can get confusing is the electron flow. Even with rectifiers, it's the same electron flow from cathode to anode (0.001 Microsecond) but (not being a physics expert) I can only assume current behaves more like A.C. at the anode of the rectifier and probably is superimposed upon the D.C. flow. Hence the logic of the A.C. test at the anode and not a D.C. setting.

As for stepping up the ripple I think this was done with cascade systems and can forward you some more info. I forget now how exactly it worked but it was pretty complex and I think was used with amps as well.

[Herald1360]

Hmmph....

The current at the anode is the same as the current at the cathode- pulses of unidirectional current or dc with superimposed ripple.

The voltage, given a reasonably low impedance source, will be sine wave ac.

With no filtering and a resistive load, the current will be half sine wave as will the dc volts at the cathode. Add a capacitor filter and it all changes. In a perfect circuit, with no load, the capacitor would charge up to peak volts on the first positive half cycle and no further current will flow. Add a load and the capacitor will start to discharge into the load as the input voltage passes its peak but if the capacitor is big enough it will only discharge far enough to drop a few volts before the next positive half cycle comes along and recharges it as soon as its voltage passes the point the capacitor has fallen to. The recharge current will be a short peaky pulse that just replaces the energy taken from the capacitor.

[sobell1980]

All very interesting. Just trying to absorb it all and take it in and try and retain it.
So do you chaps feel I should change these 2 caps C18 and C19 to a higher rating say 1kv? Or leave the 630 V in place? I rarely have the volume up more than half way. Many thanks for all your replies.
Dave.

[MajorWest]

To me, there are similarities between the rectifier and the detector, except with detectors it's an RF modulated signal that's passed through the anode - only the positive parts of the alternating cycle are passed as we know. In the diagram below, it's shown how the capacitor is being charged through the positive cycles and how it may be possible to tap into the negative D.C. for AVC at the chassis side of R.
I did think it was clever how they used to use the electro magnet around a speaker coil to smooth any rectified DC pulse and I think it was close to 100 volts that was dropped around the said coil.
I forget which battery set had the back to front rectifier valve.

[sobell1980]

If any one of you could help me make a final decision on my post #47? Some say it is fine, leave as is with 630v and some say with this high voltage back EMF they are not do sure.
Dave.

[Herald1360]

It'll be fine. Worst case if it fails it's unlikely to damage anything else.

[Herald1360]

I think posts 43-46 & 48 could well go in a separate thread in the Components and Circuits section. They don't seem relevant to this thread and only marginally to the other BC5243 one.

They are interesting as a discussion about rectifiers and detectors, but not about this set's returnto working order.

[sobell1980]

Thankyou Chris. Much appreciated.
Dave.

[sobell1980]

This set is now all complete and I'm just putting the chassis in the case. I did notice however that the voltage selector tag strip has markings for 215 - 240 v and 240 - 260 volts for the tapping on the main transformer. I've measured my mains voltage coming into the set on the live and neutral wires which 241 volts ac. At the moment the voltage screw setting is set to 215 - 240 volts. Is this OK as is? Or, should I have it set 240 - 260 volts. This is how I received the set and have not moved it. Because there are two bands of voltage selection that include 240 it's confused me which one to go for. I don't want to be overloading the set or valves etc. Set is working like a dream now. Many thanks. Looking forwards to completing the project.
Dave.

[Herald1360]

Check exactly what heater voltages are. If they're less or equal to the rated, leave it be. If they're higher, go for the 240 up tap.

[sobell1980]

Thanks Chris

Will have a look tomorrow . Kind of obvious now you say that. Especially after the Roberts RMB I did and all the trouble I had with the LT on that! I probably have already checked the heater and anode voltages in the repair after changing the smoothers, however I will go back and be doubley sure.
Thanks again.
DAVE.

[Nickthedentist]

Hi Dave,

If it's been run for a long time from mains with a higher voltage than what the selector has been set to, the valves may have been spoiled. In fact, it was a common trick to get more life out of worn-out valves.

Nick.

[sobell1980]

The valve heaters are fed by a/c from a tapping on the mains transformer. This is noted on the diagram as reference to connection "a" on all the valve heaters and dial lamps. After a short time both brand new bulbs blew and being fed the same voltage as the heaters this is bad news. The set still works but measuring the bulb holders and heaters I have 7 volts a/c !!!! Obviously this is far too high but I can't find any reference to what the heaters should be running at? The valve data chart only gives one or two cathode voltages? But nothing in a/c for the heaters? I obviously have to swop over to the highest voltage tapping but could anyone tell me what I should be seeing at the heaters? This was 7 volts a/c with my negative lead to chassis with a DMM. Many thanks,
Dave.

[Phil G4SPZ]

The typical rated heater voltage was 6.3 volts AC. Check that you are using the correct dial lamp bulbs; 6.3 volts 0.3 amps was common. A lower loading could result in a higher voltage, but it's unusual to find a heater voltage as high as 7 volts. Can you check it using a conventional moving coil multimeter, rather than a DMM?

[Herald1360]

7V is only about 10% high which isn't good but isn't disastrous either. It's still not quite as high as an on charge 6V lead acid battery. Tapping the mains up to the 250V setting will get it down somewhat- maybe even to nominal 6.3V or thereabouts- it looks like the taps are about 10% apart. It's not unlikely that the original design would have gone for correct volts at bottom of tapping range rather than allowing volts to be low at the bottom.

A lot of sets used 6.5V bulbs for longer life- they're still available, or you could look for a 6V 2W automotive bulb which will cope with a 7.2V on charge level.

[G8UWM-MildMartin]

I completely agree with Chris.
It would also be interesting, even if not directly helpful, to know the type and rating of bulb that has failed so quickly, the accuracy of your DMM (is it a cheap DT830 type reading, say, 7.0V +- 10 digits +- 2% , or something better?) and your mains voltage.
Martin.