Defiant RGSH 755 EL84 problem

[PaulR]

On further investigation I have found a .0047uf cap between pins 2 and 6 of the ECC83 which doesn't appear on the circuit diagram. I have to admit that I put it there but I wouldn't have just imagined it in the first place.

Removing the capacitor stops the oscillation but the only connection to pin 2 is then a high value (around 20m) resistor to pin 3. Connecting a 470k resistor from the end of C32 to pin 2 restores a signal to V8, but this is a little lower than that to V8 according to my ancient oscilloscope. When the signal to V7 is around 2v that to V8 is around 2.5v.

It seems that my set doesn't follow the circuit diagram and I haven't been able to find any later ones. Would the advice be simply to forget the .0047 cap and provide the signal to the second half of the Ecc83 directly from the coupling cap? Will the signal difference affect the sound quality much?

Thanks

Paul

[Station X]

Why not just wire it as per the diagram which looks correct to me?

[PaulR]

Yes, I suppose that is best. I wonder why it was altered. There are several resistors missing on mine. For example there aren't any in the signal path between the phase splitter and the output valves.

I got my V7 and 8mixed up in my earlier post. V8 is connected to the 2nd half of theECC83 and has the lower signal.

[PaulR]

I have decided I cannot wire it up as per the circuit diagram as there are too many differences. For the moment I have just connected the grid of the second half of the ECC83 to the output from the first half - post capacitor of course. I still cannot understand why a separate capacitor directly coupling the two should cause feedback.

Anyway, I have now discovered that one end of the track of the the tone control is directly connected to the anode of the first half of the ECC83 which of course brings it up to HT voltage. Is this usual? I have never seen it before.

[Station X]

Thread reopened at OP's request.

[PaulR]

I found the idea of re-wiring the set to agree with the circuit diagram rather overwhelming so I put it on the "failure" pile in the loft. I was having a clear out a couple of weeks ago and decided to have another look at it and thought I would have a go.

Mine was wired to use what I think is called a cathodyne phase splitter rather than as per the circuit diagram. Many of the associated resistors were missing. Unfortunately there isn't a Trader sheet for the set showing the component layout so I had to make it up as I went.

I fitted a new tag strip and set about it. I am sure that the result is nowhere near as professional as it should be but after realising that I had connected R35 on one of the grids of the ECC 83 to the HT rail rather than earth I managed to get sound from a CD connected the the PU sockets. Nothing from the radio section yet but at least it was a start.

I put a sine wave into it and got very similar RMS signals from the anodes of the EL84s (4.48V and 4.37V according to the 'scope) but the anode voltages are very high:

V8

Anode 328V
Screen 328V
Control Grid 0.27V
Cathode 12.16V

V7 Very similar except the control grid is 0.66V

I don't know whether these voltages are anything like right. Can anyone comment?

Thanks

[PaulR]

I have found a couple of circuit diagrams of 10w amps that have similar voltages despite the maximum shown on the EL84 data sheet showing the maximum HT being rather less than that so I am assuming it is ok. It certainly sounds good with a CD connected to the PU sockets.

I am now struggling with the radio section itself. Both AM and FM are very deaf but I haven't connected FM to a proper FM aerial yet. I have tested the valves by substitution from a working Bush FM61. I thought I would concentrate on AM first. I can get R4 on LW weakly with a lot of background hiss and nothing on MW except the strong signal from my Sstran transmitter.

The ferrite rod AM aerial mounted is on a turning mechanism controlled from a knob on the front of the set. The mounting of this was broken so I repaired it using a piece of card soaked in shellac metal varnish and drilled and tapped the top of the mount. The coils have continuity and are still firmly fixed to the rod in their original positions. I think that the trimmers had been got at as I managed to improve the signal a little by adjusting them.

I put a 470kHz signal onto the grid of the ECH81 and got some sound from the speaker. I tried to peak the AM IF transformers. I think that these had also been got at but managed to peak all but the upper adjustment of the second IF. This has a very lon,g flat "peak" lasting for most of the travel of the slug, only reducing the output towards the ends of its travel.

Can anyone offer and advice please? I am attaching a couple of photos showing my rather unprofessional work on the AF section and the rest of the set.

[ms660]

Originally Posted by PaulR

I put a 470kHz signal onto the grid of the ECH81 and got some sound from the speaker. I tried to peak the AM IF transformers. I think that these had also been got at but managed to peak all but the upper adjustment of the second IF. This has a very lon,g flat "peak" lasting for most of the travel of the slug, only reducing the output towards the ends of its travel.

Can anyone offer and advice please?

AGC action (the input signal should be as low as poss. to avoid the possibility of AGC action)

Or low Q by design or fault.

Lawrence.

[PaulR]

I suppose that faulty AGC would make it deaf.

The thing I feared about the IF itself was that one of the padding caps might have failed. Would that affect the width of the resonance of the transformer?

[ms660]

Yes, resistance affects bandwidth.

The output from the signal generator should be low enough so that the signal voltage presented to the AGC detector doesn't exceed the AGC delay voltage.

Also be aware that in most receivers it's the 1st (outer peak) you want to tune the IFT's to.

Lawrence.

[PaulR]

Right, so in view of the previous twiddling of the transformers should I wind the slugs right out and look for a first peak? I have only ever had to make small adjustments on previous sets to bring pretty well tuned transformers to a peak.

It is quite a nice set. The front control that turns the ferrite rod switches the set to "gram" at the end of its range. The knobs themselves are heavy and well made. There is a long switch on the chassis to change from AM to FM rather like that in some dual standard TV sets.

[ms660]

Normally you can tell if they've been screwed right in, if you do decide to adjust them do one at a time, starting with the last IFT, secondary first then the primary, with the signal generator input to g1 of the IF amplifier. After that connect the signal generator to g1 of the mixer and adjust the 1st IFT, secondary first then the primary, again keep the signal level low as possible to avoid any AGC action.

Lawrence.

[PaulR]

Ok, thank you.

Unfortunately the circuit diagram does not have a diagram of where the components are so I don't know which is the primary or secondary. Is there a convention as to which is which? The covers do not seem to come off them as, presumably, they have tabs underneath which would involve removing the transformers to see where the connections are.

[ms660]

I have no information about the receiver, I would suggest adjusting the top first then the underneath but if the two slugs (as found) are close to each other I would check the tuning again after the initial adjustment.

Lawrence.

[PaulR]

There is only one peak for each transformer. I have managed to peak all of them. The voltage required was a couple of mV for the second IFT and a couple of microvolts for the first one.

I have just noticed that the reception is much better if I hold the coils on the ferrite rod. Connecting a wire to the aerial sockets doesn't help much though.

I found what I think are the two trimmers for them but cannot get the signal any better. The rod was loose in the case when I got the set but still attached to its connections. The coils are firmly fixed to the rod so I think they must be in their original positions.

Does this help at all?

[ms660]

The coil or any of its associated adjustments might be out of alignment, hard to say for sure without being there....Usual rules apply when doing alignment....IF, Cursor, Oscillator, RF in that order, keep the input signal low.

Not sure about the external antenna, if it has SW maybe the external antenna is only connected on that band.

Lawrence.

[PaulR]

Thanks Lawrence

I had another go at a full alignment this morning and it is improved. I also replaced a cap that a previous repairer had fitted which was a different value to the one in the circuit diagram. The output isn't very powerful but it is ok for the amount that AM will be used.

FM is distorted but I haven't touched that yet. I will try a new discriminator electrolytic first and take it from there, being very wary of the final IF alignment.

The more I look at the set the more it seems to be high quality. Even the back is plywood rather than the usual cardboard, and it has two speakers, a larger and a smaller one. I wonder how much it cost new.

[PaulR]

A new electrolytic did not help. On looking at it, it had already been replaced so it seems as though someone has struggled with the set in the past. As there are no alignment instructions and in view of the apparent previous attempts to align it I think I will put it back onto the failure pile until I get more inspiration. Possibly some of the many disc caps used in it may be leaky.

The AF section sounds really good when a CD is played through it and I have repaired the ferrite rod mounting so I have at least progressed somewhat with it.