Ekco AC76 - deaf

[agardiner]

Hi John,

Sorry for the continued confusion. So yes, I have aligned many sets with this particular signal generator. The only reason I had experimented with the connections mentioned in post 11, was to try and test the IF coil. So, forget that, to clarify.... aligning as per the service sheet, I can pass a 130KHz signal through the set, injected at the aerial socket and a milliampmeter across the preselector. I then adjust the IF coils in the order specified to obtain the lowest current reading.

However, the reading only alters by approx 1mA maximum, from 'out of alignment' to 'correctly adjusted'. This is clearly not enough of a movement. Also, I have to have the signal generator output set at quite a high level, certainly much higher than I would expect. The audio monitored on the loudspeaker is clear, but low level (amp checked and fine).

If instead of injecting the signal at the aerial socket, I inject into the grid of each valve in order to test the individual IF stages, then the level is in fact worse. So at the moment, I believe there is a series IF problem, probably in both stages.

Hope this makes sense and thanks to all for the helpful suggestions and comments.

PS - sweeping the signal generator proves the IF aligns for best results at the correct frequency.

[John10b]

Thank you Adrian, yes that makes it clearer. It does look like an IF problem, so best of luck with your fault finding.
John

[Ian - G4JQT]

Have any components been replaced that could have a lead soldered to the wrong place?

[agardiner]

I don't believe so; the only work I have done so far is to recap the set and replace 1 valve. Obviously I will double check.

[agardiner]

OK, I have spent another few hours on this set and have not really made any progress. I currently suspect the IF is off frequency, as feeding in a sweep while using the FFT mode on my scope, suggests the IF may be low at around 120KHz. I am also not confident that V1, an FC4 type, is working properly. Voltages measure fairly close to the service sheet, but gentle tapping of the FC4 can cause very loud ringing sounds.

My current question please... Can someone please confirm good places to inject an IF signal directly, rather than through the aerial socket. I would like to independently verify each IF stage, and I am not having much luck doing this. If I carry out alignment as per the service sheet, then I can get a small adjustment on the current meter, but hanging a scope anywhere, (including across the volume control) suggest a completely different adjustment. And no amount of adjustment appears to alter the actual frequency of the IF stage. This is why I am struggling to identify where the fault actually is.

Many thanks.

PS - just to confirm, hanging the scope probe close to V1 shows the oscillator is running on at least the LW and MW bands. Not sure at the moment about SW, but need to get the basics working first!

[radiomobile]

G4 (top cap) of the FC4 valve. Also consider disabling the oscillator while checking IF alignment.

[cathoderay57]

Hi Adrian. From your Post #11 it sounds like you need to establish why the IFs are not peaking as expected. You mentioned adjusting "slugs" but according to the Ekco Service Sheet the IFs have adjustable trimmer capacitors so I assume you're referring the the trimmer screws, unless a later official or unofficial mod replaced the IFTs with tunable dust cores (not meaning to nit-pick!) I can think of a couple of reasons that might be root causes. Firstly, it is not unknown for the fine wires connecting the IF coils inside the can to the trimmer solder lugs to become disconnected or broken. Obviously you'd have to carefully remove the cans to have a look. Secondly, I wonder if the station preselect/noise suppression circuit is working as it should? Service data says "When VR1 is turned to maximum, i.e. Strong position, 5v are developed across it and applied as a negative bias to the demodulator diode". If the bias isn't set right from the outset, or for example if VR1 has gone o/c, you might get anomalous results with either strong or weak IF signal injection. If not done already, make sure VR1 is set to minimum "all stations". Hope that helps

[agardiner]

Radiomobile - Many thanks. I have indeed tried injecting it on the top cap of FC4. Very weird results indeed. Very unstable, particularly if you tap the valve, and mainly just noise that I can see on the scope with hardly any IF.

Jerry - Many thanks, I do indeed need to find out what is wrong with the IF. You are of course correct regarding the 'slugs', which are of course in fact adjustable caps inside the can. I had started to think it was time to remove the cans for a closer inspection. The station pre-selector is working correctly I think; it certainly makes a difference when adjusting. I can in fact just about get R4 LW, just virtually no signal. The pre-selector does affect that reception.

[stevehertz]

Apologies if it's already been mentioned, but are all the DC voltages present and correct on the valves? That's how I discovered open circuit coils in the IFs on my ACT 96.

[agardiner]

Yes, all the voltages are roughly what are expected.

[Station X]

In order to get the IFT's of my AD65 to peak I had to add capacitance across the windings. I assume that some kind of aging process had changed their inductance.

[agardiner]

Hi Graham,

What value did you have to add? I know it may be different on mine, but it would make a good starting point to try.

Thanks,

[Station X]

Sorry, but I don't remember. I think I started out by using gimmick capacitors made by twisting two bits of wire around each other and then, once I'd found the correct value, substituting them with silvered mica types.

I also seem to recollect that some of the trimmers shorted at some settings.

[cathoderay57]

Statement of the obvious, perhaps, but volts at the anodes of V1 & V2 only means the IFT primaries have continuity. The secondaries could still be o/c. I don't think you have any choice other than to remove those cans and do some investigation. Cheers, Jerry

[agardiner]

Certainly that is my next step.

[frsimen]

Hello Adrian,

Is the conductive coating on the frequency changer and IF valve envelopes making good contact with the valve base? You can get all manner of problems if it isn't.

Paula

[agardiner]

Thanks Paula. I believe so, but will check before going any further!

[agardiner]

OK. This set is starting to drive me a bit nuts! I put it back on the bench this morning and have carried out the following tests.... (I have not opened up either IF can yet)

1) Measured IF transformer coils with LCR meter. IF1 - both primary and secondary measure approx 14mH. Primary of IF 2 measures approx 14mH. Secondary measures 51K which is correct as it contains R8 which is an internal 50K resistor.

2) Measured voltages on the valves.... V1 anode 243V (service says 275V), Oscillator 98V (service says 75V), screen 98V (service says 80V). V2 anode 241V (service says 275V), screen 146V (service says 160V).

3) Removed top cap of V1 and injected a 130KHz modulated signal to the top cap of the valve, via a 10nF capacitor. Connected my oscilloscope to grid 1 of V2. I am able to peak the IF transformer; input level 15mV P-P and max output level I obtained is 800mV P-P. This would seem to be a good level of gain.

4) Transferred the signal generator to grid 1 of V2, still with an input level of 15mV P-P. Connected the oscilloscope to diode 1 of V3, the output of the 2nd IF transformer. Again, I am able to peak the IF transformer. The output level on the scope was 1.5V P-P. Again this seems like a very good level of gain. The preselector correctly allows me to reduce this as I turn it clockwise.

5) Connected the signal generator to the aerial input as per the service sheet. Nothing comes through when the generator is tuned to the IF frequency. However if I select a frequency somewhere in the LW band, I can tune the radio to it, although the level is super quiet.

I have confirmed the local oscillator is running and at the correct frequency, although the amplitude reduces as the frequency increases; strong in LW, much lower in MW and very low in SW.

I am at a bit of a loss right now. I am thinking that maybe I need to try replacement valves for V1 & V3? V2 has already been replaced.

Any further advice gratefully received please.

Thanks all.

[cathoderay57]

Hi Adrian, next I'd check out the aerial coils and switches continuity. Make sure you don't have an internal short in the wire to the FC4 top cap. On the Trader sheet, two additional trimmer capacitors are shown in the bandpass aerial coil section, in parallel with the main tuning capacitor sections. On the Ekco sheet they aren't shown but would be in parallel with C1 and C2. If present, make sure those trimmers are not shorting. Finally are C10 and C11 OK? If all that checks out OK then I can send you a good tested FC4 to try out if you want. I suspect V3 is OK if you are getting output when you inject an IF signal, plus signal diodes very rarely fail (except EABC80s). Unless I'm missing something the AC76 is a LW/MW only set and yet in the previous post you mentioned SW Jerry

[agardiner]

Hi Jerry,

Thanks for the reply and suggestions. Will check the FC4 top cap wire and trimmer caps you mention. C10 along with R8 is internal to the IF can, so not checked as yet. C11 is OK.

So, you are right about the LW/MW only, and this throws a curveball into the mix. Can you confirm that the wavechange switch on this set continuously rotates? IE, LW / MW/ LW / MW etc if you keep turning clockwise say? There are no stops on the one I have and I assumed SW was present as when I measure the LO, there appears to be three bands as I rotate the switch. The LO frequencies measure correctly for 3 bands, and LW repeats every 3 turns of the switch. The various contacts appear OK, but clearly something is not right here.