Ekco AC76 - deaf

[agardiner]

Hi all,

I would appreciate some advice with an Ekco AC76 that I am working on.

Replaced electrolytics; checked and changed a few other capacitors, including C24 (output grid cap), C19, C18, C29 & C12. Also replaced V2 as it was reading very weak.

On powering up the set, it does come to life, and injecting tone into the volume control proves the AF side is good. However, it is virtually deaf; just lots of RF noise, with only a very weak Radio 4 on LW in the correct position. I tried the IF alignment as per the service sheet, and signals are passing through the IF at 130Khz. They are weak however, and the mA meter connected across the pre-selector, reads 6mA with no variation at all. Without the meter connected, the pre-selector appears to work correctly, as adjusting it substantially varies the amount of RF noise.

Can someone please advise what to look for next?

Thanks all.

[agardiner]

PS - for clarity. When trying the IF alignment, my signal generator (Advance E2) had to be on the highest x10mV setting. So there is no doubt that the level of signal through the IF is far too low. It is bang on 130KHz though.

[John10b]

Just took a very quick look at circuit, I wonder if switch S6 across first IF is faulty? Just as a quick check operate the switch several times just to see if it makes any difference.
John

[agardiner]

Thanks John. Not the culprit though. I have already had a good check and clean of all the wavechange switches.

[stevehertz]

Having experienced this 'effect' recently when restoring an Ekco ACT-96, on no less than three occasions it was open circuit RF coils housed in cans. Three rewinds needed. Otherwise check and thoroughly clean all connections be it valve bases, switches, whatever.

[cathoderay57]

Hi Adrian, have you tested the FC4, or tried a replacement? If the valve is OK, has the top cap wire been replaced? I have repaired a couple of sets (albeit not round Ekcos) where audio grade coax has been used with too much capacitance which kills the mixer sensitivity. Jerry

[John10b]

Just for clarification Adrian, did you say the second IF was way ok or lacking ?
John

[agardiner]

Hi Jerry; FC4 has been tested and the emission is good. Top cap wire looks original and is in good condition.

John, I have not yet tested the IF stages separately to determine this. I don't personally suspect an IF issue, as plenty of noise is coming through as you tune. It feels like an RF issue. I suspect Steve is on to it with the RF coils.

[John10b]

Adrian can I draw your attention to your post #2, that is why I asked the question, just for clarification. Of course Steve may be perfectly correct in his diagnosis.
John

[agardiner]

Thanks John. Good point. So far I have only checked alignment as per the service sheet, which states to inject the test signal at the aerial sockets. I will however check at each IF section before starting to dismantle the RF coils.

[agardiner]

OK, so not really any progress so far. I have carried out a couple more tests though, and perhaps someone could humour me as I am probably being a bit thick.

Attempting alignment as per the service sheet - the current meter does alter as you adjust the IF coils, but not by much. If I feed a modulated IF signal into the radio, then I actually hear a louder tone if I de-tune the IF coil from the optimum, (as per reading on meter).

So, I decided to unplug the radio, and hook up the signal generator directly onto the input of the IF coil, with the scope directly on the output. I also fed the signal generator into channel 2 of the scope for comparison. Note, the coils are still in the set; I didn't remove them. I noted the following....

1st IF coil - Output signal is much lower than the input, and 180 degrees out of phase with the input. Adjusting the IF slugs, does alter the output signal, and it does peak at the expected 130KHz. However it always remains at around 50% of the input level.

2nd IF coil - Output signal is approximately the same level as the input, and in phase with it. Adjusting the IF slugs doesn't really make any difference, (it does make a small difference when aligning as per the service sheet). Sweeping the input signal also doesn't appear to make much difference until way off.

To me, this indicates that both IF coils are faulty. Am I correct?

[Silicon]

Unless you have disabled the AGC circuit, you should use a very low level of signal injection when aligning a set.

[stevehertz]

Unless there is clear evidence that someone has fiddled dramatically with the IFs, then I don't think that an attempt at realignment is going to do much good when there appears to be a fault in place as opposed to a misalignment situation on a set that's basically working. I know it's basics, but make doubly sure that there's no poor connections; all switches and valves to their bases. Use known, good valves. There does seem to be something drastically wrong other than a misalignment, and addressing the latter isn't going to cure it.

[mister valve]

If I have read the opening post correctly, you are getting signal through the I.F. strip at 130kHz but just noise at the input.... Could the local oscillator have stopped working ? You can still get R.F. noise at the Mixer input which would go straight through the IF stage to the second detector, and would be stronger on longwave band at the L.F. end, as that is getting closer to the I.F. frequency.

Have you got the correct voltage on grid 2 of the FC4 valve. Is Capacitor 'C5' (0.1uf) (Trader service sheet) in working order ?

[Sideband]

I would try injecting a 130Khz signal directly into the IF amplifier. Hopefully you'll find that you have to turn the attenuation on the sig genny right down (minimum attenuation) and prove that the I.Fs are ok. I agree it sounds like and RF fault.

[agardiner]

Thanks for the comments. I know the oscillator is working as it is strong enough to be picked up on my scope by just hanging the probe nearby. Frequency is correct and adjusts as well.

I initially thought this was indeed an RF problem, and I have not ruled it out. But right now I am more concerned about the IF, as the signal is just so weak. I have to crank the output of my generator up just to get it to pass through, despite it being on frequency. It is all a bit odd. It is has been re-capped. Will check voltages on FC4 later.

[John10b]

Just reading your posts again, unfortunately, post #11, it’s causing me some difficulty, I can’t give you further fresh advice, other than what has already been stated, but just to recap I would start with testing the 2nd IF, make sure you get a really strong output.
John

[agardiner]

Hi John,

Yes indeed; sorry for any confusion. Basically tried feeding a signal through the IF can without the radio powered, as not getting good results. So in answer to your question, no, I am not getting a strong output at all.

[ms660]

Connecting a signal generator directly across the IFT primary and a 'scope probe across its secondary will throw the tuning out somewhat, not to mention the damping effect.

Lawrence.

[John10b]

Adrian you are puzzling me further. When I said ensure 2nd IF was working I should have made it clear to you that the IF was in the radio and powered up.
Have you used your RF sig gen ( internally modulated) to carry out alignment on other sets, the reason I ask is you get to know your test gear and levels the more you us it.
John