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-   -   Eddystone EA12 - front end problem (https://www.vintage-radio.net/forum/showthread.php?t=80074)

BBRQ229 17th Feb 2012 11:35 am

Eddystone EA12 - front end problem
 
Help appreciated to identify problem with EA12 amateur band receiver. Just purchased a EA12 (made in 1966) which was working when dispatched, but alas not when received. No visible physical damage - inside or out.

White noise only until aerial fed into grid of 2nd mixer (tunable 1st i/f) when sets springs to life (but not on indicated frequencies, as would be expected). So is it fair to assume that all ok from that point on, or would failure of VFO for 2nd mixer still be possible given that signal travels from grid of 2nd mixer ok.

So I suspect something in earlier stages. Checked valves and voltages V1 to V5 - no obvious problems and voltages close enough to spec.

Stages are: Aerial Filter, Band Pass circuits, ECC189 RF amp, ECH81 1st mixer, fed from ECH81 amplifier doubler, fed from EC90 crystal oscillator.

Have tried feeding aerial into 1st mixer (by-passing RF amp). No joy. Assume its 1st oscillator problem, but not sure, or how to proceed.

Assume also that it is something caused by a jolt in transit - so probably not a component failure (cap or resistor) - is that a fair assumption?

Thoughts please.

Chris Parry 17th Feb 2012 3:46 pm

Re: Eddystone EA12 - front end problem
 
Look for silly things first. Do the heaters all light up? If the outer casing was removed, were any components on the chassis underside crunched when it was refitted? Does the HT correctly reach R18/R19 and R36/R37a? Were the crystals and valves removed during transit? If so, make sure they're all re-seated in exactly the correct positions. For example, XL1 which is confusingly shown on the >RHS< of the crystal bank on Eddystone's schematic, should be the 15550kHz rock and not the 3500kHz rock.

It does sound like a first LO problem, so there are 2 alternative ways ahead. You can inject 1V from an RF sig gen into V2 pin 7, which will over-ride the onboard LO and enliven the first mixer when injection is on the correct(ish) frequency. Or alternatively, you can connect an RF voltmeter probe to that same pin. I have no preference for these two options, it depends on what instrumentation you have locally available. BTW although not strictly necessary, it may be wise to connect these instruments via a 10nF/400V capacitor in case you accidently hit the wrong valvebase pin.

If you prefer to try changing the hardware, then put a known good ECH81 into 1st mixer V2A, which will incidentally prove the 1st LO buffer which uses its triode V2B. Beware swapping the two ECH81s. This can lead to fault propagation and cause confusion.

If still no joy, it's time to put a known good EC90 into position V3.

The good news is that you're not looking for a fault in the RF amplifier V1, because that would only cause a 40dB problem, and it sounds like your radio has a lot more deafness than that! Good luck, and let us know what happens.

Chris.

BBRQ229 17th Feb 2012 9:25 pm

Re: Eddystone EA12 - front end problem
 
Chris - thanks; that's a great help. Nothing was removed (maybe it should have been) so all valves + crystals in correct position. I checked first 5 valves on AVO, but find ECH81s unconvincing - anode current ok but not sure of gm and not sure if that's important. Will report back tomorrow and grateful if you can look at my rantings and comment again. Cheers

Tony

Chris Parry 17th Feb 2012 9:45 pm

Re: Eddystone EA12 - front end problem
 
OK Tony. I have the feeling this one is something simple - but that does not mean it will be easy to find. Instinct tells me the first LO is not running at all. I wonder if the problem is common to all the ranges? The valve types used in the EA12 are all reliable, in fact this is a very well designed set from tip to toe. The ECH81 is not a type that's inclined towards sudden failure, and neither is the EC90. Don't worry about detail GM values. This can cause slight problems as the valves age, but not the sort of double whammy you have here. Is there a possibility that the seller removed all the crystals for shipment, but then decided to leave them in position? Perhaps he then refitted them to the chassis wrongly, prior to boxing it up for the last time. Anyway please let us know what you find tomorrow. Chris.

BBRQ229 18th Feb 2012 12:27 am

Re: Eddystone EA12 - front end problem
 
I'll check out the crystals. All ranges same problem. As for test instruments; I have DMM, AMM, old sig gen that has constant amplitude goes up to 100 Mc/s and the AVO valve tester. That's it. Tried simulating 1st oscillator with sig gen set in range 1 - 1.6 with some small effect but not the answer. Feeding signal direct into aerial input (at indicated scale frequency) did nothing, but my other set at other side of the room picked it up clearly! Its very odd, but as you say probably very simple answer. Set is cosmetically exceptional, so doubt its a pup. But..........

BBRQ229 18th Feb 2012 11:31 am

Re: Eddystone EA12 - front end problem
 
Update on this. I used my 730/4 to listen for the 1st oscillator fixed frequencies and found them in the right place on 8 out of the 9 bands. So at least its oscillating. Presumably it is possible that the oscillator output is not actually getting to the mixer grid, but seems unlikely.

I have tried feeding the aerial input directly into the 1st mixer (bypassing the aerial filter, etc and RF amplifier. I guess this should still prioduce an output, albeit a weaker one, if the set was operating correctly?

Chris Parry 18th Feb 2012 12:09 pm

Re: Eddystone EA12 - front end problem
 
Hi Tony,

Not sure what you mean by: "Tried simulating 1st oscillator with sig gen set in range 1 - 1.6" because the required 1st LO frequency is selectable by the range crystals all lying between 3500kHz (XL9) and 15600kHz (XL6). None of the selectable crystals is as low as 1.6MHz, so please clarify what you were doing.

When injecting simulated 1st LO into V2 pin 7, at least 1V of CW signal will be needed. The onboard 1st LO will probably push between 5V and 10V into V2 pin 7. You don't need a level as high as that for testing purposes, but if the injection level is lower than about 100mV then maybe the mixer will be insufficiently excited to give any sensible response.

You can hear the LO, so the EC90 @ V3 is now known to be healthy. It's time to put a good ECH81 into the V2 position. Not necesarily a new one, any working example will do fine.

If you feed the antenna directly into the grid of the RF amplifier at V1 pin 7, the radio should work normally but will be deaf by between 40dB and about 60dB depending on the range selected. It will >not< be deaf to the extent of 100dB, which is what would be needed in order to hear nothing at all, even on a very strong local signal.

Chris

frsimen 18th Feb 2012 12:42 pm

Re: Eddystone EA12 - front end problem
 
Tony,

Try cleaning the contacts of the standby and calibrator switches, as both these will reduce the gain of the RF amplifier if dirty. Indeed, the standby switch will probably mute the receiver altogether, depending on the setting of the mute control RV4 (in the first 100kHz IF amplifier cathode circuit).

If that doesn’t restore normal operation, check whether you can hear the calibration signal. It is injected into the tuneable IF and will prove the operation of the IF bandpass filter.

Paula

BBRQ229 18th Feb 2012 6:49 pm

Re: Eddystone EA12 - front end problem
 
Hi all - thanks for suggestions.

The muting/standby all seems ok - level of mute (level of white noise in my case) sets ok by pot on back of set, from silent to audible, and full when not on standby.

As mentioned somewhere earlier (above) the calibration works OK and that pointed me initially to earlier in the circuit.

the 1 to 1.6 Mc/s injection was related to 2nd Mixer (my mistake) and see below

I've now tried the following:

1 connect Osc 1 anode (via small cap) to screen grid 1st mixer in case osc amplifier/doubler faulty
2 connect aerial to grid 1 of 1st mixer to bypass aerial filter and first lot of bandpass circuits and RF amplifier
3 my 730/4 picks up Osc 1 on 8 of 9 bands at correct frequencies
4 muting switch ok
5 calibration injection works ok
6 connected aerial to V1 pin 7 (RF amplifier) - not even a click

Then, and the results of this confuse me, I tried using the signal generator to simulate the 2nd Oscillator. So I set sig gen at 1.3 Mc/s - i.e. approx half way along the oscillator frequency range. I set the EA12 tuning scale at midway point which should about 1.4Mc/s (i.e. half way across the i/f tunable range), which is 100Kc/s above simulated osc frequency. Feeding the sig gen into pin 7 of the V4, the 2nd mixer, then produced plenty of output - which was tunable by varying the oscillator frequency. In this state the RF gain had no affect at all, but the IF gain worked normally. So I womnder where the broadcast signals were coming from - also from the siug gen lead?

So, the 2nd oscillator doesnt work I thought.

So to check it I tuned my other set to 1.3 Mc/s and of course picked up the 2nd oscillator.

There you have it - 'confused of East Devon'

Further thoughts welcome.

frsimen 19th Feb 2012 12:17 pm

Re: Eddystone EA12 - front end problem
 
Tony,

The results you have suggest you are right, the problem is before the second mixer stage. It’s probably best to concentrate on V2A first, checking resistors R15 through to R19. I know the voltages measured about right but just check anyway.

After that, confirm that the local oscillator is reaching pin 7 of V2A, by measuring the DC voltage there. It should be slightly negative, if all is well. A positive voltage would indicate a problem with C57, zero volts would suggest that the local oscillator isn’t getting to that point, is at a very low level or V2 is faulty.

You can check that V2A is working by injecting a signal of say 1.3MHz into pin 2 of V2A. You should be able to tune that in using the main tuning. If you hear nothing, change V2, as has been suggested by Chris earlier.

Paula

BBRQ229 19th Feb 2012 10:58 pm

Re: Eddystone EA12 - front end problem
 
Chris and Paula, thanks for help so far. Spent some more time today - still to no avail; so about to admit defeat.

Did following: checked V2A, pin 7 - voltage around -2.2. Rechecked all other voltages on V2. Only things of note are pin 8 running at 106V where spec is 78V. Pin 2 (grid) moves around - but is in the mV range, and assume that is ok. Checked all resistors - all within 10% except R16 which is 119k - within 20%.

Put 1.3 Mc/s in at pin 2 and was able to find point on scale where it came through - but not strongly. Varied frequency and able to retune it. Would have expected bigger response. (of course wavechange switch ineffective in this test). Tried feeding normal RF in after V1 (i.e at pin 1 - anode - of V1), using 7.2 Mc/s - but it didnt come through (but came through on my other set ok!). Then tried it on 'live' side of C34 - again nothing came through.

Then tried a spare ECH81 in V2 - with no improvement. Tried swapping V2 and V4, since V4 is clearly working, again with no beneficial result.

It is as if there is a break in the circuit between V2 and V4 - C43,C44,C45 ish. But since the calibrator works, that leaves C43.

So that is where I am today...

Alistair D 19th Feb 2012 11:33 pm

Re: Eddystone EA12 - front end problem
 
I know it would cost a fair bit of money but if you could ship it to Edinburgh I could sort it out for you.

Al

G4XWDJim 20th Feb 2012 11:27 am

Re: Eddystone EA12 - front end problem
 
I had a similar problem on another Eddystone that uses the flimsy peak RF control. It had had a slight bump and the capacitor vanes shorted together grossly attenuating any signal trying to make it's way through. Looks like it's C34 on the input to V2A.

Good luck,

Jim

Alistair D 20th Feb 2012 12:20 pm

Re: Eddystone EA12 - front end problem
 
Is V12 glowing? Is HT2 at 150V? The fault with your set sounds to me as if there is not enough signal from the oscillator into pin 7 of V2 to make it frequency change. The fact that V2 pin 8 voltage is high adds to my belief.

Al

BBRQ229 20th Feb 2012 1:47 pm

Re: Eddystone EA12 - front end problem
 
OK guys, I'll look at those suggestions. There has to be an answer.

What is the best way of testing circuit up to 1st FC - given that i have a sig gen with no AF modulation, and no scope. I just want to be quite sure that the problem is around V2 and the oscillator?

Alistair D 20th Feb 2012 2:23 pm

Re: Eddystone EA12 - front end problem
 
Not having a scope is the biggest drawback but do not give up. Connect a piece of co-ax to the aerial socket of your 730/4. At the other end of the cable strip back a couple of inches of the screen. You now have a sniffer probe. On the underside of The EA12 place the probe close to V3. You should be able to tune the 730 to the frequency of the crystal. Note the level of the signal. Move the probe close to V2b and tune the 730 to twice the crystal frequency. The signal level this time should be higher than before. This is a bit subjective as the position of the probe will control the actual signal level but the main point is that you should be able to detect a strong signal at 2x the xtal frequency.

Al

Alistair D 20th Feb 2012 3:12 pm

Re: Eddystone EA12 - front end problem
 
Just had another look at the schematic, C39 in cathode of V2 open circuit would also cause the symptoms you describe. Tacking a new one onto the old will be fine.

Al

BBRQ229 20th Feb 2012 11:57 pm

Re: Eddystone EA12 - front end problem
 
Alistair, thank you - I'm away for 2 weeks in Ireland, but will let you and the others who have made helpful suggestions, know what happens in due course. Thanks again

BBRQ229 10th Mar 2012 8:55 pm

Re: Eddystone EA12 - front end problem
 
OK, all ye who have been so helpful - I'm back from my travels and getting to grips with this EA12 again.

1st oscillator seems to be working - can pick it up on the 730/4 on all correct frequencies. Whether it is strong enough I dont know; but tried putting the sig gen output into V2a instead (at appropriate frequencies) but still no joy. In fact i can hear the click on the 730 as I put probe onto G1, but not on the EA12 itself!

so still at a loss, but cant do more till tuesday, when more suggestions and help will be appreciated.

Radio Wrangler 16th Mar 2012 10:10 pm

Re: Eddystone EA12 - front end problem
 
Just a quick thought, Tony.

Put sig gen into the Belling Lee antenna connector of the EA12 and use your general coverage receiver with a carefully made probe to follow the signal down the structure of the EA12. You can tune the EA12 main tuning and RF tuning knobs to check the signal peaks properly, and the GC RX can follow all the frequency conversions. Probing the input and using the RX 'S' meter will show you whether you're winning or losing on the gain front as well.

Which reminds me, last time I tried to demonstrate my EA12, it wasn't playing the game so I need to find some time and space to take a look at it.

For a probe, try a series capacitor (100pf as a guess) as a DC block and then 4.7 kilo Ohms series and 50 ohms shunt mounted in a ball-pen tube, with 50 ohm coax to your receiver. You can always wind the sig gen up a bit to overcome the losses of the probe at the beginning.

I've been spoilt for test gear with sig gens, spectrum analysers and even phase-noise measurement rigs where I used to work, so it's fun getting inventive to find ways round when such gear is no-longer available. An analogue meter, a sig gen and a general coverage RX is really all you need.

Good hunting,

David


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