Graeme -

An interim response - for now.

Fistly, thank you for your recent messages & thanks again to all the
others who have provided input on this - on this Forum & elsewhere.

1. The O/P from V4b is not a pure sinusiod - as you and I would
expect. When C68 is disconnected from its anode, so that there is no
load on V4b, the waveform is still equally distorted - but, of
course, the birdies disappear. The I/P to V4b at its grid (therefore,
O/P from the osc.) is also distorted. Again, disconnecting C65 (osc.
to buffer) makes no reduction in the amount of distortion in the
waveform. Measured with 'scope + 10x probe.

2. I will double-check the values of the Rs that have been replaced.

3. I will re-check the relevant capacitors. Yes, there are some "Red
Hunts" in there. Various Cs have been temporarily bridged during test
with a "known good test cap." - but to no avail.

4. I am yet to replace the appropriate valves with new ones. New
replacements are on order.

5. I intend to call Ted tonight.

And finally, prior to the service, no, I don't know if the birdies
were there! I didn't go specifically looking for them, prior to the
service, since I had no reason, at that point, to suspect trouble in
this direction! What I DO recall was that the overall sensistivity
was poor. It is much better now following the re-alignment.
All the above comments are applicable to both EA12s.

Just for completeness, test kit that was used / in use consists of:
Marconi TF893 power meter.
Marconi 2022C sig. gen.
HP 8640B sig. gen.
Marconi 2435 freq. cntr.
Tektronix 475 'scope.
HP 4332 LCR meter.
Fluke 77 DVM
AVO model 8
Two copies of the Eddystone EA12 Manual.

All is believed to be in a reasonable state of calibration &
functionality.

I will report later when there is more to tell!

Aside: Graeme - you know where I work ;-) At a certain radio company
that used to be at Alcester, Warwickshire; lately at Pitchill, near
Evesham! I am there employed as a Test Engineer.

73

Al. / Wed. 15-08-07 //

-----------------------



--- In eddystone_radio_users@..., "GRAEME WORMALD"
<g3ggl@...> wrote:
>
> Hello Ian,
>
> A very curious situation that Al has on his hands. A previously
unknown fault appears on different sets together! I don't think I
can offer any further advice; it's all a question of having the set
on the bench; the right test gear and the right analytical brain
(nothing personal!). I don't know if he's had his chat with Ted
yet . . .
>
> Cheers, Graeme G3GGL
>
> ----- Original Message -----
> From: ian nutt
> To: eddystone_radio_users@...
> Sent: Tuesday, August 14, 2007 11:35 PM
> Subject: Re: [eddystone_radio_users] EA12 & spurii
>
>
> Hi Graeme,
>
> I have four EA12's and none have the birdie problem Al seems to
be afflicted
> with.
>
> Sounds to me as if the VFO is operating in a non linear fashion
producing
> harmonics: I emailed him with my thoughts earlier.
>
> It will be intersting to see what the solution is in the end!
>
> 73's,
>
> Ian M0ECQ
>
> >From: "GRAEME WORMALD" <g3ggl@...>
> >Reply-To: eddystone_radio_users@...
> >To: <eddystone_radio_users@...>
> >Subject: Re: [eddystone_radio_users] EA12 & spurii
> >Date: Tue, 14 Aug 2007 17:29:45 +0100
> >
> >Hello Al; Graeme here again.
> >
> >I've just had a long chat with Bill Cooke. His simple answer
was "No, it's
> >not a problem with the EA12, it's a fault!"
> >The output from the anode of V4b (isolation amp) should be a
pure sinusoid.
> > Then there will be no birdies. He adds that he still has an
EA12 in
> >regular use and it has no birdies. Something has been
overlooked, he says.
> >He suggests (and I concur) starting by double-checking the
values of the
> >new resistors you have replaced. Have you checked the associated
caps for
> >value and leakage? (G. speaking here: the red Hunts 0.047 are
notorious for
> >going o/c, very nasty results!). And finally are you sure V4 and
V5 are in
> >top-hole condx? Something is running non-linear and a dicky
valve could be
> >the culprit. It may not always show on a tester; get new ones.
> >Ted Moore is still out of town but I'll 'mail you when he calls
me. I'm
> >sure his response will be the same.
> >Vy 73, Graeme G3GGL
> >
> > ----- Original Message -----
> > From: g8dlh
> > To: eddystone_radio_users@...
> > Sent: Saturday, August 11, 2007 11:22 PM
> > Subject: [eddystone_radio_users] EA12 & spurii
> >
> >
> > I have an EA12 on the workbench undergoing a routine service.
> > Initially it was rather insensitive on all bands - some more so
than
> > others. On measurement, it typically required about 50 - 100 uV
input
> > for a 10dB S/N at 50 mW output (should be in the region of
2uV). This
> > was corrected (and the measured result exceeded the spec.) by a
> > routine replacement of various resistors that had gone high in
value
> > and a complete re-alignment; IF, band-pass filter, 1st. & 2nd.
> > oscillators (checked with freq. counter = OK), and front-end
mixer
> > and RF stage.
> >
> > I now have an EA12 which is far from deaf! But on the 80m. range
> > there are now strong internal birdies at 3.7 and 3.75 MHz.
(Disabling
> > the RF stage does not make them go away). A bit of arithmetic
> > indicates that these could be caused by the unwanted products
of the
> > 1st. osc. and harmonics of the 2nd. osc., combining in the 2nd.
> > mixer. (I won't go into the arithmetic now).
> > Similarly, there is a very strong birdie on the 160m. band.
> >
> > It would appear that because of the design, (freq.
relationships,
> > etc.) there is little I can do about this - for example, the
output
> > from the buffer of the VFO to the 2nd. mixer looks like a
sawtooth -
> > therefore, rich in harmonics!
> >
> > Perhaps am I missing / overlooking something. Is this a common
> > problem with the EA12? If so, is there a "known fix"?
> >
> > I'd appreciate any feedback / help. etc. - 'cause this one is
causing
> > me a real headache!
> >
> > Rgds. to all,
> >
> > Al / G8DLH
> >
> >
> >
> >
>