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Old 25th Dec 2015, 6:10 pm   #121
G6Tanuki
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Default Re: Eddystone 888A.

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Originally Posted by Skywave View Post
From memory: the cathode of the valve is substantially above ground potential. Therefore, due to capacitive coupling, heater-cathode, there is a possibility of impaired S/N ratio if the heater is connected to the common 6.3 v. supply
Yes, the noise-limiter is in a high-impedance part of the circuit where audio-levels are also low. Stray capacitive coupling from the heater to the cathode (which is at signal-potential) along with the risk that the anode of the diode can "see" the heater directly (so the heater may act as a cathode) make this a tricky environment - one for which semiconductor diodes are a natural!.

In some similar circuits I've seen, the noise-limiter/clipper stage diodes are fed from a separate heater-winding which is itself raised to 50 Volts or so above ground by being fed from a potential-divider across the HT (well above the operating voltage of the diode electrodes) so the heater can't possibly emit noise-electrons.
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Old 25th Dec 2015, 6:13 pm   #122
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Default Re: Eddystone 888A.

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Hi Tanuki... There is that "Huff n Puff" synth.....which I built, but I found the software in the PIC seemed to be faulty...
Hi Wendy. The HuffnPuff I built used entirely discrete semiconductors (predominantly CMOS for the oscillators/counters/gate along with one "TAA" linear chip) - I'm not familiar with a PIC-based version. but it's some years since I last played HuffnPuff and can easily see how a PIC could - if programmed properly - be as-good as a handful of CMOS.
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Old 26th Dec 2015, 12:39 am   #123
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Default Re: Eddystone 888A.

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In some similar circuits I've seen, the noise-limiter/clipper stage diodes are fed from a separate heater-winding which is itself raised to 50 Volts or so above ground by being fed from a potential-divider across the HT.
Ah, yes! I forgot about that bit when I wrote about the 888A / 888 above. That separate heater cct. does indeed have a +ve. bias placed upon it.

In my 888A, for reasons that are many and varied (and long forgotten), I replaced the design of the noise limiter with a different cct. That, of course, freed up the then-unused 6.3 v. line for some other purpose which was then required.
I suspect that the "reasons" for that modification (and possibly a few others) had something to do with my two-metre AmRad A.M. activity back in the late 1960s ~ mid 1970s, when I used my 888A as a tunable I.F., 28 ~ 30 MHz.

Al.
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Old 26th Dec 2015, 12:37 pm   #124
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Default Re: Eddystone 888A.

Hi peeps..... Interesting and varied comments.... David, ref stability and I.F Selectivity etc.... I am not a "Serious" communication amateur, more of a "Tinker er".... thus the 888 will suffice....The 888 was a "get it going" project, which may be used as a receiver on 40 occasionally.
Al and Tanuki. Yes I had considered the 6.3V heater supply, but the main one, not the Noise limiter one...however the main heater supply seems to be 3.2 0 3.2V tapped with 0 to ground. I wondered about this and thought about asking the question ?? Can I unground the tap and use a bridge rectifier etc. ?? and Al... if you have some schematic details of that noise limiter mod, I would be interested.
Tanuki... I may look into the Huff n Puff thing again...
Many Thanks.
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Last edited by Wendymott; 26th Dec 2015 at 12:37 pm. Reason: Typo
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Old 26th Dec 2015, 1:18 pm   #125
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Default Re: Eddystone 888A.

Getting slightly more devious re. LT- you could insulate the reservoir can and wire the smoothing choke between HT centre tap and chassis for a -30V (ish) rail and work with source to -ve and drain circuit towards chassis...
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Old 26th Dec 2015, 9:46 pm   #126
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Default Re: Eddystone 888A.

Hi Turretslug, I think I will pass on that one...
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Old 28th Dec 2015, 11:13 pm   #127
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Default Re: Eddystone 888A.

Well peeps the wave change switch has been repaired with 100% success. At first look it seems impossible, but taking it steadily, it was a reasonable "doddle". Fortunately the switch shaft is not the spine of the assembly. Each wafer is supported by the die cast case walls, thus removal of the mixer section was relatively easy. The switch contacts I have are a few "thou" shorter, but there was still enough "reach". The contacts were remounted using a 2.4mm threaded bolt and nut.. not beautiful I grant but functional and not likely to break the 50 year old composite wafer using rivets.
The hard part was re assembling the front panel...all those switches and pots "floating about".
Just got to finalise the VFO supply now.
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Old 29th Dec 2015, 12:10 pm   #128
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Thumbs up Re: Eddystone 888A.

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Well peeps, the wave-change switch has been repaired with 100% success. At first look it seems impossible, but taking it steadily, it was a reasonable "doddle".
Then well done! Clearly an application of the '3 Ps' . Having said that, it should be borne in mind that sets of this vintage required intensive manual 'input' in their assembly, unlike many modern products. Nevertheless, it's re-assuring to know that that repair can be undertaken without incurring a major headache - especially considering that the 888 / 888A is not the easiest set to get at its 'innards' for any repair.

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Old 29th Dec 2015, 1:37 pm   #129
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Default Re: Eddystone 888A.

Congrats Wendy - it's good to know you got there in the end!
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Old 29th Dec 2015, 10:49 pm   #130
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Default Re: Eddystone 888A.

Hi Peeps... Thanks for your good wishes. The supply for the VFO is 2VA transformer, thanks Maplin.... "see photo". There is still a sensitivity problem on Top band and 80, but at the moment I will ignore, and retire to a darkened room and a few "wobbly pops".
Have a good New Year.....
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Old 4th Jan 2016, 12:23 am   #131
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Default Re: Eddystone 888A.

Well peeps... You may be glad to hear the 888 is finally re cased.. very stable. The BFO was retrofitted with a 2N3819. I also added some thermal stabilization in the LO area, with strips of foam rubber to stop circulating air. Not sure if it has or will have any effect, but its there. After locating a "silly error" the sensitivity on each band is very similar. Thus the 888 is cased. Again thanks for ALL your inputs and encouragements.
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Old 4th Jan 2016, 1:21 am   #132
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Default Re: Eddystone 888A.

So - after a sizable struggle, you got there in the end - well done! I cannot speak for others, but as a long-time user and maintainer of a 888A, I did learn a few things along the way which will be useful to me in the future.

Now please excuse this, but I simply have to ask: planning on doing another Eddy? If so - and if you fancy a real challenge - there's always the 770R

Al.
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Old 4th Jan 2016, 2:34 pm   #133
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Default Re: Eddystone 888A.

.... 770R.....Done that... been there .. got the Switch contacts... SOLD it on......even re valved it... heavy too.... 750 next....just waiting for some B7G plugs....
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Old 4th Jan 2016, 5:19 pm   #134
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Question Re: Eddystone 888A.

Hi Wendy; re: the 770R. When you say "got the Switch contacts", does that refer to doing any work on that mode switch (with its many double-sided wafers) which runs down the entire length of one side of the chassis? Access to those wafers - and the components and wiring adjacent to same - is a nightmare.

If you do find a source of B7G plugs, please let me know from whom they are available; thanks.

Al.
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Old 4th Jan 2016, 5:35 pm   #135
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Default Re: Eddystone 888A.

Try an EA12 next... they're very useable and quite decent inside.

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Old 4th Jan 2016, 5:41 pm   #136
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Default Re: Eddystone 888A.

"Try the EA12": yes, sure, provided that you can live with the 'backwards' tuning scale and the birdies on the 160 and 80m. bands.

Al.
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Old 4th Jan 2016, 9:05 pm   #137
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Default Re: Eddystone 888A.

I find that when I fire up the EA12, I just adapt to the backwards tuning without even noticing it. It makes a good demo station with a little QRP CW TX, built open-form on copperclad FR4, for use at special event stations. where the looks and the sounds get a fair amount of attention.

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Old 4th Jan 2016, 10:03 pm   #138
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Default Re: Eddystone 888A.

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I find that when I fire up the EA12, I just adapt to the backwards tuning without even noticing it.
David
O.K.; fair comment. I can only suppose that it is a 'personality thing', together with extended usage, the length of that 'extended usage' period depending on the user. I've always found it rather irritating.

Al.
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Old 4th Jan 2016, 10:43 pm   #139
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Default Re: Eddystone 888A.

I'm one of those people who can't easily distinguish between one situation and its mirror image. Then I'm doing a lecture I usually have a different colour pen in each hand for simple convenience, but I've learned not to have both writing/drawing at once, it 'does some people's heads in'. So I might not be representative in terms of irritation at reversed tuning. Why on Earth they had to do it that way?

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Old 4th Jan 2016, 11:33 pm   #140
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Default Re: Eddystone 888A.

I take it they were designed to be that way. I bought a 680X that had the same "feature" but that was caused by a previous owner accidentally reversing the direction when replacing the drive wire! He hadn't got the stamina to open it all up again so sold it on for a song. I couldn't live with it so bit the bullet and put it back the way it should have been. Nice receiver for the LF bands.
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