Eddystone 750 Mk2 problem

[Wendymott]

Hi Peeps. I have made an "error" with my Eddy 750 MK2 receiver. A few weeks ago it was removed from its operating location as the Eddy 888 was on the go. Somewhere along the line I removed V1 (RF Amp), possibly for the 888 assuming it was a 6BA6... At some stage I found that it was not a 6BA6, as the 888 was not functioning correctly and a 6BA6 was fitted to the 888 and worked OK.
Here's the stupid bit. I left the 750 sitting on the naughty step, till I had time to get to it. Yes you guessed... I fitted a 6BA6 back into the RF slot. It works, but not correctly, the RF control has no effect. Now I know this was working. The problem is what is the correct valve in this location. The Mixer and 2nd IF / mixer/osc were retrofitted as ECH81's, so I assume the RF stage was upgraded too.
Looking at the base wiring it seems the Cathode is Pin 1 but as anyone that knows Eddy's, it is virtually impossible to see what is going on, due to the packed area.
I have tried the Eddystone users group but they have no schematic for this version. I am relying on you good folk to get me out of the silly mess I made for myself.

[g4aaw pete]

Originally Posted by Wendymott

as anyone that knows Eddy's, it is virtually impossible to see what is going on, due to the packed area

Hello Wendy,
I'll second that.

I don't have a definitive answer, but my guess is that it was an E180F.
However, these are usually easy to spot due to their 'squat' shape.
They also have pins 1 & 6 as the cathode.

According to the 'valves used in Eddystone receivers' document on the EUG site, they were only used in the 770S.

Regards
Pete

[deliverance]

Hi I am pretty sure that V1 on the 750 is a 6BA6 I will double check later .

[g4aaw pete]

Originally Posted by g4aaw pete

my guess is that it was an E180F.

I'd like to withdraw this supposition owing to the fact that an E180F has a B9A base.

[John Caswell]

Hi Wendy,
Are you absolutely sure that pin 1 is the cathode as none of the B7G based valves that I think are likely, have pin 1 as the cathode eg, 6BA6, 6BJ6, 6BH6, EF95.

John

[Wendymott]

Hi Peeps. Deliverance. This is a MK2 version with ECH81's as mixer and second mixer, the MK1 has ECH42's, I have all the info for that one. I am going to take the base adaptor plate out tomorrow to confirm. Its so tight in there. I had to remove the switch shaft to get even to there.

[Skywave]

The RF amplifier valve (V1) in the Eddy. S.750 Mk. II is a 6BA6.

The fault you have could be down to the valve socket, especially if that socket is of the type (so heavily favoured by Eddystone) in which the contacts in the socket are of the 'flat, split pin' type. I've had no end of trouble with those (made by Cinch?) in items additional to Eddy. radios. I once has an S.680X in which I had to replace almost every one! (It had been in someone's very dusty attic for decades!)

The other possibility is sheer co-incidence. The cathode cct. of the RF amp. incorporates a desensitizing network associated with the CAL. push-button. From memory, there is a 2M2 resistor in there which goes high and I have also had the CAL. push button switch itself fail - each fault giving the symptoms you describe.

Al.

[Wendymott]

Hi Al. On my 750 there is no Cal module, no button hole either... not like the 888.
I am wondering... This set has PTFE bases, obviously not original. I am wondering if the person that had it previously retro fitted a different valve. I will take it out and see how its wired. Its a swine though as it is divided by the diecast wall.
All this because I was going to retro fit a FET in that location. As its now nearly stripped I may just do that. The Local osc and BFO have already been "Fetised" as with the 888. Onward and upward.

[newlite4]

My 750 has a 6BA6 in that V1 position Wendy, as a matter of fact it was the only valve that needed changing since it was very crackly, probably due to close proximity between electrode structures.
Neil

[Skywave]

Originally Posted by Wendymott

On my 750 there is no Cal module, no button hole either... not like the 888.

Whoops! Err, sorry about that! Looks like I'll have to amend my tech. notes on the Eddy. S.750.

Al.

[Wendymott]

Hi Peeps. It looks like the previous owner had rewired the base as follows. Pin 1 Grid, Pin 2 cathode, 3 & 4 heaters. 5 Anode, 6 G2 and 7 G3 and Screen. The 6BA6 has pin 2 and 7 reversed. After mucho open heart surgery, it is reverted to as per 6BA6.
Not tested yet as the front is still off. I want to fit a "Bandspread" Pot, but this is not possible without it making it look ugly, however the solution is to fit a dual Concentric Pot in place of the RF Gain pot. The EXTREMELY hard part is finding a Dual Concentric 10 + 10K pot. Onward and sideways...

[Skywave]

Originally Posted by Wendymott

It looks like the previous owner had rewired the base as follows: Pin 1 Grid, Pin 2 Cathode, 3 & 4 heaters, 5 Anode, 6 G2, 7 G3 and Screen. The 6BA6 has pin 2 and 7 reversed. After much open heart surgery, it is reverted to that of a 6BA6.

Hmm; strange. The only RF pentode that I could find on a B7G base that matches that inappropriate pin-out is a 6BH6. But why someone would replace a relatively common valve (6BA6) with a type much less common (6BH6) I cannot comprehend.

Al.

[G6Tanuki]

The 6BH6 is a "sharp cutoff" pentode - with a somewhat lower equivalent-noise-resistance than the 6BA6.

There was a phase in the late-1950s/early-1960s for people to replace 6BA6 etc in HF amateur-band receivers with sharp-cutoff valves (with associated bias and AGC mods - if you were lucky!) in the belief that this gave lower intermodulation in the presence of seriously-strong signals.

[Synchrodyne]

The 6BJ6, which is the remote cutoff counterpart to the 6BH6, would also have fitted. This pair was developed in the late 1940s to have better VHF performance than the 6BA6 and 6AU6, and to have lower power consumption (150 mA heaters) to suit mobile equipment.

Also the 6BZ6 had the same pinout as the 6BH6 and 6BJ6. This was a mid-1950s American TV valve of the semi-remote cutoff type, one in a seemingly never-ending series of efforts to solve AGC problems. It also gained some popularity as an RF amplifier in HF receivers, being used by Heath, Drake and others. I’d say that this is the most likely suspect.

The Eddystone 750 Mk II seems to be a rare, semi-mythical beast, sometimes referred to as the 750A. I’d guess that it came at the tail end of the 750 production run, incorporating some of the changes (e.g. ECH81 in place of ECH42, and maybe 6C4 in place of Z77 1st oscillator) that were made for the 888-to-888A transition.

Cheers,

[Wendymott]

Hi Al, Tanuki and Synchrodyne. Well... I have rewired as a 6BA6, works fine.... not too bothered about the subtle differences now....Replaced the VR150 with a 5W zener..... after removing the V3 6BA6 L/O the 150V current requirement is 4m/A for the second L/O thus I upped the series resistor to 7.5K. Just a varicap to fit for bandspread and looking for a dual pot as mentioned above.
Just one thing bothers me.... the Mains transformer has only a 230 /110V Tap..... this makes the HT too high (280) before the choke and 260 after the choke. This is about 25V too high..has anyone found problems running the Eddystone receivers at 230V AC.
THanks all for your inputs.

[deliverance]

Glad its all sorted now Wendy .

[Wendymott]

Well finally I can post a little article regarding the "Fet -ising" of the 750. The vari cap idea was dropped, as it was ok on the "High band", but caused alignment problems on the lower bands, so there it is.. back in its rightful place. Thanks for all your thoughts and inputs.