Eddystone 770R

[Hermit6345]

I am currently restoring a rough 770R. It Haas a problem on range1. If I set the LO to 165.2 MHz and feed a signal of 160MHz into the aerial socket, I can hear the signal at 160MHz but a much larger signal at about 150 MHz on the dial. I guess that the louder signal must be the wanted one and the one at 160 MHz the image. If I try to re tune the LO to put the wanted signal on the 160MHz mark on the tuning scale, I run out of trimmer capacitor travel before I get there. I am wondering what to do next. Should I replace the 12pF cap across the LO trimmer with a lower value or should I alter the inductance of the oscillator coil by bending it I don't want to damage the coil mountains or put the LF calibration Miles out.I favour the capacitor change but can anyone with more experience of the 770r help?

Ian

Looks like the aerial tuned circuit is out of line.

[Hermit6345]

Well, the aerial tuned circuit is tuned by a front panel control and that peaks up OK. I replaced the 12pF in parallel with the osc trimmer with a 10pF and that now allows the trimmer to be adjusted to bring the scale into line You could be right about the front end though. I will persevere.

[Radio Wrangler]

The front panel control is normally just a trimmer to fine tune a front-end circuit that can be pulled a bit off by whatever antenna is connected.

The first step is to find out whether the local oscillator is supposed to run above or below the signal frequency, so you can be sure which is the wanted signal and which the image.

Don't try reshaping coils just yet.

[Hermit6345]

The LO is high on all the ranges and I checked that the LO was in fact high using my counter loosely coupled to the LO valve.

In the 770R it looks like the aerial trimmer is across the RF input coil, effectively in parallel with the RF section gang. I guess that I could use a VVM and check that the RF amp is tracking correctly, but it peaks up OK, so I guess it makes up for any tracking error in the input circuits.

I don't have a scope that will go up to 160 odd MHz. At least the scale is now pretty accurate and that leaves the deafness to sort out (MDS about 12uV emf.)

I have found a 10k anode feed resistor for the LO that has gone to 15k so replacing that will help I expect. The LO anode/screen is at 70 volts vice the book figure of 110 volts. Will get there with help, thanks for your input of course.

[Radio Wrangler]

Have you seen this:

http://www.schimmel.talktalk.net/radios/770r.htm

These receivers have a bit of a reputation.

David

[Hermit6345]

Yes, I had seen that David. It certainly is a challenge mainly because of the mechanical design and the access problems that causes. Mine had the tuning mechanism seized solid and required many paper caps to be replaced. The stringing with steel wire was another challenge. Good job I have a good mate who can see and doesn't shake! All the other ranges work ell. Oh well, I can only keep at it, it worked once so can work again.

Cheers,

Ian

[Radio Wrangler]

I think they were really pushing it with that high frequency range, given the construction and the period. There was an earlier model of the 770 that went higher, but Eddystone withdrew it and replaced it with a more limited model.

A few years ago someone phoned the firm I work for, making an enquiry about a transponder for his aeroplane and remarked to the chap taking the call that 'by the way, I own Eddystone Radio' Unfortunately Mark was an Aussie with an aviation, not radio background and hadn't heard of them. He asked me because he thought the guy had said it as if it meant something special. I brought in a couple of radios to show him.

David

[Hermit6345]

I am sure that you are right David. The 770U went higher in frequency. I came across the odd 770R in the RN and I was of the opinion even in the early sixties that they were a bit deaf and nothing I have seen so far encourages me to change that opinion. It's just the challenge of trying to get it somewhere close to original. To be fair, it does make a nice 60 pound (weight) FM Broadcast receiver! A single conversion with a 5.2MHz IF isn't, in my opinion, the best design feature. However, there was a bit less stuff on VHF then I would imagine.. I do have a nice 640, 504 and 870 in my small collection and this far and away the most challenging so far.

[Andrew2]

A 5.2 MHz IF seems a strange choice for a receiver that covers up to 160 MHz. Getting enough front-end selectivity to get the image response down to something reasonable was always going to be a major headache, so we can only assume that 1) there was a good reason for choosing it instead of 10.7 or 21.4 MHz and 2) users quickly became accustomed to hearing everything twice!

[Synchrodyne]

5.2 MHz was certainly an odd IF choice and Eddystone may have been alone in using it. 10.7 MHz was standardized in 1950 by the RTMA in the US for broadcast FM receivers, and had been in use for a few years before that. 21.4 MHz (for VHF and UHF communications receivers) may have come along a lot later. But the Eddystone folks were surely aware of the 10.7 MHz number, which became the de facto worldwide standard for broadcast FM receivers and then found its way into VHF communications equipment.

That leads one to suspect that 5.2 MHz was a deliberate choice made after weighing the pros and cons. One possibility is that it stems from the fact that the 770R was designed to receive communications “NBFM” as it existed at the time, as well as (wideband) broadcast FM. NBFM then was based upon ±15 kHz deviation, 50 kHz channel separation and around 30 kHz receiver IF bandwidth. Achieving this bandwidth, much narrower than that associated with broadcast FM, using distributed LC filters and in a reasonable number of IF stages may have indicated a lower IF than 10.7 MHz. As it was, the 770R had four 6BA6 IF stages plus a limiter. As far as I know, 10.7 MHz IF crystal filters for 30 and 15 kHz (and later 7.5 kHz) bandwidths did not appear until the late 1950s, so would not have been an option for the 770R. (The succeeding mid-1960s solid-state 990R model did have a 10.7 MHz IF with crystal NBFM filters.)

Double conversion for NBFM, say 10.7 MHz followed by 455 kHz, would have addressed the selectivity problem whilst retaining an adequately high 1st IF, but perhaps Eddystone saw that as a complication too many in what had been an allegedly very tough design challenge. Images may have been more acceptable back then; and a 5.2 MHz IF for 165 MHz maximum received frequency was still a more favourable ratio than a 450 kHz IF for a 30 MHz maximum received frequency, as was typical of single-conversion HF receivers.

Cheers,

[turretslug]

Originally Posted by Synchrodyne

Double conversion for NBFM, say 10.7 MHz followed by 455 kHz, would have addressed the selectivity problem whilst retaining an adequately high 1st IF, but perhaps Eddystone saw that as a complication too many in what had been an allegedly very tough design challenge. Images may have been more acceptable back then; and a 5.2 MHz IF for 165 MHz maximum received frequency was still a more favourable ratio than a 450 kHz IF for a 30 MHz maximum received frequency, as was typical of single-conversion HF receivers.

And Eddystone had recent experience of double conversion technique in the 750, implemented neatly, compactly and straightforwardly in the form of an ECH42 sitting atop its oscillator screening can . The conversion ratio for 1620/85kHz isn't that far from that for 10,700/455kHz, so, although quite large for the LC filter era, Eddystone were obviously confident that they could get adequate 2nd IF image suppression. Maybe, as you suggest, it would simply have been yet one more thing in the development process, the complexity/bottle count was already quite impressive/alarming, depending on one's viewpoint.

[SteveCG]

I would say trying to replace all those leaking paper capacitors in the 770R is the most alarming thing about it.

When I used one I found, like others here, Range 1 to be deaf.

[G4XWDJim]

Originally Posted by Andrew2

A 5.2 MHz IF seems a strange choice for a receiver that covers up to 160 MHz.

The Hallicrafters S27/S36 also has a 5.2 mHz IF and goes up to 143 mHz. Maybe Eddystone felt that if it was good enough for Halli it was good enough for them.

Jim

[turretslug]

Would Eddystone designers have been aware of the two triode cascode technique for RF amp duties in the 770R timeframe? Perhaps the 6AK5, good as it may be generally, was a bit out of its depth here. The cascode configuration certainly featured for Band III TV use a few years on and I think some Eddystone HF sets also later featured cascode RF amp. It would be interesting to hear from those with experience of Hallicrafters VHF sets as to how they performed with their "acorn" triodes.

[Synchrodyne]

Probably Eddystone would have known about the cascode circuit. The Wallman shunt cascode circuit dated from the late 1940s, and the RCA series cascode, with its associated 6BQ7 valve, from 1951. RCA maintained that its derivative was much better for variable frequency operation, whereas the Wallman circuit was essentially suited fixed frequency IF applications. But it was not until 1953 that a suitable European valve (ECC84) became available for the series cascode case. By that time design around the 6AK5 might have been well advanced, and given that the turret was allegedly a “problem child” that took a lot of effort to get right, perhaps further redesign to use a cascode RF stage was rejected, even at the later Mk II iteration.

Eddystone did use a shunt cascode 1st IF stage (50 MHz) in the 770U UHF receiver, companion to the 770R. But its next step in general coverage VHF and UHF receivers was to the solid state 990R and 990S. The 990R looked as if it were a tour-de-force effort with germanium PNP transistors, right on the cusp of the silicon planar era, and only a couple of years before mosfets for RF work and ICs for IF strips arrived.

An ECC189 cascode RF stage was adopted for the late 1950s 880 series HF receiver, then propagated to the 830 series and the 940.

Thinking more about the dual conversion (10.7 MHz and 455 kHz) possibility, a complication here would have been that the second conversion would have been inappropriate for broadcast FM. So a full 10.7 MHz IF strip would still have been needed for the broadcast FM case, either increasing the valve count or necessitating the use of a dual-frequency IF strip in which the first or second stage could function as either a 10.7 MHz amplifier or a 10.7 MHz-to-455 kHz frequency changer.

Cheers,

[G4XWDJim]

Originally Posted by turretslug

It would be interesting to hear from those with experience of Hallicrafters VHF sets as to how they performed with their "acorn" triodes.

Over the last 20 to 30 years I've had most of the valved VHF sets. The key factor in their usability seems to be their sensitivity - or lack of.

I've spent many hours restoring them and have been disappointed in all of them, mostly for the same reason - sensitivity.

I have no firm measurements now but subjectively for what it's worth I would say that the R216, a Larkspur valve set from the 1950s had a more sensitive performance than all of the rest.

Next in line would be the Hallicrafters S27C which has a single band of about 130 to 220 mHz and with two acorn RF stages is considerably more sensitive than the other wartime models.

I used a restored Hallicrafters S27 quite regularly and found it to be deaf on the upper band in much the same way that the 770R is although I felt it was a marginally better and nicer set to use being far less complicated and easier to restore. The sound quality was very good though, almost Hi-Fi.

The 5 band R308 http://www.vmars.org.uk/r308.html has similar performance to the S27 but had a very good 'feel' in use with a half moon dial.

Using a roof mounted aerial cut for 2 metres all of these valved sets were sadly outperformed in the 115 to 145 mHz region by my Sony Air 7 portable stood on the bench nearby with its own helical aerial about 9 inches high.

However, a TV wideband aerial amplifier connected to the front end of the valved sets did improve their sensitivity a little and make them more useful.

Jim

[turretslug]

Thanks for taking the time, Jim- I suppose I suspected as much but it's a mixture of good and bad to have it confirmed! I've occasionally been tempted by a 770R but, to justify house-room here, things need to be good intrinsically before painstaking work on rejuvenating and it's a big , heavy slab of a radio. At risk of public lynching, Eddystones appeal to me for their light mechanical engineering competence- precise, neat, solid construction and the famous tuning drives and castings, rather than startling electronic innovation or sparkling performance. Ho hum.

Now, where's my flak-jacket...

[g4aaw pete]

Originally Posted by turretslug

. .Eddystones appeal to me for their light mechanical engineering competence- precise, neat, solid construction and the famous tuning drives and castings, rather than startling electronic innovation or sparkling performance.

I'm with you on that David, despite having more than a few Eddystone receivers.


Regards
Pete

Thanks Patrick.

[Patrick Dixon]

.... performance.[/QUOTE]

should do it.