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Old 22nd Aug 2022, 8:56 pm   #1
AdrianH
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Default Eddystone 870 radio question.

I know this is only a basic radio, no BFO etc, but one I have sat on the todo pile.

I do not like the universal bit, basically as I am not that sure of all the isolations from various points, possible caps going leaky at some stage etc. So I am planning on modifying this one.

The manual says that with 240 Volts input the current draw is 190mA. The dropper resistor drops it from 240 volts to 110 Volts. so should be dissipating around 24.7 Watts in heat alone, the radio section is dissipating 20.9 Watts.

So I am planning to replace the dropper resistor with a small isolation transformer 240 to 110 Volts at 25 VA. It will require me to probably make a new internal side piece, the one that holds the dropper.

It should reduce the heat in the cabinet quite a bit and hopefully mean it is safer to work on in the future?
Can anyone see any issues with this before I start, anyone done similar?

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Old 22nd Aug 2022, 10:09 pm   #2
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Default Re: Eddystone 870 radio question.

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Thought I should add a small diagram of the power input section.

I plan on basically putting the mains input after the fuses and two chokes with C44 into the primary of the transformer, connecting the secondary between V5 anode and chassis. The heater side would stay as it is again connected to V5 anode, with v3 heater going to chassis. Removing any connection between the chassis and the mains input.

Cheers

Adrian
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Old 23rd Aug 2022, 9:14 pm   #3
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Default Re: Eddystone 870 radio question.

Hi Adrian, why not use a capacitive dropper, probably smaller and certasinly lighter

Ed
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Old 23rd Aug 2022, 11:45 pm   #4
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Default Re: Eddystone 870 radio question.

Hi Ed, it was just what I know really, it would allow me to earth any part of the chassis rather than have an insulated internal chassis.

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Old 24th Aug 2022, 12:04 am   #5
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Default Re: Eddystone 870 radio question.

How much isolation does a cap give I have always understood ZERO .

I would do as you suggest Adrian.

Joe
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Old 24th Aug 2022, 7:25 pm   #6
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Default Re: Eddystone 870 radio question.

I added a transformer to my Eddystone 870A many years ago, and got the dissipation down from 50W (rather hot) to 20W (nicely warm), and a much reduced warm up time.

See https://www.vintage-radio.net/forum/...ad.php?t=66346

I had hum coupling between the new mains transformer I was planning to use and the original output transformer, and I had no space to chance the orientation betweeen the two, so I used a toroidal transformer I had available. This transformer was a 6V heater transformer with 240V and 110V primary taps.

I swapped the 4 signal valves for their (quite common) 6V equivalents, put in a few wire links across the series chain for parallel operation of the 4 valves plus the dial bulbs, and replaced the rectifier valve with a silicon diode. This removed the rest of the heater dropper, the thermister, and a hot valve.

I didn't worry about the "live chassis" because it's in an earthed metal case, so will be safe in normal operation, and if I need to work on the internals, I'd use my bench isolation transformer.

These mods are easy to remove, the original dropper resistor, thermister, etc are still in place but retired from use, the original (slightly rare) valves are stored away.

Stuart
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Old 24th Aug 2022, 7:46 pm   #7
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Default Re: Eddystone 870 radio question.

OK Stuarth, thanks for that.

The intention is to replace the end plate that houses the dropper resistor with another home made end piece, I will keep the original safely in case someone else at some stage may wish to put it back to original state.

This radio needs to have the case blasted and resprayed. I also need to replace the volume control with a double switched type as per original, it has been replaced at some point with a single pole one, it is also missing the insulators for the on/off volume control so would not be the safest as it is. The rear mains socket has already been replaced for an IEC style one which I will leave as is, it saves me trying to find any original style mails plugs.

There are a few other cosmetic things need doing with two grey and one black knobs and the lettering and edge line on the aluminium trim panel needing to be redone.

I will not be replacing valves as you did but hope the originals have life in them.

I had not considered induced mains hum into the OP transformer and will just hope I get enough isolation between the two.

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Old 24th Aug 2022, 8:02 pm   #8
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Default Re: Eddystone 870 radio question.

I reduced the waste-heat [which caused significant warmup-drift on the HF bands] in my Eddystone 840A by setting it to the 110V tapping on the voltage-plug, and using an external autotransformer.

Of course this still left it as a 'live chassis' design but provided all the insulating spacers and suchlike are present and in good condition it's probably a lot safer than a 1960s live-chassis telly.
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Old 24th Aug 2022, 8:44 pm   #9
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Default Re: Eddystone 870 radio question.

The insulation spacers could be an issue when I remove the original internal side plate and replace with a home made one, guess it depends on how well I make it?

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Old 25th Aug 2022, 5:33 pm   #10
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Default Re: Eddystone 870 radio question.

Quote:
Originally Posted by joebog1 View Post
How much isolation does a cap give I have always understood ZERO .

I would do as you suggest Adrian.

Joe
I agree with Joe, Transformer option.
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Old 25th Aug 2022, 7:43 pm   #11
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Default Re: Eddystone 870 radio question.

Well apart from watching out for inductance into the OP transformer, there seems to be no other potential pitfalls, so it is the way I am going.

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Old 26th Aug 2022, 12:41 pm   #12
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Default Re: Eddystone 870 radio question.

I'll admit to doing something similar to an Eddystone 670A sometime back, a set that sometimes gets denigrated as "DAC90A in a tin box" but it does at least have that nice tuning drive, RF stage and pretty comprehensive LF/MF/HF broadcast coverage, the bedside table insomnia distraction. I ran it for years from a 250VA BT "power conditioner"- essentially a 1:1 isolation transformer- but decided that this was still a kludge inappropriate for something that was no longer in a 1950s DC-supplied ship's cabin. I used a slice of extruded 2U rack blank as a cantilever transformer support tray on the narrow dropper shelf, making a Philips-style bolt-on power pack with rectifier, stabiliser etc., power consumption less than 30 watts even with stabiliser and 2nd IF stage. I hung onto all the original PSU components in case anyone should want to restore it to its original state. (Fully appreciating that about 99.9% of our stuff will get tossed straight into the skip when we pass on, but it's a nice idea anyway....).
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Old 26th Aug 2022, 1:14 pm   #13
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Default Re: Eddystone 870 radio question.

I had a previous larger Eddystone when I was in my early 20's, that was a universal DC/AC set. Big green style dropper resistor in the back corner of it. The amount of heat generated from it was enough to heat that corner of the case quite well. I also remember getting a shock off the thing as well, so they are not my favourite radios.

But this was picked up from Golborne swap meet and I think deserves a chance to be tidied up.

Work will start on it probably next week at the end of the 6 weeks holidays when I do not have to do jobs for the Mrs and for the house.

I am wondering what the best way to re do the small aluminium front panel, not sure if someone would be able to do a water slide for it or not Not really wanting to do Leteraset on it?

Adrian
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Old 5th Sep 2022, 3:13 pm   #14
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Default Re: Eddystone 870 radio question.

I have an 870A, and personally would be loath to make any internal modifications to it. I prefer to keep such vintage sets in original condition, to preserve whatever value it still has. If you’re worried about power wastage or shock risk, run it from an external step-down or isolating transformer. So much vintage wireless equipment has been spoilt by well-meaning amateur modifications.
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Old 5th Sep 2022, 3:23 pm   #15
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Default Re: Eddystone 870 radio question.

Good job it's mine then

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Old 5th Sep 2022, 5:05 pm   #16
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Default Re: Eddystone 870 radio question.

If it has no sentimental value it might be more economic to go out and buy a better AC only Eddystone.
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Old 5th Sep 2022, 6:05 pm   #17
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Default Re: Eddystone 870 radio question.

Up to now £10 pick-up from Golborne less than £20 for the transformer., needs caps and a case/front panel respray. Plus a few bits.
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Old 5th Sep 2022, 6:37 pm   #18
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Default Re: Eddystone 870 radio question.

Adrian, I suppose our views are totally different as we come at this from different positions.

To you, this is a cheap ten-quid radio and you’re free to do with it whatever you choose. The late Chas Miller advocated the sort of modification you are proposing all the time, which was fine when old radios were plentiful. I see it differently though; it might be a ten-quid radio to you, but to me, it’s an example of a type of set that is fast disappearing. The 870 has never been considered common as so many were scrapped when ocean liners were refitted. In my opinion, it is more appropriate to restore such sets to original condition wherever possible, and to make any unavoidable modifications minimal and reversible. It may have what we now look upon as design flaws, but it still works.

Working as I do as a volunteer in the Collections Care department at a museum, I tend to wear a conservation hat these days!
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Old 5th Sep 2022, 7:23 pm   #19
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Default Re: Eddystone 870 radio question.

Phil, Yes we are coming at it from different directions and no it is not just a £10 radio.

Lets face it to keep an old valve radio working as completely original is getting to be very difficult. I could if I wanted re stuff the tubular paper caps to keep the appearance, but I won't in this instance. That I did on my Murphy A26 of 1935.
As I have said in earlier posts the plan is to keep the original internal side panel so it could go back to original internals, It has already been fitted with an IEC connector and the rear of the case filed to allow that to happen, done by a previous owner as well as fitting a non standard volume on/off switch.

The main point for me is to get the radio working and to have a better level of user safety, my safety. As some are now turning radios into nothing more than Blue tooth amplifiers in a box, I think I am doing OK. It's better then being a static exhibit in a museum.

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Old 6th Sep 2022, 8:32 am   #20
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Default Re: Eddystone 870 radio question.

I can see both sides to this.

I had reservations about adding a transformer inside my 870A, but I wanted a radio I could plug in anywhere in the house (I actually used it in a conservatory), not a 110V radio which needed a second box with its own on/off switch, and a dire warning that it was set to 110V!

The 870 is actually a 110V radio intended for ship-board use, with a big dropper for 240V operation. In my view, when running on 240V mains, it runs too hot for long term reliability.

My modification involved fitting a toroidal transformer with one bolt through an existing chassis hole, adding a few wire links to the heater chain and a silicon rectifier diode, and swapping the four 150mA signal valves for their 6.3V equivalents (same pinout). The original dropper resistor, thermistor, etc are still in place, removing the rectifier valve effectively disconnected them. The original 150mA heater valves are stored separately from the radio, all the other unused bits are still in-situ.

With no help from me, a future restorer could work out what had been changed (the valve types would be a good clue) and take the radio back to its original state, they would just need a set of 150mA valves - assuming the valves I took out had got lost along the way.

Meanwhile, I have a nice useable radio which doesn’t run too hot, and warms up quickly.

As to the suggestion that the OP buys an AC only Eddystone, I would point out that the 870 is much smaller than most Eddystones, and has LW for Radio 4.

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