Ekco A274 - Project No 2

[Barnjet]

Flushed with success from a relatively simple restoration of my Cossor 520, I have decided to attempt the single waveband, AC-powered Ekco A274

Initial checks so far reveal:

• Mains lead needs replacing with 3 core, ensuring that chassis is earthed.
• Trader sheet acquired and these ones are particularly useful as they show the physical location of the major components.
• All valves identified in the correct locations except V5 which is listed as EL84 but I think it reads as EL85 or 86 - it's difficult to discern. See photo. Is this a problem?
• Internal foil aerial is unstuck - I imagine a drop of PVA will fix that?

To my inexperienced eye, its overall condition seems ok, and I don’t see evidence of previous repair work, but again would very much appreciate the opinion of forum member before applying power through the series limiter.

Thanks
Max

[paulsherwin]

It looks untouched.

An EL85 is a car radio valve and wouldn't work at all in that position. An EL86 is pin compatible, and should work acceptably well despite having somewhat different characteristics.

EL84s are quite common should you choose to refit one. There are cheapish Russian military equivalents available, and they remain in production for use in guitar amps.

http://www.r-type.org/exhib/aaa0028.htm
http://www.r-type.org/exhib/aad0122.htm

The internal foil aerial in these is pretty useless unless you live next door to the transmitter.

[Nickthedentist]

Have a look at my write up here: https://www.vintage-radio.net/forum/...d.php?t=117195)

Good luck. These work very well and are uncomplicated because there’s no AM/FM switching to complicate things.

[Herald1360]

EL84 and -86 are both about 78mm overall length, -85 is only 67mm. The one in the set looks like an -86 and an -85 wouldn't fit the retainer well either.

It would be interesting to know what the IC connections are in the -84, -86, just to see whether an -85 could be used in a modified socket without subsequently excluding an -84

[DoubleWound]

I restored one of these sets some time ago and thought it sounded better than the Hacker Mayflower II. I hope it goes well for you.

I wouldn't use PVA for the foil. If the foil drops down it may well touch the bare connections on the mains transformer. I used some quite expensive reinforced foil (RS from memory) and supplemented the adhesive with miniature woodscrews. This is now my standard practice where feasible.

[Barnjet]

Originally Posted by paulsherwin

An EL86 is pin compatible, and should work acceptably well despite having somewhat different characteristics.

Originally Posted by Herald1360

EL84 and -86 are both about 78mm overall length, -85 is only 67mm. The one in the set looks like an -86 and an -85 wouldn't fit the retainer well either.

Thanks - I will stick with the EL86 for the time being

Originally Posted by Nickthedentist

Have a look at my write up here: https://www.vintage-radio.net/forum/...d.php?t=117195)

Thank you Nick, your write up is very useful. There's a lot for me to digest there, as I am inexperienced and unfamiliar with some of your repair techniques such as capacitor reforming. The bottom of the large HT smoothing can (C36/C37) has a small bulge - see photo. Should i be concerned?

Also I looked around the region of the output valve V5 (EL86 in my case) and couldn't immediately identify any Mouldseal caps - see photo. I think I need to work through the trader sheets and correlate the circuit diagram to what's in front of me.

Originally Posted by DoubleWound

I wouldn't use PVA for the foil. If the foil drops down it may well touch the bare connections on the mains transformer. I used some quite expensive reinforced foil (RS from memory) and supplemented the adhesive with miniature woodscrews.

Thanks - that is a very neat job and I will ensure the foil is securely fixed.

Max

[paulsherwin]

I couldn't see any Mouldseals either with a quick look. It's possible they switched to a different type during the production run. All 0.001-0.1uF caps of that era are suspect though.

[Barnjet]

Is the small bulge on C36/C37 of concern?

[paulsherwin]

Not necessarily. They do bulge a bit with age sometimes.

A contact adhesive like EvoStik would be best for the foil.

[Nuvistor]

There are some paper capacitors in a white paper sleeve, a wax covered capacitor and some MetalMites, all suspect.
The capacitor with the bulge, monitor its temperature and if it get hot it’s faulty, try reforming it before use. They can explode from that rubber seal, don’t have it pointing at you while testing.
The small electrolytic capacitor in the ratio detector will need replacing, make sure you use a suitable voltage rating.

There is probably a paper capacitor inside the FM tuner that can get forgotten and can cause problems.

[usradcoll1]

Originally Posted by Herald1360

EL84 and -86 are both about 78mm overall length, -85 is only 67mm. The one in the set looks like an -86 and an -85 wouldn't fit the retainer well either.

It would be interesting to know what the IC connections are in the -84, -86, just to see whether an -85 could be used in a modified socket without subsequently excluding an -84

The EL86/6CW5 was used in many of the US products as a frame output and an audio output in certain TV's.
As an observation, the PL84 and the UL84 is not similar to a EL84, but is similar to a EL86!
The EL86 is probably the one that belongs in the set.
Dave USradcoll1

[paulsherwin]

The EL86 is a very rare valve in the UK, where TV sets almost all had 300mA series string heaters. I've no idea why one has turned up in this radio. It may be that a previous owner (or his repairer mate) worked in electronics and had access to EL86s like some people have access to paperclips and Post-It notes.

I once inherited a small valve hoard from someone who used to work in the GEC labs in Wembley. He used to harvest valves when test equipment was scrapped, and the hoard included lots of types rarely found in British consumer electronics, such as the ECF80 (the PCF80 was the standard TV type).

[usradcoll1]

Originally Posted by paulsherwin

The EL86 is a very rare valve in the UK, where TV sets almost all had 300mA series string heaters. I've no idea why one has turned up in this radio. It may be that a previous owner (or his repairer mate) worked in electronics and had access to EL86s like some people have access to paperclips and Post-It notes.

I once inherited a small valve hoard from someone who used to work in the GEC labs in Wembley. He used to harvest valves when test equipment was scrapped, and the hoard included lots of types rarely found in British consumer electronics, such as the ECF80 (the PCF80 was the standard TV type).

I worked on a lot of Motorola sets that used 300ma European numbered valves like PCF80, PL84, PL500 and the like, so I got used to them and I still have a few of them in my collection. They also used the 6volt counterparts in the more expensive transformer sets.
Dave, USradcoll1, trying to remember!

[Sideband]

The EL86 was used in some Philips sets in their 'Bi-Ampli' transformerless output stages. It was common in those to have an EL84 and EL86 in a 'totem-pole' configuration...the EL84 at the bottom and the EL86 at the top. The anode of the EL84 was directly connected to the EL86 (or might have had a low value resistor in series) so the cathode of the EL86 was running at about half supply around 120V. The cathode-heater insulation of the EL86 is better than the EL84 so that it can withstand the high cathode voltage. Most of these transformerless output stages used an 800 ohm speaker.

To Barnjet....don't worry about any of this, the EL86 will be satisfactory in your set! It is pin compatible with the EL84 although as stated, the charactoristics are slightly different. It probably won't make any difference in this case.

[Barnjet]

Originally Posted by Nuvistor

There are some paper capacitors in a white paper sleeve, a wax covered capacitor and some MetalMites, all suspect.
The capacitor with the bulge, monitor its temperature and if it get hot it’s faulty, try reforming it before use. They can explode from that rubber seal, don’t have it pointing at you while testing.

I've attached my earlier photo of V5, but now labeled with the cap numbers as per Trader sheet 1269. My understanding of component identification is (the functions are a straight crib from the Trader sheet) :

C27 - IF by-pass - Hunts white paper cover

C30 and C31- AF coupling - Metalmite

C33 - part tone control - Hard wax covered

C35 - V5 cathode by-pass - clear plastic sleeved cylinder

C36/37 - HT smoothing - large can electrolytic

C26 - AF Coupling - unknown type (to me) small black cylinder

C32 - Negative feed-back - unknown type (to me) small black pea-sized

Originally Posted by Nuvistor

The small electrolytic capacitor in the ratio detector will need replacing, make sure you use a suitable voltage rating.

As I understand it, the ratio detector is based on V4, but is the "small electrolytic" you suggest replacing in this photo?

Originally Posted by Nuvistor

There is probably a paper capacitor inside the FM tuner that can get forgotten and can cause problems.

I found a Hunts white paper cap there (and a lot else), but I'll address that in a separate post, after I've had a large G&T......

Thanks for your patience
Max

[Nuvistor]

I can’t see the ratio detector capacitor in the photo but could have missed it, it is C28 8uf and probably 150vw.
Not sure about the type of capacitor in the photo for C26+32.
There is an electrolytic C34 top of chassis that supplies HT to G2 V5 and anode of V4c ,another that needs testing.

[Barnjet]

Originally Posted by Nuvistor

I can’t see the ratio detector capacitor in the photo but could have missed it, it is C28 8uf and probably 150vw.
Not sure about the type of capacitor in the photo for C26+32.
There is an electrolytic C34 top of chassis that supplies HT to G2 V5 and anode of V4c ,another that needs testing.

Thanks, Frank I found C28 and C34 - see photos. I now have to research how to test these types of cap in some way without owning a High Voltage Electrolytic Capacitor Reformer.

I'll have another look at NicktheDentist's success story here t=117195http://https://www.vintage-radio.net...d.php?t=117195

Max

[Nuvistor]

Link to Nicks thread.
https://www.vintage-radio.net/forum/...d.php?t=117195

[Synchrodyne]

To digress slightly, as basically simple as it is, the circuitry does seem to have an interesting feedback cluster around the output valve. The main loop goes from the output transformer secondary to the output valve cathode (via an R//C bias circuit), and is done in such a way that the whole of the cathode current passes through the transformer secondary. I think that might be unusual, but I could be wrong. The tone control loop goes from the output valve anode back to its grid via a C and a variable R, and looks like a simple treble cut device. The third loop is described a going from the pentode anode to the triode anode via a series R-C combination, although effectively it goes back to the output valve grid. It might be there to provide some bass boost to offset the natural rolloff of the speaker/cabinet combination. That it was not used on the C273 console version (which one would expect to have more favourable natural bass characteristics) tends to support this thesis. So, perhaps it is a noteworthy critter from the circuitry viewpoint.


Cheers,