Break up or repair?

[Andrew B]

I bought this Phillips 353 chassis for £1. It has obviously seen better days. The case is missing as is the speaker and ferrite coilset, the ECH81 is missing and the EL84 is broken, but apart from that (!) it seems complete.


I snipped the HT+ wire off the smoother and isolated it, fired some mains into it and was rewarded with heaters lighting up and 305 volts on the now ISOLATED HT+ wire from the rectifier to chassis.

Mains transformer doesn't heat up, so seems like I got the basis for a 300 volt @ 60mA + 6 volts AC @ 1A power supply at the very least for my £1 investment.

I could get a ferrite premade from Maplins for £3 and the 2 valves cheaply, and I'm wondering, just for fun, should I cobble it together for practice OR strip it for parts?

I have some Hi-Z phones that I obtained from Dr Wobble and with a few more cheap purchases, I would have a good experimenters kit with the EF80's, EF85 and the EABC DDT, tuning cap, trimmers, the PSU, even the speaker O/P transformer.

I realize that the 300V HT is a bit OTT but I have 40+ years of being careful.

So, break and build or rebuild for fun??

[bikerhifinut]

looks like a bit of a basket case and I think Philips 353 sets arent so rare but willing to be corrected and appropriately admonished if in the wrong.
What's your thinking on it? If you fancy the challenge of getting it to work as a radio again then go for it. I guess you'd be able to source the missing parts with a bit of help on here.
Or
carefully strip it out so that any parts may be useful as spares for another refurbisher.
Then use the chassis and transformers to make a single ended mono amp.
Or
use the chassis and mains transformer and make a low power stereo single ended amp with a triode pentode like ECL82 and use it as a sort of "punk" iPod dock.

Ok Ready for my punishment for heresy gents...

Andy.

[joebog1]

Seriously? Get it going. The ferrite antenna can be hand wound on any ferrite rod you may have, something 6 inches or so long would work. 125 turns for the antenna and 5 turns for the coupling winding will "make it go".
Seems like good practice after it works correctly then strip it. An old set is ideal for testing yourself, and at worst costs $10 for parts and a wealth of experience

Joe

[Andrew B]

That's a bit of an idea Andy. I could make up the PSU and ECL82/84 amplifier on a common bit of chassis, although the 300 volts HT+ might be too high.

The speaker O/P transformer should match (well nearly) to the ECL82/84 Ra 4.5K. I could make provision taking off HT+, GND and heaters AND having an AUDIO input to the amplifier from any ratnest radio I construct externally. Nice suggestion that, thanks

Andrew

[Andrew B]

Originally Posted by joebog1

Seriously? Get it going.

Yes, I might do that, just to test the components. I'm interested if the FM strip would reach up to the 2M band?.

I have the diagram and alignment procedure for the chassis, so I could experiment a bit there, see if the L.O. will run up to 136 MHz

[bikerhifinut]

Originally Posted by Andrew B

That's a bit of an idea Andy. I could make up the PSU and ECL82/84 amplifier on a common bit of chassis, although the 300 volts HT+ might be too high.

The speaker O/P transformer should match (well nearly) to the ECL82/84 Ra 4.5K. I could make provision taking off HT+, GND and heaters AND having an AUDIO input to the amplifier from any ratnest radio I construct externally. Nice suggestion that, thanks

if you want to make a mono amp then stick with the EL84. As i remember the output transformer also acts as a choke for the Ht to the rest of the set.
you could make a version of this amp by using an EBC90 (I think or 91) which is hqlf an ECC81 with a double diode on a B7G base, or just parallel up an ecc81. I know this circuit works and theres an earlier version withoout the fancy CCS on the cathode (use a 270R resistor) and the zener diode feed to the screen grid (2k2 resistor from memory).

http://rh-amps.blogspot.co.uk/2013_02_01_archive.html

It's not a bad little amp and very simple, I have built a stereo version before. Dont expect the 5 watts claimed, 2 to 3 watts seems more like it.

I suggested ECL82 for stereo on the basis of the power supply being rated at 60mA and you just might get it under that. The Ht you can drop if needed with a resistor and another decoupling capacitor and get a nice smooth HT.

Joes idea is sound too.

Andy.

[paulsherwin]

Ferrite rod coils are a lot more interchangeable than is often assumed. Coils from any scrap set (valve or transistor) will probably work quite well. You can learn a lot playing around with basket case sets like these, and there's nothing to lose.

[Andrew B]

Quite right there Andy, the O/P transformer is used as a smoothing choke to supply HT to the rest of the radio via a 1.2K 3W resistor. Ia (Max) Pentode section is given as 35mA and 3.5mA on the triode section

I have squirreled away quite a few dead PC power supplies with their meaty 470uF @ 450V capacitors for just such occasions (as you do!!).

I don't know the exact rating of the mains transformer, but somewhere around 50-80mA max would seem reasonable, so I figure I have about 10-15mA spare output for external circuits. I will go on the ECL82/84 as the triode section will provide a little extra gain, the original radio used the triode section of the EABC XX as the detector/audio preamp.

The resistors are nearly all pink ceramic tubes, which I would recycle for >authenticity<, after being multimetered and their TRUE values sharpied on.

Please keep the ideas coming in everyone, fascinating stuff !!

Andrew

[joebog1]

I would suggest Andrew, that 470 uF will cook your ancient transformer and rectifier!!!
Perhaps use something of similar size?? like 32uF or perhaps 24uF.
When you increase the capacitor size ( in microfrards ) you increase the charging current peaks. In laymans terms that means the transformer will be struggling to charge the cap, let alone drive an amplifier/radio.

Joe

[G8UWM-MildMartin]

I'd start by treating it to a working EL84 or a cheap ex-USSR 6p14p rather than rebuild it for the inferior ECL82, and a new grid coupling capacitor.

[Diabolical Artificer]

If you decide to go down the amp route I have a couple of SE OPT's for ECL86's you can have cheap, think I have the hefty mains tfmr too, so in theory you could get the radio going and have a stereo amp to go with it.

Andy.

[Andrew B]

I can still use the radio as a radio/power supply/amplifier AND have it as a working I.F. strip for 10.7MHz and 470KHz An chuck in an EM 8X onto the AGC line.

Andrew

[Lloyd 1985]

I like getting hold of bare chassis like this, as then you can get it going, add a few 'improvements' (bunch of switchable inputs for example) then get really creative and build a nice cabinet for it!

I even have some empty bakelite cabinets waiting for me to build some chassis to go into them.

Regards,
Lloyd

[G6Tanuki]

Originally Posted by Andrew B

I'm interested if the FM strip would reach up to the 2M band?.

I have the diagram and alignment procedure for the chassis, so I could experiment a bit there, see if the L.O. will run up to 136 MHz

It could be done [if the LO is already "high side" for the FM broadcast-band it wouldn't need to be pushed very far if you then used low-side injection on 2M].

IF selectivity might be a bit broad though, and the recovered-audio level when receiving a 5KHz-deviation amateur signal compared to the deviation of a normal FM radio broadcast will also be very low.

What about doing something truly evil: make the FM front-end tune 118-136MHz aircraft-band (where they use AM), then hook up the AM front-end fix-tuned to receive on 10.7MHz (convert one of the FM IFTs to serve as mixer-grid coil and LO coil for the ECH81?) and downconvert to 465KHz as a "second IF". Double-conversion! If your radio's got AFC, disconnect this from the FM discriminator and instead arrange for a few volts of DC from somewhere to feed the FM front-end's "AFC" terminal via a potentiometer - this pot then becomes your 'bandspread' tuning!

[Andrew B]

Aha, I return to find lots of interesting posts Sorry no gunners on this radio (AFC) (sic). The radio is pretty primitive, When did FM broadcast on VHF start in the UK?

They could have been even cheaper skate and instead of an EABC, they could have used an EB91 as ratio detector and an ECL82 as PA.

I laid hands on a ferrite antenna assembly from Maplins for £2.99, LW and MW coils already wound out of what I suspect is Litz wire. I offered the guy a fiver for the valve set and the deal was done.

I left the caps reforming by the simple expediant of putting a 100K resistor from the OP of the rectifier onto the correct place on the audio transformer, no smoke or sparks YET.

All it needs is a few more bands, a BFO and a Xtal filter.

Air Band Radio, I don't live far from Leeds/Bradford and get an S9+30db from them, I have had a little peep at the circuit and instead of hacking it about TOO much, I will try adding series-ed up pF value caps onto C6 and C8 and see if that ups the frequency of the LO (have a Chinese frequency meter) and an antique "Advance" signal generator that could be used as the LO at a push.

I enclose the FM part of the diagram, for perusal at your leisure.

[Andrew B]

This project and the support I have received on this site is fantastic. All the suggestions for modifications etc give me inspiration to make it go !!

In total I count 2 of 2 pole 4 position wafer switches FM, MW, LW and phono input. They LOOK like the old "Yaxley" wafer switches and I am thinking of fitting more, maybe change for 3 of 2 pole 6 position for IF inputs?

I have a chance of an EM84 which could be hooked into the AGC line, any opinions?

I have found some excellent 3/8" hard balsa wood in my stock, too hard for aeroplanes, so I may magic up a case for it.

Andrew

[ColinB]

Looking forward to following this project with interest!

[Andrew B]

I consulted the oracle (manual) and have connected up the AM antenna coil, hopefully in the right "sense".

I have given the chassis a good clean, carefully polished the 5 valves that I have at the moment, given the sockets, the switches and pots a sniff of "Servisol" magic lotion.

The electrolytic cap seems to have fared up well. I have 330 volts on the main HT tail and 288 volts everywhere else, a quick current check showed that between them,the smoothing caps, the 2 EF80's, EF85 and the EABC80 were eating up about 40 mA of HT. According to the data for an EL84, it will need a max of 60mA and 10mA seems reasonable for the ECH81.

Seems good to me for go (up to now) Anyone have an EL84 and an ECH81 in Bradford area that they can loan me for a few days until mine arrive?

[AC/HL]

PM sent

[Andrew B]

Many thanks to Bill AC/HL for the valves, very pleasant to meet him.

Well, report on the radio is, FM has very good reception with a multimeter lead as the aerial, MW/LW there are some stations mixed up in the general QRN, Five live blows in easily, but radio 4 on Longwave is a lot of hum and mush, I'll wait until my neighbours go to bed and they switch off their trinkets.

I'll switch ye olde signal generator and the radio "ON" before I go to bed tonight so they won't drift all over the dial in the morning, when hopefully the local QRM/QRN will disappear and I can find the signal generators signal .

I have a 200 ft longwire antenna and a "shack" earth (a 3 foot piece of copper tube whacked into the ground), I have some ally pipe and Bakelite so I can make up a dipole for FM.

All looks good to go,

Again, many thanks to Bill and all on the forum