Philips 'Superinductance' model 274A

[Old Sparky]

Having just got back to a long delayed repair of one of the above,and, amongst other things, checked the reservoir & smoothing capacitors, C1 & C2, which, not suprisingly, as the radio is 82 years old, are u/s ( For some reason they read like two back to back diodes on one of those chinese testers!), I found a new 2 x 32uf screwfit Can Cap. in my 'stock', decided that this would replace the two original 16uf types.  I then realised that the cases of the originals are positive, as the filter components are in the negative line.  Now it's a standard resistor filter circuit, and I can see no reason why I shouldn't rewire this into the  positive side of the HT supply ( Sorry I cannot post circuit excerpts as my scanner won't work with W10!) Does anyone know any reason why The filter circuit can't be rewired as I've described it?   This will allow me to use my modern dual electrolytic in which, like most, the can is  negative.I intend to use a pair of either 1N4007 or 1N540x diodes instead of the valve rectifier as the wrong valve was in the set when I got it, and the correct type (1821, IIRC) is virtually unobtainable.

In addition, what is the best way to open the small metal can containing six capacitors?  This is a tinplate box, with the top & bottom covers turned  and soldered to the rest of the box ( I can take a photo of that & post it if that will help).  All six caps. will almost certainly need to be replaced, and it might be easier to use a new metal box in which to house them!

[pwdrive]

Is the valve bias derived from the filter resistors?

Lawrence.

[Old Sparky]

Looking again at the circuit, Lawrence, the control grid of the first valve is returned to a tap on thefilter resistor (R1), via a 2Meg resistor(R8), but as R1 is only 1260 ohms, by using a 2M2 resistor for R8, and returning it's 'earthy' end direct to the negative line. The control grid of the o/p valve is returned to the centre tap* of the mains Tx secondary via R19(640k), R14(320k), R13(2Meg), & R7 (1Meg), so the answer is, yes GB for the 1st and 4th. valves is derived via the filter circuit. (* i.e. to the negative terminal of the reservoir capacitor)

[pwdrive]

That suggests you can't reconfigure the filter circuit from HT-ve side to HT+ve side unless you make alternate arrangements for the valves grid bias supply.

Lawrence.

[Old Sparky]

Doing some more work on the 274A this morning (need to replace 2 or 3 waxies for one thing!) it suddenly occured to me that, before I removed them, theinsulated terminals of C1 (reervoir cap) and C2(smoothing cap) were connected together with a short length of solid core wire which looked original, and in fact was part of a longer insulated solid core wire. As the metal cans of C1 & C2 are connected together via the metal bracket on which they are mounted, it means that C1 & C2 were wired in parallel, which is definitely not as per the circuit diagram. Checking one of the two 16uf electrolytics again (on a different Chinese tester to the first one) showed about 15.5uf, with a low ESR an Vloss. Admittedly this is at only about 8.5v from a PP3 powering the tester, so ideally both cans, IMHO, should be replaced.

[ppppenguin]

It's amzing how old high voltage electrolytics will come back from the dead. Instead of the Chinese tester give them a dose of high voltage through a limiting resistor and see what happens.

[ppppenguin]

Forgot to add that you can get a rough idea of the capacitance by discharging the cap through a resistor and seeing how long the voltage takes to fall. For a time constant C * R it will fall to 37% (1/e) of the original voltage in C * R seconds. Any leakage will discharge it faster. It will be close to zero volts in 5 * C * R. So a 100k resistor will give a 1.6 second time constant. you'll know pretty quickly if it's anywhere near its correct value.

ESR matters in switchmode power supplies and many other applications but for the smoothers in a old radio if the capacity and leakage are OK it's unlikely that ESR will bother you.

[Murphyv310]

Hi.
The caps will be wet electrolytic type. as in my slightly later version. They can dry out and all I did with mine was to top up the caps with de-ionised water and give them a very long reform.
The set is 99% original with just a replacement mains lead and a service and its knocks the pants off many a superhet.
Please Please don't fit a silicon diode to a classic receiver, look out for a valve rectifier or a suitable substitute.

[Old Sparky]

Thanks for the suggestions and advice, guys. I don't think the reservoir & smoother caps. are wet electrolytics, as I don't see any means of adding any liquid to them, but I'll double check and maybe post a photo or two on here. The 274a is currently in a partly dismantled state so that, ampngst other things, leaky waxed paper caps. can be replaced. In point of fact this is a long delayed repair job for a member of UKVRRRDF, who is hoping to get his radio back shortly. One of the reasons for the delay is the fact rhat an incorrect rectifier valve whose heater draws 2.5Amps was in it instead of the correcr type, an 1821, IIRC, and I've never been able to source one., so, AFAIK, other than using the valve to hand, which draws 2.5 times the heater current of an 1821, which can't be good for the 82 year old mains transformer, is to fit silicon diodes, which would be under the chassis, the rectifier valve being left in situ, unconnected. I once did a similar mod. to a Roberts P5A, which enabled another veteran, albeit about15 years younger than the Philips, to work safely and reliably. I had also recapped that set , fitted a sustitute o/p transformer, and replaced the faulty Res. & Smoothing caps with modern equivalents. None of the changes were visible above the chassis.
NB I've just checked on eBay, where there is one 1821 available, but it's £89.99! Way too much - I don't think the radio's owner would pay that much in addition to other repair costs

[Murphyv310]

NB I've just checked on eBay, where there is one 1821 available, but it's £89.99! Way too much - I don't think the radio's owner would pay that much in addition to other repair costs

Hi.
He needs to consider the value of the set, it will be an appreciating asset. Although silicon has it's uses I don't like the peak voltages that will be present before the rest of the valves conduct. There is no suitable way to reduce surges in this scenario, the wrongly designated surge limiter resistor wont be of much use here as the voltages could rise well above the rating of capacitors.
I no longer estimate low for vintage repairs and restorations as it weeds out those not willing to pay for a quality repair. A set of this age deserves to be done properly without the use of 1N4007's and I am sure you will agree.
Car restorers don't skimp so why should Radio restorers?