Philips B4X26A

This "Recent Repair" was kindly contributed by Tim Pullin.


Introduction

MW and 3 SW bands. Estimated mid to late 50ís. Valve lineup ECH81, EF89, EBC81, EL84, rectifier EZ80, tuning indicator EM84. Bass and Treble controls.†

This interesting, but typical fifties looking Dutch made set had been in regular use for many years, until it was brought to me for repair with a report of "Feint". It was indeed, the volume control only having a slight effect on output.

There is a little removable panel on the bottom of this one, and with the highly polished top resting on an old piece of carpet I was able to have a quick look inside. Construction is typical Phillips, (they do always manage to cram a lot into a small space donít they?) and at first glance a bit daunting. The lack of a LW band leads me to suspect this one to be an export set or one made for the foreign market.

The innards had received attention a few years ago (judging by the style of components used, dust etc) with quite a few resistors and a few capacitors having been replaced and overall a nice job. I donít know whether the previous repairer had access to a circuit diagram or simply did a one for one replacement. They may be reading this now; such is the miracle of the Internet.


Repairs

All valves are fairly standard, although in my experience EBC81 is not too often encountered. Out came the valve data book and the- multi meter, a few quick checks at valve electrodes showed no voltage at the EF89ís screen grid. The 56k resistor feeding it had gone open circuit.

Part diagram from full service info supplied by Steve Rosenburg. Thanks Steve.

Replacement of this resistor restored the set to normal operation on all bands. Other voltage readings were taken and all seemed reasonable for the type of set and valves used. *

The grid blocker to the EL84 output valve and most of the de-couplers were those tubular ceramic types, which are common in Philips sets, and seem very reliable.††

The customer also wanted modern three core mains flex fitted, so I selected a length of gold coloured flex we thought would match with the colour scheme of the set.

Replacement is quite easy, once the chassis is removed and the conductors can be soldered directly onto the mains switch, operated by the "Piano keys". The earth conductor was soldered to a nearby chassis tag. The mains flex passes quite near to the output valve so is held clear by a twist of thick wire. This wire was also used to hold the new cable in position.†† Another clue to this being a European set, is the highest selectable mains voltage of 220v. To see if the set had been modified for use on British mains (mine is about 245v, even though we are told it is supposed to be 230v these days) I checked the heater voltage and found it was 7v AC, a significant percentage higher than it should be.

Over time this would lead to shortened valve life, due to the heaters being overrun. (* HT was also a little higher than expected but the ratios of voltages to one another was still correct.) The simplest way to get round this is to place a suitable resistance in series with the primary winding of the mains transformer. Due to the fact the value of current flowing is governed by the IMPEDANCE of the windings one canít calculate the current easily, so it's probably easier and just as quick to measure it. The best way of all is to empirically select a resistor that gives the 20-25 volts drop at the required current, and correct heater voltage. Try a stout resistor (5-10W) of around 100R and measure the AC voltage across it. Ohms law will then give the current through the resistor, and an accurate final resistance value can be calculated. From this one can calculate the power rating required. The resistor will dissipate a fair amount of heat so should be sited as far away from other components as possible.

Another method would be to use Paulís excellent calculator for capacitive droppers, substituting our voltage drop required and calculated current. Whereas this is excellent for large voltage drops in AC/DC sets itís not really designed for nutters like me that only want a series resistance of 100R, so any capacitor values would probably be quite high and fairly impractical to use. Serves me right! One canít use the Resistor diode method either; this is an AC set.

The customer wasnít bothered though, (I did advise him however) so this is really only an academic exercise.

The magic eye tuning device wasnít too clever on this set either, with a change in value of its anode load resistor from 470k to around 550k. Most of the gettering had gone from the top of the valve and the emission was also very low. A new EM84 worked perfectly, the bars opening and closing as stations were tuned in.


Cabinet

Has to have been one of the best I have seen, and you can literally "see your face" in its highly polished surface. Itís been a devil wiping off all the fingerprints! Knobs and keys were given a quick wipe.†


Other Stuff

At sometime in the past a nasty large toggle switch had been mounted at the back of the set, fixed by a bracket screwed to the inside of the cabinet. The toggle itself protrudes through a hole cut in the fibreboard back panel. This switch had been connected in series with the set's speaker, presumably to mute it while an external speaker was in use. It is never a good idea to run a valve amplifier with no speaker connected as it is likely that damage to the output transformer will result. This set being a Philips has what one could politely describe as "non standard" output transformers, some extra windings on the primary taken back via the Bass and Treble controls to form what looks like some sort of active tone network. Very nice, but not what one wants if you have to find or wind a new transformer. I wired the speaker direct but left the switch in situ to fill in the hole!†††

If you do want to do this sort of thing to a set, then one method is to permanently connect a resistor of about ten times the speaker impedance across the output transformer (a handy point is often ext. speaker sockets). Enough to terminate the output but high enough in value so as not to affect the output signal to any degree. Do study the circuit though in case any resistor connected across the output in this manner would upset the feedback. In the majority of sets it should be OK.†

The power rating can be considerably less than the power developed in the speaker, due to the increased resistance and consequent lower currents flowing.†

All in all a nice little set, and all in under two pages. Is this a record!!†


Text Copyright © 2003 Tim Pullin.




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Last updated 14th April 2006.