Philips 470A

[Sideband]

I originally restored this set about three and a half years ago but I didn’t do a write-up. Recently I did some more work on it so thought I would share the complete story.

This set was found in a store room, in fact the same one where I found my 274A Superinductance set. There was no way of telling how long it had been there but rough estimates suggest about thirty years. The 470A was originally built in 1938 and uses the B7 range of valves with 4 volt heaters…FC4 octode as frequency changer, VP4B as IF amplifier, PEN4DD as detector and output and 1821 as rectifier. The 1821 is a directly heated rectifier on a B4 base.

As found, the large Bakelite case was grubby but undamaged and looking into the back of the set, everything seemed to be in place with only a little dust. Once I got it home, I was pleased to find that the set was complete and original. The only evidence of previous work was that the pilot lamp had been rewired.

After removing the chassis I found that most of the rubber covered wiring from the mains transformer had rotted so this would definitely have to be changed first. There was a melted capacitor sitting on top of the transformer which was the mains suppressor so someone had obviously tried the set many moons ago. First I set about the rewiring. I used some appropriately coloured PVC wire for the job and spent several evenings doing a rewire. To add a professional appearance, I even laced the cables. Resistance checks around the transformer indicated that, in all probability, the transformer was fine but I thought it was worth doing a mains test so with all but the rectifier valve fitted, I applied mains via the variac set at 150 volts and switched on. The original pilot lamp was faulty but I had fitted a 6.3v type as a temporary substitute. This now glowed faintly which was at least a good sign. Slowly increasing the mains brought more life and at 200v, the valves showed a visible glow. At full mains, the valve heaters were now fully alight and I left it like that for a few minutes. The transformer stayed cold and there was no subtle essence of burning transformer!

So far so good…now on with the rest of the work. There were a number of wax capacitors fitted that had visible white mould on them. These would have been difficult to rebuild and retain original looks so I decided to just replace them with polyesters. During this exercise I found that various screened leads had also rotted. It seemed that these early screened cables have a rubber-covered centre conductor with an outer cover of tinned foil. Some have a further outer insulation of waxed cloth. The inner rubber cable on these had gone hard and rotten so I had no choice but to replace all the screened cables with new….not an easy task. However I was able to disguise these quite effectively using the existing waxed cloth covering slipped over the new cable.

So with the paper capacitors, critical wiring and screened leads replaced, it was time to do the electrolytics. There are two of these fitted to the chassis. They are of the liquid type and have a Bakelite base with a large bolt that secures them to the chassis. I did not even bother to see if they would reform as, when they were removed from the chassis, both rattled. In order to keep appearance ‘above decks’, I decided to rebuild them. If you look at my previous description of how I rebuilt the capacitors in my Superinductance radio https://www.vintage-radio.net/forum/s...t=Philips+274A I used the same method for rebuilding these. The only difference was these were easier to get into! Once they were fitted back to the chassis, everything looked original.

Before powering up for the first time it’s always wise to check everything and make sure there are no shorts from HT to chassis. The valves were fitted and a meter was connected to HT+ and chassis. Now the moment of truth! Apply mains and switch on. HT appeared after a few seconds (directly heated rec) and showed 300 volts. This started to drop down after about 20 seconds as the valves warmed up and settled at about 220 volts…..a bit low according to the Trader sheet. Advancing the volume control produced some noise and adjusting the tuning showed that the set was at least working to some extent. R4 was received reasonably well and at the correct place on the dial. Audio quality was good. Connecting the set to my long-wire aerial produced reasonable results. However after about five minutes, it started to fade and eventually all signals just disappeared. I also noticed the HT had dropped by another 15 volts or so. Time for some detective work!

Meter checks showed that the grids of the frequency changer FC4 and IF amp VP4B were going more positive (or less negative if you prefer) as the set warmed up. This pointed to the AGC line, seeing as these two valves receive their bias from there. A classic case of grid-current in one of the two valves I thought. Not so! Having checked both valves by substitution, I was left with the same problem. In fact with both valves removed, the AGC line was still going positive after about 5 minutes. Looking at the circuit, there are two diodes in the output valve. One is for detection, the other is the AGC rectifier. On a hunch, I disconnected the connection to the AGC rectifier at the valve holder and temporarily replaced it with a germanium diode. Result! Now the set ran well with no signs of fading and the sensitivity was also improved. Presumably the AGC diode in the PEN4DD was leaky in some way so I would need to find a new valve. Now, however was the chance to check the set properly and see how it performed.

Firstly a check on the I.F alignment. It uses the low I.F of 128kHz so I checked this first with my Advance signal generator coupled up to a frequency counter (so I could set the generator accurately to 128kHz) and found that it was a little out. The I.Fs appeared to tune quite sharply when adjusted and the gain was increased quite a lot. RF alignment was OK….just a nudge on the oscillator and aerial trimmers to correct the top-end of medium waves. Now the set was very lively and considering it’s age, it put more modern sets to shame. Using 128kHz I.F allowed greater gain and selectivity with the disadvantage of poorer second-channel rejection. However there is a rejecter in the aerial circuit that has to be adjusted for minimum signal at 128kHz and this seemed to tune quite sharply as well. Having set this up, I didn’t notice any second-channel problems on MW or LW.

A few days later, I collected a new PEN4DD. I removed the temporary diode and fitted the new valve. The set worked well with no signs of fading. However I started to get instability problems with the set getting to the point of oscillation. Sometimes the set would come on and break into oscillation for a few seconds. Once this had cleared, the tuning became very sharp, very much like a TRF at the point of oscillation. It didn’t do this with the old valve and temporary diode so I came to the conclusion that the new valve was faulty in some way. However the problem seemed to improve with use and after a week or so it seemed much better. Finally I replaced the 1821 rectifier. The original was giving a steady output of about 220 volts but the Trader suggested that this should be 263. I had one of these rescued from a scrap set. When fitted, the replacement was giving 260 volts at the cathode. The volume was noticeably louder with this replacement.

I was also able to replace the scale lamp with a 4.8 volt lamp from CPC.
The Bakelite cabinet was in extremely good condition and just required a good clean. I then used my method of wax furniture polish rubbed in with 000 grade wire wool for the first coat and then several more coats applied with a cloth, buffing between each coat to restore the shine. The set was then reassembled.

With use, I was aware that the tuning was a little sharp and the I.F bandwidth was a bit narrow. I wondered if I should recheck the alignment so I read through the alignment instructions again. Now here was something I had missed! The instruction requires you to damp the primary and secondary of the I.F transformers with resistors while performing the alignment. I hadn’t done this when first aligning the set. I’d just fed 128kHz into the mixer and tweaked the transformers. OK let’s do it right this time! This time I found that the adjustments were not quite so ‘peaky’ as a result of the damping resistors and it was easier to adjust for maximum signal. The second I.F needed at least half-a-turn and it wasn’t necessary to keep going back over the adjustments. Anyway when I checked the set again, the tuning was not quite so sharp, the HF response was better and, best of all, the tendency towards oscillation had completely cleared up. Obviously what had happened when I first aligned the I.F’s is that I had obtained a false ‘peak’ and made the bandwidth too narrow. Great gain but the poor old I.F amp was nearly going into orbit!! Presumably the use of damping resistors prevent the adjustment of the primary from upsetting the adjustment of the secondary by ‘flattening out’ the response. I would imagine in that case that the I.F transformers have a fairly high ‘Q’.

There are a couple of niggling faults left which,at the moment, are not causing any problems at all. Alignment is achieved by adjusting various ‘beehive’ trimmers in the I.F, oscillator and aerial bandpass coils. Most of these adjusted without problems but the one in the oscillator coil and the one in the first aerial bandpass coil are intermittent when adjusted and difficult to peak. However at present, both are in positions which don’t cause problems. I have some beehive trimmers of the correct value that could be used as replacements but the prospect of removing the coils from the chassis, especially where they are located (between the tuning gang and the front panel) and then removing the coils from the screening cans, looks like a major job which would also require the removal of the front panel, tuning drive and gang. I think while everything is working, I’ll leave well alone and it’s been OK for three and a half years so fingers crossed….!

There was quite a lot of work getting this old-timer going well again. It was all worth it though and the radio now performs extremely well with excellent sensitivity and sound quality… and it’s just had it’s 70th birthday! I suspect the original fault was the PEN4DD and the set was stored for all those years.

From my point of view, there is something extremely satisfying restoring really old receivers. Before assembling the set back into it’s cabinet, I was listening to some news items on R4. My gaze was wandering over the warmly glowing valves and it suddenly occurred to me that those very valves (well two of them anyway) had probably conveyed the announcement of the outbreak of war. Someone could have been listening on this very radio and they would have been kept up-to-date of it’s progress throughout the war years. Those same valves were now conveying information on today’s conflicts 70 years later. I wonder how much of today’s equipment will be around in 70 years to do the same thing?

[Brian R Pateman]

What a lovely job you've done!

I've always liked these sets and think that the styling was way ahead of its time. They look more like a 50s set than something from the 1930s.

Congratulations - and a good write up too!

Regards,

[yestertech]

Excellent job Rich and a lovely looking set ! I have the 371A which one can only surmise is modelled on your set. Philips, it seems to me generally seem to opt for narrow IF bandwidths, but as you say the correct way is to damp the other side of the transformer to ensure one doesn't go for just max. gain. Many of the later 50's BUSH instructions still specified this method.
Did you 'cheat' when rewiring and not reinstate all the available mains taps ? Although purists may scream, it seems a touch pointless in this day and age

Andy

[Nickthedentist]

Nice work, Richard!

And as Andy says, the big, elder brother of the 371a.

I'm sure I noticed that Alf Garnett had one of these in his bedroom

Nick.

[Sideband]

Originally Posted by setsappeal

Did you 'cheat' when rewiring and not reinstate all the available mains taps ? Although purists may scream, it seems a touch pointless in this day and age
Andy

Ummmm.....Yes! I didn't see the point rewiring taps that will never be used. Another point is that the voltage selector was very intermittent so although the wiring is still present, it's not actually doing anything and it's been rewired for permanent 240v.

Just for interest, I thought I'd take a close-up of the tuning dial. It's doubtful that the radio would ever have been able to receive all of these but I think at the time, people were impressed by seeing lots of stations marked on the dial. Hardly a familiar station listed, it's before the days of Light, Home and Third.


Rich

[petervk2mlg]

This radio put in an appearance in the film, "The spy who came in from the cold".

[howard]

Well done Richard

That's a very pretty little set, I wouldn't mind one myself.

Interestingly, this model was built all over Europe in 1938/9, in the UK, Germany, Holland, France and in Czechoslovakia.

Howard

[Sideband]

Originally Posted by howard

That's a very pretty little set

Thanks Howard. It's not that little though!



Rich.

[johnn]

........and what a good write up! My 727A had a narrow frequency response until I damped things down & recalibrated the IFs. I am still looking for a set like yours though.

John.

[maitiustandun]

hello rich, marvelous set , it brings a tear to my eye , for i remember it well in my childhood , wish i had one now , and believe me it was well capable of receiving all those stations marked and more besides , especially with a good long outdoor aerial and an earth , good look with it and many years of pleasant listening , best regards maitiu.

[geofy]

Great restoration to a very good radio, it is curious that my Philips 650A is of the same year and yet it has the more conventional 470Kc/s IF and more 'modern' valve line up of 6.3 V E series rimlock valves. Both have the same very attractive art deco grill cloth pattern, probably from the same roll of cloth! It is interesting that your radio has a couple more stations such as Montpelier and Viipuri on the MW dial though the 650A has a more comprehensive SW list of such stations as Chicago and Pittsburgh. Nice large speaker, which probably has an inner static 'tweeter' cone made from plastic (bakerlite?). I also only wired my set for 245V AC operation. I an quite sure the radio brought the news during those dreadful dark days of WW2, and how many where destroyed in all the bombed houses. I have noticed the London Region on both dials of 342 meters is not in the same place as more recent radios of 330 meters, the present Five Life position.

Geof