Philips B2G81U

Introduction

Photo of the set as I bought itI used to own one of these sets some time ago, but sold it with several others when I was short of money. This was the only one I regretted selling, so I was pleased to see this example at the NVCF in May 1999. It was originally priced at 10 but was reduced to 6 towards the end of the afternoon. The seller accepted my offer of 5 right, at the end of the day.

Although it was dirty and tatty, the set was complete apart from the back. The knobs still had their chrome trim, and the clear plastic tuning scale "glass" was in good order. The only small problems were that the "PHILIPS" logo was missing the first "I", and the white paint on the front was a bit chipped and scratched. Internally the PCB, components and cabinet floor were covered with a thick layer of dust. All the valves were present and appeared to be correct. The only sign of previous repair work was a RadioSpares resistor connected across a faulty section of the dropper resistor. It was clearly a good restoration project. I have several other sets here awaiting my attentions (some have been waiting over two years now), but this one was so tempting that it jumped the queue and work started on it the following weekend!


Electrical Checks

Ignoring my own advice about meticulous checking, I decided to connect it to the mains and see what happened. I did connect it via the Lamp Limiter however, since I know what the smoothing electrolytics in these sets are like for reliability...

On the first test, the sound of arcing was heard from the mains switch. This would probably need replacing, but for now I quickly switched it on and off a few times and it seemed to clear up. The valve heaters started to glow, but nothing else happened. There was no sound at all from the speaker.

Rear view of set as I bought itI connected a test meter between the chassis and the HT terminal on the main smoothing electrolytic - just over 200V. However, the positive terminal of the next smoothing capacitor (the other cap in the same can) was at zero. The decoupling resistor was open-circuit. This is another section on the dropper resistor, and (according to the service data) should be 600-Ohms. The closest I had was a 560-Ohm wirewound component from a scrap Bush set.

With this fitted, I tried the set again. As the set warmed up the lamp on the lamp limiter unit started to glow much more brightly than it should. A quick check on the HT showed that it was now around 50V, and the electrolytic can was getting warm. Clearly the electrolytic was very leaky and this had caused the demise of the dropper section.

This is a PCB mounting electrolytic, with two 50 MFD 275V sections. This had five tags (three for the negative/can connection), and all of them were bent under the PCB. Removing this from the PCB, without damaging the tracks, was somewhat tricky. I used a solder sucker to remove most of the solder, then desoldering braid to remove the rest, before carefully straightening the pins and lifting the component out. A couple of the tracks had lifted a bit, but not too seriously. I did not have a direct replacement, but I did have another 50+50 MFD 275V PCB mounting component with a different pin layout. For test purposes, I temporarily connected this to the PCB with three lengths of flex.

Time for another test. Now things looked more promising. The HT rose to just over 200V, then dropped to around 190V as the output valve warmed up. The speaker produced a buzzing sound with a bit of hiss, which suggested the set was doing something. However, the tuning control would only turn around ten degrees at one end of the band, and the waveband switch did not seem to do anything. I connected a loop of wire to the signal generator and placed it close to the aerial. The set responded enthusiastically around 465KHz (the IF) but nothing else.


Tuning Drive

The first problem to tackle was the jammed tuning drive. On this type of set, the clear tuning scale cover is retained by a bracket and screw inside the cabinet. This must be removed before the tuning knob and pointer can be removed; all of which must be done before the chassis can be removed from the cabinet. On this particular example however, the retaining clip and screw were missing. Instead the clear cover was held in place with epoxy resin adhesive. This stuff sets very hard, and is difficult to shift. Fortunately the chrome trim around the edge of the clear cover came away easily, which allowed me to get at the adhesive. I spent some time trying to remove some of the adhesive with a craft knife and other implements, with very little success. I decided the only option was to apply a bit of force to the clear cover and hope that the retaining lug (which is where the glue was) would break away from the rest of the cover without anything else breaking. Crossing all available fingers, I inserted a screwdriver into the small gap between the clear cover and the cabinet, and twisted it gently. This time I was lucky, but I really do not like using that approach to dismantling sets!

The tuning knob them simply pulled off. The cause of the limited rotation was now obvious - the pointer had been pushed too far down the shaft and was fouling on the tuning capacitor fixing screws. It is retained by a rubber grommet, which had gone hard and crumbled away when I tried to move it.


Further Testing

With the pointer removed, I could rotate the tuning control fully, so it was time for another test. Still gentle buzzing and hissing right across both bands, and no crackles or anything from the waveband switch. Touching the UCH81 mixer-oscillator valve bought about a lot of crackling so this was clearly not making good contact in its socket. I removed it, applied a small amount of contact cleaner into the socket pins (by spraying the cleaner into the can lid then applying a small amount to each socket pin with a small artists paintbrush), then refitted the valve. I also gave the waveband switch a quick shot of the contact cleaner.

Time for yet another test. Success! The set now picked up a reasonable number of stations on both wavebands. A couple of quick voltage checks around the UCL82 output valve suggested that the normally troublesome coupling capacitor was OK. The set was still not completely sorted though, there was a rattle from the speaker when the volume was turned up, and the sound quality was fairly shrill. But it was working.

I connected the original faulty electrolytic can to my capacitor reformer, and left it for a few hours. I was not expecting it to be successful, but it was worth a try. After a couple of hours the current had dropped from 11.6mA to 9.8mA, so I decided to leave it reforming overnight...

... but the following morning it was still at around 9.5mA, so I gave up with that idea! Clearly I would need to fit the similar replacement capacitor in its place, but I decided to give the whole PCB a good clean first. There was also the shrillness to be attended to, and from further listening, it was apparent that the hum level was excessive, so this would need to be investigated too.


Disassembly

To do all this I had to remove the chassis from the cabinet. Since the tuning control and pointer had already been dismantled this was a fairly quick job. The on/off/volume and waveband knobs on this set are fitted with grub screws, which pass through threaded holes in the shaft. The tops of the grub screws are supposed to be covered with small plastic grub screws for insulation, and surprisingly one of these was still in place. With this lot removed, the speaker leads unsoldered, and the two screws inside the cabinet released, the whole chassis lifted out easily.

Before doing anything, I gave the PCB and chassis a good clean. The dust was easily removed with an old toothbrush, but some waxy residue remained in a few places. This responded well to contact cleaner and paper tissues, and soon the whole assembly looked much more respectable. One resistor lost its markings during the cleaning and although it was still electrically correct, I replaced it to avoid confusion later. I gave the valves a quick clean with some paper tissues and a bit of saliva!


Electrical Repairs

I started with the hum, and immediately suspected the replacement electrolytic can (it was salvaged from another set). I replaced it with a brand new can, but this made no difference. Grounding the grid of the output pentode section of the UCL82 got rid of the hum, as did grounding the grid of the triode amplifier section (via a 0.1uF capacitor). However, the hum remained when grounding the wiper of the volume control (as would happen when the control is set to minimum). The only things in between were one capacitor and a couple of resistors, all of which checked out OK. I noticed that this area of the PCB, which is fairly close to the rectifier valve, was a bit discoloured. I became suspicious that the cause of the problem was the PCB becoming slightly conductive, due to the results of years of dirt and heat. This is apparently quite common with this type of set. To prove the point I removed the three components from the PCB and wired then directly between the grid pin of the valve holder and the wiper of the volume control. Much better. The area around the base of the valve holder was discoloured too, so I removed the tracking to the grid pin (by applying a soldering iron until it came away) then cut away a small amount of the PCB around the pin using a burr attachment in a PCB drill. The pin was then sleeved where it passes through the PCB with silicon rubber sleeving, and the components were reconnected directly between the volume control and the pin. Success! No hum at all!

While I had the PCB drill out, I made the extra holes in the PCB necessary to fit the replacement smoothing electrolytic can (it had been dangling on wires up until now). I was able to position the holes so that the tags on the replacement component could be bent over underneath the PCB and soldered directly to the original tracks.

Circuit SectionI had been running the set with a test speaker and, without thinking, disconnected this and reconnected the original speaker while the set was on. Bad move! Valve amplifiers do not like running with their speaker connections open-circuit. The effective impedance of the primary of the output transformer rises dramatically, resulting in huge voltage excursions as the output valve tries to deliver the current into no load. In this case the voltage excursions caused the tone correction capacitor (which is connected between the anode of the output valve and ground) to go short-circuit. Fortunately, I was still running the set through the lamp limiter, otherwise this mistake might have resulted in more serious damage.

With the capacitor replaced and the original speaker reconnected I decided to do something about the shrillness of the sound. I had been here before with the example of this set I owned previously, so I referred to my notes and applied the same modification. The shrillness is due to third harmonic distortion, and my modification adds some negative feedback to reduce this, and also adds a bit of tone correction to improve the bass response.

Circuit SectionThe negative feedback is picked off the cathode of the output pentode (which is not bypassed) and applied to the bottom of the volume control. The direct connection from the bottom of the volume control to ground is replaced by a 1K5 resistor. A 10K resistor and 0.022uF capacitor are connected in series and wired between the cathode of the output valve and the junction of the 1K5 resistor and pot. The resistor values result in a significant amount of negative feedback being applied, while the capacitor value is such that this effect is reduced at lower frequencies, giving a bit of bass boost. I assembled this lot temporarily, and found it worked just as well as it did on my previous set. I had experimented with the component values with my previous set, so I knew they were good! The two diagrams here show the original and modified circuit.

Some people might object to the idea of modifying the circuit in this manner. However this particular modification could easily be reversed at a later date if required, and makes the set much more suitable for listening to today's MW broadcasts (if you can find anything worth listening to). I am much more likely to use a set that sounds good - and one of the pleasures of collecting vintage radios is using and enjoying them!

Components around volume controlI now had quite a few components tacked temporarily around the volume control, and it was time to fit everything tidily. Before doing so I decided to replace the volume pot/on-off switch. The switch had been behaving properly after its arcing earlier, and the volume pot had responded to a shot of contact cleaner. However, there was quite a lot of work to be done in that area and I did not fancy having to do it all a second time if the control failed later. As I mentioned before, the original control has a threaded hole through the shaft, for the knob fitting grub screw. My replacement control had a plastic shaft, so I cut this to the correct length and drilled a 2.7mm diameter hole through it (this is easy with a bench drill and drill vice, but would have been very fiddly with a hand drill). This hole was about the right size for the 6BA grub screw to cut its own thread into (I do not have a set of 6BA taps). I also replaced the mains flex at this stage.

With the electrics complete, I turned my attention to the rattling speaker. This was due to the cardboard edge ring having shrunk, and therefore lying across the cone. This also resulted in the cone not being secure around part of the edge. I cut away the offending part of the edge ring, then re-fixed the cone with EvoStick adhesive. I then cut the removed piece of edge ring into two pieces and glued these back into position. This left a couple of small gaps, but these did not affect the sound quality.


Cabinet

With the chassis out of the cabinet, I could get to the lug that I had to break from the tuning scale cover. I removed most of the epoxy resin with a craft knife. The lug then seemed able to move slightly, but would still not come out. I applied a small amount of WD40, left it for a few minutes, then pressed it carefully from behind with a screwdriver. It slid out fairly easily, and I was then able to remove the remaining epoxy resin from the hole. It seems that, although the WD40 does not directly affect the epoxy, it does penetrate and undermine the bond. I glued the lug back onto the tuning scale cover with some superglue, then made a fixing clip from an old metal P-clip, so that the whole thing could be assembled properly later.

The next stage was to clean the cabinet, and repaint the front white section. I removed the speaker (again) and its hardboard spacer. The tuning scale (printed on thin aluminium) was glued in place, but with some very gentle easing, I was able to remove it without bending it. The PHILIPS logo was removed by pressing on its fixing pegs from the inside of the cabinet. The cabinet, knobs and logo were then washed in warm soapy water, rinsed, and left to dry. Everything that did not need painting was masked off with masking tape and newspaper, and the front was given a fairly thick but even coat of "Rover White" aerosol spray paint. I also sprayed the tuning pointer with the same paint.

Repaired PHILIPS logoWhile the paint was drying, I turned my attention to the missing "I" on the plastic PHILIPS logo. With the logo face down, I laid a straight piece of 22SWG tinned-copper wire where the "I" would have been, and gently pressed it into the plastic with a hot soldering iron. I trimmed the wire slightly shorter than the surrounding letters, then slid a short piece of thin black PVC sleeving over it. The end of the sleeving had to be carefully trimmed so that it closely fitted the logo with no visible gaps. The other end of the sleeving was then trimmed to the same length as the other letters, and the whole lot was secured with a small amount of superglue If one looks closely at the finished set (and knows where to look) the repair is visible because the repaired "I" is round section whereas the other letters are square-section. You can just about see this in the photo. However, from a normal viewing distance of a few feet no one would notice. It is certainly much better than a missing character!

Once the paint had dried, it was time to reassemble everything. The PHILIPS logo was pressed into place and secured with a small amount of EvoStick on the pegs from inside the cabinet. The same adhesive was used to secure the tuning scale onto the cabinet, and the chrome trim onto the tuning scale cover. The speaker and chassis were then refitted.

Fortunately my idea of drilling a small hole through the volume control shaft for the grub to tap a thread into worked! I fitted the one remaining plastic insulating grub screw into the waveband switch knob. Because the volume control now has a plastic shaft, it does not need an insulating screw.

The completed setI fitted a new grommet in the tuning pointer, and positioned this approximately on the shaft, then fitted the tuning knob. At this stage I tested the set, and aligned the tuning pointer accurately. This was a good stage to leave the set on an extended soak test, to make sure no other faults developed. After a few hours, I was happy that all was well.

The final stage was the most fiddly (this is why I left it until after the soak test)! To refit the tuning scale one needs to position the retaining clip (which I had made) and the screw inside the set, in the small gap above the ferrite rod aerial; while at the same time holding the cover in position from the outside. After some grumbling and swearing, I found the easiest way was to magnetise the screwdriver blade. The screw will then stick to the blade and the clip will stick to the screw. It is still a delicate operation, but it can be achieved with just two hands!

The only item outstanding on my set now is the lack of back. I am hoping something suitable will appear on a scrap set in an auction, but if not I will make one from hardboard.

This was an interesting restoration, because of the unexpected and unusual problems that cropped up, which required some thought and ingenuity to overcome. It was also satisfying because it has transformed the set from a piece of sad looking junk to an attractive and functional piece of equipment.


Update

After adding this repair to the website, Dave Goddard emailed me and very kindly offered to give me the correct back from a set he had scrapped.

I am really pleased to have the correct back for the set - it is much nicer than some home-made effort. Thanks Dave, for your most welcome donation.




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Last updated 14th April 2006.