This "Recent Repair" was kindly contributed by Tim Pullin.
A wooden cased 5 valve all wave AC superhet, circa 1949. Valve line-up† 6K8G (X63*), 6K7GT, 6Q7GT (EBC33*), 6V6GT. Rectifier 6X5GT. An attractive wooden cased offering from KB, using the well proven late forties valve line up. Typical late forties feel too with a waveband indicator cursor operated from the waveband selector shaft and the on-off switch incorporated with the tone control.
The case is very tidy with all knobs and fibreboard back, and the chassis not too dusty. A couple of valves have also been replaced, (those found are shown inside brackets in valve line-up) the most obvious a red Mullard EBC33 in place of the 6Q7GT. The only problems apparent on brief visual examination were a broken dial cord and probably original 3-core rubber-insulated mains flex.
I found info for this set on Trader Sheet 906, which covers the DR10, but the two chassis are identical. Judging by the picture so are the sets!
With the set was a "Jiffy" capacitive aerial, a cardboard tube with a few inches of cloth covered wires hanging from each end. Although I am unfamilliar with the device I assume it provided some sort of load, simulating a long line aerial.
This set had a coating of brown nicotine residue, covering almost everything, including the rear of the tuning scale glass, (it condenses out presumably) cabinet, valves and chassis. I suppose it's potentially hazardous in theory, so you should really wear gloves and wrap your waste in a plastic bag for disposal.†I use "Grimex" wipes, while really intended for hand cleaning they are exellent on any smooth non-absorbent surface, particularly glass and plastics.
All mains tests passed with no problems, and in order to renew the mains cable the chassis was removed from the cabinet. This is extremely quick and easy, one only having to pull off the four knobs and remove two screws in order to slide out the whole thing, which is then able to be placed in almost any position for servicing.† The 16-24uF HT smoothers gave no kick when checked with an ohmmeter, so I wasnít too hopeful. A section at a time was connected to the bench HT power supply, via the usual 100k resistor to reform.† I had heard this set working though with an audible 100Hz hum. The finder had plugged in the set in to try.... still it is sometimes possible to bring back HT electrolytics from the brink..
While this operation was proceeding, the dust was brushed from the chassis (try a pastry brush - they are ideal) and the original mains flex was replaced with a new length of black 3 Amp 3-core. In order that the metal P-clip gripped properly and provided adequate strain relief, a rubber sleeve was fitted over the mains cable outer sheathing.
A quick resistance test to chassis revealed a lower reading than one would have expected, probably due to the handful of wax paper de-couplers (mostly 0.02uF, 350v). Most were indeed leaky, including "that" capacitor, the DC blocker to the output valves grid. While the HT capacitors were being re-formed all the wax paper capacitors were replaced. The wiring is a mixture of rubber and fabric insulation, the former used for the heater and dial lamps and the latter for HT etc and all in good order. With these capacitors renewed, HT to chassis resistance measured in the order of megohms.†
With temporary HT capacitors connected for testing (I used 16 and 32uF axials, the values indicated in the service data & visible in the view of the underside) the set was plugged in, a few feet of wire forming an aerial. Both dial lamps lit, followed soon by all valves. A station was just audible on the MW band, which became distorted but no louder when the volume control was advanced. (Sound familiar?) Tuning "manually", with a finger on the flywheel connected to the tuning gang, it appeared the only station able to be received was BBC Wiltshire (our local station).
The HT was around 20 volts lower than quoted - probably due to the 6X5GT rectifier being worn by the strain of the extra HT current caused by all those leaky capacitors. Its rated current output of 65mA is barely adequate for the set under normal conditions, let alone with an extra few milliamps caused by leaky capacitors and possibly extra anode current in the output valve (A few sets of this period with similar valve line-ups use the 53KU/5Z4G instead, which would seem much more sensible, provided you have the space of course). The tired rectifier was replaced by a used good EZ35, which corrected the low HT.
With voltages now tallying with the Trader sheet comparisons would be much easier. I decided to tackle the output sage first, at least then one can hear what is (or isnít!) happening once the output stage is correct. The 6V6GT valve as used in sets of this period tends to be quite reliable, all voltages around this valve were correct and indeed a signal applied to the grid appeared loud and undistorted from the 8Ē speaker.
However when a signal was fed to the grid of the preceding stageís triode (the detector amplifier-EBC33/6Q7GT) the output was barely audible and horribly distorted. A check of the anode voltage showed a reading of around 12v, as compared to 39v given in the service data. The 470K anode resistor was checked (with the set switched off!) and found to have risen in value to 880K. (Sound even more familiar?)
These valves donít take much current so I think the problem often encountered with anode resistors is due to the low anode voltages, or more accurately a higher voltage drop. I have found that the old carbon resistors donít seem happy with any more than about 80-100V drop across them, any more and they will often go high or open circuit. This is some times found with screen grid resistors (read on!) and anode resistors of tuning indicators too. A replacement resistor did the trick, producing my one station much louder and better sounding.
But why only one station? What made this poor old set so deaf? Touching the top cap grid of the mixer valve with my finger brought the set to life with lots of well received stations on all bands, so it was likely that the fault lay in the aerial or first stage, as there appeared to be plenty of IF gain with the finger applied. I found it a little awkward using one hand so a length of wire took the place of my finger and worked just as well).
The voltages around the mixer valve were not quite right with both the mixer anode and screen grid measuring low. The oscillator anode measured 10v high at around 100v. The mixer anode and screen grid resistors were also found to be high in value and were replaced. (The screen grid of the mixer and IF amp share the same resistor, voltage was quoted as 100v. See above comments re. resistors)
With this done and all voltages except the oscillator anode normal, and the set still not performing correctly I became more convinced of a fault in the aerial section, as it was obvious the local oscillator was running and the set worked fine with an aerial connected to the mixer grid.
The aerial coils all checked OK for continuity, but while I was doing this I noticed a 0.003uF Hunts Mouldseal capacitor connected between the tail of the aerial primarys and chassis. What if this had gone low capacitance/open as they are prone to do? I replaced it with a polystyrene capacitor of similar value (3300pF) and HEY PRESTO! Now we had a good range of stations, at good volume but only with a colossal aerial. I would have expected a set using this valve line up to give lively performance with about a 10í wire aerial, but I had to use a large mass of metal to achieve satisfactory results. The steel shelves in the workshop worked very well, but an old fireguard hung from the ceiling was best. I might keep it up there, it really was good, and itís even directional!
Initially I had assumed the X63 fitted was an equivalent to a 6K8G, but further investigations with the valve equivalents book revealed the X63 isnít an equivalent, (it is actually equivalent to a 6A8G) and according to my data doesnít even have the same pin-out as the 6K8G so itís a wonder the set worked at all! The existing valve was replaced with a brand new CV1944/6K8G. Just to be on the safe side the connections to this valves socket were also checked in case they had been modified for the substituted valve. Oscillator anode voltage now tallied with service data. Much better. Performance was almost normal now, and certainly acceptable with stations being received with about 4 feet of wire.†††
Here is the chassis undergoing alignment on the bench. At this point all wax caps have been replaced; also screen grid resistors and tuning dial cord.
The set was still a little deaf on the LW band but with a broken dial cord I had no way of knowing whether the tuning was accurate, and hence alignment so that would have to be the next job. A previous repairer, having replaced the 6K8 with the X63/6A8 may have needed to align the set to achieve acceptable performance.
The old cord, made from black linen thread, had merely stretched and come off the various pulleys. I tried numerous times to re-thread using this cord but it eventually broke.
Re-cording one of these sets is not to be attempted if you cannot spare a few hours. The trader sheet makes it sound easy but it really is a stinker, one having to start by fastening the middle of the cord in the tuning capacitor drive wheel and threading both ends round all the pulleys, pulling the slack up afterwards (and took me nearly three hours!). Adjustment is achieved by loosening the shaft fixing screws of the tuning capacitor drive disc and the capacitor shaft. The designer obviously had three hands and assumed every one else had too. This one makes a Philips B3G63A look easy!
I followed the procedure in the Trader sheet and aligned the pointer with marks provided on the tuning scale back panel, (only visible once the layer of nicotine had been removed!) and thankfully the pointer was spot on when the chassis was replaced in the cabinet, with stations appearing in the correct positions on MW and SW, but slightly out on LW (Radio 4 appeared at about 1350-1400m).
Tim's Tip: The thin nylon cord of the type used for kites works very well. I have used it in a few sets and have had no problems, although an extra turn or two around tuning spindles etc. is sometimes required to ensure the cord grips properly. Melt the cut ends with a soldering iron to stop them fraying. Loops can be made by tying a loop in a knot - or if you need to thread a loop through something, an end can be folded back and passed through the cord a few times.
With a working tuning pointer, alignment could proceed. As is usual, the first step is to align the IF stages, by injecting a signal into the mixer grid at the receiver's IF and adjusting the IF coils for maximum output. It's best to use an AC millivolt meter across the speaker or volume control, but most analogue multi-meters will do the job. I actually used a 'scope with its timebase set deliberately slow, so the displayed waveform gives a broad stripe, making it easier to judge maximum.
So, with the sets local oscillator disabled (short oscillator grid to chassis) I injected 465khz and was a little surprised when the set did not respond. Swinging the output of the generator a few kHz either side, a tone was heard from the set at 480 kHz, so was it possible the IF could be at the wrong frequency? That would certainly explain the problems with long wave.
Thanks at this point to Mike Horne and John Berry for their help and advice. John helped me to align the set to the correct IF frequency, allowing the line up to continue. John suggested the following:
Set your signal generator to the setís IF frequency (in this case 465kHz) and turn the modulation on. With an AC millivolt meter connected across the speaker, and the set's local oscillator disabled (I simply shorted the oscillator triode grid to chassis) apply the generator output to the IF stage grid (6K7G) and adjust the final IF coil for max output on the meter. Then apply the generator output to the mixer grid, tune set to the low frequency end of the MW band and adjust the oscillator anode coil for maximum output. Keep the generator's output low (to avoid AGC action) and the set volume high enough to show clearly on the meter range you are using.
With this done the set was realigned in accordance with the process detailed in the service information, and responded very well with its performance now much improved.
LW was now much more sensitive, with the tuning pointer now accurate (Radio 4 now at 1500m) and very much louder. As you might expect short wave was hardly influenced at all, but that was fine anyway! Variable capacitors are used for most adjustments, the threads locked with Loctite or similar. A little too much had been applied to the LW aerial trimmer and unfortunately the brass nut sheared during alignment. I wasnít THAT heavy handed, so it's certainly possible this had been strained at sometime in the past, possibly when the 6A8G was fitted in order to get some reasonable performance.
The trimmers are fixed to a small angle bracket, which is in turn screwed to the chassis. By carefully removing the bracket from the chassis it was possible to loosen the screw under the trimmer and remove the damaged portion of thread. A new 1" 6BA BRASS screw and star washer were fitted, leaving about 5/8" of thread protruding from the top side. The whole assembly was then refitted and a brass nut and washer used for the adjustment. When alignment of the LW band was complete the nut was locked with correction fluid.
It is ideal for this purpose too. Just enough to lock the thread but not too much to stop it being adjusted if required. And it has many other uses, such as marking the alignment of parts prior to dismantling etc. Of course you can even write on it!† †
The tuning scale in this model remains in the cabinet when the chassis is out, and one is supposed to rely on measurements taken between the tuning cursor and a datum line on the painted panel behind it. These were only found when a layer of dust and nicotine had been cleaned away. I found this measurement process specified in the trader sheet awkward and fraught with possible errors. The chassis is so quick to replace that this may as well be done every time. It worked for me!††
Overall this one is pretty good - just covered in Nicotine! I tried all sorts in an effort to shift it but found the best thing was a lightly dampened rag, which also brought up the grain nicely.
The wood was left overnight to dry and then lightly waxed and polished. The tuning scale was carefully removed and cleaned with baby wipes, making it instantly brighter when lit and the waveband indicator clearly visible through its window.
The four knobs were cleaned in the dishwasher (donít forget to remove the metal spring collars) on a low temperature programme, as they are made from plastic rather than Bakelite.
The gold lettering on the control knobs is slightly recessed, so was traced over with a fine point gold paint pen, the ink left to dry and then the excess scraped off with a thumbnail. The Knob was cleaned further and polished again. Virtually undetectable!†
Although negative feedback in the output stage is derived by a winding on the output transformer, an improvement in sound quality can still be achieved by connecting a 22uF capacitor across the output valveís cathode resistor.
The HT smoother didnít reform so two new axial capacitors were wired in permanently, using a piece of three way tag strip mounted on one of the transformer fixing bolts.
DO BE CAREFUL IF YOU ARE MOVING THE CHASSIS WHILE PLUGGED IN. The mains cable terminates at bare terminals on the voltage selector at the back of the chassis. These terminals are easily touched if your fingers are curled under the chassis to get a good grip for removal. UNPLUG FIRST! †
It just goes to show you never can tell. What I thought was going to be a nice little job turned into something more. Still, that's half the fun of our hobby.†
I have come across one or two KB sets before and they seem to be of a good build quality. All in all a nice looking and not bad sounding set now it's finished. I might just keep it......
Text and Photos Copyright © 2003 Tim Pullin.