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VINTAGE RADIO By RODNEY CHAMPNESS, VK3UG The Healing 412E: an Australianmade PC-board valve radio Valved radio receivers on PC boards were not all that common in Australia, mainly because few manufacturers took the plunge and adopted this technique. Among those that did were Healing, Pye and Admiral. There’s no doubt that PC board production methods simplify production line manufacture. However, with valve radios, there was the problem as to how to mount the heavy power transformer. Mounting it on the PC board wasn’t really a proposition, since the board could easily be cracked if a minor mishap occurred in handling the receiver. For this reason, the power transformer was usually mounted either on a small metal sub-chassis (eg, as in the Healing 412E mantle receiver) or on a substantial chassis which also accommo­dated other heavy items (eg, as used by Admiral). In the latter case, the PC board was mounted in a cutout on the chassis. Healing 412E The Healing 412E is a small mantle AM receiver which used three valves: a 6BE6 converter, 6N8 IF and detector stage, and a 6BM8 for the audio output stages. The power supply consists of a transformer and a half-wave solid state rectifier – see Fig.1. The dial system on this set is rather crude, being a di­ rect-drive system with the tuning knob mounted on the end of the tuning gang spindle. I had expected that it would be touchy to tune but the knob is big enough to make tuning easy. The dial leaves a bit to be desired, however – it consists of nothing more than a piece of gluedon cardboard with station markings (some radios apparently have the dial calibrated in frequency only). My radio has no cover over the dial although some units were fitted with a clear plastic cover. In this set, the power transformer was mounted on a small sub-chassis The PC board, mains transformer and the transformer sub-chassis are all removed from the cabinet as one piece. Note the earthing wire from the 6BE6 shield to the top of the adjacent IF transformer can – a necessary modification to improve sensitivity. 84  Silicon Chip at the righthand end of the cabinet (as viewed from the front). A PC board running across the remainder of the cabi­net width took care of most of the circuitry, while a multiple-turn loop antenna was wound on the back wooden board cover of the receiver. Interestingly, the more fashionable loopstick antenna was not used in this set. Restoring the Healing 412E So how did I come by the set? Well, a friend was cleaning out his garage and wanted to get rid of it. When I saw it, I could understand why – it was the grubbiest little mantle set I had seen in years. Despite this, I happily accepted the receiver although I had no idea at the time what I would do with it. Eventually, however, I decided to restore the set and I began by dismantling it so that it could be cleaned up. It was covered in oily, greasy dust-impregnated muck and had also had water through it if the rust was any indication. Initially, I decided to clean the PC board and chassis metalwork using a brush soaked in household kerosene. This did a reasonable job but some spots were difficult to get at, so the cleaning was not to perfection. It was then that I made my second mistake (the first being accepting the set) – I tried using methylated spirits to give the chassis and PC board a final clean but found that this removed the screen-printed track pattern on the top of the board. This was something I hadn’t expected. The screen-printed track pattern is handy because it mirrors the copper track pattern on the underside of the board, which makes it easy to trace the circuit. I dried the metho off as best I could and was relieved to find that most of the gunk was removed but not much of the print. The cabinet was an even worse disaster. It was scrubbed in the laundry tub using water and detergent to get the gunk off. This was a slow process because I had to be careful to avoid splashing water onto the paper dial scale. The front decorative grille is a real challenge to clean. It consists of many 5mm square holes which are around 5mm deep. It was extremely difficult to clean the sticky gunk out of these recesses, so I tried using methylated This view shows the Healing 412E before cleaning and restoration. The cabinet was covered in an oily dust-impregnated muck and was scrubbed clean in a laundry tub using water and detergent. spirits to help loosen the gunk. Unfortunately, the grille started to dissolve – or perhaps it was some paint (I’m not sure) – so I promptly stopped doing this. The grille returned almost to normal once the methylated spirits had evaporated but it left a dirty white-looking finish where the metho had been. Because I had nothing to lose, I decided to spray-paint the grille using several coats of white enamel. I had to hand-paint some bits around the dial and the end result was less than per­ fect but it was a definite improvement on the original. The rest of the cabinet needed a good cut and polish. I started by using a fine grade of wet-and-dry paper to get rid of the deep scratches but some were just too deep and I had to be content with getting rid of most of them. I then polished the cabinet with automobile cut and polish and it now looks quite reasonable, although not up to my normal standards. Water damage The thin composite wood panel used for the back of the set was also a problem. It had buckled due to water damage at some stage, although this The PC board was also covered with dirt and grease but responded quite well to a cleanup using a kerosene-soaked brush. FEBRUARY 2001  85 The composite wood panel used for the back cover had suffered water damage and was restored by spraying it with matt black paint. This panel also supports the antenna coil, which is mounted on the inside. hadn’t adversely affected the antenna loop that was glued to the back. This panel was sprayed with matt black paint to get rid of the water stains and this really improved its appearance, so that it now looks acceptable. When looking at a receiver, restorers should always ask themselves, whether the set is worth restoring in terms of time, effort and money. In this case, having restored the cabinet and cleaned the chassis, I was beginning to question the wisdom of tackling this particular project. Overhauling the circuit The next job was to get the set to operate. First, I checked that the power transformer had no shorts from any winding to earth using a high voltage tester. All was well, so I then had a good look at the power cord. It was a 3-core lead that someone had fitted with a bayonet connector, so that it could be plugged into a light socket! Naturally, the person who did this had cut the earth lead off which isn’t exactly the smartest thing to do. The bayonet connector was quickly removed, a new 3-pin plug fitted to the cord and the earth reconnected. Next, I checked the paper capacitors and found them all to be too leaky to leave in the set. These were all replaced, along with C7, a .01µF 25V redcap ceramic capacitor, even though it showed no sign of leakage. Redcaps have had a poor reputation for reliability and I believed it was cheap insurance to replace it. R7, the plate resistor for the 6BM8 triode, was also replaced as its value had increased from 220kΩ to 320kΩ. At this stage, all appeared to be in order and so the set was connected to power and switched on. The high tension (HT) voltage was around 140V which was close to normal. After a short time, the receiver showed signs of life and I was able to tune in a couple of the stronger local stations but the set’s performance was really quite poor. It was time to go through the alignment procedure and see if this would improve matters. Alignment To start the alignment, the tuning gang was closed and a 455kHz signal Fig.1: the circuit uses three valves: a 6BE6 converter, a 6N8 IF and detector stage, and a 6BM8 for the audio output stages. The power supply consists of a transformer and a half-wave solid state rectifier. 86  Silicon Chip This is the view inside the set after the restoration had been completed. An alignment, some valve changes and a couple of modifications turned it into a reasonable performer but it’s not as good as the Kriesler 11-99 mantle radio (July 1998). from a signal generator was applied to the grid of the 6BE6. I then attached a digital multimeter to the AGC line and increased the signal level so that some AGC could be meas­ured. This done, I was able to peak the alignment of the four IF windings, which were only slightly out of adjustment. An inter­esting feature of the IF windings is that they are all adjusted from the top. The slugs are hollow so it is possible to push an alignment tool through the first slug and adjust the second slug – nifty. Aligning the antenna and oscillator circuits is also fairly straightforward in this receiver. First, I closed the gang and applied a high-level 530kHz signal from the signal generator to the antenna terminal. I then adjusted the oscillator coil slug until the signal was audible. Next, I opened the gang, tuned the generator to 1630kHz and adjusted the oscillator trimmer capacitor until the signal was heard once more. I then repeated these adjustments at both ends of the dial, until the set would tune from 530kHz to 1630kHz. This set has no inductance trimming adjustment for the loop anten- na, so it can only be peaked for best performance at the high-frequency end of the dial. To do this, a relatively weak signal was coupled into the loop antenna and the tuned circuit adjusted at around 1400kHz for best performance. This was done by ear but a digital multimeter could again be attached to the AGC line to accurately indicate maximum sensitivity. Because there is no inductance adjustment for the loop antenna coil, the set’s sensitivity with a signal generator attached to the antenna terminal and earth varies across the band. A noisy signal could be heard at 10µV on 530kHz but only 3µV was required on 1600kHz to achieve the same result. And these results were obtained only after the problem described below had been solved. Improving the performance After the alignment had been completed, the set was still noisy and stations were weak at the low frequency end of the dial. The receiver certainly was not performing as well as I would have expected. Initially, I suspected that the 6BE6 was noisy. I’ve never liked 6BE6 Silicon Chip Binders  Each binder holds up to 14 issues  Heavy board covers with 2-tone green vinyl covering  SILICON CHIP logo printed in gold-coloured lettering on spine & cover REAL VALUE AT $12.95 PLUS P & P Price: $A12.95 plus $A5.50 p&p each (Australia only; not available elsewhere). Buy five and get them postage free. Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. FEBRUARY 2001  87 stalled and the back bias in­creased to -7.6V, indicating that the original valve lacked performance. I decided to replace it even though it probably had a reasonable amount of life left in it. Using a multimeter to monitor the AGC line and the back bias resistor is a relatively simple method of testing valves in a receiver to assess their performance. It’s worth remembering if you don’t have a valve tester. The bottom line The restored receiver looks quite presentable although it doesn’t exactly take pride of place in my collection. The tuning knob is direct-coupled to the gang. valves as they tend to be noisy due to their design. I tried another valve but no improvement was observed. I then placed my fingers around the valve and I noted a decrease in noise. It seemed that there were some strange radia­tion effects occurring, so I made a metal shield out of tinplate from a discarded fruit tin and placed it over the 6BE6. The shield was attached temporarily via a short length of wire to the gang and to the nearest IF transformer (both are earthed). By experiment, I found that the earthing point for the shield was critical. I ended up with around 50mm of wire running from the top of the shield to an earth point I made in one corner of an IF transformer. This can be seen on the photograph of the board. The radio now works very well on the low-frequency sta­tions. Why was it necessary to do this and what caused it? I have found that some radios are conditionally stable and it is neces­sary to do some remedial work on them to achieve good perfor­mance. In this case, I believe that the problem is caused by inadequate shielding due to the use of the PC board with its long, thin earth tracks. This causes the IF signal to radiate around the set and into sections where it shouldn’t, such as the aerial circuit which is resonant just above the IF frequency. The 6BE6 has no integral shield to stop radiation from its plate, so it will radiate signals on 455kHz. It’s likely that the set was regener88  Silicon Chip ative and on the verge of oscillation prior to the fitting of the earthed shield. It certainly sounds much better with the shield in place. Bypass capacitor Another small modification that also helped the general sensitivity of the receiver was to fit a 68pF ceramic capacitor on the underside of the PC board between pins 2 and 3 of the 6BM8. This bypasses any 455kHz signal that remains after the IF signal filter capacitors (C6 and C8). If this isn’t done, radiation from the 6BM8’s plate and screens finds its way back into the front end and tends to desensitise the receiver. Simple valve testing Despite my work so far, the radio was still somewhat lack­ing in performance. It was time to become a “valve jockey”. I left the digital multimeter attached to the AGC line and tuned the set to a strong station. The voltage reading on the AGC line was about -1.5V. I then tried a replacement 6BE6 which made no difference but when I changed the 6N8, the AGC increased to -3V. Obviously, the original 6N8 was rather sick and so it was con­ signed to the bin. The performance of the 6BM8 was checked by measuring the back bias across R11. This was around -7V but is supposed to be -7.5V. I removed the 6BM8 and the voltage decreased to -3.5V. A fresh 6BM8 was then in- This receiver appears to be similar in concept to American sets of the same vintage. The PC board, like many of that era, has the components going every which way. By contrast, modern PC boards invariably have the parts neatly placed and much more thought goes into making sure they are not overcrowded in any area. Mounting the PC board horizontally also created problems in this type of set. These receivers were destined to operate in the kitchen. The air flow due to the heat of the valves draws air through the set and cooking grease and dust settle on the board. If the boards had been mounted vertically, there would be much less grime on them, making it easier to identify components and printed tracks. However, I am not aware of any manufacturers that mounted PC boards vertically in their valve radios. The Healing’s performance doesn’t rival the Kriesler 11-99 that I described in July 1998. However, with the small modifica­ tions mentioned earlier, it is now quite a good performer. It’s a shame that the performance of many receivers suffered because manufacturers cut corners. Even the most prestigious manufactur­ers goofed from time to time. In summary, the Healing 412E is an interesting little re­ceiver designed for the lower end of the market and it does quite a creditable job. Cost cutting seems to have been one of the design objectives but despite this, it did all that was required of a kitchen set for that era. Personally, I prefer the Kriesler, which is a superior radio designed for the same market segment but I’m still happy to have this little unit in my collection. However, because of the less than pristine cabinet restoration, it won’t be sitting on the front row SC with my favourites.