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Restoring Amplifier In HMV Radiogram & Futile Attempt To Restore Its Radio
This HMV Radiogram model # HMV5231CA belongs to my school classmate, and was lying dead for quite some time. When I chanced to meet him a few months back, after a gap of many years, and could spend a few minutes chatting, I mentioned about my electronics hobby as a part of my updating news and ‘self-marketing’ statements, and he just casually enquired whether I could look into this set, which was lying in Mumbai. I said I would give it a try and gave him hopeful assurances, siting some of my ‘great’ achievements in this field. These are occasions when some of us use, to do self-pat, boost oneself to the sky and bulge, and I belong to this ‘some’!
He brought only the chassis, as he had removed it himself from the Radiogram for troubleshooting purposes, having given it elsewhere. Here you can have a look of it:
The valve numbers used were totally different except the ECC81 in this set and that baffled me in the starting itself. It used two N78 in the output, ECC81 for pre-amp, 6AT6 for volume control, ZD152 for inter-IF-Amp, X79 for RF mixer & Oscillation and X79 for RF Boosting. EM34 was in use for Tuning Indicator (A lovely one as you can see later in the article). You can see how complex the wiring is from the second picture above and without a schematic, it was almost impossible to track the circuit. My repeated attempts to fetch a schematic from the web and my contacts of antique radio hobbyists all over the world, turned negative. My own attempts to track and draw a circuit ended me up in such a confusion and madness that there were moments that my wife would call an ambulance from the mental asylum! Well, that’s not an exaggeration! It’s mainly because of the complex band switch. This switch was fit in such a way, that only a portion could be seen from the top, but no accessibility. The switch had poles and ways on both sides of each plate. In addition, there was a pull/push switch on the Tuning Knob, which changes the frequencies, perhaps for a different range. The RF section was also found tampered and a few wires cut off and left open.
Anyhow, my preliminary duty was to make the set work in audio section. When I gave power to the set, the dial lamps lit, and there was voltage present in the B+. Incidentally, one of the technicians who had done the service earlier had used two rectifier diodes in place of the Rectifier Valve. From its base left as it was, fit on top of the Mains Transformer, I could make out that it was an Octave Large size valve, which was in common use many years back. I downloaded the pin details of all the valves, many from Radio Museum. When I checked up the voltages at the pins of N78, these were ok though not good enough, but no output was coming, which made me turn my attention to the output transformer. When I touched, I almost burnt my fingers as it was very hot, indicating partial shorting. I could hear some spark sound too coming, which I first presumed could be from it. Here are some pictures to give a pause to the lectures:
You will notice a burnt mark on the second picture, which needs an interesting explanation. The top plate which gave the details of the transformer was made of such an inflammable material that it caught fire when I was desoldering the pins! It was only prudence and timely use of a thick sheet that I grabbed quickly to cover it, put the fire off. Though it took be totally unawares, my quick reflex action saved myself from burns and/or other damages! I still have no idea as to what sort of a dangerous material that was! The transformer was given out for re-winding, and my friend shared the winding details with me, as given below:
I reconnected the transformer and tried. The sound was coming, but there were frequent crackling noises and burning smell coming out from the output area. Then I traced where the output wires were going and found another small transformer on the side of the chassis, which had a capacitor. I thought the cap might be arcing inside and replaced it. But even then the crackling noise continued. I learnt on contacting my friend that there were two speakers connected to that section, which were in series. Just for the sake of checking, I removed the plate from the chassis. Just see what I found there in the second picture below:
The point where B+ was coming from a tapping primary winding of the main output TX (for a second output through this transformer) was arcing to the chassis through the punctured cardboard beneath! As the PCB itself was in bad shape, I replaced it with a cut piece of common PCB that I had. Measured and marked the windings of the small TX on the top for future guidance to any technician.
With these works, and after replacement of a few resistors and capacitors in the output section, the audio was restored very well and it was giving a good result when I played a song from my USB player. While replacing resistors and capacitors in Valve sets, which are wired externally, I do not cut the leads. I bend them to the required size and use a sleeve so that the bent ends will have more surface area for better bonding. Just see the following two pictures:
Then I turned my attention to the bass and treble section, where I saw a few capacitors and resistors connected were of vax coated type and as a routine in restoring old Valve sets, we need to replace these. Removed these two units carefully from its mounts, (but it cracked like a fried papad), scraped the wax gently to see the value hidden by the dirt collected on the surface and replaced each one of them. When measured, the values were way out, as expected.
Provided extra filter capacitors of 100n/1000V across the mains input and across the diodes. Rewired the input section by new shielded wires, patched-up all dry solders in that section. Here are a few more pictures, in the last one of which you will see how the look of the audio section changed beneath.
Enough of these pictures and let’s continue with lectures! I made a crude circuit diagram of the audio section, which I myself cannot follow afterwards, though it was useful for my rewiring at that time. I do not wish to reproduce it here, for fear that whatever little regard you all have for me, should vanish on sight of it!
Having done what looked best at that time in the audio section, turned my attention to the RF section. The voltages at the pins could not be counter-verified in the absence of a circuit diagram or a similar circuit. But the variation in grid to cathode to anode seemed to be ok, except under ZD152 Valve. The biasing resistors and filter capacitors were replaced to obviate the chances of any defect due to these. There indeed were differences in values from the markings. After checking configuration of ZD152, I substituted it with EBF89. Then injected a signal at each input of the section. IF inter-stage was not working. Checked and found that the winding was not ok. So removed it and took out the IF transformer and rewound the coils, after measuring the gauge and matching it with a coil that I had. The winding machine you find is a temporary fixture made by me using a stepper motor of 120RPM/12V DC.
Last picture you see above is the complex wiring of RF section and the most complicated double sided band switch.
After putting back the IF, injected the signal and adjusted it to 455Khz. In the absence of its original service manual, that’s what I could do; stick to a known standard.
Then injected the signal to the Oscillator-cum-mixer IFT and tried to adjust it too. But the output from it was very poor. So, removed that IFT, opened it and noticed that it’s not the usual one. There were extra tappings which were connected to the pull/push switch to vary the IF frequency to match the band! Well, let me tell you this was something new to me and I was at my wit’s end as far as this Radio was concerned. All that ‘bulging’ statements that
I had made to my friend at the time of my re-introduction got deflated instantly and I was like an innocent electronic child, flattened to the core!
I forgot to mention that in between, after I repaired the second IFT, I tried tuning in a station and when I gave the aerial to the input of this IFT, I got the local station. But that was just a freak accident and though I tried it again several times, much later, it did not work. Now the only option in front of me was to modify that section like a regular Radio for 455Khz signal, which I did. Rewired the IF coils to suit that and tried feeding signal and it did work fairly well, though it was not to the required peak level. But the RF signal from the aerial output was not reaching the input of the RF boosting Valve, X79. In spite of trying all my very best to trace the reason for it, I could not solve it! So, all the residue of my ‘populated’ ego died and here I was at a dead end! But let me mention one thing here that my friend was very patient and tolerant and kept me encouraging and pushing, whenever I turned out to him helplessly (through whatsup messages only). I made several futile attempts to revive that section, by giving gap of several days to weeks to months, each time of which I attacked the problem with fresh minds! My conclusion was that either RF coils were all very bad, or the band switch is the culprit, or some other unknown causes still remained there. I even tried by-passing the RF booster valve. The Signal was never reaching the input from any of the RF coils of different bands! I even varied the modulated input frequency. Because of the difficulty in removing the band switch, due to its short and tight wiring, some of which were single strand wires and inaccessibility for doing any measurements or studying the tracks, I was at a total loss as to further course of action. Then I tried feeding an RF signal locally to the input of the mixer valve, but the IFT there did not support and no stations could be tuned. I tried even with a different IFT. So, it looked as though the whole RF section was so peculiar and unique for this Radio, any slight deviation would not work. After updating my friend about my helplessness to do anything further on this, I sought his advice. He not only pacified me, but also suggested to provide an external USB player so that he could use it as an antique amplifier. So I made one USB player for him. Removed the costly replaced components from the RF section and shut that section off, by cutting off all supplies except the filament of the valves. Put back the old IFT as it was before. While checking the output of the external speakers, I noticed that the output was not good and the TX was not able to deliver the required output. So, after consulting my friend, removed it and replaced it with a two-way-network, for which purpose, I had to drill special holes. I kept a magnet underneath to capture all the iron dust, so that it does not get into valve bases. I provided sleeves for the fixing screws so that it does not come close to the chassis and once again create shorting problems like the previous connection did. Powered the Amp and checked its working, which was found to be excellent. Now, time for a short break through pictures:
My attention finally (a big relief, eh?) was on the magic eye Valve, EM34. As the Radio section was shut off, the tuning indicator would not work. So, I decided to convert it as Audio Signal Level indicator. I followed the instructions and diagram of converting EM84 Valve as a VU Meter.
Just see how lovely the indicator is! The green portion shrinks and expands at both sides responding to the output, which was connected to the speaker output.
Watched the VU Meter playing around with amusement like a child (Remember I had already become one, as mentioned earlier!)
Here are the defective components that I replaced in this Radio (some of which were though removed later!)
After testing the Amp for a few days, it was handed over to my friend. The story, however, do not end here, as it came back within a couple of days, because its power transformer failed! That was another jolt in the jaw for me! But my friend was cool, and gave me a chance to rectify it too! I got it rewound, then provided EZ80 Valve after changing the base and did a few modifications in the Power Section to suit that and the set started working once
again very well. I also hunted the market and fetched a couple of bulbs for the dial lamps, which were not lighting up. However, I had to put in a lot more efforts in adjusting the filament voltage, as it was slightly higher, by providing a suitable current dropping resistor. You can see that in the picture below, near the 2-way network.
The set was taken back by my friend and this time he ventured to fit it back in its case. He did it so quickly, that drew a lot of my admiration and once again I regretted to have boasted myself in front of this great man, who was never demonstrative of his skills, knowledge, experience and expertise! The old adage goes that empty vessels make noise, and therefore, the full one ………………. I expressed hats off to him, who not only tolerated all my ‘misdoings’ on his Radio, but came out to appreciate the result, in spite of the fact that the Radio was a memento of his loving father.
So, the long story comes to an end. I have cut short many steps that I had taken along with the connected pictures, in order not to make this article look like a ‘never ending’ TV Serial!
Could I add this to my satisfaction list, I am not sure! So, you all can conclude!
This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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