Last Summer a novice newcomer to another forum sought advice about restoring a Stella ST105U AC/DC set, similar in circuit and valve line-up to the Bush DAC90A. As with the DAC90A, being live chassis, not the best set on which to cut your teeth, but the number of successful restorations of DAC90As is ample evidence that given sensible advice and precautions, such restorations are within the scope of novices. He got the usual dispiriting remarks from certain quarters along the lines of 'if you don't know what you're doing, leave it to the experts'. As soon as I see that, my eyes glaze over and I just resort to sending advice by PM - not within the spirit of open forums, true, but I simply refuse to be drawn into threads in which self-styled 'know it alls' seem to want to use the forum not to give sound advice and encouragement, but to bounce their ego off the walls.
I had the makers' service sheet for that model, which is excellent and runs to ten pages, with clear diagrams of component location. It was evident that there were nine paper caps - not 'waxies' but pitch encased - which would need changing. I advised him not to plug in the set and turn it one until the caps have been replaced, and talked him through how to go about restoring it, and what precautions to be taken, offering to help if he got stuck.
I didn't hear anything for a while, then got a PM thanking me for the advice, saying he'd decided that it was too much of a challenge, and that he'd like it to go to a good home FOC, offering me first refusal. I'd have been happy to have bought the set, but he wouldn't hear of it. As he lived in Warrington, I picked it up on the way to the Golborne Swapmet last November, and on the basis that fair exchange is no robbery, turned him a little bowl in sycamore, which was well received.
I've just got round to restoring the set and am waiting for a couple of caps to finish it off. As I've got a working DAC90A, I can test the valves in that. Meanwhile, some comments on progress to date might be of interest. Firstly, though few restorers can resist plugging in and seeing if a new acquisition works - if only through a lamp limiter - the advice I gave to not turn on the set until examined turned out to be wise advice. For some reason, the mains lead had been cut inside the set, yet the same mains lead had the wires stripped and twisted together, such that they could touch.
I always start by making a cradle from scrap wood, in which to stand any chassis on which I'm working - it only takes a fw minutes and makes life much easier - far better than trying to balace the chassis upside down propped on whatever is to hand. Then I make a few preliminary continuity checks of such things as the heater chain, transformer windings, 'switchery' and the speaker and dial lights. In this instance, it immediately came to light why the set had been dead and consigned to a loft or garage to gather dust. The on-off switch ganged to the 0.5Meg volume control was open circuit. But this wasn't any old 0.5M volume control - it had an extra tapping at 50k as illustrated by the circuit. Forum member 'OldJohn' who is a chum who lived about 15 miles to the east of me had kindly given me a 105U 'scrapper' for spares. I checked the VC on that, and it too was open circuit, so it seems to have been a feature of that set. I removed the VC and carefully bent back the three tabs which hold the switch to the pot, with the switch in the 'off' position so that the internal contacts would be revealed. I twizzled a small piece of 'Duraglit' wadding into the end of a cocktail stick, and cleaned the contacts with it. I gave it a light spray with Servisol, and also the pot track and reassembled the switch. It now works perfectly. It might be worth bearing in mind if anyone else has a similar problem.
The cleaned up contacts can bee seen in the attached pic.
The pitched covered paper caps, one of which had blown its top, can be seen on the underside of the chassis, and one on the top. They're a monstrous size compared to current replacements. I've now replaced all but three, which I'm awaiting delivery of. The little 'Col wire winder' came in very handy and made the task of replacing the caps far easier. Some of the caps were in awkward places on valve-holder tags etc at which there were several other components that I didn't want to disturb by trying to unsolder and remove the cap leads. Hence, I cut the old caps off, leaving about 1/4" of wire still in place, coiled the ends of the new caps, slid them over the wire, making a perfect joint. Out of interest, I checked the values of the removed caps and they were all way over their original value, mostly by a factor of ten. I haven't bothered to check them for leakage on a neon at their working voltage, but no doubt it would glow brightly!
I always make a list of the resistor values, and check each in turn to see if it's within 20% - the usual tolerance when the set was made. Surprisingly, all were well below 10% and some were spot on, so that was good news.
Today, I'll reform the two 50uF smoothing caps on my reformer, and start on the cleaning up the cabinet, the front and back of which will need a re-spray. This set uses a 19V dial light bulb at 0.1A, which I assumed would be unobtainable, but no, here they are, large as life, at 75p each, the only snag being a minimum order value of £10, and a minimum quantity of six, so I ordered some other stuff as well to make up the order:
http://www.lampspecs.co.uk/Light-Bulbs-T...A-1-9W-E10
I've attached a few pics of the set and the unusual volume control with it's fourth tag, and where it featured on the circuit, for reasons that elude me!
The set came out in June 1953 (the year and month of the Queen's Coronation) at a price of £11.8s.0d, + 33% Purchase Tax - about £16.00, which equates to about £340 in today's terms. Thus, as with other sets of that era, few people would have bought it outright, but would have resorted to 'easy terms' - the polite term of the era for HP - the less polite term being the 'never never'!
Hope it's of interest.
David
I had the makers' service sheet for that model, which is excellent and runs to ten pages, with clear diagrams of component location. It was evident that there were nine paper caps - not 'waxies' but pitch encased - which would need changing. I advised him not to plug in the set and turn it one until the caps have been replaced, and talked him through how to go about restoring it, and what precautions to be taken, offering to help if he got stuck.
I didn't hear anything for a while, then got a PM thanking me for the advice, saying he'd decided that it was too much of a challenge, and that he'd like it to go to a good home FOC, offering me first refusal. I'd have been happy to have bought the set, but he wouldn't hear of it. As he lived in Warrington, I picked it up on the way to the Golborne Swapmet last November, and on the basis that fair exchange is no robbery, turned him a little bowl in sycamore, which was well received.
I've just got round to restoring the set and am waiting for a couple of caps to finish it off. As I've got a working DAC90A, I can test the valves in that. Meanwhile, some comments on progress to date might be of interest. Firstly, though few restorers can resist plugging in and seeing if a new acquisition works - if only through a lamp limiter - the advice I gave to not turn on the set until examined turned out to be wise advice. For some reason, the mains lead had been cut inside the set, yet the same mains lead had the wires stripped and twisted together, such that they could touch.
I always start by making a cradle from scrap wood, in which to stand any chassis on which I'm working - it only takes a fw minutes and makes life much easier - far better than trying to balace the chassis upside down propped on whatever is to hand. Then I make a few preliminary continuity checks of such things as the heater chain, transformer windings, 'switchery' and the speaker and dial lights. In this instance, it immediately came to light why the set had been dead and consigned to a loft or garage to gather dust. The on-off switch ganged to the 0.5Meg volume control was open circuit. But this wasn't any old 0.5M volume control - it had an extra tapping at 50k as illustrated by the circuit. Forum member 'OldJohn' who is a chum who lived about 15 miles to the east of me had kindly given me a 105U 'scrapper' for spares. I checked the VC on that, and it too was open circuit, so it seems to have been a feature of that set. I removed the VC and carefully bent back the three tabs which hold the switch to the pot, with the switch in the 'off' position so that the internal contacts would be revealed. I twizzled a small piece of 'Duraglit' wadding into the end of a cocktail stick, and cleaned the contacts with it. I gave it a light spray with Servisol, and also the pot track and reassembled the switch. It now works perfectly. It might be worth bearing in mind if anyone else has a similar problem.
The cleaned up contacts can bee seen in the attached pic.
The pitched covered paper caps, one of which had blown its top, can be seen on the underside of the chassis, and one on the top. They're a monstrous size compared to current replacements. I've now replaced all but three, which I'm awaiting delivery of. The little 'Col wire winder' came in very handy and made the task of replacing the caps far easier. Some of the caps were in awkward places on valve-holder tags etc at which there were several other components that I didn't want to disturb by trying to unsolder and remove the cap leads. Hence, I cut the old caps off, leaving about 1/4" of wire still in place, coiled the ends of the new caps, slid them over the wire, making a perfect joint. Out of interest, I checked the values of the removed caps and they were all way over their original value, mostly by a factor of ten. I haven't bothered to check them for leakage on a neon at their working voltage, but no doubt it would glow brightly!
I always make a list of the resistor values, and check each in turn to see if it's within 20% - the usual tolerance when the set was made. Surprisingly, all were well below 10% and some were spot on, so that was good news.
Today, I'll reform the two 50uF smoothing caps on my reformer, and start on the cleaning up the cabinet, the front and back of which will need a re-spray. This set uses a 19V dial light bulb at 0.1A, which I assumed would be unobtainable, but no, here they are, large as life, at 75p each, the only snag being a minimum order value of £10, and a minimum quantity of six, so I ordered some other stuff as well to make up the order:
http://www.lampspecs.co.uk/Light-Bulbs-T...A-1-9W-E10
I've attached a few pics of the set and the unusual volume control with it's fourth tag, and where it featured on the circuit, for reasons that elude me!
The set came out in June 1953 (the year and month of the Queen's Coronation) at a price of £11.8s.0d, + 33% Purchase Tax - about £16.00, which equates to about £340 in today's terms. Thus, as with other sets of that era, few people would have bought it outright, but would have resorted to 'easy terms' - the polite term of the era for HP - the less polite term being the 'never never'!
Hope it's of interest.
David
