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Why Aren't There More Aspies Into Electronics?

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Gone away

Long live valves! ... they still have quite a following ... there is a facebook page called 'vacuum tube amplifier'

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Dr-David-Banner
23 hours ago, chris@chris said:

Long live valves! ... they still have quite a following ... there is a facebook page called 'vacuum tube amplifier'

This is true. Unfortunately I discovered a lot of the websites devoted to tube engineering were disappointing. I now try and get hold of really good books and work by myself. One I have is from the late Thirties and I found it really helpful. It was interesting to see how the engineers back then found ways to do tests without digital meters. Anyway this is a particularly useful book as the set I'm working on now is a Thirties model with a (so far) untested Battery Eliminator.

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Nesf
On 2/2/2016 at 7:52 PM, chris@chris said:

Long live valves! ... they still have quite a following ... there is a facebook page called 'vacuum tube amplifier'

I have a vacuum tube amplifier :)

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Dr-David-Banner

Yesterday was a good day for soldering. The work went well. I'm taking out frayed 1930's wire and soldering in new, safer wire. I may upload a pic some time later. The tubes are really pretty big like pepper pots but they obviously got smaller in the Fifties.

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Dr-David-Banner

Here is the Thirties radio receiver I'm working on. The first attempt some months ago was pretty poor standard so I redid all my work from scratch. I was more confident to dismantle more of the set so I could really get in with the soldering and replace all the worn out wire. I was inserting some modern capacitors today and it can be time consuming to solder on old terminals.

Whether it will work remains to be seen. I don't yet know if the tubes still work after so many years and also you fear any possible error. I guess all the circuits will have to be checked repeatedly before I apply voltage. It needs 120 Volts D.C. for the circuit and 2 volts for the tube filaments. This set would have once announced the start of WW2.

DSCF0747[1].jpg

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Pinky and his brain

@Dr-David-Banner I don't know if you already do this, but if not, then using a little bit of liquid flux will help to solder on those old terminals. The flux will help remove oxidation and dirt, so that the tin will flow better and make a good contact.

The only problem with liquid flux, is that you need good ventilation, it smells really bad. And is not likely to be good for your health, so make sure to let the smoke/fumes out of the room.

 

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Dr-David-Banner
6 hours ago, Pinky and his brain said:

@Dr-David-Banner I don't know if you already do this, but if not, then using a little bit of liquid flux will help to solder on those old terminals. The flux will help remove oxidation and dirt, so that the tin will flow better and make a good contact.

The only problem with liquid flux, is that you need good ventilation, it smells really bad. And is not likely to be good for your health, so make sure to let the smoke/fumes out of the room.

 

One or two wires and capacitors were removed today and redone. A little of what you see is old soldering. The very small capacitor was a no go as it turned out to be only rated at 50 volts but, back then, I had to rely on the staff at the shop for purchase. I changed these for Wima capacitors. These were sleeved in insulation, tinned and soldered on. I tend to scrape the old lugs and clean after with a dab of surgical spirit. I have known a soldering joint to come off but not as much now as in the past. The trick is not to let the iron get too hot either as then you do have issues where solder won't take and, of course, a heat sink is always a good idea. Bad joints tend to move as you pull at the joint so I tend to pull it a bit and finally test for Ohms. It should be zero resistance.

Soldering takes a while to get good at and I experimented with various methods. I settled on the following for capacitors:

You take two rigid strands of wire and tin both the radial capacitor and the wire end. Best to get some heat on multi-strand wire so the solder covers it. Meld the wire to the component. Presto, you now have an axial instead of radial capacitor.

Thick wire is harder to solder than thin but more rigid. Multi-strand I find really a bit hard and have to take my time.  The perfect wire is medium gauge and I got some once from a fridge that solders great.

Below is a tricky job where I had to solder in around a kind of ladder - all the red capacitors are polypropylene.

DSCF0746[1].jpg

Edited by Dr-David-Banner

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Dr-David-Banner

I sort of have my doubts this 1939 receiver can be made to work again. It's been riddled with short circuits due to aged and split wires that run in all kinds of virtually unreachable nooks and corners. It can take ages to solder in a new wire if the joints are cramped in somewhere. I found one major but not obvious dead short in the cable that runs from the valve to B+ (there was a break in the wire shorted to the chassis).

All those coloured wires shown here are new. The red and yellow capacitors are also new. You can see one valve on the left hand side but I've no idea if they will still be functional. I do know the bases are a bit loose and will have to be fixed.

At the end everything will have to be tested with a meter to check the circuit is intact.

My battery eliminator now turns out to have a damaged transformer winding so probably I'll have to contrive one of my own. I guess an American 120 VAC transformer would get me on the way to 120 VDC.

I can only hope this may work at the end of it all. If it doesn't, it's been good practice along the way.

dt.jpg

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Pinky and his brain

Do you have the right test equipment ?

There are many ways an old circuit like that can fail. The old carbon composite resistors do have a tendency to increase in value over the years. So what may once have been a 100kohm resistor, can easily be 1 or 2 Mohm today. Another source of trouble could be conductive dust or carbon residue on the brown "main board". It could easily make a connection between some of the terminals. Maybe not a dead short, but still an unwanted leakage/injection of current.

From my point of view, you might want to start by creating a schematic, and then disassemble the whole unit and test every part outside the circuit. And then start to assemble everything in a clean and orderly way.

The old power tubes are most like dead, in the past people usually used their devices until they died, and then put them away somewhere. But signal tubes (preamp) usually have a much longer lifespan than power tubes, so they might still be good. So you need to find out what tubes are in there, and if you can source some replacements. Because if they are no longer available, then there's not much point in restoring the whole unit. Except for the practice of course.

You might also want to research how to test tubes. Because if you like working with old tube amps, then you will sooner or later be forced into testing tubes. Often you can find old tube testing equipment online, but the prices vary a lot, and some of them are not in good condition, so you have to be careful. But you will need one, if you want to work with tubes in a serious way. It's not easy to inspect a tube from visuals only. Only a broken vacuum is visible by the eye, it leaves a white/greyish residue inside the tube.

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Dr-David-Banner
22 hours ago, Pinky and his brain said:

Do you have the right test equipment ?

There are many ways an old circuit like that can fail. The old carbon composite resistors do have a tendency to increase in value over the years. So what may once have been a 100kohm resistor, can easily be 1 or 2 Mohm today. Another source of trouble could be conductive dust or carbon residue on the brown "main board". It could easily make a connection between some of the terminals. Maybe not a dead short, but still an unwanted leakage/injection of current.

From my point of view, you might want to start by creating a schematic, and then disassemble the whole unit and test every part outside the circuit. And then start to assemble everything in a clean and orderly way.

The old power tubes are most like dead, in the past people usually used their devices until they died, and then put them away somewhere. But signal tubes (preamp) usually have a much longer lifespan than power tubes, so they might still be good. So you need to find out what tubes are in there, and if you can source some replacements. Because if they are no longer available, then there's not much point in restoring the whole unit. Except for the practice of course.

You might also want to research how to test tubes. Because if you like working with old tube amps, then you will sooner or later be forced into testing tubes. Often you can find old tube testing equipment online, but the prices vary a lot, and some of them are not in good condition, so you have to be careful. But you will need one, if you want to work with tubes in a serious way. It's not easy to inspect a tube from visuals only. Only a broken vacuum is visible by the eye, it leaves a white/greyish residue inside the tube.

I already found the anode load resistors are a bit out. The max resistance for this set is 5 MegOhm although the data will have 5000,000. My anode resistors were around 6 and a bit MegOhm. At the time when I was taking part in electronics forums people told me it didn't matter if such a resistor was "out" by a Megohm or so. I'm not so sure about that and sought to change the resistances till I found they're not sold over the counter at the store I go. Sure, you can get around 1 K but the store did 1 Meg max in resistors. So, for now, I extended the resistor with longer leads and insulation for the time being.

I have had the schematic a while although part of it is no longer available so I don't have the current data. I do have the circuit details. It's a TRF with an SG RF amplifier, a pentode detector and then a triode and output Tetrode. It uses grid leak detection and regeneration.

I've managed by now to replace around 80 per cent of the wiring and have a touch more to go yet. It's doable with patience.

I was definitely puzzled over the L.T. circuit which also uses a fuse bulb in the negative line. The L.T. for this set it 2 volts. I spent quite a lot of time working through the pins so I could trace the bias and grid leak resistances. The idea, of course, was to locate the heater pins and check out the resistances.

There were many short circuits and the worst was an anode wire that had somehow broken at the insert of the chassis, despite the rubber grommet. This will have to be replaced. Of course, I do a resistance test between the H.T. lines an I also made a dim-bulb tester which is a great way to protect a circuit.

The tubes may or may not be functional.

It looks probable I'll have to make a new H.T. unit as the original one seems to have a dodgy transformer winding. I'm toying with the idea of using a capacitor mains dropper but time will tell.

I can't say for sure whether this will go again. So far, I've always had success but not quite with anything so battered by age as this set has been.



 

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