HMV Barn Project

[Nowhere-Man]

I'm working on a radio I picked up last year from a barn in the Moorlands. It's an HMV. It probably had quite a few Winters in zero temps and came to me as a sorry sight. It's been stored in my boat for a time but I decided now is a good time to have a go at it. I don't feel any pressure from this radio as it's quite hard as a project so I figured I'll just give it a shot and do my best.

First thing I find is the volume pot is kaput. Resistance was infinity between live and neutral so I feared the transformer primary was shot. It soon turned out there was actually no feed via the switch to the transformer and when I did locate the actual transformer primary leads, Ohms was about 40 R.

I won't go into details about how hard it was to get all the knobs off for chassis removal. Just imagine 15 odd years in a barn and no slot left in the grub screws (as well as hollowed shafts). A lot of profanities and curses resulted and a few aches and pains the day after.

The external speaker plug in socket may also be kaput as its resistor measured 0 ohms across the OT secondary but as normal when removed. I'll look into that later. Not sure as of yet.

Soldering seemed formidable. It was a large busbar that's trapped into place with capacitors and resistors going across. I decided in the end not to remove the busbar but to try and solder across it. Otherwise there'd be a risk of clipping out so many wires, error could take place and the circuit is very jumbled to trace as easily as normal.

[Nowhere-Man]

I tried to add a pic of the chassis but it won't let me. I think pixel size adds up somehow so I used it all up on the first pic. Anyway I cleaned up the chassis and even the valve cans. These cans were easy to remove so I took them off.

[Nowhere-Man]

I've been having problems today with soldering on this set. The problem lies in the fact the designers chose to mount the main capacitors and resistors on a small ladder. Each side of the ladder has small holes and then rungs. The idea is to actually thread in leads via the holes, twist and then solder so all the connections are ultra safe. That's all well and good at the time of manufacture but - 60 years later on it's a big problem.

There are two ways to approach the ladder layout. One option is to take the ladder off so you have full access. The snag with that is it will involve snipping at least 10 tangled and barely visible leads. Some of these go to valve pins or chassis lugs. Should you in any way get mixed up as to what goes where, tracing the circuit will be a nightmare. Lots of wires have been crossed tied together and some may disappear into holes and emerge with others, knotted up. Plus the original caps are so dirty, you can only read the ratings after cleaning with chemicals.

Second option is to leave the ladder where it is and attempt to solder regardless. That is the way I've tried to do it. I had no success at all with my usual soldering method as the further end of the ladder has a small surface area and is designed to solder and thread leads through the small perforations drilled out. I finally have found a system that offers some hope. I got some stiff, insulated wire and used lots of solder to fix one end of it to the furthermost end of the ladder where the original capacitor was. Then draw the wire forwards and loop a couple of turns around the near end of the ladder to take up any stress in the joints and provide a firm base. Next, solder the capacitor one end to the knotted wire and the other end direct to it's lug. It's not as good as the original layout but I found after a few attempts I ended up with a pretty strong joint.

Normally you'd just solder one lead to one valve pin and another lead to wherever it may go and you can see it all in context with the circuit. In this case, it's much more awkward.

I'll need a 5000 K volume pot a bit later on.

Definitely much harder than the GEC I was working on. Hopefully now I worked out a viable approach it may be easier tomorrow.

[ppppenguin]

I was struggling a bit with terminology in this thread. I think what you are calling a "ladder" is tag strip. There are various types but here's a picture of one type: http://creativity103.com/collections/Tec...al2620.jpg There is also tag board such as: http://www.brimal.co.uk/components/com_v...988106.jpg

Both tagstrip and tagboard are commonplace in sets that pre-date printed circuit boards. It was also common proactice to make a solid mechanical joint before soldering. This makes it hard to remove a component. Many of us will simply cut the lead of the old component and solder on the new one. We can debate the merits of making a mechancial joint - I usually hook the component lead into the tag but don't wind it round.

First thing I find is the volume pot is kaput. Resistance was infinity between live and neutral so I feared the transformer primary was shot. It soon turned out there was actually no feed via the switch to the transformer and when I did locate the actual transformer primary leads, Ohms was about 40 R.

This reads strangely. When you say "volume pot" I think you mean the mains switch which is commonly combined with the pot. It's quite common to get an infinity reading even when the switch is closed. This is usually due to oxidation of the contacts. The low voltage of the meter won't punch through the oxidation. When you apply mains, even with a lamp limiter it usually works just fine. Though I've see some that need some Servisol and repeated switching on/off to get them to work.

Of course it's possible for the switch to fail completely. Sometimes it's possible to get a repalcement pot with switch, other times you may have to do transplant surgery and graft on a good switch from another pot. The last resort is to bypass the switch and rely on the mains switch on the wall. Not ideal but at least the set can work. It's a good idea to switch off at the wall anyway.

[Murphyv310]

In addition to Jeffreys post I'd say 90% of pre PCB radio and TV's used tag strips in construction. I always snip out the offending components first then either use a solder sucker or desoldering braid to remove excess solder and then unravel the residual wire.
This can even be done with care on B9A or B7G valve sockets. It's always much better to feed the new component into the tags and do the mechanical joint rather than just relying on solder or splicing into existing components leads, that is untidy and leads to dry joints in my view.

[ppppenguin]

In addition to Jeffreys post I'd say 90% of pre PCB radio and TV's used tag strips in construction. I always snip out the offending components first then either use a solder sucker or desoldering braid to remove excess solder and then unravel the residual wire.
This can even be done with care on B9A or B7G valve sockets. It's always much better to feed the new component into the tags and do the mechanical joint rather than just relying on solder or splicing into existing components leads, that is untidy and leads to dry joints in my view.

Trevor has much more repair experience than me and I fully respect this. (I design electronic stuff for a living, I'm OK at repairs but not a pro) Having said all this I've found out the hard way that B9A etc valveholder contacts can be quite fragile and sometimes prefer not to stress them by unwinding a component lead. A splice to an existing component lead may not look so nice but it's possible to make a decent mechanical joint before soldering. Another alternative is to cut the old component lead close to the tag and wrap the new lead around the tag once. Again not so elegant as Trevor's best practice but normally perfectly OK.

If you're not experienced in dealing with replacing components a bit of practice on a scrap set will pay dividends. You will learn when things will break, how to get decent soldered joints etc.

[Murphyv310]

Yes indeed, actually dismantling a scrapper and rebuilding it or even using the chassis for another project is not only great for improving soldering skills but a great learning curve.

[pwdrive]

Trevor's covered it really regarding removing old components that have been wrapped around tags etc, a fine pair of decent quality side cutters also makes life a lot easier, I've always removed the bulk with a solder sucker then finished of with some solder wick then unravel/cut using a sharp pointed tool and fine side cutters.

Lawrence.

[ppppenguin]

This tool is very useful for generally twisting, prodding, poking, unwinding when soldering. The metal is an alloy that doesn't take solder. Don't know what it's called or where I got it.

[Nowhere-Man]

I was struggling a bit with terminology in this thread. I think what you are calling a "ladder" is tag strip. There are various types but here's a picture of one type: http://creativity103.com/collections/Tec...al2620.jpg There is also tag board such as: http://www.brimal.co.uk/components/com_v...988106.jpg

Both tagstrip and tagboard are commonplace in sets that pre-date printed circuit boards. It was also common proactice to make a solid mechanical joint before soldering. This makes it hard to remove a component. Many of us will simply cut the lead of the old component and solder on the new one. We can debate the merits of making a mechancial joint - I usually hook the component lead into the tag but don't wind it round.

First thing I find is the volume pot is kaput. Resistance was infinity between live and neutral so I feared the transformer primary was shot. It soon turned out there was actually no feed via the switch to the transformer and when I did locate the actual transformer primary leads, Ohms was about 40 R.

This reads strangely. When you say "volume pot" I think you mean the mains switch which is commonly combined with the pot. It's quite common to get an infinity reading even when the switch is closed. This is usually due to oxidation of the contacts. The low voltage of the meter won't punch through the oxidation. When you apply mains, even with a lamp limiter it usually works just fine. Though I've see some that need some Servisol and repeated switching on/off to get them to work.

Of course it's possible for the switch to fail completely. Sometimes it's possible to get a repalcement pot with switch, other times you may have to do transplant surgery and graft on a good switch from another pot. The last resort is to bypass the switch and rely on the mains switch on the wall. Not ideal but at least the set can work. It's a good idea to switch off at the wall anyway.

Actually there was no electrical contact through the two fuses due to the grime and dirt you referred to. On this set they fused the incoming mains before rectification but the metal was so aged, no ohms reading showed. I had to start by scraping the fuse-holders.

As to the switch, basically the on and off function was clapped from the start. No click, I'm afraid. There was also no feed through  the switch to the transformer primary. Given the wires were so tangled, I ended up scraping a tiny bit of insulation off what I believed to be the transformer wires and then finally I got a 40 Ohm or so reading. So, the primary may be O.K.

It would be nice to get a pot and switch as a unit and just rewire it as it should be. Then I should get 40 Ohms reading between live and neutral with the switch on.

The "ladder" as I call it had me thinking very carefully about how to approach the soldering. I gave it lots of thought and tried to weigh up the chances of snipping the ladder out so I could thread in the new capacitors. I did think of dabbing different colour paint on all severed leads. I suppose it's possible but you have to ask whether there might be an easier way around. In the end, I decided to just have a got at soldering across the ladder. That means, if the soldering joint isn't good enough, you'll have more stress later on. Anyway, here is how I did it:

I have some thick cable that's insulated and very rigid. If you start by tinning the end of the wire and tinning the lug on the furthermost end of the ladder, you can get lots of heat and solder and virtually weld it on. You can then tug the whole wire forwards, loop under the opposite rung a couple of turns (needs a bit of force) and you end up with an overhang. Snip off a bit of insulation and you then have a large soldering base. It's taken the rear to the front. You can then just solder the capacitor at the front of the ladder rather than across. Again, lots of solder and heat helps. You do have an inconvenient possibility of a future short this way but I'll insulate these joints later on and take safety measures. I'm afraid there is always a chance of a joint coming off because you lack the secure mechanical anchor of the original method, but it's the best I can do at this point. I've tugged on my joints and they seem O.K. so I'm off to Maplins tomorrow to get some 0.047's.

So, the chassis has been hugely cleaned up and looks a lot better. The transformer and valves might be O.K., we'll see. I could do with a 500 K switch/pot. There is surface rust on the waveband switch.