An Intimidating Experience

[Nowhere-Man]

A most unusual and unexpected experience took place yesterday. You see I have this ACDC Ultra Bakelite radio that does work but very poorly. It belonged to a woman who used to use it regularly but later found it didn't work so well as time passed by. I took it, connected up a big home-made aerial and managed to get one station. I put it aside for a rainy day.

Well, yesterday wasn't a rainy day and it was quite sunny. I thought while I was working on my other projects, I might as well just have a look and see what the issue was with the Ultra.

Having taken the back off, it soon became obvious the dial string was knackered. No drive of the tuning vanes. I also noticed this was a far older set than I had thought - about 1947 and there was even a spider living quite cosy inside.

There were two wires that were downright dangerous. Bare wire stretched through a hole in the chassis. So, the plan was to quickly renew the wire and get ready for a major clean-up and change the capacitors.

Anyway, changing those two wires was much harder than I assumed. The soldering was really going tough. I was using strong
multi-strand cable (a touch thicker) but tinning it was problematic. Also, a bit hard for me to see the lug on the tuning cap where I was placing the iron. It took a few attempts to get a proper joint. I tend to leave the old wire on till last so I can test continuity between both wires and then I snip off the old one.

Both of these wires led to (1) a large lug on the variable tuning capacitor and (2) a lug on the volume pot. From those points the 2 wires led through a hole in the chassis to the scale-light bulb wedged in a bracket with the wires soldered above and below.

Here is the part where you will maybe gasp. My plan was to do a bit more rewiring and gradually change the brown caps and I decided to just test all was well. The radio chassis was propped up and I plugged in the radio but was very careful to be ready to shut down if necessary.

All was quiet for a few seconds and then I immediately noticed faint traces of smoke from  a brown wax cap. Something didn't see right so I shut down immediately. As I did so the cap started to get blacker and it was oozing traces of wax and then a smell of burning and a bit of smoke. Then it calmed as the power was off. Damage was confined to about 3 wax caps, one particularly must have been on the verge of spitting wax. I dread to think what could have happened if I'd not been really quick killing the supply. No sign of burned wires or actual damage elsewhere so can only hope the set is still O.K.

I tested the main reservoir cap for voltage and then had a careful look around the chassis. My first thought was I had done something very silly with the wiring but, in time, it seemed all I'd really done was connect 2 new wires back to the lamp fuse.

Later I decided to test resistance between neutral and live and the reading seemed to be fine and not indicating a short circuit.

I have just printed off the service manual for the Ultra which should help me as I'm going to have to go through this whole circuit and test. Also I need to replace all the wax caps (many O.O1 Mf) and, yes, a Variac is going to be essential.

Anyone have an idea why this happened? Maybe something was disturbed on removal of chassis but again the ohms reading between neutral and live seems fine and the valves were lit.

[Radio Fixer]

You don't do it like me. I replace (actually re-stuff) all the wax caps and stuff the electros with new (forget all the reforming stuff just more work later) and change any across or down mains caps for X or Y. Then test all the valves and go to stores as needed. Get rid of all old rubber covered cable, and line cable, and look for dodgy (over heated resistors) and measure and change as needed. If its a set I'm going to keep I actually measure everyone even if it means disconnecting (most don't).

Check the mains on off switch for low Ohms and insulation. Check all pots with an audio genie and a pair of headphones.

Then power it up, don't need the Variac.

Each to there own way of doing it. First thing when powered up is to measure HT (have a meter hanging on there from the moment I go "click") is to measure cathode volts, O/P valve is vital. And I have a plug in cheapo mains meter measuring current and watts, one eye on that. They may be cheapo but actually have DSP and measure RMS.

Gotta go: dinner bell Gary

[Radio Fixer]

Well back from dinner and Edit has timed out: why is this?

But I should have said I always measure the HT line with an Ohm meter before powering up.

As to pots you can usually tell if they are likely to be any good: if they looked old, tired or have been abused just change them. Blore Edwards do great metal backed pots in any value with old 1/4" shafts if you want them or 6mm if more modern. They are currently making some for me with 4mm diam shafts.

Have fun Gary

[Mark Hennessy]

Well back from dinner and Edit has timed out: why is this?

There is a 20 minute window where you can edit posts - after that you can't, but a friendly staff member can help.

It's been that way for as long as I can remember. I believe that initially (back in 2011), there was no time-out, but pretty quickly it was found that occasionally, confusion arose in threads by folk editing earlier posts. The 20 minute window was added by the staff at the time (I was a regular member then), but on balance, I think it was the right decision, and it brings us into line with most other forums.

Aside from that, excellent advice in your replies . I can think of one thing to add - the classic lamp limiter: http://www.vintage-radio.com/projects/lamp-limiter.html

All the best,

Mark

[Refugee]

The oozing wax capacitor was most likely the mains filter.
Back in the 1980s a work mate had a TV in as a private job. The fault was that it had gone bang and some smoke had come out and the owner was worried about it.
The mains filter capacitor was split clean in half.
It is not unknown for them to blow up and smash a valve or even a tuning dial.

[Nowhere-Man]

The oozing wax capacitor was most likely the mains filter.
Back in the 1980s a work mate had a TV in as a private job. The fault was that it had gone bang and some smoke had come out and the owner was worried about it.
The mains filter capacitor was split clean in half.
It is not unknown for them to blow up and smash a valve or even a tuning dial.

I had a look around the big reservoir cap and took you all a pic of the infamous "wax oozing cap". Still mystified as to how the onset of smoking and melting came about as the bulb socket I wired to is only 3.5 volts. That was the only change I made, given the former wires had no insulation in parts. Maybe it was just disturbing the chassis sort of pushed the ancient mains supply over the edge and it was due to blow at any time soon.

Here is the cap.

[Nowhere-Man]

You don't do it like me.  I replace (actually re-stuff) all the wax caps and stuff the electros with new (forget all the reforming stuff just more work later) and change any across or down mains caps for X or Y.  Then test all the valves and go to stores as needed.  Get rid of all old rubber covered cable, and line cable, and look for dodgy (over heated resistors) and measure and change as needed.  If its a set I'm going to keep I actually measure everyone even if it means disconnecting (most don't).

Check the mains on off switch for low Ohms and insulation.  Check all pots with an audio genie and a pair of headphones.

Then power it up, don't need the Variac.

Each to there own way of doing it.  First thing when powered up is to measure HT (have a meter hanging on there from the moment I go "click") is to measure cathode volts, O/P valve is vital.  And I have a plug in cheapo mains meter measuring current and watts, one eye on that.  They may be cheapo but actually have DSP and measure RMS.

Gotta go: dinner bell   Gary

I find the service manual help me a lot. I don't have the exact manual as it's not listed but I do have a very closely related model by Ultra of the same era and appearance. And I'm familiar with the All American Fives. I think I may start my way from the power supply and outwards as I try and fix up dodgy wires and semi melted wax caps.

These were pretty clever designs of the time. Live mains supply directed via rectifier valve to supply H.T. plus DC and then the non rectified live branching off to heat the heater valves filaments in series. Eliminates any need for a transformer,

The scale lamp layout always had me a bit confused as I think the design varied a bit. On my Ultra the scale lamp runs through neutral I think and many sets used a shunt resistor.

In the following pic you can see above a sooty brown cap that's even worse. I will definitely not be powering this up again till it's vastly safer.

[Nowhere-Man]

Well back from dinner and Edit has timed out: why is this?

There is a 20 minute window where you can edit posts - after that you can't, but a friendly staff member can help.

It's been that way for as long as I can remember. I believe that initially (back in 2011), there was no time-out, but pretty quickly it was found that occasionally, confusion arose in threads by folk editing earlier posts. The 20 minute window was added by the staff at the time (I was a regular member then), but on balance, I think it was the right decision, and it brings us into line with most other forums.

Aside from that, excellent advice in your replies . I can think of one thing to add - the classic lamp limiter: http://www.vintage-radio.com/projects/lamp-limiter.html

All the best,

Mark

Here you can see where I made the brief start. The brown and blue wires are new but I had some difficulty for a while getting a decent solder joint. It was a touch thicker wire than the original. I had removed part of a Mica cap to get access to the lug and then put it back.

The plan now is not to do anything silly again and just make good use of the workshop manual. I'll be taking my time and will start off fixing up wiring and removing caps. I'll need some very high voltage caps, the resevoir cap is rated at 1000 volts.

[Radio Fixer]

Cant see why that is rated at 1000V, unusual for an electrolytic. Sure it is the reservoir? If so the 500V ones sold by the BVWS would do you just fine.

Gary

[ppppenguin]

AFAIK, no electrolytics are rated at 1000V. Are you sure you're looking at the reservoir cap? In an AC/DC set the reservoir cap worst case will have no more than 360V across it. The only caps likely to be rated at 1000V in an ordinary domestic radio are:

Mains suppressor (across mains) or snubber (across switch). Replace with a Class X or Class X2 part. These can often be salvaged from scrap computer PSUs. You can safely run without this fitted at all.

Tone correction cap, if fitted. Usually from the output valve anode to chassis. A modern 630V part is normally adequate.