HMV 1124 - Has Been Tested And........

[Nowhere-Man]

O.K., I finally forced myself away from the boat work and have taken the bull by the horns as it were. A fair bit has taken place over the weekend.

First test was to see if the transformer worked O.K.. After some thought, I decided it might be wise to do this test without the valves so I removed the lot. You see, I've no idea how long this wireless has been out of service while stored in a farm garage. I used my lamp-limiter and plugged into mains. The transformer was kicking out about 300 VAC. I left it on for a bit to dust away the cobwebs.

Next time I decided to see if the scale lamps still worked. These are at the front of the case so I dragged that over to the chassis and connected it in circuit. Powered on. Zilch! No light at all. That meant taking the front cabinet apart to investigate further. I found 4 small bulbs (very silvery) but dead. Likewise the lamp holders were in poor shape. I removed some bulbs for direct testing and these were found to be stone dead. I'll need some new 6 volt bulbs later on and maybe use Coca Cola to clean up the lamp holders.
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I dropped a tuning slat also because some genius in the past didn't worry about making a fixing pin, just in case. This was plain glass and shattered on the floor. I was kind of upset. However, there wasn't much lettering on it and I was able to cut an identical slat with Perspex. This slotted back in and hopefully I'll be able to find some way to letter it as I saved the broken slat for reference.

Finally, today, the real test. This was a bit nerve-wracking. I double checked all my work, put all the valves back in, got the lamp limiter and powered up. Silence. Silence. Profound silence. Yet no big bang or sparks or indication of doom.

At first, I thought this was a major serious problem and was checking fuses and switches. After a few moments, I finally realised the radio was silent but definitely alive with juice. The valves were clearly glowing and I got a 400 VDC reading from the H.T. Smoothing Can. I also noticed on powering off the can drops kind of slow and gradual. So, definitely not wise to touch anything with power off as the caps charge quite well.

So, yes, disappointed but was I seriously expecting it to work first time? Nope, nobody is quite that lucky. And at the very least. the valves that haven't worked in many years have glowed again and it seems my rectifier is O.K. as well as the transformer.

Why is there no sound at all? Well, I checked the speaker works and it does.

I haven't yet investigated actual valve anode voltages, except the rectifier. Especially the KT61 must be tested. Sure, it glows fine but I'd best test anode voltage.

I tested speaker voltage with the set on and no voltage apparent. I'm a bit worried about the so-called plug and socket device Chas Miller has warned about in his book. This is a three pin socket at the back of the chassis. With the speaker socket plug out, the set is silenced so external speakers can be used. Could this socket be muting my internal speaker due to a fault? Also, although I cleaned the waveband switches and they seem O.K.. many years in a farmyard garage can create problems (you should see the scale lamps when I upload a pic later).

So, I guess I now have to soldier on and try and get to the bottom of this. Any comments on speaker mute sockets welcome in case someone has had issues with this.

P.S. I ought to test the output transformer too.

[Terry]

... After a few moments, I finally realised the radio was silent but definitely alive with juice. The valves were clearly glowing and I got a 400 VDC reading from the H.T. Smoothing Can. I also noticed on powering off the can drops kind of slow and gradual. So, definitely not wise to touch anything with power off as the caps charge quite well ...

P.S. I ought to test the output transformer too.

The exceptionally high HT voltage plus the slow discharge points to there being no load. 

It would be a good idea to check the primary of the OP Transformer, as you say, but I suspect that this not the fault.

Having no idea of the age of the set, the connection between the smoothing and the reservoir sections of the smoothing can may be either a resistor or a choke, and I would guess that this is where the problem lies.

Older HMV sets used what is often incorrectly called a 'mains energised speaker'. This is not connected to the mains at all but what makes it different is that it doesn't have a permanent magnet like most speakers do. Instead it has an electromagnet which also doubles as smoothing choke.

If your set has one of these, you should check it for continuity.

Do you have a circuit diagram for this set?

The model number will also help others to help you - just HMV on its own is of very little use.

[Nowhere-Man]

Here is a pic of part of the radio. It's possible a lot of people wouldn't be too keen on my lollypop Wima capacitors but they're a fair bit cheaper than maybe ordering from JustRadios and, so far, I find they work very well so far as audio quality goes. However, they are very different from vintage era. I did use a few Spragues as well. Given I've done a fair bit of radio work lately I try to keep the costs low if it's a very old set so I use these large red caps.

You'll notice there are black insulted wires that loop over the rung. These are the capacitor leads and the loop anchors them in place. The actual connection is right at the back so on this set the work was very demanding. I'm not sure why they cramped so much of it together with valve bases really hard to get at.

[Nowhere-Man]

... After a few moments, I finally realised the radio was silent but definitely alive with juice. The valves were clearly glowing and I got a 400 VDC reading from the H.T. Smoothing Can. I also noticed on powering off the can drops kind of slow and gradual. So, definitely not wise to touch anything with power off as the caps charge quite well ...

P.S. I ought to test the output transformer too.

The exceptionally high HT voltage plus the slow discharge points to there being no load. 

It would be a good idea to check the primary of the OP Transformer, as you say, but I suspect that this not the fault.

Having no idea of the age of the set, the connection between the smoothing and the reservoir sections of the smoothing can may be either a resistor or a choke, and I would guess that this is where the problem lies.

Older HMV sets used what is often incorrectly called a 'mains energised speaker'. This is not connected to the mains at all but what makes it different is that it doesn't have a permanent magnet like most speakers do. Instead it has an electromagnet which also doubles as smoothing choke.

If your set has one of these, you should check it for continuity.

Do you have a circuit diagram for this set?

The model number will also help others to help you - just HMV on its own is of very little use.

This is the HMV 1124. Should you wish to take a look at the schematic or any schematic, just google up Radio Workshop and it's easy to access any valve radio.

O.K. so now I've found out what is wrong. You are basically right that the voltage was way too high. It now turns out I have no H.T. output across the H.T. smoothing caps. This has never happened to me before and is a new experience to get to grips with. I did test the inline smoothing resistor to be sure this wasn't a factor. That resistor is a few Ohms out of spec but not crucial. The bottom line is when I power up the radio, the rectifier sends a high voltage to the first H.T. Smoothing cap but there is no output. There should be a voltage through the wiring from the first to the second H.T. cap whereas there is nothing.

The schematic gives 3 smoothing capacitors. The two big cans are 16 MicroFarads at 400 volts DC. The first can is showing a high voltage on only one terminal but the outgoing terminals show no voltage and it's the red wire from that point that powers the set. What could have happened?

My job now is to make a new smoothing circuit and take it step by step.

I've sort of tucked this thread away from the main Valve Radio Section with a view to making the odd observation or maybe even digressing to some theory discussion on old radio design. Not sure if it's of much interest but this is where I'm at the moment - a basic H.T. issue.

[pwdrive]

Check the voltages and check for continuity and resistance, start at the rectifiers cathode connection and work through from there.

Lawrence.

[Nowhere-Man]

I started with the output valve and there was nothing there. I did find the grid stopper resistor too and found that was a touch out of spec in Ohms. Anyway the starting point was no voltage on the valves.

Transformer is O.K. Tube heater circuit is O.K. Scale lamps look as if they kicked the bucket 20 years ago! They're six volts but the feed wires are fine and do measure 6 volts.

Rectifier it seems must be working as voltage will be D.C. from cathode to H.T. Smoothing cap. And at the cap on one lug there is voltage measured in D.C. There is, however, no voltage from this can on the opposite lug where an HT supply wire leads to the next smoothing can. There are actually 2 HT cans and not just the usual one. And no voltage anywhere on the other can where more wires radiate to the circuit.

Continuity is O.K., though. It's just voltage is not reaching those wires. There is a bit of rust on the top of the cans so maybe damp and cold over the years has had its impact.

I'll be working on it a bit more tonight...

[pwdrive]

Normally an HT problem like you describe would be an easy fix pending any component availability if it's component failure...Keep at it.

Lawrence.

[Nowhere-Man]

Well, fingers crossed, I guess.

[pwdrive]

You can do it.

Bye the way, looking at the picture in post #3, long leads on some capacitors can lead to problems in some designs, hum/feedback in some audio circuits and all sorts of problems in IF/RF and Mixer Osc circuits, best avoided if possible, just a tip.

Lawrence.

[Murphyv310]

Sorry but I'd be chucking out those red caps, buy an axial kit from justradios and start again.
For reliable operation caps must be similar in physical design with cable routing done as was originally if at all possible. As has been said hum and instability can rear it's ugly head.