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

[ppppenguin]

Trevor is right from an appearance and workmanship point of view. He is also right to say that for some caps the physical layout and cable routing can be critical too. But in many cases any suitably rated cap will do the job, even if the result looks ugly. For example an audio coupling cap should never rely on critical layout.

[Murphyv310]

 For example an audio coupling cap should never rely on critical layout.

Unless of course the replacement is positioned very close to a mains transformer or the output stages, then hum or feedback is a distinct possibility. 
Neatness of course  can make fault finding much easier especially with man made errors.

[Refugee]

Those yellow tubular ones are cheap enough even from the big suppliers and you can even use a brown magic marker to make them look like old wax capacitors.

[Terry]

... 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 ...

... but not the  HMV 1124!

By a lot of hunting around I finally found out that it is similar to the HMV 1121, for which there is a Trader sheet.

Now may I respectfully refer you to my signature line?

... 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.

In this case there are two. R19 is 750Ω and R20 is 680Ω.

   

[Nowhere-Man]

Right, there has been a very unexpected twist in the account related so far. On reflection, it did seem a bit odd I was getting high voltage on the first smoothing capacitor but also voltage was lingering at the terminal after powering off? Normally that shows the smoothing cap is in pretty good shape.

Here now is what's happened and I imagine the smoothing caps will be O.K:

Below the rectifier mounting platform, there is one of those small connection brackets - a tiny strip board with a few soldering lugs.. It seems some very small wires or possible a resistor has come off and left the whole second H.T. line out of circuit. Basically the second H.T. connects to a terminal where the smoothing resistor is by the first large H.T. cap but the other end has no feed from the cathode of the rectifier. This is also deceptive because with the looms of wires, it looks as if all is well but there's a link wire involved. In actual fact, this may be a resistor as I recall finding one disconnected in the chassis but the value wasn't listed in the schematic.

This happened to me once before with a silent set. In that case it was one disconnected red wire from a transformer....

[ppppenguin]

With the set switched off and the caps thoroughly discharged you can measure between the tags of the smoothing cap with a resistance meter. This will tell you if there are resistors present and working.

These 2 resistors work fairly hard for a living and get hot. They are good candidates for failing. Assuming that one or both have failed the real question is whether they died of old age or due to a fault. For example if the capacitor C25 on Terry's circuit clip is leaky then the audio output valve will take excessive current which could easily kill either or both of those resistors. Can also damage the valve and the output transformer.

[Nowhere-Man]

Trevor is right from an appearance and workmanship point of view. He is also right to say that for some caps the physical layout and cable routing can be critical too. But in many cases any suitably rated cap will do the job, even if the result looks ugly. For example an audio coupling cap should never rely on critical layout.

These large Wima capacitors are pretty hard to miss. I've now come to quite like them. They were never intended for valve radio work at all but there are ways and means. One system is to solder copper wire to the pins and sleeve the wire. This makes it a bit safer. If the copper wire is the right thickness there will be a rigid, solid span where you connect. In most cases you don't need long lengths.

On the HMV, I had to experiment with flexible wire instead which was also rigid enough for support. There are some leads that are longer than the norm although a far few of the original caps had longish leads as well. I would only remove 3 of the old caps at a time because it would be really easy to get mixed up. That would be fatal as the HMV is very awkward to trace.

The Wimas are pretty affordable and easily picked up at a local Maplins branch. Most are rated at high voltages.

In terms of audio quality they're impressive. On the G.E.C. the audio quality is terrific so long as there is some grounding of the earth wire.

I think they're also very robust with soldering heat. I do try and use a heat sink but find the Wimas take a bit of stress.

They look a bit "loud" and that's why it's not everyone's cup of tea.

[Crackle]

Here is the circuit for the HMV1124 the 2 resistors Terry mentioned are arrowed, on the manufacturers circuit they are listed as R17 680R and R20 750R.
My bet it is either the resistor R20 or a dry joint. Try re-flowing the solder joints with good clean multicore 60/40 solder.

   


By the way I use these types of capacitors. You cant get much cheaper, and they do have a fairly decent length of lead, it saves having to solder wires onto those little stubs on the Wima caps. The caps you are using are intended for printed circuit use.
http://www.ebay.co.uk/itm/291569440252
They are also available from UK sellers http://www.ebay.co.uk/itm/261820249903 with free postage.
Mike

[Nowhere-Man]

With the set switched off and the caps thoroughly discharged you can measure between the tags of the smoothing cap with a resistance meter. This will tell you if there are resistors present and working.

These 2 resistors work fairly hard for a living and get hot. They are good candidates for failing. Assuming that one or both have failed the real question is whether they died of old age or due to a fault. For example if the capacitor C25 on Terry's circuit clip is leaky then the audio output valve will take excessive current which could easily kill either or both of those resistors. Can also damage the valve and the output transformer.

I'm a bit confused here. Are you saying the H.T. supply from one tag on the H.T. cap passes through to the other tag inside the can itself through an internal resistor?

Basically as I recall, on the top of the first can is three tags. One I think is a ground. One of the tags has a definite feed from the rectifier cathode. It shows a good 400 volts D.C. The tag opposite has no voltage showing but it supplies H.T. to the set. In fact only one terminal of the can shows a voltage.

The tag that shows no voltage at all has no supply from the cathode of the rectifier.

I think I need to get all of this absolutely clear before going ahead.

[ppppenguin]

In terms of audio quality they're impressive. On the G.E.C. the audio quality is terrific so long as there is some grounding of the earth wire.

What makes you think these caps are responsible for good sound quality? Even in the realms of top quality hifi the "sound" of capacitors is mostly snake oil. On a humble radio the exact type of cap will make no difference. Obviously a faulty (usually leaky) cap can give problems with sound quality.