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Old 12th Jan 2023, 1:41 pm   #21
stevehertz
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Default Re: What did I get myself into? Murphy A40C.

I have a factory service manual for this radio that I'm about to put on the for sale section.
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Old 15th Jan 2023, 10:34 pm   #22
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Default Re: What did I get myself into? Murphy A40C.

Bought!

Lots of work prepping cabinet this weekend, mainly gluing, filling and sanding but its all done now. Every time I thought I was ready to get it stained I found a crack or something else to fix.
3 small woodworm holes from a long time ago so not too bad. Top thuya burr veneer cracked in few places and lifted in a couple more, thin too but I think its salvageable and I'll see how it re colours

Started cleaning chassis too but damn that's going to be a LOT of work to get all this greasy dust off, tempted to just hose it all down

Anyone has any ideas where I could possibly get these rubber feet holding transformer board? I think I'll try halfords tomorrow, see what I can find there.
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Old 20th Jan 2023, 7:53 pm   #23
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Default Re: What did I get myself into? Murphy A40C.

Lots of work done on this past few days. Cabinet is finished and drying. Rails painted, grill mesh cleaned a number of times and varnished too. I'll probably assemble the cabinet innards today, also done initial chassis check and cleanup now(that was not fun!) and I think I've got 3 dead valves.
The MU14 in the transformer board is definitely dead, does not light up and I can see a bit of glass in the valve so something in there cracked/exploded
I can hear something inside one of the AC4/PEN and AC/VP2 valves.
Is there any way to test these without a dedicated valve tester?
Powered it on the freshly made bulb limiter for a minute or two. Everything lights up so I guess transformer is fine. Green eye does not come up.
I'll get the replacement valves and start recapping it.
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Old 20th Jan 2023, 8:43 pm   #24
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Default Re: What did I get myself into? Murphy A40C.

Valves can easily be tested in the circuit by taking voltage readings - you'll soon know what's going on. If voltages don't agree then you first check for any faulty surrounding components before condemning the valve. It's very unlikely (but not impossible) that you'd have three faulty valves in the same set. Many old valves rattle and appear to have bits of glass rattling around inside, it's quite normal, but can occasionally indicate a fault.

So replace any critical capacitor such as the one feeding the control grid of the output valve and give it full mains and take voltage readings. Feel the can of the smoothing capacitor after a few minutes for any sign of warmth. If it feels at all warm then switch off immediately, as this indicates that it has internal leakage and will likely pop its guts.

When buying valves over the internet, beware of any that don't look new, but are in original boxes, as they're likely to be old faulty ones that the repair man put back in the box that the replacement came in and which never got thrown away. If the valve doesn't look clean and new, then I prefer to go with un-boxed dirty looking ones, as these are likely to be pulls from scrapped sets and will probably still be in working order.
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Old 20th Jan 2023, 8:50 pm   #25
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Default Re: What did I get myself into? Murphy A40C.

Just as an addition - the rectifier that doesn't light up sounds like it could have an open circuit heater, so check it for resistance between the appropriate pins. Otherwise check for heater voltage at the underside of the base - it'll be AC. Be very careful not to get an electric shock when working on a live set!
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Old 20th Jan 2023, 8:54 pm   #26
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Default Re: What did I get myself into? Murphy A40C.

Rattles are often loose bonding material inside the base and not an issue. I agree that you definitely don't need a valve tester if you have the valves in the equipment they are to be used in. Just measure the voltages and note any that are different from the service data keeping in mind that you will be using a high input impedance DMM and the service data is usually based on an AVO that will add some load to the point under test.

If you need a replacement rectifier I would suggest a Mazda UU5 as they seem very reliable.

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Old 20th Jan 2023, 9:05 pm   #27
Leon Crampin
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Default Re: What did I get myself into? Murphy A40C.

I would fit 2 rectifier valves of the same type in an A40. The twin rectifiers need to be reasonably matched to share the (substantial) load. UU5 rectifiers are reliable - anything from MOV less so. Many good valves rattle...

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Old 20th Jan 2023, 9:12 pm   #28
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Default Re: What did I get myself into? Murphy A40C.

I think...you've got no HT.

Without a working MU14 rectifier, nothing in the set will work!

Check its heater for continuity/resistance with the valve removed from the set.

If the heater shows continuity, then it could be that it's not getting any voltage, so check for 4 volts AC, but be careful - there's high AC voltage direct from the transformer around the base of that valve and it can kill, so be very careful to measure the correct pins or you'll blow your meter if you have it on a low voltage range and you get it wrong.

The MU14 takes 2.5 amps for its heater and at this sort of current it's quite common to get oxidised connections both in the valve base socket or the actual valve pins themselves.

Edit: I see it has two rectifiers - I'm not familiar with the set. Check for heater voltage all the same. Just seen the post by Leon.
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Old 20th Jan 2023, 9:48 pm   #29
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Default Re: What did I get myself into? Murphy A40C.

You need two UU5s.
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Old 20th Jan 2023, 10:10 pm   #30
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Default Re: What did I get myself into? Murphy A40C.

Ah I see, so schematics showing two UU4 in there, I assume UU5 is direct replacement, on mine I have one UU5 and one MU14 and I should replace the latter with UU5 to match the other one.
The MU14 is definitely faulty, no continuity on the heater and there is literally a chunk of glass inside, it does not light up either so I'll replace it with a UU5 and see what happens, hopefully the other two are ok.
Thanks all!
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Old 21st Jan 2023, 12:18 am   #31
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Default Re: What did I get myself into? Murphy A40C.

I agree regarding the UU5.

I've got data books, but you can look up all you need to know about most valves on-line.

Here's a link to the valve museum, where there's data on thousands of valves, including your MU14 which I've linked to...
http://www.r-type.org/exhib/aaa0703.htm

You can probably glean from the writeup on that page that it wasn't the best of valves, being a re-badged MU12 that happened to just meet the specifications. Another thing is that the UU5 is a lower heater current, so less stress on the base pin connections and the transformer. You can see all the equivalents listed at the top of that page and click on them to see their specifications, including heater current and peak anode voltage etc. I quite like using the Brimar R3, not for any other reason than I happened on a stash of them a few years ago. They seem to be a good valve with 500 volts anode rating. However, they do draw 2.5 amps heater current, so perhaps not ideal for your set. Although the R3 is shown in the equivalents list on that web page, there's no on-line data for it, so I had to use one of my data books to check the exact specifications.

It's worth noting that the original valve being a UU4 only has a 2.2 amp heater, so your set has had its transformer slightly overloaded with that MU14 with its 2.5 amp heater.
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Old 21st Jan 2023, 3:08 am   #32
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Techman View Post
When buying valves over the internet, beware of any that don't look new, but are in original boxes, as they're likely to be old faulty ones that the repair man put back in the box that the replacement came in and which never got thrown away.
Yes. If the valve label/writing is in green, it will very likely be the faulty original valve put back in the box. New replacement valves tend to have black labels/writing.

Genuinely NOS valves can often be stored in damp conditions, which can cause the metalising (the grey coating on the valve) to stain or come off, or cause some corrosion on the pins. But is usually easy to tell the difference between damp storage and dirt from being exposed in the radio set.

Mazda changed the design of their valve boxes a number of times between the 1930s - 1960s, and the labelling on the valves changed too. So with experience it is possible to check they match. A 1940s style valve label in a 1960s box is an obvious give-away it is the old valve.
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Old 21st Jan 2023, 3:36 am   #33
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Katar83 View Post
I can hear something inside one of the AC4/PEN and AC/VP2 valves.
Is there any way to test these without a dedicated valve tester?
You can't check emission or how good the valve is (mutual conductance), but, you can check two common faults with a multimeter. Open circuit heater and shorted (low resistance) electrodes. Basically the heater pins should have a low resistance between them. Any other pins should have a very high resistance between them, e.g. heater and cathode (which will indicate a fairly common heater cathode short which will produce hum), or between cathode and anode, cathode and grid, anode and grid etc.

If the rattling isn't just some of the cement becoming loose in the base, but part of the electrode supports, the above may determine if the electrodes have shorted as a result.
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Old 21st Jan 2023, 10:51 am   #34
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Techman View Post
I agree regarding the UU5.

[...]

You can probably glean from the writeup on that page that it wasn't the best of valves, being a re-badged MU12 that happened to just meet the specifications. Another thing is that the UU5 is a lower heater current.
It's worth emphasising at this point that the UU5 has a lower heater current wrt the incorrect MU14, but it is 0.1A greater than the originally-specified UU4.
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Old 23rd Jan 2023, 3:03 pm   #35
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Default Re: What did I get myself into? Murphy A40C.

Got my BVWS bulletin today with 10+ pages about A40C and what a great reading, it has everything about this radio, including modifications, official and unofficial, lots of pics, schematics and what not including the AC/SP1 to TH41 official Murphy update which I couldn't find anywhere else!

The article also states that Murphy recommended using AC/VP1 as a drop in replacement for AC/SP1 in V5 or using VP4B with just the anode and control grid connections swapped over.
Definitely a must have magazine for anyone owning one of these radios!
Oh and there is a colour pic of what looks like a mint A40C with a dark walnut finish, I think I've matched it well. Very happy with how it turned out.

Also, in post came original Murphy service instructions, got to love the detailed schematics for this. Thanks Steve!
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Old 23rd Jan 2023, 3:13 pm   #36
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Katar83 View Post

Also, in post came original Murphy service instructions, got to love the detailed schematics for this. Thanks Steve!
No problem, sorry it took so long due to my error. And thanks to our friend who sent it on.
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Old 24th Jan 2023, 2:43 pm   #37
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Default Re: What did I get myself into? Murphy A40C.

While waiting for the new rectifier tube I thought I'd have a look at the big capacitors on the transformer board, one is shot (8uF, exploded a long time ago), the other is leaky and has about 50% its rated capacity.

Any rules on replacing these big caps? I see they are rated at 500V so I guess I'll have to go with something at least that and can't use widely available 450V? Can I use 10uf and 22uf instead?

I think there's enough space underneath the cans to leave the big caps as they are and hide something newer there to keep the original look and not bother with restuffing too?
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Old 24th Jan 2023, 2:54 pm   #38
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Default Re: What did I get myself into? Murphy A40C.

10 and 22 uF 450V should be fine.
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Old 24th Jan 2023, 10:31 pm   #39
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Katar83 View Post
Got my BVWS bulletin today with 10+ pages about A40C and what a great reading
Which issue of the bulletin is this, doesn't seem to be the latest?
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Old 25th Jan 2023, 8:24 am   #40
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Default Re: What did I get myself into? Murphy A40C.

Not the latest, no. It's BVWS Bulletin Volume 44, Number 1 (Spring 2019).
Caps ordered, sadly didn't have anything close to correct ones in my parts bin but will cobble something up for testing. I also found 8.2uF caps, didn't even realise that such value is available.
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