McMichael 472 overhaul and M34 to 5E6C conversion problem

[squinancy]

Hello!

I am asking for help from those in the know. I have recently done what seemed like a good job on a McMichael 472 (see attached pic). Electrolytic and paper capacitors all replaced, valve seats cleaned, moving parts properly lubricated and so on.
One big problem came with the M34 magic eye. The original was as good as dead, perhaps 15% of the original brilliance. A totally brilliant new one costs at least £40-50 on the Internet, unreasonable I would have thought. What I found in my collection instead is an unused 5E6C Russian magic eye that uses the same 8 pin socket, except that pins 1 and 4 are not in use. I also found the following instructions for conversion on a German Internet site which was translated by the radio enthusiast who posted it:

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EM34 -> 6E5C

Usually both screens of the EM34 are used parallel. Therefore you´ll find two resistors mounted from PIN5 (L)to PIN3 (a1, d1) and from the same PIN5 (L) to PIN6 (a2, d2).

6E5C has only one screen hence only one resistor ( 1 MegOhm) will be used:

One Resistor is mounted from PIN6 (L) to PIN3 (A).
PIN6 (L) gets the wire which was originally mounted to PIN5 (L).
PIN5 (G) gets the wire which was originally mounted to PIN4 (G).
The socket has to be turned by about 45 degrees since the 6E5C Screen is aligned different to the EM34 ones.

We recommend keeping a sketch of the original wiring with the set.

Pinning EM34:
PIN
Ident.
1
not used
2
f
3
a1, d1
4
g
5
l
6
a2, d2
7
f
8
k, g


Pinning 6E5C:
PIN
Ident.
1
not used
2
F
3
A
4
not used
5
G
6
L
7
f
8
K


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I did the prescribed re-wirings (removed the existing resistors and inserted one across pin 6 to 3), but left all other wires in place, including those that lead to the scale bulbs:

"6E5C has only one screen hence only one resistor ( 1 MegOhm) will be used:
One Resistor is mounted from PIN6 (L) to PIN3 (A).
PIN6 (L) gets the wire which was originally mounted to PIN5 (L).
PIN5 (G) gets the wire which was originally mounted to PIN4 (G)."

The result was a mini-disaster. The 5E6C showed no sign of life, but smoke started to emanate from the power pack at the bottom of the set (at least I think it came from there). I quickly disconnected everything and restored all the original wire connections, including the old resistors. The outcome is another setback. Nothing, not even the 15% brilliance is produced by the old M34. Something somewhere must have been blown. Could you please try to give me a hint what may have gone wrong and how I could restore the magic eye in this set?

----
The other issue is the volume control. It starts very loud, almost maximum loud at minimum setting after switching on, but increases slightly further by turning the volume up. Do you think it is a simple issue of changing the pot because of a worn or broken track or could there be another snag involved? So it looks like a promising job, not entirely botched, but still some way to go.
I'd be grateful for all suggestions.
Many thanks indeed. Miklos

[Steve_P]

I'd look at the Volume Control first. It sounds to me like the earth side is faulty - i.e not connected. It's either bad soldering or a disconnection inside the pot. If you've not got the manual, you will get it up top there.

The magic eye problem - the smoke is probably a 2k 6 Watt resistor which may have survived. There are two resistors up top there. They may be on the valve base or in the set. They both go from Pin 5 to pins 3 and 6 respectively.

The valve may have gone, but try it after checking these to make sure.

The EM34 is here: http://www.r-type.org/exhib/aaa0331.htm

There is a Russian Equivalent this is true and it needs changes. You'll get a new one and the changes here: http://www.pasttimesradio.co.uk/spares/spares.html

Cheers,

Steve P.