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EM34 Faulty and 'repaired'

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Forum » Valves / tubes - Semiconductors » VALVES/TUBES / SEMICONDUCTORS in RADIOMUSEUM » EM34 Faulty and 'repaired'
           
Michael Watterson
 
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18.Nov.11 23:51

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Initially the tube was completely illuminated, though dim apart from a bright part flickering shadow open and closed about 1 or 2 degrees, but very intermittent. Mostly fully closed.

The AVC/AGC and drive to grid fixed (a separate story) and the opposite shadow now fully open with no signal and closing with stronger station. But the "occasionaly flickering slightly open" shadow remained fully closed.

The working shadow seems to be pin 3 (sensitive? closed g = -5V) and "broken" flickery shadow (fully closed) the pin 6 (coarse closed g = -12V?).

Repair
So I disconnected the 1M on pin6 and the bottom other shadow still worked. Disconnecting 1M on pin 3 no moving shadow.

It seemed like leakage or something so I momentarily shorted unconnected pin 6 to 0V and got a "crack" of a spark.

The EM34, while still dim, now works perfectly. One sensitive shadow and the formerly broken shadow is less sensitive and fully open instead of closed on no signal. A really strong shortwave station will close both shadows (the rarely used part being brighter).

Background
The EM34 tube is in a possibly 1949 or 1950 Pye 39 JH/E  (no mistake!) It was found walled up under a staircase during renovations of a house in West Limerick. I'll write up on the radio set repair/restoration after I get the model created. These are similar but no Magic Eye PYE 39J/H (closest) and PYE A39 J/H.

If the repair is only consideration  then a 6e5s (Cyrillic 6E6C) could be fitted with wiring change or adaptor. Pure "Restoration" would see the EM34 retained. However I want to listen to this set regularly. So I may fit a 6e5s (Cyrillic 6E6C) and attach the EM34 with a note and circuit changes inside the cabinet.

update:

It was intermittently loading HT line, so I unplugged it and between the various electrodes apart from grid (a1, a2, T, k) in every combination connected by capacitor leakage tester with a 1uF capacitor charged on its connector (nearly 360V at very low impedance for short period). The Magic Eye is now performing perfectly though dim but just usable.

"Repaired" EM34 with weak and strong signal
(Top shadow needs very strong signal to close, bottom is more sensitive)

 

This article was edited 21.Nov.11 13:46 by Michael Watterson .

  
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