8mm Forum: Bell & Howell 642 Lamp Holder

This is topic Bell & Howell 642 Lamp Holder in forum 16mm Forum at 8mm Forum.

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Posted by Paul Mason (Member # 4015) on September 03, 2014, 04:19 AM:

After replacing the exciter and pilot lamps on my 642, the main lamp has stopped working. The lamp is OK as it lights up in my 644 projector and I rely on one mains lead for both projectors so it can't be that. This leaves either the lamp holder/base or the control switch. has anyone had trouble with the switch or the lamp holder? Is it straightforward to replace? Many thanks.

Posted by Martin Jones (Member # 1163) on September 03, 2014, 04:26 AM:

If this has the "valve" type base with pins, try working it up and down in the holder to clean the internal contacts of the holder. A little Servisol contact cleaner on the lamp pins first will assist with this.

Posted by Simon McConway (Member # 219) on September 03, 2014, 07:28 AM:

I had to replace the whole switch which was a fiddle. Starting currents are high with this lamp which can produce arcing across the contacts. This causes damage over time. The lamp holder could also have been damaged by arcing if a lamp had previously not been pushed fully home during the past life of this machine.

Posted by Paul Mason (Member # 4015) on September 03, 2014, 09:57 AM:

Thanks Martin and Simon,
I tried moving the lamp up and down to rub the pins as Martin suggested and it does now light up. As Simon says it is easy to not push the lamp fully home. I suspect corrosion in the base was always the most likely as the lamp didn't come on in forward or reverse whereas a switch fault would probable only affect forward. I may try gentle cleaning with a needle file as well

Posted by Maurice Leakey (Member # 916) on September 03, 2014, 11:08 AM:

The pins are very fragile. Handle with extreme care.
Don't forget that the lamp is always live, the switching is on the neutral. Only handle the in/out of the lamp with the mains plug disconnected.

Posted by Terry Sills (Member # 3309) on September 03, 2014, 11:22 AM:

Maurice
Is that true of all projector lamps?
Terry

Posted by Maurice Leakey (Member # 916) on September 03, 2014, 01:14 PM:

Fragile pins, or live connection? I assume you mean the latter.

This applies to all 16mm Bell & Howells from the 641 onwards, until the TQ models appeared. They can be distinguished by their 4-pin mains sockets. By the use of a special transformer 115 volt lamps can also be used. The wiring is so designed for this. The separate wire direct to the lamp can also be used for a change-over arrangement.

I am referring to the A1/205, A1/206 and A1/207 lamps.

Posted by Paul Mason (Member # 4015) on September 03, 2014, 02:17 PM:

The Bell & Howell 643/644 instructions warn that the mains supply should be disconnected before removing or replacing the lamp. I remove the plug from the projector or the mains socket. There is a shock risk with lamp voltages over 30volts but it applies to all projector lamps since an accidental short-circuit can weld metalwork if a high current is passed similar to that caused by car batteries. The pins of G17 type bases are delicate but with care should allow the lamp to be moved vertically in and out of the holder without damage.

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