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Beamec CRT tester and reactivator.

 
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Beamec CRT tester and reactivator.

Post by Till Eulenspiegel » Fri Mar 27, 2015 2:12 pm

I've brought this instrument down from upstairs to have a look at it with the intention of restoring it to working order.
The restoration job is a joint effort with VRAT forum member Alistair who also has one of these instruments.
I haven't opened up the Beamec yet but I'm informed it's quite a complicated affair, lot's of complex switching. A Mullard EN91 thryratron is employed, the circuit will have to be traced out to find out to what function it is used for.
Information for the equivalent 2D21 can be found here: http://www.radiomuseum.org/tubes/tube_2d21.html

Till Eulenspiegel.
Attachments
Beamec_0065.jpg

 
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Re: Beamec CRT tester and reactivator.

Post by Alistair D » Fri Mar 27, 2015 5:21 pm

Here is is a slightly different version in use by member Vintage Rob.

viewtopic.php?f=5&t=10576

I started to put together some rough notes about the Beamec with the intention of doing a comprehensive write up of what it does. Below is a copy of what I have so far.

I am pretty sure I put a copy of the user instructions in the library. I will check that later. I also have a more readable OCRd copy somewhere if anyone wants it

NOTES
Octal plug
Pin1 = Pin 7 via 330k
Pin2 = A1
Pin3 = Grid
Pin4 = Cathode & +ve Volts
Pin 5 = Heater
Pin6 = Heater
Pin7 = A2
Pin8 = -ve Volts

CLEAR LEAK
A 32uF capacitor, charged to HT, is discharged across the named set of terminals.
When reforming capacitors prior to powering up the unit. The main 16uF and the 32uf cap up beside the meter must be done. S2 in the NORMAL position includes this capacitor. In CLEAR LEAK the capacitor is isolated
Meter rating
The oprating instructions state a meter sensitivity of 250uA. My 1958 dated meter is 500uA FSD. I do happen to know that tere is an internal shunt in the meter so perhaps the movement is rated at 250uA

Valves
EN91 85A2

Emission
Triode/ Pen -Tet Switch
Triode Pen - Tet
UUT Anode 0V 260V
UUT Grid 10V 4.75V

A2 Supply
700Vrms is connected to a full a full wave voltage doubler. The +ve output is connected to the UUT A2, the -ve to UUT cathode???? The centre point of the doubler caps is connected to HT-ve(0V)

P.S. Sorry about the emission table. Not played with the table generator yet.


Al

 
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Re: Beamec CRT tester and reactivator.

Post by Till Eulenspiegel » Sat Mar 28, 2015 2:03 pm

Pictures of the internals of the Beamec tube tester. As you can see there is some complex switching for us to deal with. In the screening cans are the two tubes employed in the instrument, a Mullard 85A5 stabiliser and the thyratron which a 2D21 = EN91.

Till Eulenspiegel.
Attachments
Beamec_0070.jpg
Beamec_0071.jpg

 
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Re: Beamec CRT tester and reactivator.

Post by Alistair D » Sat Mar 28, 2015 3:30 pm

As far as I remember, in the emission mode the 85A2 is permanently lit. In the process mode it is tuned into a relaxation oscillator. The anode is connected to the junction of a resistor and the 32uF capacitor on the tag board. The cathode is connected to the locknutted pot just behind the mode switch then off to G2 of the thyratron. The idea of the pot is to control the current through the stabiliser and thus the on time of the pulse. The thyratron is used in conjunction with the 700Vac supply(I presume current limited) and is applied between the cathode and grid of the CRT. When the tube is not conducting the is a large -ve voltage present.

I do not have any old CRTs here at present but when I tried the process mode on an old valve I had lying around there was a purple glow inside the valve during the non conducting pulses for the first couple of cycles. So I wonder if one of features of the process mode is to try and 'burn off' any air in the CRT. The user manual does not describe what is supposed to happen during this test.

Al

 
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Re: Beamec CRT tester and reactivator.

Post by Till Eulenspiegel » Sat Mar 28, 2015 6:03 pm

Hi Alistair,
there is a description of a CRT rejuvenation process in the booklets that accompany the Muter CRT tester. I'll try to find this information. The process might be similar to system employed in the Beamec.
What is known about the Muter rejuvenation process is that during the time the button is pressed the heater supply to the CRT is disconnected. The theory behind this idea is that the cathode cannot be damaged because the CRT self regulates the rejuvenation process.

Till Eulenspiegel.

 
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Re: Beamec CRT tester and reactivator.

Post by Doz » Wed Apr 01, 2015 11:04 am

Till Eulenspiegel wrote:Hi Alistair,

What is known about the Muter rejuvenation process is that during the time the button is pressed the heater supply to the CRT is disconnected.


Same as the B&K 467

 
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Re: Beamec CRT tester and reactivator.

Post by Alistair D » Fri Apr 03, 2015 3:37 pm

Beware of leaving the function switch in the A2 position until you have checked the 2x 0.1uF 1000V waxed paper caps on the underside of the machine. On mine they started to get quite warm due to leakage.

Be aware that when in the A2 test mode there is not far short of 2kV between the A2 and cathode terminals. Also there is no bleeder resistor to discharge the caps.

Earlier I mentioned that pin 1 of the octal plug is connected to the A2 output via a 330k resistor. I wonder if the original tube base adapter had a switch that used the 330k to discharge the tube and the capacitors.

Al

 
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Re: Beamec CRT tester and reactivator.

Post by Alistair D » Tue Jul 14, 2015 9:17 am

With the help of David and Mikeymushradio I now have a service manual for my Beamec. An OCR'd and tidied version of it has been put in the library.

Al

 
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Re: Beamec CRT tester and reactivator.

Post by Alistair D » Wed Jul 22, 2015 10:31 am

The attached schematic shows how the Beamec does processesing on a tube. Basically the un-rectfied 240V HT winding with a 38 ohm current limiting resistor in series is connected between the tube grid and the Com -ve. The Thyratron connects the cathode to Com -ve for short periods every 5 seconds or so. This seems a bit brutal to me. What do you think?

Al
Attachments
S3-8.jpg


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