Bush 62 Series heater chain anomalies
 

Hi all
I've been working on a TV62 (the one with the cracked case!) and when powered up, the tube looked a bit tired. A check on the positioning and ion trap assemblies (it's an electrostatically focused tube) improved matters a tremendous amount but it still looked rather dim. I decided to measure the volts across the tube heater. 3.62V!! I then measured the volts across the RF chassis heater chain - this calculates at 69.1V - and got 80V. Why? I decided to open the chain and measure the current. This was a little high at 312mA but this, theoretically, doesn't give 80V. As the tube heater has the right current flowing through it, can I assume that the heater is getting the correct "heating effort"? Help please.
Cheers
Brian

Re: Bush 62 Series heater chain anomalies
 

Hi Brian,
Unfortunately it's a common fault with Mullard CRTs, one half of the heater goes short circuit leaving only half the heater voltage at pins 1 and 12.
In a transformer fed 6.3 volt supply the heater would appear abnomally bright.

DFWB.

Re: Bush 62 Series heater chain anomalies
 

Hi David - So it is probably the tube! The set isn't turning out to be the bargain I thought it was. What type of tube is fitted to the TV36 as there are no labels etc. on the one fitted and the service sheets don't mention that particular variant? I can then post a "wanted" thread.
Cheers
Brian

Re: Bush 62 Series heater chain anomalies
 

Hi Brian,
The TV36 was fitted with the Mullard magnetic focus CRT MW36/24 or MW36/44. The former has a tetrode gun and the latter has a pentode gun. They are interchangable.
I have in the shop a TV62 which belongs to a forum member. The original AW36/20 has the same problem with it's heater as your has.

Being unable to find an AW36/20 or 21 a MW36/24 is being considered as a replacement.
In order to employ the MW tube in the TV62 a focus magnet assembly will have be found and fitted. The one that comes to mind is the compact focus magnet that was fitted in the Ekco T311/327/330 TV receivers.

DFWB.

Re: Bush 62 Series heater chain anomalies
 

Hello Brian,
This link may explain what David has described with another Mullard tube using a similar gun assembly.
https://www.vintage-radio.net/forum/...ad.php?t=12710
Regards John.

Re: Bush 62 Series heater chain anomalies
 

The original tube in my TUG-12 got like this, the heater is made in an "M" shape and if the bottom middle bit of the "M" touches either of the side bits then this is what happens. Sometimes a sharp tap on the neck with a screwdriver handle (careful now!) can provide a temporary cure.

What ***does not*** work is trying to force 6.3V across the heater by using a small transformer, this will just open-circuit the half of the heater that you have left!

Re: Bush 62 Series heater chain anomalies
 

Thanks for that tip. I will try that and see what happens. Knowing my luck with this set, the other half of the heater will go O/C!!
Cheers
Brian

Re: Bush 62 Series heater chain anomalies
 

GOOD NEWS!! I tried Studio263's suggestion and gently tapped the tube neck with a rubber-handled screwdriver while the AVO was still connected across the heater and the voltage rose from 3.8V to 7V!

Fantastic. I then changed the mains tapping to the more appropriate setting and now the reading is 6.3V. However, it is a bit touchy and sometimes drops to the lower level with mechanical shock (another knock!).

The picture does improve in contrast, and is watchable in bright daylight without overload, so I'm happy about that. Maybe I should fit a coherer type solenoid & hammer with a push-button mounted on the back-board!

Thanks again Studio263, Dave & John for the help.
Cheers
Brian

Re: Bush 62 Series heater chain anomalies
 

Hi; another trick is to turn it upside down! This sometimes works but you may have trouble with the eht lead, and you must also turn the base connector to fit the tube; but if anything can save an otherwise usable crt, try it.

Danny