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BRC What Watt

 
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BRC What Watt

Post by crustytv » Fri Jan 30, 2015 6:01 pm

Can any of the EX TV techs remember what the wattage should be for the modified/uprated resistors R751 & R752, on the 3500 convergence panel.

 
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Re: BRC What Watt

Post by malcscott » Fri Jan 30, 2015 7:18 pm

15/17 Watt. Only need to change one of them.

 
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Re: BRC What Watt

Post by Jayceebee » Sat Jan 31, 2015 7:35 pm

The upgraded replacement for R751 was 10 Ohm at 20 Watts and was a must change if it was an original what ever the service call was for along with the pincushion correction transductor if it was marked "Portugal".

R752 was 18 Ohm at I think 15 Watts, it was later supposed to be uprated to a 20 Watt version also but to be honest as long as the original is a white cement type and not black manufactured by Osbourne it should be OK.

John.

 
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Re: BRC What Watt

Post by crustytv » Sun Feb 01, 2015 1:38 am

Hi John,

That's both interesting and confusing.

I always thought the board on the left (see below) had the uprated resistor, for example R751 10R( without wattage marking) is very large Wire-wound hollow core.

Whereas the board on the right, ( below) has the white cement, these are both I believe 9W and match the original service data ratings.

res.jpg

 
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Re: BRC What Watt

Post by malcscott » Sun Feb 01, 2015 1:44 am

The 10 ohm resistor in the picture was the only mod i knew about.

 
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Re: BRC What Watt

Post by crustytv » Sun Feb 01, 2015 2:16 am

Looking through my BRC Bulletins of August/September 1971, I found a picture (see below) of the newly released 3500 convergence panel. It clearly shows the original R751/R752 10R & 18R, are different to the ones fitted to my examples.

I might suggest that in the photos above, the left hand board indeed has the uprated 10R (15-20W). The board on the right has had the originals replaced but used cement at original wattage of 9W.

3500MK1.jpg
First Gen 3500 convergence Panel

As an aside, an interesting note:
As of Aug/Sep 1971 nearly half-a-million 3000 series chassis had been produced in two years ( 3000 released in 1969) continuous production. In this month BRC released two new versions of the chassis. The first being the BRC 3000 MKII which was stated to have better performance. The second was a variation of the MKII to cover the different performance requirements of the 26" 4:3 tube, this was to be known as the 3500.

 
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Re: BRC What Watt

Post by Jayceebee » Sun Feb 01, 2015 10:27 pm

Hi Chris,

Sorry about the confusion, I had originally put 10 Watts (close) as the original spec but changed my mind for some reason and edited it to 15 Watts. :aab

The LH picture shows the correct uprated replacement for R751 and Malc may be right at 15/17 Watts although I always thought it was 20, . The same picture also shows R752 as the troublesome type of Osbourne manufacture, the most unreliable version of R751 was identical other than the value of course. The upgrading of R752 may not have been on a BRC technical bulletin but it certainly was in DER and possibly RR fairly late in the chassis life span and certainly a couple of years after the R751 mod was made mandatory.

If either of these went open circuit it could case major damage to multiple components and to the paxolin destroying the PCB. To be honest with the white cement types fitted in the right hand pic, R751 did fail but very rarely but I don't recall R752 of that type failing at all. I once tried an experiment on a scrap set running with R751 o/c, there certainly was quite a lot of smoke but no damage occurred to the PCB material until around the 45 minute mark. ttt:

I certainly remember those resistors shown in the 3500 board pic, they were green in colour in very early production and later grey. I don't remember any failures of that type either.

Hope this helps.

John.


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