Power resistors, heat on PCBs - was Re: [TekScopes] 2236 Focus range.


toby@...
 

On 2021-08-26 10:19 a.m., n4buq wrote:
I recently had a 1/2W film resistor flame-out on the LV Supply board in my 2445 and it burned a significant divot in the board. I mounted the replacement about 1/8" or more off the board to give it some breathing room. Probably unnecessary as the cap that caused the resistor to burn out is now replaced but I did it just for good measure.

Thanks,
Barry - N4BUQ
In the Hewlett-Packard 'scope I have been working on (1340A), they have
spaced every power resistor _at least_ 5-6mm from the board. This seems
like a very good idea.

There is also a nice ventilation measure using 1/2" circular holes where
the components are on the bottom of the board (so the hole provides an
upward exit for warm air right next to the hot compoents). They have
done this in the X/Y/Z amplifiers where there are 2-4 transistors and
power resistors clustered around the hole. Photos on request!

--Toby


n4buq
 

I wish HP would have done that in the 410B. Several higher-wattage resistors in that model tend to discolor the circuit board where they're mounted due to excessive heat. I space those as well when I replace them.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: toby@telegraphics.com.au
To: TekScopes@groups.io
Sent: Thursday, August 26, 2021 9:33:24 AM
Subject: Power resistors, heat on PCBs - was Re: [TekScopes] 2236 Focus range.
snip<
In the Hewlett-Packard 'scope I have been working on (1340A), they have
spaced every power resistor _at least_ 5-6mm from the board. This seems
like a very good idea.

There is also a nice ventilation measure using 1/2" circular holes where
the components are on the bottom of the board (so the hole provides an
upward exit for warm air right next to the hot compoents). They have
done this in the X/Y/Z amplifiers where there are 2-4 transistors and
power resistors clustered around the hole. Photos on request!

--Toby









Geoffrey Thomas
 

In such cases I always used ceramic stand-offs to mount the replacement, such as these form Farnell:

https://uk.farnell.com/search/prl/results?range=exc-obs&st=ceramic%20standoffs&pageSize=50&sort=P_PRICE

Geoff.

On 26/08/2021 15:33, toby@telegraphics.com.au wrote:
On 2021-08-26 10:19 a.m., n4buq wrote:
I recently had a 1/2W film resistor flame-out on the LV Supply board in my 2445 and it burned a significant divot in the board. I mounted the replacement about 1/8" or more off the board to give it some breathing room. Probably unnecessary as the cap that caused the resistor to burn out is now replaced but I did it just for good measure.

Thanks,
Barry - N4BUQ
In the Hewlett-Packard 'scope I have been working on (1340A), they have
spaced every power resistor _at least_ 5-6mm from the board. This seems
like a very good idea.
There is also a nice ventilation measure using 1/2" circular holes where
the components are on the bottom of the board (so the hole provides an
upward exit for warm air right next to the hot compoents). They have
done this in the X/Y/Z amplifiers where there are 2-4 transistors and
power resistors clustered around the hole. Photos on request!
--Toby


Jim Strohm
 

You can get ceramic beads from the big-box hobby stores that work just fine as standoffs. Some manufacturers in the 60s actually did this after it was suggested in 73 or some other magazine.

73
Jim N6OTQ

Sent from my quenched-gap spark transmitter.