[TechAssist] Re: Mod RCA F32650----------Ch# ATC113 CA1

  • From: "majorthom@xxxxxxxxxxxxx" <majorthom@xxxxxxxxxxxxx>
  • To: techassist@xxxxxxxxxxxxx
  • Date: Sat, 11 Jun 2005 14:13:23 -0700 (PDT)

Hi Joel,I thought the leading edge indicated turn-on time and the trailing edge 
the turn off?No?
The coll pulse seemed OK.Still looking for pulse width answer,tho.
Best,Pete
 

Joel Jacobs <samurai@xxxxxxxxxxxxxxxx> wrote:
You want to look at the collector pulse, not the base. Set your scope to
max v/div and use a 10x probe. You want to look where the flyback pulse
starts. The corner of the waveform where the pulse goes from ~ground and
starts upward to 1200v or so should be a very sharp transition. If it's
rounded and not well defined then the transistor is turning off too slow.
If it's turning off slow it will dissipate huge amounts of power during
turnoff. That's usually why they run hot.
Causes of slow turnoff are 1. insufficient base drive and 2. poorly
matched transistor in a marginal design. Look for bad electrolytics in the
drive circuit and if you don't find any try a different transistor. I'm not
afraid to use a substitute part as long as I can verify the collector pulse
is ok. I had allot of CTC187's that used a BU517 that ran scorching hot for
no apparent reason. Sub a 2SC5148 and they run cool.

Joel Jacobs
Ohio Consumer Electronics
Pickerington, Ohio

----- Original Message ----- 
From: 
To: 
Sent: Friday, June 10, 2005 5:23 PM
Subject: [TechAssist] Mod RCA F32650----------Ch# ATC113 CA1


> Got in w/ SMPS squeal,replaced HOT & fly.
>
> Tested and noticed a creeping temp of HOT.Did electrolytics around
driver,L4401 resolder
> After mucho time noticed the drive pulse to HOT base @ half-power points
to be something like 10 usec.Can anyone refresh me[?] I've been all over
NTSC sites -no help.
>
> My question is: What is pulse width in usec @ base of HOT in a sample
television receiver?That would be @ 1/2 power point or .707 of peak
> Regards[?]
> Pete
>
> Peter Neal
> Neal Electronics
> 1905 E. Wm.Cannon
> Austin,TX 78756
>
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Peter Neal
Neal Electronics
1905 E. Wm.Cannon
Austin,TX  78756
 
 
 







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