Digest for sci.electronics.repair@googlegroups.com - 7 updates in 3 topics

8:55 AM Penedão
Cursitor Doom <curd@notformail.com>: Feb 21 02:34PM

On Sat, 20 Feb 2016 15:45:34 -0500, JC wrote:
 
Have you posted this before on another thread related to this? Either you
have or it's deja vu. Sorry if you did, I didn't note it properly.
 
> had lying around, 6 x 470R 5Watt w/w resistors.
 
> You can pull one of the connectors out of the scope for a connection.
> (See photos)
 
Yeah, those Stocko connectors. On another Philips manual I have for a
different scope, they actually publish the proper values for a dummy load
which would be really helpful to have on the current problem I face.
 
> Just for fun (and I'm running this off an isolation transformer), pull
> V1812 and scope T2 with T1 as probe ground. you should see a nice drive
> waveform for a few seconds and you can check the frequency is 20KHz.
 
Not sure why you say pull V1812, but here's the waveform I got between
those two points when I did this test a few weeks ago:
 
https://www.flickr.com/photos/128859641@N02/24538703002/in/photostream/
 
But then it goes downhill. Here's V1812's C/E junction:
 
https://www.flickr.com/photos/128859641@N02/24020257733/in/photostream/
 
And the B/E junction of the main chopper transistor:
 
https://www.flickr.com/photos/128859641@N02/24538689342/in/photostream/
 
Not surprising it doesn't work properly with control voltages like that!!!
JC <Chipbee40_SpamNo@yahoo.com>: Feb 21 09:46AM -0500

On 2/21/2016 8:36 AM, Cursitor Doom wrote:
> I've faced with this repair.
> The EHT multiplier has been totally disconnected all through my tests
> except where explicitly stated otherwise.
 
Hi, Its been some time since I worked on these but I'm pretty sure we
ran these with the board out, turned round so you can get the connectors
on and I guess without the HT connected. Alternately put a suitable load
on the PSU.
 
> I like your thinking! But no, the one nearest the bridge is a BY208-1000
> alright, the other one to the side of it is a BY134. They both tested
> fine out of circuit.
 
That might be one problem, the sine voltage around T1801 is 800v, your
BY134 is a 600V diode. Also HV diodes can go reverse leaky, try a high
ohmsmeter on it (10-20 meg range). Shouldn't be any reverse leakage.
 
I guess you saw my next post on this? Try a load on the board before you
do any more work. It will tell you if the PSU runs silent or not under
load. The one I tried was screaming like heck then silent with a load.
JC <Chipbee40_SpamNo@yahoo.com>: Feb 21 09:52AM -0500

On 2/21/2016 9:34 AM, Cursitor Doom wrote:
 
> And the B/E junction of the main chopper transistor:
 
> https://www.flickr.com/photos/128859641@N02/24538689342/in/photostream/
 
> Not surprising it doesn't work properly with control voltages like that!!!
 
Pulling V1812 lets you see the clean output from the driver chip without
all the crap feedback from the transformers. Your frequency looks good.
 
I got the same crap and ringing/distortion on my PSU without a load. Put
a load on it. switchers don't work off load.
Cursitor Doom <curd@notformail.com>: Feb 21 03:24PM

On Sun, 21 Feb 2016 09:46:52 -0500, JC wrote:
 
> That might be one problem, the sine voltage around T1801 is 800v, your
> BY134 is a 600V diode. Also HV diodes can go reverse leaky, try a high
> ohmsmeter on it (10-20 meg range). Shouldn't be any reverse leakage.
 
Will a DVM suffice or should I do this with my faithful old analogue AVO?
 
> I guess you saw my next post on this? Try a load on the board before you
> do any more work. It will tell you if the PSU runs silent or not under
> load. The one I tried was screaming like heck then silent with a load.
 
You're a late-comer to this party, so you will be unaware that even when
tested under full working conditions with all the loads plugged in, this
twitcher/switcher still hisses and the 20 Ohm power resistor R1814 (just
below right from the chopper transistor on the schematic) quickly starts
to burn up.
 
I take your point on the dummy load, though. I must rig one up before
doing any more live testing.
legg <legg@nospam.magma.ca>: Feb 21 11:12AM -0500

On Sun, 21 Feb 2016 13:36:49 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:
 
<snip>
 
>I like your thinking! But no, the one nearest the bridge is a BY208-1000
>alright, the other one to the side of it is a BY134. They both tested
>fine out of circuit.
 
The BY134 is a lower frequency part with 2uS recovery time and is
probably unsuited to replacement of BY208-1000 in any of the snubber
or conversion positions indicated on the schematic primary. It should
be soft recovery, medium speed (200-600nS) avalanche-rated part with a
minimum 800Vprv.
 
I'd avoid the use of anything advertised as 'ultrafast' (ie UF4007),
as this circuit may need a modest recovery time in order to reduce
power loss and EMI, but they could be used temporarily in
troubleshooting.
 
RL
Wayne Chirnside <mark@faux.com>: Feb 21 12:13AM

On Fri, 19 Feb 2016 20:38:18 +0100, Sjouke Burry wrote:
 
> old ones 0 to ~100 milliamps.
> Very useful when you compare old- with new battery. Dont try this on big
> ones!!!!!
 
Or just buy a really cheap voltmeter from Harbor Freight.
Mine cost $6.00 and HAS a loaded battery test function with the milliamp
reading to be expected for a good battery right on the case.
 
I like them as cheap disposable meters but never use them on high voltage
other than the simplest tests and then with great caution as they
obviously aren't built with much or any real safety features.
"Ralph Mowery" <rmowery28146@earthlink.net>: Feb 21 10:18AM -0500

"Steve" <loft@centurylink.net> wrote in message
news:87egc65xql.fsf@centurylink.net...
> the
> < iron.
 
> That just sounds like another excuse to the use the butane torch :)
 
Most of the rg-6 uses aluminum instead of copper or tinned copper. The
normal methods of soldering will not work on it.
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