This set is chassis FL1.6, last summer a bad solder joint blew the PS
switching transistor, I repaired solder joint and replaced transistor 7216
(ON4827) by a BUT12AF. All fine until winter came, the set started to have
difficulties to start-up in a cold morning. It's been sitting since then and
now I'm trying to repair it. Using a hair drier to heat up the supply
section would fix the set and work fine until turned off and let it cool
down. When doesn't start, indicator 'on' comes, and after one minute
indicators 'mute', 'stereo' and 'st-by' flash (error 99, protection). No
power at all out of the supply, not any noise, nothing at all, just these
indicators on front panel.
B+ is 0V it should be 141V. All other outputs dead.

Okay, it took hours of work, but finally found the temperature sensitive
component is the switching transistor BUT12AF.
If I warm up it, then starts normally and works fine all the time. Any
mechanical stress has no effect and solder joints are fine. Could it be the
start-up base drive is too weak and so has difficulties to switch the
transistor when is cold?. Could it be the characteristics of original
transistor ON4827 were slightly different than the BUT12AF. Before I replace
that transistor again I would like to know if anyone has some experience
like that. Does some circuit need some tweaking after that replacement
transistor?
I'm really confused with this one, the BUT12AF transistor checks fine with a
diode tester and is almost new!! I tried disabling the MAIN SOPS and EHT
protection according to service manual with no difference.

Thank you and Merry Christmas.

If you are using the original type transistor, also check for high ESR
capacitors. You will need an ESR meter to find these, or change all the
caps to find it. The caps can also become a bit temperature sensitive.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"Jeroni Paul" wrote in message
...
This set is chassis FL1.6, last summer a bad solder joint blew the PS
switching transistor, I repaired solder joint and replaced transistor 7216
(ON4827) by a BUT12AF. All fine until winter came, the set started to have
difficulties to start-up in a cold morning. It's been sitting since then and
now I'm trying to repair it. Using a hair drier to heat up the supply
section would fix the set and work fine until turned off and let it cool
down. When doesn't start, indicator 'on' comes, and after one minute
indicators 'mute', 'stereo' and 'st-by' flash (error 99, protection). No
power at all out of the supply, not any noise, nothing at all, just these
indicators on front panel.
B+ is 0V it should be 141V. All other outputs dead.

Okay, it took hours of work, but finally found the temperature sensitive
component is the switching transistor BUT12AF.
If I warm up it, then starts normally and works fine all the time. Any
mechanical stress has no effect and solder joints are fine. Could it be the
start-up base drive is too weak and so has difficulties to switch the
transistor when is cold?. Could it be the characteristics of original
transistor ON4827 were slightly different than the BUT12AF. Before I replace
that transistor again I would like to know if anyone has some experience
like that. Does some circuit need some tweaking after that replacement
transistor?
I'm really confused with this one, the BUT12AF transistor checks fine with a
diode tester and is almost new!! I tried disabling the MAIN SOPS and EHT
protection according to service manual with no difference.

Thank you and Merry Christmas.

"Jeroni Paul" a écrit/schreef/wrote le/op/on 23/12/2003 :
This set is chassis FL1.6, last summer a bad solder joint blew the PS
switching transistor, I repaired solder joint and replaced transistor 7216
(ON4827) by a BUT12AF.

Sorry, Paul, but this is "asking" for problems.
Use the original one, ON4827, order nr Philips 4822 130 63239
The BUT12AF is not heavy enough.

Like Jerry wrote, check the caps in the vicinity of the LOT and also
the solderings of the Hor driver tranformer and the E/W small
transformer. But replace that BU :-)

Merry x-mass

--
Alain Beguin
mailto: http://cerbermail.com/?XiEiqwImaa
Newsreader: MesNews http://mesnews.no-ip.com

Jerry G. expuso:
If you are using the original type transistor, also check for high ESR
capacitors. You will need an ESR meter to find these, or change all
the caps to find it. The caps can also become a bit temperature
sensitive.

Thank you Jerry, that's the first thing I checked but in this set there are
no electrolytics in the supply mains side. I checked the secondary side caps
they are fine.
The BUT12AF is not the original one but is a suggested replacement. It seems
like this is the problem but the original ON4827 might be hard to find
around here...

Alain Beguin expuso:
Sorry, Paul, but this is "asking" for problems.
Use the original one, ON4827, order nr Philips 4822 130 63239
The BUT12AF is not heavy enough.

Like Jerry wrote, check the caps in the vicinity of the LOT and also
the solderings of the Hor driver tranformer and the E/W small
transformer. But replace that BU :-)

Merry x-mass

Ok, thanks, I'll try to get the original one...

"Jeroni Paul" bravely wrote to "All" (23 Dec 03 18:18:26)
--- on the heady topic of " Philips 28ML8805 intermittent start-up"

JP From: "Jeroni Paul"

JP Jerry G. expuso:
If you are using the original type transistor, also check for high ESR
capacitors. You will need an ESR meter to find these, or change all
the caps to find it. The caps can also become a bit temperature
sensitive.

JP Thank you Jerry, that's the first thing I checked but in this set
JP there are no electrolytics in the supply mains side. I checked the
JP secondary side caps they are fine.
JP The BUT12AF is not the original one but is a suggested replacement. It
JP seems like this is the problem but the original ON4827 might be hard to
JP find around here...

A common failure in the AC startup side are metal film resistors that
open up. I think this is due to them being basically under rated to
handle powerline spikes. You might sometimes see a faint ray of soot
pointing out from under them but mostly they look just about normal.
The simplest way to find them is to shotgun with a dmm on the ohms
range and any unusually high reading is unmistakable.

.... No electrons were harmed in the posting of this message.