[sebhc] H89 Power-up problems.

Barry Watzman Watzman at neo.rr.com
Sun Jul 9 12:41:09 CDT 2006


Re: " My DMM has a 2K/(diode picture) in the resistance selection. At that
setting, for those two diodes, the resistance would start in the hundreds,
then slowly (~10 secs) work it's way up past 2K."

Wrong diagnosis (maybe).

When you see that (an initial low resistance that increases) you have a
capacitor in the circuit that you are charging with the battery in the
multimeter.  As it charges the voltage rises.  And the electrolytic
capacitors are polarized, so they won't necessarily behave the same if you
swap the leads.  The point being, in order to test the diodes in the bridge
rectifiers, you have to disconnect the leads going to the rectifier (all of
the leads).  Otherwise, you are not measuring the characteristics of the
diodes, but rather of the transformer windings and filter capacitors to
which the rectifier inputs and outputs are connected.


-----Original Message-----
From: sebhc-bounces at sebhc.org [mailto:sebhc-bounces at sebhc.org] On Behalf Of
Mark Garlanger
Sent: Sunday, July 09, 2006 1:10 PM
To: sebhc at sebhc.org
Subject: Re: [sebhc] H89 Power-up problems.

On 7/9/06, Lee Hart <leeahart at earthlink.net> wrote:
> Another possibility is that your bridge rectifier is bad, and that only
> 2 of the 4 diodes in it still work. This makes it a halfwave rectifier
> instead of fullwave, with a resulting drop in in voltage and current
> capabilities.
>

This appears to be the issue. They were a little hard to get the probes to,
but it appears that D103 and D104 are bad. My DMM has a 2K/(diode picture)
in the resistance selection. At that setting, for those two diodes,
the resistance
would start in the hundreds, then slowly (~10 secs) work it's way up past
2K.
The other diodes would almost always started greater than 2K, although D101
and
D102 would sometimes show 1.8K but quickly show over 2K.
I'll just replace all 4 of those diodes when I take it apart.

Thanks for everyone's help.

Mark
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