Hp 6227B power supply smoking

[Applemac]

I plug in my Hp 6227b today and noticed it was smoking. After switching it off and taking a look noticed burn marks on R8 on Master crowbar board. Was wondering if amyone had any ideas? Nothing was attached to the outputs as I was doing the manual startup checks.

So i removed both the master and slave board snd switched on again, no smoke this time. As the R8 on the masyercrowboard was orginañly smoking this leads me to believe that something is wrong on the master boad causing the smoking of the r8 resistor on the master crowboard.

[Specmaster]

I plug in my Hp 6227b today and noticed it was smoking. After switching it off and taking a look noticed burn marks on R8 on Master crowbar board. Was wondering if amyone had any ideas? Nothing was attached to the outputs as I was doing the manual startup checks.

So i removed both the master and slave board snd switched on again, no smoke this time. As the R8 on the masyercrowboard was orginañly smoking this leads me to believe that something is wrong on the master boad causing the smoking of the r8 resistor on the master crowboard.

Sounds like the circuit is seeing over voltage so the crowbar is trying to clamp it down. I'd check the circuitry prior to the crowbar.

[Applemac]

Hi Ive checked the 20 volt  regulated rail that goes to the crowbar. But testing test point 33 is less  than 1 volt when it should read +20 volts. Made this test without the master or slave boards plugged in. Still need help diagnosing the problem, this is my first repair so any help is well appreciated.

[MaxFrister]

This is not really a beginners repair.  You might want to put it aside and work on simpler things for a while.  Here is what I suggest

#1 Read about power supplies and crowbars (Art of Electronics 2nd ed).
#2 Figure out how to test caps, transistors, and triacs
#3 Make sure you understand the safety risks of working on high-voltage electronics.
#4 Make a simple linear power supply including a crowbar
# 5 and then try to find someone local to help.

Perhaps more helpfully, the main filter caps are probably bad.  I always begin these kinds of repairs by checking the (internal) power supplies.

The crowbar should not have fired without a load on the machine.  That probably means that the output was overvoltage.  That implies something upstream has failed, maybe a pass transistor. 

Be aware that once the crowbar fires, it can take other things with it (check the pre-regulator triac if it has one).

[Applemac]

Thanks for the info, let me restate things im not a beginner to electronics its just my first But I will heed your advice.

[bd139]

Check VR2 isn’t short. Had a shorted Zener in an HP supply a couple of weeks back. Also the filter caps as already mentioned. These are definitely EOL in these supplies.

[Applemac]

Thanks for the info, let me restate things im not a beginner to electronics its just my first repair, I will heed your advice.