My Take on X-BOSOZ

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#1,143
Thanks Brian,

I have tested the two PS boards. One tested correct at +79.9 and -20.9. The other one has me perplexed. As I remember the readings, they are something like -34 and +39. I attempted to visually compare the two boards component by component and they appear identical. So I had to remove the copper coloured zener on the + side to see if it was the correct value as I soldered both of them with the value down to the board - a lesson learned. It was the correct 75v zener. I initially thought that I may have reversed the two.

Any ideas?
 
#1,144
I think that I may have found the problem. In switching the tranformers between the ps boards I dislodged one of the blue common transformer wires from the Marr connection and it was loose.

What is the best way of discharging the capacitors so that I can remount the zener that I removed. I am hesitant about shorting the circuit with the forcepts that I use to hold the zener if the caps are not emptied.
 
#1,146
You might not want to wait long enough for a 10K R to drain a big cap... Unless you are permanately attaching it... And, at 10K you prbably don't need 2W, a 1/2 would probably not burn your fingers or itself... I would use more like a 100-200 ohm if I were just wanting to discharge them on the bench..
Granted, 10K wont' make sparks or burn your fingers but, I use a 25-100 ohm ussually without worry. A screwdriver or roach clips will blind you and melt stuff! Not to mention the anxciety it will cause...
 
#1,150
I only suggested the 2W 10K as you could just leave them there even during normal use without having to worry about them getting very hot.
Click to expand...
Thats the normal approach, and nothing wrong with that. There is nothing wrong having bleeder resistors installed permanently, I think. I just dont like the idea😉 Especially not when we are talking class A gear! The bleeder res. will dispose a little current all the time, that could be used by the circuit😉 For testing purposes, I allways have bleeder's installed🙂 A 2 watt 10K resistor could reside in a preamp with no negative effect, just so we are clear🙂

Steen😎
 
#1,151
Oh sure, by no means was I suggesting that the 10K 2W was wrong or anything. I just don't like waiting so long on the bench. I always have at least one bleeder somwhere too on a system that is close to finished... And, it's ussually time to hit the fridge or something when you shut down... So, a minute or two might not be so bad... In the begining though, I tend to make several quick changes to get going before something smokes...
Yes, those aligator things... Someone suggested Forceps I beleive... That's Just a term that has trouble leaving my brain from days gone by... Sorry, very politically incorrect of me... the important thing there is that shorting out big caps is risky bis... like a miniature lightning bolt. I have melted 1/2 of the end off a #2 screwdriver doing that on a power amp supply... And then there is the ever reminicant., "It wasent the arc, it was my hand hitting something when I pulled it away..."
 
#1,152
Thanks to all for your advice. I just purchased the bleeder resistors and have successfully discharged the caps. BTW, the forcepts that I was referring to were clamping needlenose forcepts to hold and stuff the zener down inside between the smaller and larger caps. I realized that if the plier ends touched the adjoining jumper leads that a short would occur and that the caps would need to be discharged before attempting that.

I reinstalled the 75V zener and tested the ps board again. But I am still getting incorrect values on one board, correct values on the other. The +ve side reads 24V and the -ve side begins at -38.9 on the faulty one. These values decrease slowly as I leave the tester probes connected. Visually the boards look identical. any ideas?
 
#1,154
I'm still trying to debug my ps boards. I used the following post as an assist:

http://www.diyaudio.com/forums/showthread.php?s=&postid=886427&highlight=#post886427

Metalman asks for some voltage readings and I have taken them but I was not able to add the loading resistors that Metalman specifies, so I just took the readings unloaded.

1) Before the bridge rectifier diodes:

Baord 2 (only) -- +32.4 and -13.0

2) After the bride diodes:

Board 1 -- +96.6 and -38.9
Board 2 -- +96.8 and -38.8

3) At the P/S output posts:

Board 1 -- +80.1 and -21.0
Board 2 -- +9.8 and -38.8

4) Across the zener stacks: (I measured from the zener to the GND Faston- I hope that this is correct)

Board 1 -- +84.8 and -24.4
Board 2 -- +92.x and -24.0

5) Across R3 and R5:

Board 1 -- R3 is -24.4 and R5 is 0.0
Board 2 -- R3 is -24.7 and R5 is 0

Can anyone interpret the values? TIA
 
#1,156
Yes, I would have said that too (of Coarse, after the fact). Bipolars do this too. I believe the actuall failure mode is "punch thru" or something like that. A tiny, not totally destructive hole as been "zapped" through the oxide of a FET... Measure the Gate to source and/or drain and you may see leakage... Just an idea from past experience... Sorry...
 
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