Hi John :
It is overall power handling issue for the PSU , U1275 is the controller for the shut down action, 5V light is NOT only output,? there is feedback to R1304 to controller too !?
On the other hand the total current sampling from T1235 to U1275.
I did check C1316, C1317 and C1318 are normal .?
My PSU was damaged by miss-align one pin of SENS input on LV Board ( P1483 ) :
pin 1 ------- Beam I Sens? input on LV board? NO Connection.
pin 2 ------- GND Sens input on LV board connected to P83 pin 1 -------Beam I sens. --------------- Beam I sens short to GND.
pin 3 ------- +5V Sens input on LV board?connected to P83 pin 2 ------- GND sens, -------------means +5V short to GND !
pin 4 ------- -50V Sens input on LV board?connected to P83 pin 3 -------- ? +5V sens, ------------- short -50V to +5V !
pin 5 ------- +50V Sens input on LV board?connected to P83 pin 4 --------? -50V sens? -------------- short +50V to -50V
pin 6 -------- -15V Sens input on LV board connected to P83 pin 5 -------- +50V sens ------------- short -15V to +50V
pin 7 ------- +15V Sens input on LV board connected to P83 pin 6 -------? -15V sens? ------------ short +15V to -15V
The results is all ( +5V, -50V, +50V, -50V, -15V, +5V and Beam I sens? short to GND ). may be serious damage for the PSU or main unit ?
Anyway it is under repair for the damage !
RegardTony CheungAPR 25 2018
From: JJ <jajustin@...>
To:
[email protected] Sent: Wednesday, April 25, 2018 5:26 AM
Subject: Re: [TekScopes] 7904 Mainframe damaged
Tony,
If all you can draw is 100ma on 5v lights, you could have a problem with
that power supply's pi section filter - I had to change C1318 to get my PSU
up. It measured 40uf instead of 100uf. That cap may be right on the edge of
failure at certain loads. There are also 2 other caps in that filter -
C1316, C1317. Note that 5v lights is not regulated on the LV regulator
board - it just goes through the board to the connectors - so the problem
is localized to the cap rectifier board.
I would first change C1318 with any 100uf cap (doesn't have to be tantalum
to just test). And, do the same sequentially for the other two - swap out
then test. They're pretty easy to change.
Best,
John
On Tue, Apr 24, 2018 at 2:35 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:
Hi John :
Yes, I did put it back to 7904 , but find something NOT working !
1. Graticule Light always on ----------- I did check NO -15V at R1451, but
PSU was tested with full loading pass .2. Front panel push button's light
not function.3. For Horizontal Mode Switch ALT , CHOP will shut down the
PSU. A or B is OK !
Regard
Tony Cheung
? ? ? From: JJ <jajustin@...>
? To: [email protected]
? Sent: Wednesday, April 25, 2018 1:21 AM
? Subject: Re: [TekScopes] 7904 Mainframe damaged
Tony, have you tried putting the PSU back into the mainframe to see if it
works in its normal load and interaction environment? You may be chasing
the proverbial wild goose! :)
Best,
John
On Tue, Apr 24, 2018 at 12:16 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:
Hi:
It is a good idea ! But I leave my workshop already ! May check day after
tomorrow. It is midnight in Hong Kong !
One thing I find at least, I disconnect-15 V loads, it become normal. So
that my next step to check the resistor!
Regard
Tony Cheung
從我的 iPhone 傳送
dadhills@... 於 2018年4月24日 下午11:40 寫道：
Have you checked R1286, 1K, 5%?? It is a carbon composition resistor
and
if it has drifted high in value, it will lower the "total" load current
limit threshold.
Dave
On Tue, Apr 24, 2018 at 07:59 am, Yiu On Tony C wrote:
Hi Harvey :
My PSU are test alone and connected to Loading resister and " sens "
return to
LV board.
For the current limiter was controlled by U1275 , and I find the zener
VR1297
---- 9V are opened , I replace it and now always shut down again ,
even
I open
the zener again , still shut down ! It means cannot return as before ,
I did
check all the diodes on A12 , but no defective !
There may be something wrong again !
RegardTony CheungAPR 24 2018
? ? ? From: Harvey White <madyn@...>
To: [email protected]
Sent: Tuesday, April 24, 2018 9:41 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged
On Tue, 24 Apr 2018 13:27:01 +0000 (UTC), you wrote:
Hi :
My 7904 still have problem for PSU !
+/-50V, +/-15V, +5V , +130V are normal with loading power on !
Once I connect the loading to +5V light , it cannot load even 0.1A !
it will
shut down PSU. (? If normal , It can output up to 3.5A ! )
The diode for +5V lights are defective ( shorted ) and replaced by
fast
recovery diode FR607? ( junction Cap 80pF )or HER208 ( junction cap
14pF ).
Sounds as if the supply is working normally for regulation, but the
shutdown circuitry is too sensitive.? There's a current sense resistor
in there, and some associated parts.? Could the current sense resistor
be too high in value?? That will certainly cause this supply to shut
down on a lower current.? I'd also check the amplifier for that sense,
to make sure it is working properly.
Harvey
Yiu On Tony C