Tony, does the value of 5V lights fall below 4.5V when you increase the load to the point that the PSU shuts off? 5V lights is monitored by the Inverter Control IC U1275. A fraction of the 5V lights voltage is fed into the BAL Sense input of U1275. I would monitor TP1302 on the rectifier board and 5V lights. There is also a sense current from T1310 that enters that Bal sense node and can shut down the PSU. On page 3-58 of the manual, it discusses the Balance Sense input. Best, John On Thu, Apr 26, 2018 at 12:05 PM, Yiu On Tony C via Groups.Io < tonycheung_hk@...> wrote: Hi JJ:
TP1625 are normal 40V +/-3V ! ( it must be connected to maim unit , it is
related to CRT HV CKT !
If PSU alone , such voltage reading will more positive ( + 130V ) ).
On the other hand , I did measure the value of R1286 and 1287 , it is OK !
When PSU connected to main unit , I do not understand ,for CR1311 and 1312
, I use HSR208 ( 2A 1KV CJ=14pF ) will shut down PSU . but use FR607 ( 6A
1KV CJ= 80pF ) will not shut down the PSU ! both setting are Control
ILLUM must OFF, and GRAT ILLUM are always turn on ( not controllable ! ).
Still finding the problem !
RegardTony CheungAPR 27 2018
From: JJ <jajustin@...>
To: [email protected]
Sent: Wednesday, April 25, 2018 7:04 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged
Tony, did you do the Power Supply test on page 5-8 of the manual : Adjust
Inverter Control R1293 for a meter reading on TP1625 of +40V? Maybe your
operating point for the Inverter Controller is way off. Mine was way off in
the negative range until I adjusted it.
Best,
John
On Tue, Apr 24, 2018 at 10:52 PM, Yiu On Tony C via Groups.Io <
tonycheung_hk@...> wrote:
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
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