J
Jens Petersohn
Guest
Hi,
I have a Macintosh Color Classic analog board that I am trying
to repair. The symptoms are that the screen brightness will
periodically flicker between a darker and a brighter setting.
The size of the raster is not affected by this flickering.
Here is an ASCII diagram of the G2/G3 circuit:
SCREEN to CRT PCB FOCUS to CRT PCB
| |
| 2M | 2M
--/\/\/\/\-----------------/\/\/\/\--
| | |
|----||-----------| |
| 103z 2KV | |
- | -
- | -
-------|<-----
P02-20? |
|
----------------PIN on LOPT
|
C on HOT (Motorola 16206)
If I measure the voltage on the center tap of the screen pot
it fluctuates with the screen brightness flicker. Unfortunately
I can't measure the hot side of the pot since my DMM ends at
600V. The collector of the HOT is also tied to some other logic
(horizontal deflection, I assume?). The base drive of the HOT
comes through a small isolation transformer.
I don't have a pinout of the LOPT (It's label 157-0146 MTI-LTC-002).
I found two pins on the LOPT that have low resistance (< 1 Ohm) to
the collector of the HOT. Note that the FOCUS/SCREEN are completely
external to the LOPT.
The other problem is that I can't identify the diode P02-20. Has
anybody ever heard of this thing? It's in position DL6 on the board.
So there are several possibilities:
1) There is an intermittent fault in the SCREEN circuit on the
tube side of the center tap of the pot. If I could measure the
primary side of that pot I could probably confirm/deny this since
the pot in it's normal position is around 1M from the primary (hot)
side. This should provide enough current isolation so the primary
(hot) side doesn't change.
2) The diode at DL6 is whacky. The fact that the HOT doesn't appear
affected by the screen flicker (no other symptoms, no change in
raster size) would at least isolate the fault to the downstream side
of the diode toward the tube. Is this an accurate analysis or am
I off in the weeds?
3) LOPT/HOT is broken or something near those (lots of caps,
resistors, etc.). Since the raster isn't affected, is this likely?
4) B+ is screwed. Seems unlikely for the same reasons as 3).
If someone could supply some insight I would immensely appreciate
it.
Jens Petersohn
P.S. I would assume that FOCUS has to go through some sort of
voltage multiplier on the tube PCB. Unless this tube is really
odd (it's a miniture (10") Sony Trinitron). The tube PCB component
side is encased in a metal box that soldered to the board in
about 15 places. I haven't removed it yet.
I have a Macintosh Color Classic analog board that I am trying
to repair. The symptoms are that the screen brightness will
periodically flicker between a darker and a brighter setting.
The size of the raster is not affected by this flickering.
Here is an ASCII diagram of the G2/G3 circuit:
SCREEN to CRT PCB FOCUS to CRT PCB
| |
| 2M | 2M
--/\/\/\/\-----------------/\/\/\/\--
| | |
|----||-----------| |
| 103z 2KV | |
- | -
- | -
-------|<-----
P02-20? |
|
----------------PIN on LOPT
|
C on HOT (Motorola 16206)
If I measure the voltage on the center tap of the screen pot
it fluctuates with the screen brightness flicker. Unfortunately
I can't measure the hot side of the pot since my DMM ends at
600V. The collector of the HOT is also tied to some other logic
(horizontal deflection, I assume?). The base drive of the HOT
comes through a small isolation transformer.
I don't have a pinout of the LOPT (It's label 157-0146 MTI-LTC-002).
I found two pins on the LOPT that have low resistance (< 1 Ohm) to
the collector of the HOT. Note that the FOCUS/SCREEN are completely
external to the LOPT.
The other problem is that I can't identify the diode P02-20. Has
anybody ever heard of this thing? It's in position DL6 on the board.
So there are several possibilities:
1) There is an intermittent fault in the SCREEN circuit on the
tube side of the center tap of the pot. If I could measure the
primary side of that pot I could probably confirm/deny this since
the pot in it's normal position is around 1M from the primary (hot)
side. This should provide enough current isolation so the primary
(hot) side doesn't change.
2) The diode at DL6 is whacky. The fact that the HOT doesn't appear
affected by the screen flicker (no other symptoms, no change in
raster size) would at least isolate the fault to the downstream side
of the diode toward the tube. Is this an accurate analysis or am
I off in the weeds?
3) LOPT/HOT is broken or something near those (lots of caps,
resistors, etc.). Since the raster isn't affected, is this likely?
4) B+ is screwed. Seems unlikely for the same reasons as 3).
If someone could supply some insight I would immensely appreciate
it.
Jens Petersohn
P.S. I would assume that FOCUS has to go through some sort of
voltage multiplier on the tube PCB. Unless this tube is really
odd (it's a miniture (10") Sony Trinitron). The tube PCB component
side is encased in a metal box that soldered to the board in
about 15 places. I haven't removed it yet.