30 Aug
2015
30 Aug
'15
4:36 p.m.
Hi Tony,
I have checked the horizontal signal from the MDA using an
oscilloscope. It is pin 8 like you mentioned. I get a regular 18KHz or
so signal out of that. Pin 9 gives the vertical signal at roughly
50Hz.
Then I traced that on the board and it seems ok (as in I can see the
18Khz signal on other components). One thing that I noticed is that
Pin 6 of the CRT neck comes pretty much from one of the pins of the
flyback transformer (via some diodes and resistors). So I checked the
voltage on the pins of the flyback transformer itself, so here is how
it looks
1 and 8 - Not connected
2 and 4 (I think those are inputs) - 15V
3 - GND
5 - 15V (this one goes to pin 6 of the video board where you mentioned
it should be hundreds of volts)
6 and 7 - about 200mV.
Does that look correct to you?
Regards,
Vlad.
On 30 August 2015 at 16:29, tony duell <ard at p850ug1.demon.co.uk> wrote:
OK.... Given the CRT voltages you give later, I would expect that.
Well, all the electrode votlages are very low.
They all come from the flyback transformer in the horizontal output stage. Now the 5151
is
unusual in that it doesn't have a horizontal oscillator. The horizontal signal from
the MDA
card goes straight to the horizontal driver which, IIRC, is transformer coupled to the
horizontal output transistor.
So, start by checking you have a horizontal signal from the MDA card. I think it's
pin 8 of
the DE9, if not it's pin 9, the other being the vertical sync signal. If that's
present and correct
check through the driver and output stages, there are not that many components.
If it is transformer coupled, then I have had those driver transformers go open-circuit
in
monitors -- not in the 5151 yet, but it could happen.
-tony
Regards,
Vlad.
>
> -tony
Do you
mean no picture or no raster? To me the 'picture' is the video
modulation (that makes different bits of the screen light or dark). If you
turn up the brightness and contrast controls do you get any illumination on
the screen?
The screen is entirely black no matter what I do with brightness and
contrast controls. The CRT
pins (from memory) are as follows (All voltages guessed wrt ground):
1: Control grid (10's of V, +ve or -ve)
225mV
2: Cathode (10's of V +ve)
170mV
3,4 : heater. One is ground, expect 12V or so on
the other
3 : 12V
4: Ground
5: Control grid (see pin 1)
225mV
6,7 : I call them anodes, you call them grids
:-). Expect a few hunded volts on each pin.
6: 7.33V
7: 10.29V
What voltages do you measure?
I checked all those with respect to ground ( pin 4).