Tek 577 D1 Repair (B07xxxx)
1. 2020-Oct-24 :replaced a leaking electrolytic capacitor C124 on the A2 collector supply circuit board.
2. Oct-26 :The Collector supply disabled light turned on right at power on.
only one small spot could be seen on the screen.
Power consumption dropped from normal ~60W down to 51~52W.
Checked A3 power supply board:
U732 IC(7812,the -12V regulatpr IC on A3 board):
Input pin:+21.18Vdc ok! (should be +9.5V under normal conditions)
Common pin:+0.72 NG! (-12V)
Output pin:GND(0v) ok!
Replaced the U732(uA7812,1.5A) with a ST L7812CV(1.5A) on Oct-27 and fixed the issue.
I also replaced the U722 IC (+12V regulator IC,7812) and adjusted +30v/-30V to within +-0.2%(specs:+-0.5%).
3. Oct-29: A1 main board shorted
The traces disappeard suddenly after power on for about 30min,only an fine slant line about 2mm long could be seen.(it's a good news,much better than nothing on screen)
After inspections I found:
Power consumption jumped to ~90W(normal:~60W)
A3 power supply board:
U732 (7812,+12V regulator Ic) ok!
The resistance between +12v and GND: 0.5 ohm!
Some shorted part brought the +12V down to +0.6V and consumed about 30W!(it's about 2.5A if all come out of +12V rail)
After disconnecting P722 from A3 board the resistance between +12V and GND became about 2.3K ohm.this led me to A1 main board.
today I found the shorted part,it's C297 (15uf/20v,20% electrolytic capacitor).
I replaced the C297 with a panasonic 22uf 35V capacitor.
The Tek 577 D1 is in good working condition now except some noises(high frequency ripple) on the traces it comes and goes from time to time,
I will deal with it later.
4. (B108xxx,My 2nd 577 D1)
2020-Nov-2~8: Max 8 traces displayed on screen(9 lines included the baseline)
Checked the staircase signal:
Saw only 8 steps, 660mV /per step <-------NG (should be 10 steps,500mV/step)
Adjusted the step amplitude VR(A1 Board) and got the staircase of 10 steps (500mV/step).
5.(B108xxx,My 2nd 577 D1)
2020-Nov-9~15: The Collector supply disabled light turned on right at power on.
Vb=+1.3V <-------NG Vbe=+1.3-(-12)=13.3V>>0.7V
Fixed this issue by replacing the 2N2222 with a 2sc1384(50V,1A).
2020-Nov-15~30: Broken Trace
The last trace's broken into two traces:1st half of last trace and 2nd half of base trace.
The solution:Mod c265 1uf --->0.27uf on A1 Board,to improve discharging timing.
7. B072023 & B108637 (Mod:11 Steps)
2020-Dec-1~15: Retrace / Double Trace issue(even steps)
I prefer to eliminate the double trace and keep the 10th step,my solution is 11 steps(12 traces)
why 11 steps(12 tarces)?
a.it will eliminate double trace.
b.refresh rate/per trace:
11 traces(10 steps): 240/22 traces(11x2,double trace)=10.9 times/s
12 traces(11 steps): 240/12 traces(no double trace)=20 times/s
c.12 traces is 3 times of 4 (zero crosses):this will improve flickering.
The solution is simple and easy(eliminates double trace issue only at Fast step rate):
Put a 68K ohm resistor(>=1/4 W,5%) in parallel with R319(9.76K,A1 board)
new R319=68kx9.76k/(68k+9.76k)=8.535K ohm
New voltage of Pin 3 of U305B(adjustable by "Number of Steps"):
Min:30mV(1 step) to Max 4.75Vdc(10 steps)------->Min:26mV(1 step) to Max:5.28Vdv(11 steps)
Before and After:
By changing R319 we can get more max steps(max 12v/0.5v=24 steps).