avcom psa 35

[electro1]

It was powering on and not giving any vertical difflection now the screen is not lighting up. I see the crt neck is lite. What should I check I have a schematic and test equipment but I need help with the schematic.

[budm]

Re: avcom psa 35

It may have blown fuse in the high voltage (about 2000Vdc or more) section. But check the power supply voltages first.
Post the Sch will help.

[electro1]

Re: avcom psa 35

Here is the flyback Hv power supply schematic

[budm]

Re: avcom psa 35

Are you getting the -15Vdc output from that 7915 Voltage regulator?, if you do, the transistor that is driving the transformer is working, the switching frequency is done by the 555 timer setup as an oscillator for driving this power supply circuits. High voltage rectifier is integrated into the transformer, it is drawn as a single Diode but more likely to have more than one Diode in series to be able to handle thousand of volts. You will need DC high voltage probe to check that high voltage output since it may have open winding.

[electro1]

Re: avcom psa 35

I tested the 7915 no - 15 volts I put negative probe on chassy and tested the 3 pins on 7915 I only get 5 volts from the center pin can I replace this 7915 with a 7915cv I went back in my mind and two things i desodered there is a 1 uf 160 v cap and a 330uf 16 cap if that 1uf cap fails would this cause the -15 volts to not be present. 330 uf 16 v cap brand is sm the esr is .10 ohms and the cap reading is 269.8, the 1 uf is reading 1.16uf and 3.6 is this a good reading. Alot of the componets except the round electrolytic caps and resistors that have the color bands on them tubuler style ,(2) 3 pin transistors I need help with identification of these rectangle orange colored componets , red tubuler parts, the blue with the red and black dots part . thank you for info.

[budm]

Re: avcom psa 35

You cannot leave the cap 1uF160Vdc out of the circuits, it is part of the blanking circuits.
The 330uF16V is part of the DC filter circuits to give smooth power supply for the driver transformer and the high voltage transformer, this may explain why there is low/no output on the secondary side due to high ripple AC.
You can use PANASONIC FM/FC/FR series from Digikey. The scope this old should be recap anyway. Never leave parts out off the circuits, they are there for reason.

[electro1]

Re: avcom psa 35

got the screen working now the 7915 is not reading neg 15 v rear of transistor facing me top pin 0v , middlepin, 3.08 , 0.38 last pin my question is if there is a short should i still be getting -15v? there is a trace on the screen just no vertical difflection

[budm]

Re: avcom psa 35

The two rectangular orange components are Polyesterfilm caps. There should be some printing on them.
You should be able to read the voltage on the 7915 as shown in the picture attached using Ground pin as a ref point, see what voltage you are seeing at the input and output.
If have no vertical scan, then you have another problem some where else.
By the way, are you getting 12vdc on that 330uF cap (I see you have noted on the diagram that the supply voltage is 12Vdc). There also a lot of DC filter caps for that -15V power supply section.

[electro1]

Re: avcom psa 35

7915 is not giving any voltage meter probe on ground pin and red probe on other pin also 2.2v input pin output + 0.47v also there is a disc style capacitor on top of a diode the disc cap is red hot to the touch what is this 7915 in the circuit for? The 330 uf cap I will have to solder a wire to the plus side of the cap there is no room for the probe

[budm]

Re: avcom psa 35

That disk style cap is CERAMIC cap, when they go bad, it shorted out. you should take that out and check the resistance.
Can you show that cap in the picture and in the diagram?
Well, I just look at the picture of the board, it does not have any designators for the component so we can identify them with the diagram, the designer should be shot for not labeling the component.

[electro1]

Re: avcom psa 35

if i disconnect the 2 connectors on the sweep board the 7915 then puts out -15 v the center pin is +30.7v and the red disc cap is cool not hot to the touch and the 2 big 2 pin metal round transistors I noticed are not hot now can leave my finger on them. Should I still check to see if the 330uf cap + side is getting 12 volts?

[budm]

Re: avcom psa 35

Can you circle the picture of this cap. Yes make sure you do have 12V for driving this high voltage circuits.
The diagram does not show where this -15V goes to, but it looks like something is loading down the -15V, but still do not understand about the cap getting hot, I need to see what this cap is used for.

[electro1]

Re: avcom psa 35

in the picture the cap is right next to the flyback and cap is soldered to a diode you will also see the transistor thats the 7915. If the sweep board is removed the 7915 output -15 works input pin is 30.7 does that sound right and that red cap is cool to the touch im thinking there is a short on the sweep board do you agree red disc cap reads like this 75f does this site have a chatroom i'm sending you a pm for the server. 220k

If i replace the 7915 will the pinouts be the same im thinking of using a ka 7915 or a 7915cv

[budm]

Re: avcom psa 35

That cap is for Diode switching noise suppressor, the only thing that will cause that cap to get hot is that it has resistance leakage, so instead of the current flowing through the Diode to the load, the current is flowing through the cap, you should left one leg of the cap from the Diode and check the cap resistance, it should show infinite resistance. You also check the Diode itself to make sure it is open in both direction.
7915 TO-220 Case style all have the same pin out, the input voltage of -30Vdc does not not sound right, that means it will have 15V across the input and output terminals.
By the way, do you the rest of the diagrams of this scope?

[electro1]

Re: avcom psa 35

this is the sweep board schematic for a similar unit

[budm]

Re: avcom psa 35

By the way, that diode and the cap picture, is that the CR802 Diode and the 270pF capacitor?
I see that they use the -15v for -Vcc for the Op-Amp IC's.
Can you take the resistance reading on the output pin and gorund of the 7915 with all the board hooked up, I want to see if it is feeding into low resistance load.

[electro1]

Re: avcom psa 35

what resistance setting should I put the meter on is 20k good enough?

[budm]

Re: avcom psa 35

2K, then 200 will be good since we are looking for low resistance.
What did you find on that cap that gets hot?

[electro1]

Re: avcom psa 35

I disconnected one of the cap leads meter is displaying a one all the way on the left side of the digital meter I did test with the esr tester but it does 1-22,000 uf I hope I didnt damage the cap with the peak atlas meter reading on peak was open circuit or low capacitance on other tester I get 230-236 resistance 1.80-1.9 ohms 245-246pico farad 1.3 r it fluctuates
You had asked if that diode and the cap in the picture, is a CR802 Diode and the 270pF capacitor?
the schematic shows em together i would say yes nothing else on the board sodered together like that

[electro1]

Re: avcom psa 35

this cap is now completly removed to test im getting resistance reading reading 231.5 -849.9 ohm for esr and cap 231 using intelli tweezers 229.230.70 flutuates now the diode removed completly to test
1n4934 tester says its a diode or diode junction the forward voltage is 0.55v and test current is IF=4.75ma im soldering back in the diode the red disc capacitor is it bad?