Tektronix 2230

[Sarojman]

Trying to repair 2230 tektronix oscilloscope. When I checked it I found two rectifier diode shorted. Cr907 shorted, u930 blown, q908 shorted, cr908 shorted and of course q9070.
Replaced all the component and power on the equipment in this case it took away fuse, two rectifier diode and q9070.
I Google regarding the issue, most of the site suggest to change the old capacitor in the high voltage and low voltage side of the power supply.
So I changed all the electrolytic capacitor and c907.
Those capacitor are not tectronix original.
Changed the blown parts again.
Power on the oscilloscope it again took away rectifier diode q9070 and cr 907.
People are also suggesting to remove molex connector on q9070 and directly conennect wire with the transistor pin.
How important to do this. Will connecting wire directly to the pin of q9070 help a lot in my case.?
I wanted to change all the capacitor in the line filter module but could not because it is hard to find in my area.
Right now the scope is in following cndition
1changed all the capacitor except C900, C902, c903, C904, C922, C919, c908 andC919.
2 I have not connected the molex connector. Take
3 in this case I can safely power on the scope.
4 I measured the voltage in the preregulator it seems OK.
5 I measured the voltage at base of q908 and I get pulsating voltage which drag me to conclude that u930 is oscillating.
6 measured the resistance at ground (tp950) and tp950 which is around 8ohms.
7i am very much reluctant to connect molex connector to q9070 thinking it will take again those transistor and diode.
I would appreciate if some one guide me on this issue.

 

[Martaine2005]

Use a current limiter bulb. It will save you keep blowing components. There is obviously a dead short somewhere.

Martin

 

[Sarojman]

Thanks for the response
What shall be the wattage of the bulb?
Thanks

 

[Martaine2005]

40W, 60W or 100W incandescent.


Martin

 

[bertus]

Hallo,

Do you have the service manual?
http://www.jabse.com/L1Rla3Ryb25peC8yMjMw
It might give you some support in the search.

Here is the powersupply schematic from the manual:



Bertus

 

[Sarojman]

You mean to put a bulb in series with ac line?

 

[Martaine2005]

Yes, exactly. It will light up full brightness due to the short circuit without blowing components.

Martin

 

[Sarojman]

Thanks for the schematic. I have
also down loaded the service manual which has lots of information including schematic.
Today I will put on the scope with limiter bulb. Let's see what happens.

 

[Sarojman]

Hi Martin
Put on the scope with limiter bulb.
Did not take away the diode and FET.
The bulb is flashing.when the bulb lights it is quite bright.
I will measure the voltages at major point as described in the schematic and report here.
I forgot to tell you that while I changed the u930 I changed it with tl494. I could not find the one in the manual.

 

[Sarojman]

Measured the voltages at different points they are as follows:
1 voltage at output of main rectifier varies from around 100v to 285v.
2 voltage at tp 940 varies from 0v to 2.5v which should be around 43v according to manual.
3 voltage at base of q928 with respect to Pin 7 of u930 varies from 3.5v to 4.9v which should be around 6.8v according to the manual.
4 voltage across C925 varies from 11v to 20v.
Thanks once again at least I could measure these voltages with your help.

 

[Sarojman]

Hi Bertus
Do you have any idea with the measurement I have done.

 

[bertus]

Hello,

When the voltage is jumping, it looks like the circuit is in protection mode for over current.
When overcurrent is detected Q935 will act and try to lower the voltage.
There must be an overload somewhere.
It can be defective transistor, capacitors or chips.
I have seen shorts caused by tantalium capacitors.

Bertus

 

[Sarojman]

Hi Bertus
Thanks for the response
I disconnected R934 to disable the protection. The situation is same. The limiter bulb flashes around 60 times per minute. And there is a ticking sound synchronized with flashing of limiter bulb.
I even took out Vr 935 and q935 from the pcb board and checked.
I checked q935 applying 12 volt to it, just a finger touch at the gate caused conduction of that scr. Tektronix categorize this scr as sensitive gate device. Is it the normal behavior.
While checking Vr935 I applied 65volt in series with 10k resistor and measure the voltage across zener, I got around 59v.

I have disconnected all the low voltage side jumpers that supply 5 and 8 voltage to the different circuits.
Please guide me.
Thanks

 

[Sarojman]

According to the manual
Q935 conducts in tow situation:
1 when ever the voltage at tp 940 increases beyond 51v
2 when ever the emitter current of Q946 and Q947 increases which in turn causes sufficient voltage drop across R949 to trigger the gate of Q935 through CR948.

 

[Sarojman]

Spelling mistake it's two not tow

 

[Sarojman]

Hi Bertus
When I measured the resistance between tp950 and tp940, the multimeter reads around 9 ohms. Is it a normal value?
I checked each and every transistor in the power board and seems OK.