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help tek 475 power supply

Roberto
 

hello everyone after changing the electrolytic capacitors on my teak 475 I have a problem that turned on after a few minutes it turns off I checked that the transistor Q1446 2n2222 was defective due to the fact that it looks like a 24v zener diode with a 180 series resistor hom that heats up and makes you break some ideas?
Roberto

Chuck Harris
 

Because you don't get a lot of room to route the heavy
traces needed in a power supply, with a two layer circuit
board, designers very often used cheats, like using the
fact that all of the negative lugs on twist lock electrolytic
capacitors are connected together, to make no cost
jumper wires used to make circuit board routing easier.

If you didn't make sure that all of the twist lugs on the
capacitors you replaced have wire jumpers to make those
connections, your supply will be very confused.

-Chuck Harris

Roberto wrote:

hello everyone after changing the electrolytic capacitors on my teak 475 I have a problem that turned on after a few minutes it turns off I checked that the transistor Q1446 2n2222 was defective due to the fact that it looks like a 24v zener diode with a 180 series resistor hom that heats up and makes you break some ideas?
Roberto



Roberto
 

Thank Chuck Harris
I checked the tracks where the negatives seem to be going well, the tensions would all be there and the only problem is the one exposed in the previous message
Roberto

Roger Evans
 

Can you explain what you mean when you say 'after a few minutes it turns off'? Do you mean that initially there is a trace on the CRT but after a few minutes it disappears? The 475 should show a light behind the V/div knob of the vertical channel that is selected - does this light stay on when the scope 'turns off'?

Can you measure the voltages on the low voltage DC supplies, +50V, +15V, +5V, -8V, -15V and also '+15 unregulated'. Do any of these these change between first turning the scope on and the 'turn off'? If the only problem is with the regulated +15V then we can trace the voltages through the regulator.

Measuring Q1446 in circuit may be misleading since there are many parallel components.

Regards,

Roger Evans

Roberto
 

hi Roger
Yes, the track is there that performs all its functions, but after a few minutes the CRT backlight turns off and the reason is missing the + 15v, the voltages are all up when 2n222 fails even the 24v zener I noticed that it heats up
thank
Roberto

Roger Evans
 

Roberto,

If everything works before Q1446 fails then the simplest explanation is that Q1448 has failed with its collector open circuit and Q1446 is passing all the current to the +15V output through the base emitter junction of Q1488. The regulator will still function to provide the correct +15V. Much more current will be drawn from the output of U1454A since there is less current gain in the regulator and U1454 will draw more supply current via VR1418 so VR1418 will also be hotter than normal.

Regards,

Roger

Roberto
 

hi Rogers thank for indications I will control the transistor
Roberto

Roberto
 

Hi all SJE924 NPN transistors with which I can replace it?
Thank
Roberto

Roger Evans
 

If you look at this thread;

https://groups.io/g/TekScopes/topic/7642352

you will find possible replacements but the ordering of the pins is different so you will need a little ingenuity in fitting it. Basically you cut some of the pins short so they don't connect to the existing PCB pads and then solder wire with a sufficient current capability between the short pins and the PCB pads.

Regards,

Roger

Roberto
 

hi Roger thanks, info I have resumed the repair in these days after a pause, however measuring the transistor outside the pcb would seem good at least that it is not defective and an attempt to replace it
Roberto