TEKTRONIX 2213 Power Supply Dead

[beanhauler]

This is an interesting one.
when i got the scope i put some juice on it and nothing.
Slid it apart and its fitted with the pre-regulator option board A18 rather than A19 current limit board and triacs.
Unscrewing the board i note that the spade connectors are are just dangling around and the Mosfet has exploded along side some other components.
hmmm time to find out why its gone to the wall.
This scope is very similar to the 2215 and the 2235 i recently repaired although there are some subtle differences in the mosfet mount on the A18 board
The 43V rail is jumpered across to the main board where the inverter circuit is mounted. The driver and associated control is slightly different to the 2235 , its simpler however does the same job,
First up is to try the 43V ruse connecting a power supply to the spade terminals that come from the main board and will connect to P803 and P804.
803 is plus 43v and 804 is the ground as far as the inverter is concerned.
Now if you are lucky to have two adjustable 30V supplies that are isolated you can create your 43V supply in series and feed to these test points.
Lets have a look what happens.
Strange looking trace with noise on for channel 1 and no vertical display when connected to its internal square wave.
No channel 2 either despite flicking buttons etc.
Check all voltages on the secondary side of T940. They are all correct...odd why no display on ch2 and no vertical on ch 1. Both channels faulty unlikely but possible.
Switch off the supply and dead check all the resistances on the output of T940.
-8.6 114 ohm @TP500
+8.6 95 OHM @W975
+5 330 OHM AT W985
+30 905 OHM @W968
+100 12.5k @W966
did a quick check looking back into the HV winding on T940...About 300ohm so looks good.

Well all the resistances seem ok except the 5V rail. It measured 5V on the voltmeter but the o/p said 30ohm....hmmm.
Its fed from a jolly old 7805 regulator which is good for an amp but probably upsets Mr Pre regulator. its fed from the +8.6 rail.
Cap short...nope lift one leg and still the same.
Follow the 5v rail round and it goes to three links. Lift them and see the effect.
Lifting W982 got me back to 270 ohms...not the 330 but closer ...so what does it feed?

I traced from the front of the attenuator using another scope and found that the O/P feeding the vertical pre-amp was completely missing on ch 2 and one leg of ch1 was also missing. Is this a switch error or an amp problem?

The 5V rail feeds a few TTL logic circuits that control the switching of channels
U317B chop circuit and U310 switching circuit.
Desoldered both chips and measured pins. sure enough 60ohms to ground looking back in not good
TTL is not very tolerant of over voltage but these are fed from the 7805.....has someone been playing i wonder?
Anyway ordered some new chips and added in the other two TTL chips in this part of the board just in case.
While we are waiting lets get back to the pre regulator and see if we can fix that up.
Now we really need an isolating transformer for this kiddy and a variac to test things gently and ensure things are in the right ball park.
To be continued.

This A18 unit is looking quite sad.
Q931 BROKEN SHORT EMITTER BASE. COLLECTOR TO EM
Q933 LEGS DISINTEGRATED
C 933 BURNT FROM BEING ALONSIDE R933 WHICH HAD MELTED
R911 GOT HOT. R934 CRACK IN IT
CR931 BUST.CR933 BROKEN SHORT.
To make matters worse the main track had partially fried so some rework there as well.

Q931 IS THE PNP DEVICE AND SUGGESTION IS TO REPLACE WITH 2N2907 .THIS SEEMS TO WORK WELL AS A REPLACEMENT.
So replaced all these items but Q933 is SHOWN as npt3n40 which some list as IRF710
OTHERS SUGGEST irfbc40ap. Anyway selected the 40AP and wound it slowly up on the variac. This is a 400V drain source device.
with no load the output settled at 43.3V which seemed ok but putting the scope ground on pin 4 of the TL494 and connected to one end of C933 gave a very odd waveform.
it had a strange sinusoidal ripple.
I let it settle at 140V even though the variac was not up to max as this is the uk and mains rectified is 300+.
Wondered if because no load. gave it about 75 ohms load and the mosfet slowly caught fire...something odd here.
Checked all the pins of the TL494 and missed a large short across a few pins and a large solder blob across a couple of tracks....must have been tired.
Cleaned that off and checked the damage to Q933.
Now the mosfet is held in a little plastic holder with a heat sink strapped to its back via an insulating block. The 40AP has a metal tab which is at drain potential and although i sprinkled it with heat sink compound it looks like it had conducted across the mica where the compound had bridged it. It had cooked the IRF as well. I didnt have another doh!
So plan B. Looked at a few old switched mode power supplies i had and saw that one of them had a FQPF6N80 Mosfet. Checked the spec and its an 800V device which is good plus can handle 6.6A which is another plus. Gate drain threshold a little higher but the TL in conjunction with Q931 throws out 15V of gate drive which is more than adequate. BUT THE BIGGEST bonus is it comes with an insulated tab...bingo just the job.
At this point i decided to replace TL494 just in case anyway. It may have had a stressful life driving that shorted Q931 pnp driver transistor anyway.
After this ,time to fire it up again and see what happens. Plugged my 75 Ohm resistor across the output[2 X 150 OHM 50W] and monitored current voltage and waveforms.
Of course this is all via the isolating transformer so dont try this at home without...or else you will have two broken scopes.
Waveforms are as below. Checked voltages at pin 12 which is 14v or thereabouts and pin 14 which is the reference for the opamp current and voltage amp.
My two ceramic 150 ohm resistors as a load were pushing 25W @43.3v which is getting on for 600mA and they were getting rather hot.
The FQPF6N80 though was taking things in its stride and given the low vds was barely warm to the touch. Seems like this part may be good to go.
Checked the on pulse width changed as i wound up the variac which proves the  current and voltage sensing are good.
look at the different waveform now. Slightly different. Note that the manual list this as a 175V pp but i reckon that s for 110V not 220V AC.
pin 5 of the TL is producing a nice sawtooth of 3V at 32.8Khz.
Finally plugged the unit direct to the mains to check full voltage applied causes no issue...all fine and dandy.
slight delay on switch on of about a fraction of a second but is probably due to R911 in series with 100u of C913 which gives a slight soft start.
The waveform ripple2 ,jpg is the ripple on the 43v line pulling 570ma of current.10x probe is showing under 100 mV or so which is about 0.2% but may come down to something a bit lower when it sees the inverter capacitance via C937 and a bit more load.

Lets get back to the main scope. i'm getting rather lazy now and safety conscious and like working with the 43V on the primary side even though you have to watch your fingers on the 2KV side of things.
So U310 and U315 replaced lets see what happens now.

Well that's fixed the channel switching side of things but aside from the main power supply dead i still have a problem with both channels of the scope not producing an o/p unless both gain controls are set to max and then you get a very small waveform if fed from the 500mV test waveform.
It also has an issue with intensity as it cant be varied.