Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Redwire I have not replaced any capacitors yet.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

You put in new C924/C925?
R926 I think could only smoke if diode CR924/CR925 are shorted. If the cap C924/C925 arced/shorted, could have taken out the diode(s).

Tektronix 152-0242-00 is 1N486A Si low-leakage switching diode 0.2A 225PIV; superceded by 1N486B

Or there is a short on the PCB. Maybe also multimeter test Q924 and Q925.

I cut out the page we are talking about: TEK434 s/n below B500000 schematic page 173

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

C926 reads approx 62.1 kohm

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Did you check C926? If it's shorted that would be 115V through a 100ohm resistor......

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi gents, latest on Tek 434.
Received all components but been busy.
I turned scope back on to identify problem areas R926 smoked.
I unplugged P126 on Z Axis board this is a split plug I removed the left hand plug looking at schematic pins1/2/3. It's the 115v line.
Turned scope on panel lights on fat trace line and primary voltages present turned back off.
What would cause R926 to smoke?
By the way hope everybody fit and well.
Kindest regards, John.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Golf was not great but weather was lovely no t consistent enough but happy plenty of room for improvement just like electronics. Evening refreshments was even better ha ha slurp!!!

Mouser email order arrival 28th March.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Critical Question - How was golf?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi lads, just to let you know the capacitors are on back order so awaiting arrival.
Cheers John.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

Old HV disc caps are pretty huge compared to modern ones.
New Sprague/Vishay 564R60GAD10 is 1,000pF 6kV Z5U +/-20%
New Sprague/Vishay 564R60GAS10 is 0.01uF 6kV Z5U +/-20%

I find C924, C925 as 1,000pF 20% 5kV Tektronix P/N 283-0021-00; DHR17Y55102M5KV
I find C934, C990 as 0.01uF +80/-20% 5kV Tektronix P/N 283-0162-00; DHA42Y5S103Z5KV

The subs looks good to me.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Been trying to source HV caps nearest I can find are mouser.
C924, C925, C935, C 990 all 5Kv.
Old originals are approx 30mm x 22mm.
New replacements are much smaller.
Will these be ok.
Typical mouser number 564R60GAD10 higher KV ok.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Enjoy! I've only been able to hit a few balls into the net so far....

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Well guys turned on scope this morning and she started to smoke. Turned of waited for caps to drain off voltage removed Z Axis board could smell 1 of the 5kv caps so I will send to mouser for new caps.
Good news for you guys I am going for a few days golf so scope will be on hold for a week or so.
I would like to say thanks for all your help.
Be back soon.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

C1130 page 169 primary circuit A10 far left near waveform 9.
When scope shorted there was no loud bang just low crackle checked case no arcing found.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

If you're going to put a new zener in we don't need to build up a quick test rig. Let us know what happens....

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

It's not making sense to me, unless there's an open connection. If the kabang/arc was really loud it might have melted a connection.
You really need 51V across the zener and getting to Q1030 base then Q1032 for the PSU to start up.
Where is C1130 you are asking about. The PSU primary schematic all are C10xx

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Q1025 swapped same result. Will fit new 51v Zener tomorrow but I am sure this is ok.
What does c1130 one side is negative the other side is positive don't understand this how does it operate?
Took secondary board out everything seems good refitted turned on still shorted.
-15v to ground zero ohms. +15v to ground 172ohms.

I have a 240vac variac and a 30vdc 1amp power supply.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Sorry haven't been able to respond earlier, had to deal with a 15 hour power outage in the neighborhood and a lot of stupid neighbors with generators and suicide cords.....

The other possibility besides Q1025 being shorted out is if ZR1025 shorted. John you said that you tested it in circuit. Did you by chance check it out of circuit? If you want to test it out of circuit to make sure it's dropping the 51 volts, let us know what voltage your power supply is and we can tell you what size current limiting resistor to use.

If you wanted to be brave you could just try the scope without Q1025 in it. Just remember that this COULD cause a CATASTROPHIC failure.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

Q1025 Tektronix 151-0250-00, 2N5184 TO-39 package; subs to NTE-154
In this PSU section of the scope, the transistor is used as a switch and not critical- although the original is a HV part (300V video) it only sees ~60V. I think any NPN Si say 80V or better will work there, in a TO-92 package.

I suggest substitutes 2N5550, 2N5551, MPSA06, more possibilites here

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Can anyone suggest a suitable replacement for Q1025.
I will swop Q924 from Z Axis board same as Q1025 to see what happens.

[redwire]

Re: Replacement caps for Tektronix 434 Power supply board.

VR1020 (100V Zener) should have 65-100V across it, assuming the neon lamps are still blinking, post-putting the lid on short.

I'd say Q1025 is shorted C-E to stop VR1025 from getting 51V across it.
Any leakage current across neon lamp DS1024 could also turn on Q1025. I'm hoping the pcb is spotless (no flux etc.) because it is a sensitive node in the circuit. But Q1025 not looking happy.