Tektronix 2221 : Repair of Power-Supply-Issue

[Pak03841]

Hey Guys,

Im new to the forum. Im an electrician but my elektronik skills r kinda low.
I got a the Tektronix 2221 from a friend it wasnt working. After inspection, i plugged the powercord in. The Power-Unit makes a squeaking noise at a rate of 1 or 2 Hz. No function at all, even the status LED is dead. I would imagine that there is somekind of short that changes the cycle of the Switchmode? Maybe?
I measured some Voltages. Confusing part is that the Voltage after the Full-Bridge-Rectifier in the plan is 145V. My DMM reads like 300V DC. The Volatage over C925 is not like stable 14,3 V it ranges from 12 to 20V DC.
I attached the circuit diagramm of the powerunit and marked the Voltages.
I would be very grateful if u guys could give me some help or advice.

Thanks

[jdragoset]

Looks like the preregulator in the 2235.
Should produce 43 volts at 1 amp to the power supply inverter(Q 944, 946 & 947, T 944 & 948), which can be provided with an external DC power supply to TP 940.
If current is too high (as continuously tested across the 0.2 ohm resistor) it will shut down and restart repeatedly, chirp, chirp, chirp.
Over voltage will trip the SCR crowbar (Q 935), over current load with the same effect.
Be careful with this circuit, as there are no valid earth potential points so a scope ground cannot be connected unless the AC supply is provided through an isolation transformer.
Download the preregulator trouble shooting guide from the manual
https://w140.com/tekwiki/wiki/2221

[bdunham7]

The 145V is based on 120VAC line voltage.  The back of your scope should read "90-250VAC", and the 145V point will be higher, of course.

Do you have access to a DC PSU that could put out 40-45 volts at several amps?  That might help in the troubleshooting process.

[Pak03841]

TY for the Help and the Information. The failure is in the secondary side. When i disconnect q9070 the power unit is working. I went for the troubleshootguide. Q9070 is dead. Sadly i released the magic smoke out of q908. So I need to order some parts.