Tektronix 2246A power supply repair and safe bench testing


Jon Harrison-Hughes
 

Hi all,

I've been working through my collection of broken test equipment and have got to a 2246A. On initial power on I found it to be completely dead; nothing from the CRT, no HV crackle and nothing on the front panel only a whistle and a feint chirp from the power supply.

I've checked the common failures and found Q2209 to be shorted C-E. I've replaced the original Motorola 476-03 fitted for Q2209 and Q2210 with TIP41C devices using thermal grease to the heatsink as this wasn't originally used (it's an early serial number). The service manual states that these devices are selected and a description of the operation of the power supply states "one transistor should have a slightly higher gain than the other and its collector voltage will decrease more than the other". Having purchased ten TIP41C devices from the same batch I've selected two by gain. I've also replaced the surrounding 5R1 and 2R7 resistors with 0.5W parts.

I've replaced all of the 1N4936 (ZS) diodes with STTH2R06 and all of the electrolytic capacitors as the orignals were at least 30 years old. I've also replaced the input Y filter caps (the infamous Rifa 271) which hadn't cracked and exploded yet but they will and the X2 caps (for peace of mind) so that I know they are all new. I've also changed the 0.1R sense resistor, again for peace of mind as much as anything else, as once I've finished I don't plan on opening this 'scope again.

Now it's time to power it up and test it again. With the A18 pcb out of the 'scope I'd like to power it on the bench before refitting it. Isolation transformer is a given but how do I power it up safely with U2230 producing 13.7kV and -2.7kV ? Do I just remove link W2201 and check the voltages and waveforms on the primary side and then hope for the best on the secondary side when it's back in the 'scope ? Am I worrying unnecessarily or should I try and disconnect pin 23 of the transformer from U2230 first before powering it up ?

Thanks,

Jon


Jon Harrison-Hughes
 

So having repaired the power supply I've powered the 'scope up and tested the LV outputs on J1204, supplies are as follows

+5.24VDC
+5.24VDC
-5.27VDC
-15.96VDC
+14.81VDC
+7.41VDC
-7.50VDC
+58.5VDC
+131.9VDC
+159.8VAC

I had to adjust the trimmer to get the -7.5V within spec which in turn increased the +5V nearer the limit of +5.3V but the supplies are all within spec. I'm going to leave it set here whilst I investigate and repair other issues with this 'scope.

Jon