low voltage, Blown transistor I think

[keithp]

I assembled my BeePre and completed the resistance check successfully. After inserting the tubes and turning on the power, all 8 diodes on the C4S boards glowed as expected. During the voltage test I must have touched something I shouldn't have touched because I saw a spark and heard a pop. The two diodes on the B side of the C4S board went out. I rechecked my resistance readings and everything was good except terminal 11 which showed 0 resistance instead of the expected 30 kOhms +. The voltage at terminal 11 was down to 128 V instead of the expected 146 V. Several other voltage readings were also out of spec. I believe I blew the PN22907A transistor because two of the leads were shorted. Do I need to replace the diodes when I replace the transistor? Does the blown transistor explain the low voltage?

[Paul Birkeland]

Yes, replace the transistors.  The diodes are likely OK, but it won't hurt to replace them as well.  Can you post all your voltages that are out of spec?

[keithp]

Thanks for your help. The other voltages that were lower than spec were D7 and D9, both were 122 V vs. the specified 147. Terminal 17 was higher than specified at 117 (90 to 110 specified).

[Paul Birkeland]

Is POS out about where it should be? 

[keithp]

I am having a really bad day. POS is 9.7. I remeasured some of the terminals. D9  had increased to over 180(?) While probing D7 I accidentally hit the ground lug and now the other two diodes on the left C4S are also out. When I get the replacement parts, I will check the rest of the C4S  board for collateral damage. I guess I should be prepared to replace all four diodes and two transistors. Is there anything else I need to check before ordering the replacement parts?

[Paul Birkeland]

Yes, that's a good plan.  One thing you can do to make probing easier is to wrap all the metal except the tip of the probe in tape.

-PB

[keithp]

That's a good idea. Thanks for the tip.

[jdm]

Keith,

Heat shrink works well on the probes too.

Jim