[Johnson] Thunderbolt problem

Mahlon Haunschild mahlonhaunschild at cox.net
Sat Jun 21 22:59:07 EDT 2008


OK, John, let's get some more information here.  Is it really an early 
Thunderbolt (8 uF oil-filled capacitor in the HV circuit, 4-400As in the 
PA) or is it a late Thunderbolt (stack of electrolytics in the HV 
circuit, PL-175s in the PA)?

Was your friend testing it without anything connected to the bias socket 
in the back (a SPDT switch of some sort is mandatory on an early 
Thunderbolt whenever HV is applied)?

Did he check all of the VR tubes for correct voltage before he started 
testing?

regards,

Mahlon - K4OQ

> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 20 Jun 2008 09:56:00 -0600
> From: "John Lyles" <jtml at losalamos.com>
> Subject: [Johnson] Thunderbolt problem
> To: johnson at mailman.qth.net
> Message-ID: <96856609b277f730ca1689c6ae3b8883 at losalamos.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> A friend just got a Thunderbolt and was testing it, and hearing HV breakdowns. Unfortunately, he ignored them until the big one got him. It appears that the multimeter got changed, in that the meter movement is no longer a mA but needs like 10 mA to get full scale. So the calibration is off by a large amount. He said that the sparks would occur only when he switched on the HV power. This sounded like a slight overshoot on the HV, and a marginal component or insulation somewhere. Anyway, after finding the breakdown, he will need a new meter or a repair of his. I suspected that the high peak current (of the 8 uF charged capacitor) was dumping through the plate current meter (which is a high current movement) as well as the HV meter/grid meter to get to ground in the HV position. Any advice here? Anyone got a parts unit that they'd be willing to sell the meter out of? 
> 
> 73
> John 
> K5PRO


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