[Johnson] Thunderbolt repair

[John Lyles]

Mahlon
It is an old one with the 8 uF cap. BTW, this cap is marked 2 kV WV, yet the power supply runs about 2300 volts key up. Anyone else note this with Thunderbolts? 

He was testing with the bias socket connected to a relay which ran it either to operate or standby bias, not open circuited. Apparently he heard the sparks during switch on of HV. This past weekend, I convinced him to hi-pot test the components for the PS to the RF output. All the caps, (blocking, and bypasses) held off their rated 5 kV, but he said that the four mica padders across the switch for the output loading all ran warm when hipotting, and drew some leakage current, so he may replace them (even though there is no DC here, only the RF voltage across 50 ohms). 

Nothing indicated breakdown, no burn marks. I am wondering if this amp is capable of parasites, I see it has parasitic suppression all around the connections to the 4-400s, grid, plate, cathode. 

I don't know if the VR tubes were checked first, but he said the screen voltage and bias were close. The bias was 90 instead of 75 volts as the VR tube was changed. 
Thanks, know where we can find a meter? 

John 

> Message: 2
> Date: Sat, 21 Jun 2008 21:59:07 -0500
> From: Mahlon Haunschild <mahlonhaunschild at cox.net>
> Subject: Re: [Johnson] Thunderbolt problem
> To: johnson at mailman.qth.net
>
> 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
>