[Johnson] Thunderbolt repair

[Mahlon Haunschild]

John,

My oil cap is the same rating.  I understand that oil caps can withstand 
more than their rated votage to some extent, but I don't know how much 
trust I'd put into that.  But if the arcing was audible, it wasn't the 
cap.  The mica capacitors used in the loading padder don't have DC on 
them, so that's not it either.  By the way, mine are somewhat lossy as well.

Check the plate blocking capacitors.  Better yet,just replace those 
cheesy TV doorknobs with some real Centralab transmitting capacitors.

The amp is most certainly capable of parasitics; if you look around, 
you'll notice many suppressors in the grid compartment as well.

It appears that someone swapped out the grid bias regulator tube; it's 
supposed to be a VR75 for an early T-bolt with 4-400As, per the book. 
The other VR tubes regulate the screen voltage.  Check 'em all.

Oh, one other thing:  connect a 270K 2W resistor across terminals 5 and 
6 of SW102.  Doing so will prevent the amp from having no bias if 
switched to "LINEAR" which will be most kind to your bias supply choke & 
transformer.  This change was made for all of the late T-bolts, apparently.

I'd try to find the source of the problem before looking for another 
genuine meter.  And make sure that you bypass them with some big honkin' 
  back-to-back rectifiers to (hopefully) keep this from happening again

Good luck.  Let us know how things go.

regards,

Mahlon - K4OQ

> ------------------------------
> 
> Message: 6
> Date: Mon, 23 Jun 2008 21:47:16 -0600
> From: "John Lyles" <jtml at losalamos.com>
> Subject: [Johnson] Thunderbolt repair
> To: johnson at mailman.qth.net
> Message-ID: <916cd2b71936d31e5f632f9b2920bbf0 at losalamos.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> 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
>>
> 
> 
> 
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> End of Johnson Digest, Vol 53, Issue 14
> ***************************************
>