[TenTec] Centurion 422b troubleshooting help needed

[Gary J FollettDukes HiFi]

Thank you for the information. This is a very interesting read and leads one to believe that many things would improve with these chokes in place, especially in noisy settings. 

As far as a transistor for the keying circuit, one might first want to determine the parameters I had suggested earlier. What is the collector to emitter voltage across the keying transistor when unkeyed?

What current (if any) flows through the transistor when it is unkeyed. It won’t be much buy will likely be non-zero.

R, a 150K Ohm 2 watt resistor is in fact in place across the emitter to collector points of the 2N3055 (Q1 as shown in the TT schematic). I am certain its intent is to suppress just exactly the charge accumulation I mentioned before. This amp is in reality a Cathode keyed design and would be expected to show key clicks but for the presence of R2 and, to some extent, C1 (0.1 uF). Without measuring, I would be hard pressed to guess what the voltage on Qi collector would be.

2 Watts is pretty big for a 150 K resistor. In order to heat this resistor to 2 watts you would need 550 volts applied! TT had a reason for using a 2 watt resistor here so I am guessing one might expect a few hundred volts to be possible at this point. This would make a 2N3055 quite uncomfortable, even a perfect one.

The MJL4302AG <http://www.futureelectronics.com/en/Technologies/Product.aspx?ProductID=MJL4302AGONSEMICONDUCTOR3062501&IM=0> from Future Electronics would be a nice choice. Collector to Emitter rating is 350 volts. It is NPN. Current rating is 15 amps, more than enough to handle the Cathode current for the tubes. It is a power tab, not an SMD so installing it should be easy (relatively). In this circuit, gain is not much of a factor since the thing is only operating as a switch anyway.

Some of the horizontal output transistors from CRT color TV’s should also work as they have high CE voltage ratings and sufficient currant capacity. Many are TO-3 and virtually all are NPN.

Gary

W0DVN




> On Feb 22, 2016, at 7:07 PM, Stuart Rohre <rohre at arlut.utexas.edu> wrote:
> 
> Allen,
> Look for higher collector breakdown voltages to mitigate any transients.
> 
> The 2n3055 is a bit puny in the voltage breakdown dept.
> 
> Pick silicon with higher PIV, and similar Beta as the 2N3055.
> I will have to look up the part nos. for what I have used in industrial circuits.
> 
> Stuart Rohre
> K5KVH
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