[TenTec] Paragon Finals and other woes

[MadScientist]

Mainstream sellers are hard to come by, that’s for sure.

I have had great luck with eBay seller Littlediode. I have gotten many parts from them, all of them worked.

Now as to your specific problem - It seems your bias is out of control, or you have a leaky transistor in the amp module.

Try leaving the power amp connected but unhook cable 11. This removes bias from the final and driver transistors.

To narrow things down before performing surgery, I suggest following this path:

In operation, the “TX” voltage goes through U1B, a simple non-inverting voltage follower for the bias pot R15. This in turn drives emitter follower Q6 to put something like 0.6 volts on the bases of Q3 and Q4.

This same cable 11 provides bias voltage to the drivers via R19, U1B and Q5.

If the power amp still goes to current limit with cable 11 unhooked, you have bad finals or bad drivers.

If the amp does not go to limit in this condition, then look at parts in the bias supply.

Now I believe you mentioned that the radio goes to limit after you turn it one for a while - how long does it take? Is the radio in receive when this happens? If so, again remove cable 11 from the amp and see if this corrects things as there may be an issue with the “TX” voltage.

If you still go to limit with cable 11 unhooked, remove the heavy red wire on the final module from the 13.8 volt line and measure the resistance to ground from that wire to amp chassis (radio powered off). It should be quite high, thousands of ohms. Anything less suggests a leaky (not yet shorted) transistor somewhere.

Leakage current in the finals (or drivers, for that matter) could thus be a cause because this would cause the amp to draw current even in standby (RX). As the offending transistor heats up, its resistance drops fairly quickly at first, then very quickly to the point of over-current limit, in an exponential profile. This should take only a few seconds, depending upon the amount of leakage current.

If this turns out to be the case, you can remove L7 and do the test again to determine if it’s the driver or the final stage.

Hope this helps.

Gary

W0DVN

> On Jan 29, 2019, at 9:10 AM, Ben Hall <kd5byb at gmail.com> wrote:
> 
> On 1/28/2019 8:35 PM, Bwana Bob wrote:
>> on the power control board was open.  I fixed that, but found a short on the 13.8 V bus.  I haven't dug into it yet, but fear that the Astron supply zapped the finals.
> 
> I've got a Paragon here with a short in the RF power amplifier somewhere.  It's strange - the radio will power-on for a few seconds before current on the supply (proper Ten-Tec supply) ramps up shutting it off.  I know it is in the power amp module - if I disconnect the heavy gauge red wire to the power amp, it will power-up without any issues.  The power supply is fine - no issues there.
> 
> I'm almost thinking that with this behavior, it's not the transistors themselves, but something goofy in the other components leading to a turn-on of the finals?  Not sure.
> 
> I too am wondering if I should fix it or not.  Before it broke it was a good running Paragon - sent and received fine.  It's sentimental, it was my father-in-laws rig before he became an SK suddenly in a motorcycle wreck.  But like some others have already posted, it seems like these radios have a lot of issues over time so that if I fix the problem in the finals, what's next?
> 
> FWIW, if the problem is in the driver MRF476, haven't found a mainstream place that sells those yet.
> 
> Thanks much and 73,
> ben, kd5byb
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