F5UL: PA blow-up again

[Brian Broggie]

Sent to the reflector on behalf of Robert, F5UL in Senyod, France:

Hi the list
 
Some months ago, I got a PA blow-up and with the help of the list and
particularly of Gary, it was solved for I thougth a long time. Well it
happened again and the results are differents ! I replaced both 2sc1969
and also Q13 who was shorted CE which was the cause of PA loss!

I end up with a power out of 12/13 w continus with an collector curent
aroud 5 A, and after search an auto-oscillation around 2.5 Mhz was
found. The power control have no action CCW or CW position it's the
same. It's the 2SC1969 the culprits, or at least their presence, if I
remove them the instability is not there, and the driver output is about
5 volts PEP on each base ( 9/10 V PEP base to base). At this point the
signal is clear in the control receiver CW or SSB.

I tried other transistor (2sc1969, 2sc1971, MRF476) to no avail, always
this square wave signal with frequency variation from 2 Mhz to 3.5 Mhz,
but always full +/-12V PEP.
I removed all components, up to Q5 circuits, put them back in after
rewinding the cores with no results.

So I look for some help with this new problem. Can the DAC converter be
involved?
The receiver is working OK, the PA bias is staying with 0.62 volt. With
only the driver the curent is around 0.480 A but more than 5 a with the PA.
 
73/72 from F5UL
 

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[Don Wilhelm-3]

Robert (and Brian),

If the current is high during receive periods as well as transmit, check
first to see if R45 is hot.  If it is, then your problem is likely a faulty
Q5.  You can confirm the condition that I suspect by measuring the voltage
on the 8T signal line during receive periods (you can measure the 8T voltage
easily at the collector of Control Board Q1 - it should be near 0 volts
during receive).

I have found at least two bad Q5 transistors in K2s that I have
repaired/analyzed recently - the transistor develops (or originally had) a
collector to base leakage which turns it on and backfeeds voltage onto the
8T signal line during receive.

Since it was working for you, I will assume that you have checked all the
components for proper values and good soldering.  The circuit does work fine
with the proper component values installed properly.

73,
Don W3FPR

> -----Original Message-----
>
> Sent to the reflector on behalf of Robert, F5UL in Senyod, France:
>
> Hi the list
>
> Some months ago, I got a PA blow-up and with the help of the list and
> particularly of Gary, it was solved for I thougth a long time. Well it
> happened again and the results are differents ! I replaced both 2sc1969
> and also Q13 who was shorted CE which was the cause of PA loss!
>
> I end up with a power out of 12/13 w continus with an collector curent
> aroud 5 A, and after search an auto-oscillation around 2.5 Mhz was
> found. The power control have no action CCW or CW position it's the
> same. It's the 2SC1969 the culprits, or at least their presence, if I
> remove them the instability is not there, and the driver output is about
> 5 volts PEP on each base ( 9/10 V PEP base to base). At this point the
> signal is clear in the control receiver CW or SSB.
>
> I tried other transistor (2sc1969, 2sc1971, MRF476) to no avail, always
> this square wave signal with frequency variation from 2 Mhz to 3.5 Mhz,
> but always full +/-12V PEP.
> I removed all components, up to Q5 circuits, put them back in after
> rewinding the cores with no results.
>
> So I look for some help with this new problem. Can the DAC converter be
> involved?
> The receiver is working OK, the PA bias is staying with 0.62 volt. With
> only the driver the curent is around 0.480 A but more than 5 a
> with the PA.
>
> 73/72 from F5UL
>
>

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