Regarding Q508 failing with a short:
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There can be voltages from the transmitter that exceed the BS170's? Vds-max of 60v.
Several forum posts have told tales of woe when the four BS170's in the final got blown.
In addition or perhaps instead of blowing the final, this could take out your Q508.
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Blown finals is generally attributed here to operating into a bad SWR.
But this old post from Chris (KB1NLW) had a now gone plot showing that Vds on the BS170 finals
can exceed the max of 60v when on 160m and 80m and operating into a 50 ohm dummy load:
>? The plot does not address power. Only peak voltage across the BS170s. However the 160m has higher power than the other bands.
>? You can see that 160m and 80m exceed the 60V ratings of BS170 and are marginal on 60m and 40m.
>? When evaluating the effects of ATU switching (used my Elecraft T1) I was able to capture ~100v very fast spikes.?
>? Breakdown from these spikes may cause cumulative damage and ultimately total failure.
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I'm beginning to think that L502 may be the culprit.
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The four BS170's and T501 are arranged in a standard voltage mode class D push-pull amp.
The center tap of T501's primary is normally a DC voltage source, as it is on the 50W QCX amp.
When one set of BS170's pull one side of T501's primary to 0 volts, the other side pops up to 2xVdc (so 24v if operating from 12v).
And then the other set of BS170's pull the second side low, ?creating a square wave at the transmit frequency?
with a pk-to-pk voltage of 2xVdc across T501's primary. Since the LPF on the secondary of T501
allows through only the fundamental to the 50 ohm external load, the harmonics are blocked by the LPF
on the far side of T501, and the current through T501 is a sine wave.?
The current drawn from Vdc at the T501 primary center tap is a rectified sine wave.
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Seems to me that adding L502 would enforce a constant current into the T501 primary center tap,
not a rectified sine wave. ?And that the extra current with nowhere to go would be piling up,
causing excessive drain voltages that might blow the BS170's.
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I'll try simulating the final in LT-Spice, see if I can figure this out any further.
If my guess is right things might behave better if L502 is replaced with a short wire,
plus 0.1uF cap to ground to handle those rectified sine wave current peaks at double the transmit frequency.
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Any of you real life RF engineers care to weigh in?
RF design and especially magnetics is not my strong suit.
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Jerry, KE7ER?
