Re: ubitx v5 down: IRF510 heatsink very hot immediately after turning on the radio #ubitx #ubitx-help

Jens Kaemmerer

Hi Jerry,

I have now been able to start with the 'noise injection' test:

I first connected the wire to TP21 and could hear noise increase
(VOL set to high). This was not much of a surprise because I
had already established that the LM386 is working.

Then I connected the wire to TP20 (output of the de-modulator?)
and found that I could not hear any difference in noise level.

If I read the uBitx v5 schematic correctly, this could mean that
the Q70 transistor (type 2N3904) or the relay K3 might be damaged.
I assume Q70 acts as an audio pre-amplifier(?).

I continued by measuring continuity between C51 (M1) and TP21 (M2)

with the uBitx turned off: continuity exists (expected)
with the uBitx turned on: continuity does not exist (?) (not expected)

I expected that continuity continues to exist when the uBitx is
turned on.


with the uBitx turned on *or* off:
continuity exists between TP8 (TX path) and ANT (antenna center conductor)
and TP1/TP2 and ANT

This is strange - based on the schematic it should be:

Either there is continuity between TP1/TP2 and ANT *or* TP8 and ANT - not

My conclusion:
Maybe the relay K3 is damaged (or somehow 'stuck').

I was almost ready to send this email when I tried one more thing ...

I noticed in the past that a relay in the uBitx rapidly switches back and forth
when I startup the radio *and* have theĀ  MIC cable (coming from the
soundcard) already plugged in.

So I figured, if the relay is 'stuck' - maybe I can 'free' it by plugging
the MIC cable in again and force the rapid back and forth switching.

Bingo - it worked, RX is working again.

Next up: fixing TX ... (don't have time for this today as this will require some surgery ...)

I am very thankful for everybodies help in this endeavor,

-jens (KM6ZJV)

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