[Elecraft] K2 receive problem

[Brian Mury]

I've found the problem. Gary at Elecraft told me that VFRDET (pin 2 of
the control board processor) is pulled low, and should be close to 0
volts when in receive - if it's not, the processor will not assert RX.
After a couple email exchanges with him, I found that when I had
inserted the IC into the socket, pin 1 bent over and shorted with pin 2.
Straightening the pin and reinserting the IC solved the problem.

My thanks to Gary, and now I can finish my radio!

VRFDET input pin 2 of 
> the 18C452 is allowed to float, it will not assert the RX pin 28 to 5V
> and the 8R line for the RX will remain off.

On Mon, 2004-06-14 at 23:34, Brian Mury wrote:
> Not sure if my previous email made it to the list - I haven't seen it
> yet, and I've had other emails not make it (while some just take a
> really long time to show up).
> 
> Anyway, one more bit of data to add. In my previous email I said:
> 
> If I short Q2's collector and emitter to force 8R to 8V, or short Q4's
> > source and drain to turn Q2 on and make 8R go to 8V, then the S meter
> > starts acting normally, but I get no audio out - so I may have more than
> > one problem. /MUTE maybe? I tried monitoring /MUTE with my scope, but
> > putting a scope probe on that pin mutes the audio when the receive is on
> > - the scope appears to be loading the circuit somehow. Floating pin?
> 
> If I short Q2's collector and emitter, and pull /MUTE high (5V), then
> the S meter works *and* I get audio, so it looks like a problem with
> both RX and /MUTE not getting asserted properly. 
> 
> Once more, any ideas anyone has would be appreciated - I don't know
> *why* RX and /MUTE are behaving incorrectly.
> 
> On Mon, 2004-06-14 at 23:28, Brian Mury wrote:
> > No replies to this one yet, but I have spent a bit of time looking over
> > the schematic and poking around with my DMM & scope. Here's what I've
> > found so far:
> > 
> > TX (RD4 on the control board MCU) is normally low, 8T is 0 volts. When
> > transmitting, TX goes high, Q3 turns on Q1 and 8T becomes 8 volts.
> > 
> > RX (RD5 on the control board MCU) is normally low, 8R is 0 volts. When
> > going into one of the CAL menu items or the ATU menu item, or between CW
> > elements when transmitting (see my original message below), RX goes
> > high, Q4 turns on Q2 and 8R becomes 8 volts.
> > 
> > I haven't found a description of any of these signals anywhere, but it
> > looks like TX and RX are MCU outputs; that TX should be high and 8T
> > should be 8V when transmitting; and that RX should be high and 8R should
> > be 8V when receiving. Is this correct? 
> > 
> > If I short Q2's collector and emitter to force 8R to 8V, or short Q4's
> > source and drain to turn Q2 on and make 8R go to 8V, then the S meter
> > starts acting normally, but I get no audio out - so I may have more than
> > one problem. /MUTE maybe? I tried monitoring /MUTE with my scope, but
> > putting a scope probe on that pin mutes the audio when the receive is on
> > - the scope appears to be loading the circuit somehow. Floating pin?
> > 
> > Assuming TX and RX are indeed MCU outputs, and that RX does get asserted
> > under certain conditions, does this point to a bad MCU? Since RX *does*
> > get asserted, I would suspect that it's *not* an issue of something
> > pulling it low that shouldn't be.
> > 
> > Any ideas?
> > 
> > 
> > On Mon, 2004-06-14 at 07:20, Brian Mury wrote:
> > > I've just completed part 2 of my K2 RF board. I seem to have a problem
> > > on receive. I get no audio, and the S meter is reading maximum. If I
> > > select any of the CAL menu items or the ATU menu item, the receiver and
> > > S meter work normally. As soon as I exit the menu, it goes back to no
> > > audio and the S meter reading max. I can also receive between elements
> > > when keying.
> > > 
> > > Anyone have any ideas what to check?
-- 
73, Brian
VE7NGR