Deafening click on Mohawk

[AB3FL]

When I switch my Mohawk from standby to receive(or the other way around) I get a loud click in the speaker/headphones.  I would assume this is because of the way the switch works.  Is there any easy fix to eliminate this such as putting a cap across the switch

thanks

Tom - AB3FL

[w3jn]

How are you switching it from stby to receive?  If you're switching the B+, that's a lusing way of doing it.  The PS voltages raise up plus it's really dangerous.  Ask me how I know this 

Best way is to open the RF gain pot's path to ground and switch that.  Shouldn't be too big of a "pop" when you switch to receive, if you put a 47K resistor in there to keep a bit of DC current flow going.

Also could be a relay sequencing issue.  If the transmitter's still "on" during the transition from TX to RX, you'll get a big thump due to the strong signal the receiver sees.  Generally this isn't a problem if the coass relay is switched from the xmitter.  The xmitter relay turns off, kills the xmitter, and the coass relay (because it has a short inherent mechanical delay) switches the antenner and the RX from stdby.

Which begs the question, are you using the aux contacts on the coass relay to switch the RX to stdby, or some other method?

[AB3FL]

The Mohawk mutes by cutting off the if/rf and audio tubes.  It unmutes by grounding the lines.

Not a sequencing issue as it does it with the transmitter off

Tom

[w3jn]

OK, I downloaded the sckiszmatic.  Once again Heath screwed the pooch on the design.  They're switching the cathode circuit of the audio output as well.  No wonder yer getting a thump.

There's a 330 ohm resistor paralleled with a 2 uF cap from the cathode (pin 2) of the 6AQ5.  Remove the ends of those 2 components that are going to the stby/RF gain return circuit and ground them.  You may need to increase the value of the 15K resistor that provides DC return if you get some audio in the 'phones or spkr when xmitting.  Sometimes that's useful, you can monitor yer own signal.

[WQ9E]

JN,

I agree that switching the audio stage that way is a little strange but that was during the era when lots of strange muting techniques were used (like shorting the speaker output with the relay).  But neither of my Mohawks (both basically stock) have this thump on switchover.  One is used with a Marauder/HA-10 setup and the other is used with an Apache, SB-10, and KL-1 "Chippewa" and switching for both is done with a dow key relay.  So I suspect something else is still going on.  I wonder if the 15K resistor has drifted way up in value.

I just tried both Mohawks briefly just to see if they had this issue.

Rodger WQ9E

[w3jn]

Doubt the 15K resistor is the issue.  It's more likely associated with switching 30-50 mA of current thru the audio tube with the resulting transient.

As I said, a terrible design.  And the Mohack has plenty of other examples of inattention to proper receiver design.

[WQ9E]

JN,

No argument from me on some design flaws.  I think they started by being closely inspired by the SX-101 design and if they would have "cheated" a little more it would have been a good idea.  The IF gain control really should have remained an internal alignment adjustment like it is on most receivers since run wide open it greatly reduces overall performance.  In addition to some design flaws Heath overestimated the ability of typical users and it would be much worse today.

But, I do use both RX-1 stations a bit and they really aren't that bad.  The received audio on AM, CW, and SSB is fine and neither has the drifting problem described in ER.  The only real RX-1 problem I had was a damaged 10 meter coil from a previous user and it was easy to use the 11 meter coil form as a replacement so unless we get 11 meters back I won't miss it (of course since many of the previous 11 meter users are now on 75 SSB maybe it will come back one day).

Rodger WQ9E

[AB3FL]

Thanks Johnny

This worked perfectly.

Tom - AB3FL

OK, I downloaded the sckiszmatic.  Once again Heath screwed the pooch on the design.  They're switching the cathode circuit of the audio output as well.  No wonder yer getting a thump.

There's a 330 ohm resistor paralleled with a 2 uF cap from the cathode (pin 2) of the 6AQ5.  Remove the ends of those 2 components that are going to the stby/RF gain return circuit and ground them.  You may need to increase the value of the 15K resistor that provides DC return if you get some audio in the 'phones or spkr when xmitting.  Sometimes that's useful, you can monitor yer own signal.

[w3jn]

Cool!  Glad to help.