HRO50T-1

[k7mdo]

Has anyone had experience with the B+ switch causing a significant "snap" when being switched on after a coil change?

It is a recently acquired set but it worked fine until recently.  The snap is an audible "pop" in the speaker when the b+ switch is closed.

Thoughts?

Thanks, Tom

[KA2DZT]

The pop you're hearing may be louder on different bands.  Put the first coil back in and see if there is any change.  Also try turning the volume control all the way down and see if any change occurs.

I have a HRO-M and there is some pop when I change coils

Fred

[Jim, W5JO]

Move the switched B+ on the first audio tube plate and screen and phase inverter plate to the same buss as the screen of the audio output tubes which is unswitched.

[k7mdo]

Thanks for the thoughts....  I will first try the audio all the way down...  thought I had done that already.

Circuit modifications would be easy enough but it still seems to me that the problem didn't at first exhibit itself.  It just seems to be a recent thing ...

I will try the volume reduction first and report.

Tom

[k7mdo]

Well, tried the "audio control" off during switching and the pop is still there.  It is there with both the audio and rf gain controls completely down.

Another clue might be the tone control as when the radio is first powered up and the b+ is appplied (with its attendent pop) and then the tone control is switched on, it also makes a very good pop.  But, subsequent tone control switching on and off does not pop...  in fact the only way to get the tone control to pop again is to go back to a full restart of the rig.... ie power down and then up again.

Btw, the only caps that have not been replaced have been the the main filter caps... I have ordered them but not yet received them...  possible they are exhibiting some failure mode that results in the popping?

Thanks again for the help.  Tom

[k7mdo]

It doesnt look like anyone with an HRO 50 has tried or reported a similar problem to the one I have described.  I am "lounging" on Kauai for this Christmas so further testing will have to wait a little while. 

The "popping" didn't happen until very recently so I know it is not normal.  I don't plan on rewiring the audio section without some hope of identifying the cullprit first.  Seems like a bypass cap in some location is failing?  It isn't tube related as those have been checked and swapped. 

The funny thing is that I can't say when the popping started... it just finally got bad enough that it became annoying.

When and if it is fixed I will post the answer.  Tom

[Jim, W5JO]

Another person with the problem

http://www.emmittsfixitshop.com/Projects_HRO_thump.html

[WQ9E]

The link posted by Jim looks like the proper solution.  

I don't use B+ switching for T/R operation wiht my HRO receivers since it is noisy and leads to drift but you still need to switch B+ off for coil changes.

For T/R I lift the grounded end of the RF gain control and put a resistor equal in value to the pot in series between the lifted end and ground with a line from the junction between the pot and added resistor brought out to be shorted to ground by the auxiliary contacts on the T/R relay when in the receive position.  You can use a variable resistor for the added resistor if you want adjustable gain during standby (could be used for CW monitoring).  Hallicrafters did this in the SX-88 and perhaps other receivers.

[k7mdo]

Eureka!  Thanks so much...  I begin to think tha the reason that the "thump" was not as prevalent before was due to a speaker change that was made.  I think the move to a larger speaker may have accentuated the noise.

Will work on the circuit change for both the thump and the T/R switch.

Will report results, Tom

[k7mdo]

Progress!  The emmitsfixitshop wiring change was easy and the result was at least a reduction of 75% of the thump that had been so irritating.

Now just the snap noise of the switch itself is the predominant sound.

Thanks to all for the help.

Tom

[W7NGA]

it seemed a shame that I had a 'like new' HRO50T-1 languishing in the closet, so after finding a new HRO dial on eBay, I installed the dial and did a quick listen on 75-meter AM. the 'pop' you described was present and normal. glad you found an easy fix .. I think I have some caps to change to do the push-pull 6v6's justice. just as soon as my back recovers!  

W7NGA
San Juan Island, Wa.