Heath Mohawk Check-up

[WA2ROC]

It is about time I pull the old Mohawk off the table and open it up to do some work.  Things like alignment, new tubes, voltage and resistance checks and maybe a good cleaning are on the list.  Cosmetically it looks good, chrome knobs and all.

I have a few questions:

1-  Anybody recommend a reliable tube dealer?  The tubes are probably as old as the receiver, so I won't bother testing them, just replace all of them.

2-  Any substitute tubes that would perk up the sensitivity without major circuit mods?

3- I know there are lots of updates and mods that can be done.  As of right now all I want to do is get it hearing a bit better and sounding a bit better.  Suggestions?

4-  Yes I want to keep it on the station inventory to match the other Heath stuff as long as I can do the work myself.

Thanks in advance

[WQ9E]

I would never do wholesale replacement of tubes NOR do I rely upon a tube tester.  The RX-1 operates the tubes at a reasonable voltage and it has sufficient ventilation so most of the tubes will have a very long life.  You could easily replace a current high quality tube with a new one that has some defect or shortcoming or may have a very short life.  Small receiving tubes operated within recommendations will generally last many thousands of hours and I doubt if your RX-1 has that many hours on it.  Even in low quality and excessively hot television sets the majority of tubes had long lives.

If you want to do a good check-up I would go ahead and replace any original or old electrolytic caps and old paper caps (includes all of the various plastic encased paper variety), check resistors (can be done in circuit in almost all cases since carbon resistors typically go up and not down in value when they age poorly) and take care of mechanical items (clean and lubricate gears as needed, lubricate variable cap bearings,  clean switches as needed, remove dust).  Now test the receiver and consider taking voltage readings if performance is clearly abnormal.  At this point you may want to consider tube issues but substitution is the best test because even high quality testers are not testing the tube in an operating RF circuit.  LIGHTLY tapping tubes will uncover microphonic and intermittent shorts/opens. 

Only after you have done all of the above would I undertake alignment.  During alignment you MAY uncover other problems (i.e. bad mica padder cap indicated by a transformer that doesn't show a normal peak during alignment). 

When operating the RX-1 the #1 thing to remember is keep the IF gain control set well back from maximum.  A properly operating RX-1 has far more IF gain than required.

[WA2ROC]

That sounds like great advise, and it'll save me some time and $$$ too.  Caps are cheaper than tubes, not easier to replace, but do-able. 

I'll follow your recommendations.  One of the reasons I want to do this is that sensitivity on 20, 15 and 10 has decreased a lot, but 160, 80 and 40 seem to be fine.  On 20, I have to run the RF and IF gain almost to max.  Could be the RF stage needs perking up, or maybe something else.  Time, and a lot of unscrewing, will tell.

Thanks!

[WQ9E]

Your reduced sensitivity on the highest bands could be a tube issue and often a weak HFO tube (12AT7) in this case is the culprit but the RF amp and mixer are also suspect.  But this could also be a result of alignment.  Does the antenna trimmer show a definite peak with gradual fall off on either side of peak on these higher bands?  Alignment of these stages is also highly impacted by the load presented by your antenna so if it is far different than what was used during original alignment then that usually has a bigger impact as frequency increases.

[WA2ROC]

I was thinking the same thing too.  Yes, there is a peak, when using the calibrator rather than the QSB-influenced signals on 20 meters, but not a really large definite peak.

I'll start off getting an RF amp, HFO  and 1st mixer tube, install them and redo RF and mixer alignment.

I guess I should hook up the antenna to be used under standard operating conditions rather than leave the antenna terminals open when peaking the RF and mixer adjustments.  I believe I did not do it that way last time I was inside the receiver.

Thanks!

[WQ9E]

Although there are standard dummy load values you can use for the receiver the most accurate front end alignment for your setup will result when you use the actual antenna that will be used with the receiver.  I generally use an external receiver (small synthesized shortwave with memories) to listen for and align the HFO set points since this is easier than moving a signal generator and also avoids the possibility of wrong-side tracking which can easily happen with single conversion receivers on the highest range.  The HFO in the RX-1 is pretty well isolated but for a lot of receivers mixer (and sometimes RF stage) adjustment is best done with a broadband noise source coupled to the antenna to avoid the effect of oscillator pulling when doing alignment.  A CFL (compact florescent lamp) provides a suitable broadband noise source when an antenna lead is run into the curl of the bulb.

Like the Hallicrafters SX-101 from which the RX-1 borrows some ideas, alignment of the low IF is critical to proper performance so take your time there.  Also make sure that the two crystal controlled conversion oscillator frequencies are dead on, if the crystals have drifted slightly the alignment procedure can be slightly modified to account for this drift and still provide proper operation.  So carefully check these oscillator frequencies (1632 and 1732) before doing the final IF alignment.  The final IF is aligned at: ((absolute frequency difference between the two oscillators/2) +. 4) which results in a slight asymmetrical response for proper SSB operation.  If one or both of the conversion oscillators is off then the IF alignment frequency must be adjusted to compensate.

[wa3dsp]

The latest Electric Radio has an article on updates to the Mohawk. There are also prior Electric Radio articles on it.

See - Nov, Dec 97, Jan 98, May 99, Mar 03

[WA5VGO]

Applying AVC to the last two IF amplifiers will make a dramatic improvement. This is especially true on the lower frequency bands where the receiver has excessive gain.

Darrell