Results of HRO-60 overhaul, and questions

[Dennis Gibbs]

Greetings everyone,

Back around November 1, I submitted a posting in which I asked for possible solutions
to what I believed at the time to be a problem with the B+ circuits of my HRO-60.  The
symptoms of the problem I was experiencing was that the HF Oscillator seemed to
stop working (intermittently) on the 'A' band (14-30MHz).  Also, the overall sensitivity
of the receiver seemed to be lacking.  The strongest signals would only read 2/3 scale
on the signal strength meter.  I had had other HRO-60s in the pass that did not exhibit
this behavior.

I received quite a few responses offering possible causes.  I tried to send individual
notes to each person who responded.  The response which "pushed" me into finding
the real problem came from Hank van Cleef.  I'm paraphrasing his response, but
basically he suggested to stop looking for a "single cause" to the problem.  He
believed that the problem could be due to several causes, such as a combination of
leaky capacitors and resistors that had drifted in value.

I decided to go through and completely re-cap the receiver, and replace all paper and
electrolytic capacitors.  In the process, I also checked the value of each and every
resistor in this radio.  I just completed this work a couple of weeks ago.  Here's what
I found:

Almost every paper capacitor was leaky.  None were disasters, but would measure 10-
20 Megohms of resistance.  I replaced these with Mylar capacitors.  Also, I found MANY
resistors that had drifted WAY out of value.  Several were 2-3 times the value they
should be.  In those cases, I doubled or tripled the power rating for the replacement unit.
The worst case of drifted values was in the 1st RF amplifer circuit.  A 47K Ohm resistor
actually measured 900K!  None of these resistors showed any significant signs of overheating.
It looks to me that the resistors were simply of poor quality.  I also replaced any resistors
that had to be unsoldered in order to replace other parts, on the theory that the combination
of age and heat (from my soldering iron) would cause it to drift.  All replacement resistors
were 2% tolerance metal-film resistors, whereas the originals were 10% tolerance carbon-
composition resistors.

By the time I finished with this unit, I had replaced 48 resistors and 45 capacitors!

The result is that now, after a complete alignment, this is the best HRO-60 I have ever used.
The strongest signals easily peg the S-meter.  The Audio quality has improved, as well.  Now,
I am really enjoying this receiver again.

I would recommend that all HRO-60 owners who are experiencing problems to give their
receivers similar treatment.  In my case, it appears that drifted resistors were the greatest
contributor to my problem, more so than the paper capacitors.

After overhauling my HRO-60, I have several questions that I thought I would throw out for
the collective wisdom of the list to consider:

1) This receiver has two 6H6 tubes.  For both of these, I measured that the receiver is only
supplying about 5 volts for the filaments of these.  At first, I figured it was a problem, but when
consulting the schematic and the voltage charts, 5 volts on the filament is intentional!  Anyone
have any idea why National would deliberately run the 6H6 filaments 20% below their proper
rating?

2) Has anyone figured out a way to determine the exact (or even approximate) date of manufacture
of a given HRO-60?  I looked thoroughly, and did not find any date code anywhere, not even on
any of the components.  I suspect that mine is one of the later production models.  Does the
serial number give any clues?  The serial number format on HRO-60s that I have seen seem
to be NNN NNNN, in other words, a three-digit number and a space, followed by a 4 digit number.
Could the first three digits be a date code of some kind?

3) Has anyone ever built (or obtained) a proper jig for alignment of the coils?  Exact alignment
of all coils is nearly impossible with the coils in their normal operating position.  I was able to
get mine in excellent alignment, but it took many hours of trial and error.  I'm considering
building such a jig to enable alignment of the coils outside the receiver, and was wondering if
anyone else has attempted this.

4) On two out of 13 bands on my overhauled HRO-60, I noticed some behavior that is odd.  If I
peak the antenna Trim control, as I approach the peak, the receiver S-meter begins to peg
and the receiver makes a motor-boating sound, even with the RF-gain control turned all the
way down.  I can make it stop by detuning the Antenna trim coil slightly.  This does not happen
on any of the other bands.  Performance on the two bands it does happen on is still good
provided I keep the Antenna Trim control de-tuned a bit.  Can anyone enlighten me as to what
is happening?

Best Regards, and Happy Holidays!

Dennis Gibbs
dgibbs at rational.com

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