HRO-500 Rebuild Notes

This is an early (75-series) unit that I picked up as part of a package deal, along with a second HRO-500 in similar (very poor) condition.  I decided to work on the poorer of the two first.  I succeeded in putting it back into operational condition.  It will meet all spec’s after the rebuild and looks great on the operating desk.  Here is what I went through to make it work. 

I first removed the cabinet and transistors.  Some transistors were the incorrect PN, and some were misinserted.   This was followed by cleaning up the dust and dirt from the chassis.  I tested all the transistors on my Beckman for hfe; most were good.  I replaced some of them with NTE equivalents.  Note that the 2N2672 = NTE160, 2N2429 = NTE102A.  Also note that some NTE equivalents have different bases, so the leads need to be bent in an odd way to insert them into the sockets.  However – you CAN get these germanium transistors, and they do work.

All switches and external contacts were cleaned with DeOxit D5.  When inserting the transistors into the sockets, I inserted them several times to wipe the sockets’ contacts.  This is important if you want a reliable radio, BTW.  Past comments regarding poor switch surfaces, transistor voltages, etc are likely other problems, such as poor synthesizer alignment, etc.  I have had no special problems with the switches and other contacts in this radio once cleaned up. 

I brought up the power to the radio with a variac.  Power supply voltages looked ok.  Radio was dead as far as receiving signals.  Began the alignment per the manual and worked through several problems – finally began to hear some signals and managed to achieve synthesizer lock on some frequencies. 

The S meter was found to be sticky beyond repair.  I found a similar meter in a junker NCX-3 and exchanged the scales.  This was very delicate.

Found a loose/unsoldered 100 ohm resistor on the power supply TB.  It measured 220 ohms.  Replaced with the correct value and resoldered loose red wire near it.

Found loose/unsoldered wire at Q311 collector R349 (330 ohm).  Appeared to be a factory defect.  Resoldered it – resulted in much more stable gain – can flex chassis without losing everything.

Noted noisy gain – tweaked bandwidth filter trimmer caps – noise disappeared (more later)

BFO ceased operating – slowly went to zero.  Replaced crystal with unit from the other HRO-500.  Peaked L24 for max output.  Sounded better.

SYNTHESIZER NOTES:

Mechanical:

Removed synthesizer drum: noted that it had been binding and sticking.  The Synthesizer Tune control is a pincher drive – the disk was oily – cleaned it.  To remove drum – followed instructions in Para 5.6.5.1 (light bulbs), NOT the Addendum:

Removed side bracket

Removed passband cap shield

Removed meter/bracket

Rmoved right drum bracket and drum

Cleaned drum, cleaned excess plastic that was preventing smooth rotation

Cleaned drum mask and washed it

Cleaned plastic drum bearing surface

Replaced light bulbs with correct PN’s; some are 6.3 V, some are 14 V – do not substitute.  The bulb under the drum can melt the plastic drum overlay.  Both receivers were found with the bulb socket tilted up to light the kHz red jewel brighter.  Loosened the nut and mounted the holder horizontal – to move the bulb away from the plastic drum.  Keep wiring away from the main tuning capacitor, however (see hum discussions).

Followed the manual to reassemble.  Note – the small metal blocks to left and right of the drum both have setscrew holes – use only the right side setscrews!

Aligned the “-“ mark on the drum by tuning full CCW – BOTH C-332 and C373 MUST be fully meshed – accurately (see manual., p. 44)

When reassembled, verify all setscrews tight, rotation of the caps and drum smooth, accurate, repeatable.  If not – alignment is very difficult.

SYNTHESIZER SIGNAL CHECKS:

If possible, replace all 0.1 black disk ceramics – known failure items.

Cleans all switch contacts.

Verify correct transistors used.  Can use 2N2222 for Q-302, but must use 2N706 for Q-301.

Aligned per the manual Addendum.  Verify on scope that Figs 22-24 are being reproduced.  Blanking pulse period is important to image rejection.  Noted some excessive sine wave damping on some syn settings – but generally looks like the book.

Note – BEWARE small transformer leads near Q302.

Verified 500-kHz crystal absolutely accurate – had to change it out and used the other receiver’s crystal – this affects a lot of dial and synthesizer settings and must be dead on.

Aligned synthesizer per Manual.  Banks 1 and 2 are toughest – the bottom and top range caps interact.  Used my IC 756 spectrum analyzer display using the HFO output to observe correct HFO frequency for each segment.  Monitored Synthesizer generator output carefully per Addendum – SB 300-500 mV p-p.

Results: achieved solid  lock on all bands and segments.  Most synthesizer readouts accurate, but Bands 1 and 2 are a bit spread out compared to the drum span markings.  Band 5 aligns very well for all segments.

Note – when the drum is out, it’s a good time to replace the VFO transistors and align the VFO – since the L and C are accessible.  Used the IC 756 to align, using a small loop of wire with a test lead from the transceiver antenna jack (disabled TX on the transceiver, of course).

MAIN DIAL REFURB

The HRO-500 dial was found to be loose, and the drive had considerable backlash.  The clutch did not function.  Warning – if you are mechanically challenged, don’t try this at home!

The main drive has four large ball bearings that ride in holes in the main shaft.  The balls set into a groove in the inner shaft.  The balls are retained by two retaining rings.  The ring towards the radio can be adjusted in/out by the main collar (2 setscrews), which can be turned in/out with respect to the main tuning mechanism and its stationary ring and the bearings.  The collar adjusts the compression on the main ball bearings.  Too tight – not smooth.  Too loose – will not turn VFO drive train.

I removed the main drive and the clutch, took them apart, cleaned them, and rebuilt them.  I also lubricated the VFO main tuning capacitor and used DeOxit on the spring contacts.  The clutch can be adjusted for “bite” by loosening one setscrew on the larger, front cylinder portion and rotating it w/respect to the rear cylindrical portion.  The clutch must operate correctly, or you will not be able to align the main dial easily, if at all (the dial lock will not work properly). 

After rebuilding and getting the slop out of the ball bearings at the rear of the entire drive train, the drive train, including clutch, should rotate smoothly without backlash through the worm drive ear, which has one small ball bearing at the ear, riding in a small setscrew through the U bracket.  This adjustment screw is held in place by a small nut at the rear.  The nut can be backed off a bit and the the screw adjusted to keep the ball bearing in place firmly but not too tightly.  Note that the two Phillips screws that hold the main hex nut in place at the front set the pressure of the entire drive against that small ball bearing at the rear. 

After rebuilding (cleaning, regreasing lightly) the drive train, including the clutch, I replaced it into the radio.  The anti backlash split gear was compressed one tooth (at least) retained with two strong alligator clips on the two split gears – then the drive train was put into place, with the worm gear against the split gears. 

Result of all of this (it’s tough to describe concisely) – smooth, backlash free tuning with proper clutch action (dial lock control works).  Note that the HRO-60 manual describes how to take apart and reassemble the famous HRO dial assembly (avoid 180 deg mistakes!).  After reassembly, the main tuning cap capacity (main plates, not the end plates) should increase during dial rotation to higher numbers.  If you get this wrong, the receiver will tune backwards.  Ask me how I know.

C24 SLUG REPLACEMENT

The slug in the can was stuck.  I could see it had fractured along the main axis.  Used progressive drill bits to remove it – hard to get all the pieces and dust out.  Use a Q tip.  Replaced with a slug from the donor NCX-3 coil – perfect fit.

26- MHZ OSCILLATOR (USED ON BANDS 1,2)

During alignment, found it to be free running at 29 Mhz.  Verified correct frequency using IC 756.

AGC NOTES:

Found bad C-134 (black 0.01 disk ceramic) was leaky.  Replaced with mil spec.  Could not find C131.  Both receivers seem to be missing this capacitor.  Replaced C130 (was 25 uF, now 33 uF).  This slows down the decay constant a bit – to my taste.  This cap is on the side of the chassis, near the gain pots.  Found miswired AGC circuit at S-1F.  Could have been previous attempt to repair the unstable AGC (ie, the bad cap replaced above). 

With AGC threshold off (meter should pin hard), RF gain control R54 should apply ~8.75 to 11.75 V DC to R123, thence to Q12 base (voltage divider – most voltage will be at base due to high value of R59.  Increasing voltage should lower gain. 

Symptom – at times, meter would not pin with AGC threshold “OFF” (manual gain, not AGC, max gain on RF gain control, 50 uV signal).  Finally found C138 (black 0.1-uF disk) bad – replaced with mil spec.  Gain now stable, all bands {this cap failure probably led to the original AGC miswiring.} 

After fix – checked AGC per 5.5.17.  S9 = 50 uV, 50,000 uV = 85 dB on meter). 

HFO COMPONENT CHECKS

Found several OOT components near Q303.  Replaced R312, R313, R315, C339, C338. 

Q1 (RF AMPLIFIER) REPLACEMENT

Originally tried a 2N2222A for fun.  Finally used an NTE107, which resulted in much better gain.

HUM PROBLEMS

After many hours of troubleshooting, found multiple sources of hum on CW/SSB:

VFO will pick up AC hum via lead wires to light bulbs in vicinity.  Solution – can’t make it go away – so turn off lights if needed.  [Probably a lead dress problem – see light bulb replacement].

Performed final frequency calibration per 5.5.18.1 (NOT in the Addendum).

Wrapped several turns of small insulated wire around Q302.  Could zero beat the spectrum oscillator to WWV easily.  Attempted to “slope detect” the 4.75-MHz residual tone per manual.  After doing this, tuned in WWV and and tweaked the 4.75-MHz. trimmer very carefully.  Tone improved.  Probably sounds as good as any of them – pretty good, but not perfect.  Can easily copy CW/SSB signals, no hum on AM.

GAIN CHECKS

            The S meter is arbitrary in that it can be set to a wide range of sensitivities.  The receiver can be pretty deaf and still read S9 for 50 uV.  I found that the different bands had different gains.  Bands 1 and 4 had the highest gains, and 2 and 3 had problems.  Found a slipping preselector dial – tightened loose set screws on the fiber shaft and two large set screws on the main haft (accessible from the top).  Realigned the preselector function (again).  Inspection of the circuit shows that low gain on bands 4-10 and 10-20 likely due to a bad cap in the dual-ganged preselector switches.  Measured C1 – C7.  These are 2% caps, because they operate in pairs and must track.  C7 was 879 pF, SB 910 pF.  Will replace it someday.  For now, will live with slightly lower gain on 40 m. 

FINAL RESULTS

This receiver works to specification and looks great.  I cleaned up the knobs and main dial (which was oxidized).  They work well on all modes, the VFO is very accurate and stable (for this vintage), and are great general purpose HF receivers.  This one will rival a Collins 51S-1 in most respects.  The HRO-500 will overload much easier, but using the attenuator properly will allow almost any signal to be received on a chosen band.  The styling is pure 60’s and very, very sharp!