National HRO inquiry

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KM1H:
Quote

Drift can be mitigated to a good degree with a TC style ceramic cap across the oscillator tuning cap as written up by Loren Windom, W8GZ, ages go. I dont have the reference handy at the moment but will dig it up later.

I dropped a word and also forgot a bit of details.

The 3-4pf N750 TC cap goes across the BS trimmer in the oscillator can for 40, 20 and 10. The article is in QST for June 1949. I dont see why placing it right across the main tuning cap wont work on any coil set. Use that one to establish a stable 80M and either say its finished or tweak the cans on the other bands.

The oscillator screen/plate should be zener regulated and voltage set just above whats needed for sufficient injection to the mixer.

Ive been using a 100pf coupling cap on one of the HQ-129X and 140X here for several years since both are a little drifty and that brute force lock works well.

k4kyv:
It's been over 30 years, but as I recall, I had to regulate the screen voltage to the oscillator stage.  With strong signals, the AVC controlled tubes would pull enough current to cause the voltage line to sag. With a strong CW signal, the pitch would vary over each CW character, giving a whoop-whoop sound.  It could be eliminated by turning the rf gain down, but with the modification, the frequency would not shift with the strongest signal. I used a pair of VR-150s for the regulated voltage. They fit well into the rack mounted power supply.  That would have been a problem with the dog-house supply.

KA2DZT:
I have a HRO-M that I use as a main station receiver on 75M AM.  It has square IF cans.  It uses 6C6's and 6D6's with a #42 output tube.  It is a rack mount model.  I have all nine coils (50Khz -30Mhz) along with two steel rack mount coil housings.  I made a power supply for it, 250V at about 80ma.  I have the original manual which I think is dated around 1941.  The chassis is gray and I don't see any serial number.

Fred

k4kyv:
For the  serial number, look on the chassis through the  rectangular opening in the cabinet near the antenna terminals.  No nameplate; it would be stamped on the chassis with black paint. At least that's the way it was with the pre-WW2 civilian models. As I recall, the ser # is 2 or 3 characters long, a number and a letter.  The same ser # should be stamped on the coil sets.

The earlier rack models came with the same cabinet as the table model, but with a rack type front panel.  I believe the later rack models came minus  the cabinet.

Collectors will pay more for a  receiver with a complete set of coils all with matching serial numbers. I have heard that the coils were custom matched to the receiver at the factory so you are better off with an original coil with the same serial # the receiver, but I'm not convinced that the electrical match-up would be anything beyond normal alignment procedure as described in the manual.

KM1H:
With the early pre WW2 chassis the serial was stamped, not painted, into the metal in front of the antenna terminals. During the war it could be anywhere in the area including by the audio tube. Post WW2 was stamped beind the crystal filter and continued there thru the HRO-60. I suppose some were ink stamped if that step in the process delayed production.

When the radio and coils were relatively new the alignment mattered mostly for oscillator tracking but after 60-70+ years of moisture, grime and banging around they all need realignment. at least the basic A-D coils. I have all 9 coils for my E-75 set plus a few extras setup on bandspread, some were original and some I restamped.

I also have a D-25 (I may be off on the # as I havent looked at it in years) table model with all 9 original coils, PS, speaker and coil boxes that is likely the set Millen gave to MIT as it was discovered there in a storage room not too long ago and appears to be either unused or very very little.
This one is remaining a virgin and is stored in padded boxes in an unused bedroom closet. All I did was blow most of the dust out and it had plenty. I'll let my kids put it on Fleabay for my funeral expenses ;D

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