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 Post subject: HRO 60 RF ALignment
PostPosted: Nov Sun 19, 2017 5:53 pm 
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Joined: Jan Mon 25, 2016 3:13 am
Posts: 122
Location: Allen, TX
I'm looking at a friend's HRO 60 and trying to figure out why some band coils sets have good gain while others don't. The problem coil sets cover are B and C covering 80 and 40 meters in either band spread or General coverage.

I have probably tweaked on those padders and inductors 100 times, following the alignment instructions in the manual. No combination of tweaks that I make raise the gain (background noise) to the level of the other coil sets. The alignment procedure is one of the most round about ones I've seen and putting a meter on the speaker to judge tuning results is a joke in my opinion.

If I put a probe on the HF oscillator tube socket, pin 6 going to T4 in the coil set, the gain jumps way up. I've replaced all of the capacitors in the oscillator circuit.

Are there any tricks or hints out there that would make this alignment easier, or is it just a real pain?

Has anyone seen failures within one of the 4 circuits within each coil set?

Thanks in advance!
Rick
W5RAA


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Sun 19, 2017 6:18 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4804
Rick,

Take a close look at the contacts on the offending sets to see if they all look clean and not severely worn. Since presumably A and D work and the problem sets, B and C, are on both the single and dual conversion ranges the problem doesn't lie in that circuitry.

Is the oscillator section working OK? (oscillator is at correct frequency for the dial settings at the lower and upper ends of the range)

Try injecting a signal into the converter grid, second RF stage grid, and first RF stage grid; this will help you to narrow down which stage is the problem. Since you note increased signal when touching the oscillator stage it appears there is an issue with either the first or second RF stages on these coil sets and by injecting at the aforementioned points you can determine which is the problem.

I have found damage to HRO coil sets which was likely due to shipping/handling damage.

Rodger WQ9E


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Sun 19, 2017 6:23 pm 
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Joined: Jan Mon 25, 2016 3:13 am
Posts: 122
Location: Allen, TX
Thanks Roger,

I have cleaned both the internal and external contacts thoroughly.

Can the Band coils be damaged by hot switching?

Rick


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Sun 19, 2017 6:25 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
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Rick,

Typically the only damage would be to the contacts from arcing. In theory the front end tubes could be damaged by hot switching since the screens of the RF tube remain powered while the plate potential is removed but I have never heard of this actually occurring.

I have found parts broken loose inside the coil covers and wires broken in several cases. Pull the shields off the suspect coil drawer compartments and see if there is apparent damage.

Rodger WQ9E


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Mon 20, 2017 6:01 pm 
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Joined: Jan Mon 25, 2016 3:13 am
Posts: 122
Location: Allen, TX
I took several of the offending band coils apart and couldn't find any issues. The inductors have continuity. The variable caps are in good shape.

There is 5X gain through the first RF stage and another 5X through the second. The mixer has high gain and is working properly.

Some of the coils won't adjust the HF oscillator at all, others coil trimmers seem to have no effect either. The bottom line is that some coils provide great gain and others very little, telling me that the problem of low gain lies within the plug in coils.

I may have to admit defeat on this one!

Thanks for your help!

Rick W5RAA


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Mon 20, 2017 6:32 pm 
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Rick,

The oscillator adjustment should definitely have a major effect; the HRO series is pretty stable but any movement of adjustments in the oscillator section of the coil drawer should result in a very noticeable frequency shift.

Double check that the shorting links are all correctly turned to the general coverage setting for initial alignment and that the link is making good contact. The adjustments for the general coverage inductance and capacitance impact both the general coverage AND band spread setup regardless of whether the coil shorting links are set for GC or BS operation but the BS adjustments will NOT affect the coil when it is set to the GC position. So align the GC setup first and then switch to BS, making sure not to touch the GC settings during this part of alignment or you will have to start over.

Bands A, B, and C use the shorted turn loop inside the coils to vary inductance for the general coverage setting; band D has a disc for changing inductance.

Be careful when following the chart because the layout of the band spread vs general coverage adjustments is different for the oscillator vs the other coils and can easily lead to mistakes.

If there is a problem in the oscillator section of the receiver it will cause a reduction of apparent gain because there is insufficient drive to the mixer for normal conversion gain. This problem may only show up on certain frequency ranges and start by trying a substitute 6C4, also measure the plate voltage at the 6C4.

I had this issue of low conversion gain with the first piece of vintage gear I restored, a NC-183D but in its case it was a fault on a single band due to a damaged oscillator coil.

Good luck and don't give up :)

Rodger WQ9E


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 Post subject: Re: HRO 60 RF ALignment
PostPosted: Nov Tue 21, 2017 7:11 am 
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Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5320
How is it that you are tuning the inductors in the coil packs, they aren't accessible when installed in the receiver.

Image

If you're simply moving the tuning links (9-16), and sticking the coil pack in the receiver to see "where you are" I suspect you now have a real mess on your hands. My understanding is National had special tuning fixtures for initial alignment of the coil packs...

Image

...and did the final touch-up using capacitors (1-8) with the coil pack installed in the individual receivers, and to do that, you need a long non metallic screwdriver to adjust the padders.

Image

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