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HRO-60 Crystal Filter




 
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Author Topic: HRO-60 Crystal Filter  (Read 859 times)
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KW4H
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« on: August 14, 2021, 07:03:53 PM »

National's alignment instructions for the crystal filter (T-5) mention L-13, but are silent about L-12.  Does anyone know what approach should be taken for L-12?  Should it be peaked along with L-13 or at a different point in the process?

73 - Steve
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MikeKE0ZUinkcmo
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« Reply #1 on: August 16, 2021, 07:48:56 AM »

L-12 is the plate load for both the single and double conversion 6BE6's, and should be added to either the L2 through L11 list or to step 11 of the second converter tuning procedure, and tuned/peaked for 455KC.

interesting they didn't say anything about it.   
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Mike KE0ZU

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KW4H
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« Reply #2 on: August 16, 2021, 09:58:17 AM »

L-12 is the plate load for both the single and double conversion 6BE6's, and should be added to either the L2 through L11 list or to step 11 of the second converter tuning procedure, and tuned/peaked for 455KC.

interesting they didn't say anything about it.  

Thanks very much!  By 455kc, I assume you're referring to the the true frequency of Y-1 (which may be slightly different than 455kc, plus or minus).  I've run into a curious and unexpected problem with the crystal filter -- if you go to position 1, you can hear the filter engage, but positions 2 through 5 produce no discernible difference in selectivity.  During alignment, L-13 produces a maximum meter indication, but C-38's peak is very vague, if at all.  In response to this, I opened up T-5 and performed an inspection -- S-5 had a lot of tarnish and I cleaned that up.  Everything else, however, seemed just fine -- no mechanical or electrical issues that I could identify.  The problem persists.  At this point I believe it's a possible alignment issue and am revisiting T-5.  If you have any comments or suggestions it would be greatly appreciated.

By the way, I don't recommend working with T-5 for fun.  There are a lot of things I really like about the HRO-60 design -- but the physical construction of the crystal filter assembly ain't one of 'em!

73 - Steve, KW4H
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KW4H
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« Reply #3 on: August 16, 2021, 03:10:00 PM »

Well, when it rains it pours.  There's something amiss with with the crystal filter -- I'm not able to get a peak on L-13.  Adjusting C-38 has equally no peak found.  I've already performed an inspection of T-5 by removing the chassis from the case and then opening up T-5's metal shield.  The components inside appear to be fine (although the switch needed cleaning).  I tested C-36 and it adjusted smoothly along its capacitance range.  C-38 was extremely difficult to get to; therefore, I can't attest to its condition electrically.  My gut feeling at this point is that C-38, a 3.5 - 35 pf mica trimmer, may be shot, and this is one of the most difficult parts to get to in the HRO-60.  

UPDATE:  T-5 has to come out for deeper testing of all the components. 

73 - Steve
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KW4H
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« Reply #4 on: August 17, 2021, 09:48:17 AM »

T-5 is now out of the radio and opened up on the bench. At least one of the problems is definitely C38 -- it's a 5 to 35 pf mica compression variable cap, and the mica is degrading and flaking, causing erratic readings and poor range (it would only go down to about 15 pf). C36 is also defective. I found what appears to be nearly exact replacements on eBay and ordered them. In the meantime I'll continue isolating each component and testing them to see if anything else in T5 is unhappy. I have a TinySA coming in and, since I already know the exact frequency of the crystal, I might try pre-aligning the crystal filter before reinstalling it. Could be a good way to become familiar with TinySA as well.

By the way -- pulling T5 was surprisingly straightforward and did not require removing the radio chassis from the case.  I'm planning to write an ER Mag article about this HRO-60 and will include a discussion about T5.

73 - Steve, KW4H
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KW4H
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« Reply #5 on: August 29, 2021, 04:53:23 PM »

T-5 has been repaired and the crystal filter is functioning again!  Both of the mica trimmers (C36 and C38) had to be replaced, and that means the phasing (related to C36) needs to be realigned.  The National manual mentions making "small adjustments" to C36 if the phasing is a little out of alignment, but doesn't say what to do in the event that C36 is replaced entirely.  I've played around a little with the new C36 and am not able to find the sweet spot where there are two nulls, 180 degrees apart.  Would appreciate any/all advice.  Thanks!
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KW4H
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« Reply #6 on: August 29, 2021, 06:36:16 PM »

Found the spot!  The phasing is now working correctly.  I wondered why on earth someone would have drilled a peephole through the bottom of the chassis and into the crystal filter assembly.  Now I know why -- it allowed a view of the positioning of the phasing variable capacitor.  In the interim, I found a posting online that mentioned putting that variable into the halfway position for the alignment.  I tried that and it aligned.  Today is my lucky day.   Grin
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