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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Fri 14, 2014 1:53 am 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 14393
Location: Southern NH, 03076
What those weaned on silicon often dont realize is that there has been germanium device leakage since manufacturered. The procedure today is to determine if it still falls in the acceptable range on a decent transistor tester.

Carl


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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Fri 14, 2014 2:40 am 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6628
Good point Carl!

The British transistors like those in my Eddystone EC-10 seem to be prone to hard failures from a short to the metal case unlike the U.S. types that generally have a soft failure from reduced performance. I bought a couple of NIB Chinese military receivers (139B, 339) that came with a complete set of replacement transistors and given the age I imagine they are germanium. In a pinch those could probably provide some suitable spares.

For me the bottom line is for receivers I really like I will find original or substitute parts to keep them operational.

Rodger WQ9E


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 Post subject: Re: HRO-500 challenges
PostPosted: Sep Thu 29, 2016 9:45 am 
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Joined: Sep Thu 29, 2016 9:39 am
Posts: 1
Carl

I am taking a wild shot at contacting you out of the blue as you may know more about the HRO-500 than I have been able to glean elsewhere.

I have been slowly working my way through the restoration of a pristine HRO-500, replacing many resistors, cap’s, etc., and more than a few failed or almost dead transistors.

I would like to ask you a couple of questions, if you have the time to answer it will be greatly appreciated.

At first, I was unable to get the synthesizer to lock anywhere, but soon discovered that the HFO buffer output was too low at input (R324) to the 4.75 mHz IF mixer stage (Q306), averaging only -65 dBm (All measurements taken with a spectrum analyzer using a FET probe). With careful rework of the bias resistors and bypass cap’s with new close tolerance components in the HFO oscillator and buffer stages, I was able to get the average signal level up to approximately -26 dBm, which is sufficient to drive the final IF transistor (Q308) and feed the DC comparator with enough output to get a lock on most, but not all, 500 kHz segments.

Still, it is marginal. I noted that Stuart Rumley’s restoration article has spectrum analyzer snapshots that show the HFO input closer to -12 dBm. Unfortunately, all of the trimmers on the HFO buffer in my radio are unable to provide this amplitude, and a few of them are against their stop point. Any suggestions on anything else I should look at?

Second, my S-meter is dead, and I know why. Do you have any suggestions of a way to change Q24, which is the S-Meter amp, located underneath the synthesizer IF? I can’t see any other way to access it without removing the entire synthesizer frame or disassembling the front of the radio. Also, according to the schematic, the final S-Meter amp is Q-23, but in actuality, these two transistors are labeled incorrectly on the schematic.

I should mention that I have probably forgotten 90% of what I learned in the 60’s about transistor biasing, and have had to work from rough calculations as the voltage chart in the manual is simply wrong. The values would only be rational if the 4.75 mHz IF transistors were biased from the 11V supply. This radio is a model wired to use the 12V supply for the synthesizer IF, not the 11 V, and the voltages in the chart are not even close. I have a manual for both versions, and the schematics show this change, but the voltage chart is identical in both. I was also encumbered by the fact that all the parts, except the transistors, for the entire 4.75 mHz IF circuit are not included in the parts list in the manual.

Any wisdom you can provide would be extremely useful. Thank you in advance for taking any time to reply.

Regards

Dave
KB7JS


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 Post subject: Re: HRO-500 challenges
PostPosted: Sep Thu 29, 2016 4:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 22951
Location: Somers, CT
Dave

Carl is no longer active in this forum. You might be able to track him down on QRZ.com or over on eHam.net.
He has posted there more frequently.

Pete k1zjh

_________________
Just because it can be done doesn't mean it should be done.


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 Post subject: Re: HRO-500 challenges
PostPosted: May Tue 07, 2019 7:03 pm 
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Joined: Sep Wed 28, 2016 9:48 pm
Posts: 5
I just picked up a HRO-500 and LF-10. The rcvr has issues but can't find the s/n and I think it was just ink stamped on it the back. Its been wiped clean.
Anywhere where else I can check?

Thanks
Paul
N6FEG


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