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Re: R-388 51j mechanical timing, and other problems.




 
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Author Topic: Re: R-388 51j mechanical timing, and other problems.  (Read 1094 times)
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Scott SWL
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« on: July 19, 2021, 06:53:37 PM »

Yesterday I bought 2 R388 receivers, 1 slightly works(receives faintly on several bands) the other seems deaf, but there is some rush at low volume, neither has been rebuilt.
The one that is deaf looks like the RF deck cams don't match the "working one"
In the manual there is a section that describes using small holes in the side plates to synchronize the cams, but it does not say which band this corresponds to,
So I'm looking for info as how to tell if the cams are correct for the frequency.
My SX-71, and Hammarlund SP-400 blow these out of the water in their present condition, I'm surprised by this, but the bathtub caps may be some of the problem.
Also, what kind of alignment tool is supposed to be used on the IF's?
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Scott SWL
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« Reply #1 on: July 20, 2021, 05:30:09 PM »

Ok, fixed the timing problem, have bands 2 & 3 working, but band 1 picks up nothing, not even the calibrator. Is there a common failure for this?
several other bands also don't work. How reliable are the ceramic caps, and crystals?
The VFO or Calibrator is off across the band, I have to figure out which, I have a frequency counter, so that should be easy to find, but may be e real pain to fix, if it is the VFO.

The working radio with a better antenna is picking up on all bands, but the S/N ratio is very poor compared to my other radios, there is much hush, even with the antenna leads shorted. it has much more volume than the first radio.


* band 1.jpg (2469.3 KB, 4320x2432 - viewed 99 times.)

* band30.jpg (2218.97 KB, 4320x2432 - viewed 104 times.)
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WBear2GCR
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« Reply #2 on: July 21, 2021, 07:20:10 AM »

Check the tubes first.
The R-388 is not the quietest receiver as far as self noise. More than say an R-390A.

However they are super reliable.
Work fantabulous on AM!

A scope is going to be needed, and a copy of the manual to see what oscillators are running
at what freq, then go through the alignment procedure, IF you have bands working, the tubes
are good and the oscillator(s) are running for each band.

I haven't worked on any for a few years - even though I have a few - so off the top of my
head, without looking at the manual again, I don't recall much of the scheme or details right
now.

                   _-_-

PS. best to state what freq bands ur speaking of - not just numbering them... tnx.
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Scott SWL
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« Reply #3 on: July 21, 2021, 09:50:02 AM »

Tubes have been tested, first band 1mhz, 4,5,6,7mhz 29,30mhz
One problem I found is a bad trimmer on the 15mhz, in the slug rack(it came apart)
I have to look deeper in the manual to see where to measure the frequencies.
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WB6NVH
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« Reply #4 on: July 25, 2021, 09:38:32 PM »

Postage stamp mica capacitors are becoming notorious for failure in the R-388. Most problematic for some reason are the 100 mmf ones used in the DC blocking mode.  They break down and start leaking DC.  I forget the part number ID's but you will need to meter the voltages on the tube pins against the manual.  Which is a pain. One of the manufacturers was Micamold, the most problematic for me, and others were CDE.

The PTO's were terrible, honestly.  The core slugs drifted out of spec and many sets even by the early 1960's could not be brought into endpoint adjustment without removing a turn from the endpoint corrector coil.  This is described in a Bill Orr article from years ago which should be on the web.  There are also several skinny "bumblebee" paper caps in the PTO that need to go.  It is best, if you can, to avoid messing with the corrector cam stack in the PTO as getting that set right is tedious to say the least and means building a test jig for powering the PTO outside of the receiver and connected to a frequency counter. The good news is after doing the necessary work the PTO should be fine for a very long time.  There is a Ham Radio Magazine article on PTO overhaul, can't recall the issue.

As far as 65 year old crystals, I suppose you could expect some bad ones by now.  In my shop when I was working on these I had a crystal tester feature on one of the frequency counters and you would plug in the crystal and read the frequency out on the display.  Not having that, you need to determine if the appropriate oscillator is running with that crystal.  A scope or amplified sniffer on a counter come to mind.  Or another receiver nearby.
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Geoff Fors
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Scott SWL
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« Reply #5 on: July 25, 2021, 10:16:20 PM »

I will check the voltages as soon as I can,and likely replace the caps as I go through it. I think the 1mhz band should be the easiest, because it shares most of it's circuit with the working 2 and 3mhz bands
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