[Collins] Cosmos PTO for 390A newbie - Help requested

[Chris Kepus]

All,

This may be a bit long but I would really appreciate a response if you have
taken a Cosmos PTO apart.  I haven't taken one apart...yet.

I just picked up a Cosmos PTO for my R-390A. I liked the fact that Cosmos
came up with a design that didn't require messing with a corrector stack.
Others have pointed out that repairing and calibrating the Cosmos isn't for
the faint of heart, but it can be done. (I know, fools rush in..) 

Although the seller said it was dusty (it is), he didn't say the outer cover
of the PTO had numerous scratches suggesting a rather "rough" storage
environment.  The fragile elements on top of the output transformer were in
good shape with no observable damage... Anyway, in addition to scratches, it
was evident that someone had pulled the outer cover since only one of the
screws had the accompanying lock washer.  And the heater was possibly
removed because the screws that retain the heater (and insulation) were
missing.  If it is possible to remove the heater without unsoldering any of
the leads, it might have been removed. The heater wires did not look as they
had been unsoldered (could they have been cut at the top? ) My leads have
what appears to be shrink wrap over about an inch of them where they go
across the rear of the heater "can" assembly.  Photos at
http://www.flickr.com/photos/w7jpg/.

 Lastly, how should the PTO shaft feel when rotated by finger power?  I
expected low resistance and a smooth feeling rotation.  Mine is rather stiff
and when rotated, has an odd feeling, similar to rotating a shaft with
"soft" detents...not smooth at all.
  
So what do you think?  According to the writing on R390.com and elsewhere by
authors like John Harvie, Dallas Lankford, and Tom Marcotte, there doesn't
seem to be many conditions that cannot be repaired.  Is there anything I
mentioned that causes you experienced folks to be concerned (other than my
inexperience?)  Is there anything else that I should be looking for before
diving in? 

Thanks es 73,
Chris
W7JPG
 



-----Original Message-----
From: collins-bounces at mailman.qth.net
[mailto:collins-bounces at mailman.qth.net] On Behalf Of Dr. Gerald N. Johnson
Sent: Friday, November 16, 2007 12:40 PM
To: collins at mailman.qth.net
Subject: Re: [Collins] 321B-4 watt meter, accuracy of

On Fri, 2007-11-16 at 13:57 -0600, w9hak wrote:
> I have just compared my 312B-4 watt meter against my HP-410C with HP455A 
> rf adapter using an 50 ohm non reactive load. My watt meter reads low on 
> all bands of my 32S3 compared to the E2/50 values. Error values range 
> around 25%, with the watt meter reading low. The transmitter is putting 
> around 110-130 watts output depending on the band. The plate current is 
> being set at 230 ma.
> 
> The 312B-4 hasn't been abused and has been in service since new. It 
> reads reflected power if it's present. My initial thoughts are to leave 
> the unit alone. Absolute power output isn't high on my priority scale. 
> Having said that, what are the members of the reflector doing with their 
> 312B-4/5 control consoles? I would think the 1N82A diodes have seen 
> better days and wonder if anyone has replaced them and the results of 
> having done so.
> 
> Smith Bradford
> W9HAK

Then 1N82A was never a robust diode. Its a diode that can be damaged by
soldering it, surely could be damaged by too much RF or lightning.

Be careful in your measurement that the RF probe isn't changing the load
Z seen by the 312B-4 if the voltmeter is on the load and that the 312B-4
isn't changing the impedance at the voltmeter if the probe is before the
wattmeter.

Then check the calibration resistors, if they are carbon composition, be
very suspicious of them having drifted from age and humidity over the
years. And its not beyond possibility of the meter movement having
gotten stiffer with age. I see in my Yellow Book that the calibration
resistors were chosen at assembly to make the meters read correctly.
Changing diodes could mean needing to readjust all four of those
resistors and changing the type of diode could affect the meter scale at
all powers other than the one calibrated at.

My Yellow Book says the accuracy is 10% of full scale for the 302C
wattmeter. With the 200 watt scale of the 312B-4, that's 20 watts +/-.
Being 25 watts low at 100 watts isn't much beyond that 20 watt range.

I'd be happy with it.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer

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