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Trapezoidal Rehash & Transformer Insulators




 
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Author Topic: Trapezoidal Rehash & Transformer Insulators  (Read 1363 times)
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WV Hoopie
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« on: October 17, 2009, 01:53:10 PM »

I'm back with more questions:

First, the project box with the voltage divider circuit is built! It has two taps on the resistance, one at 3megs and the other at 6.25megs. That takes care of the low & high power issues of the Johnson Desk.

Rain has started here in the Pacific Northwest and the electronics of the pedestal will be on the dolly real soon. So while the beast is all out in the open, the coupling cap for audio sampling/trapezoidal display will be installed.

Question: Seems like a good time to mount some of the HV transformers on insulators. HV Plate, Modulation, HV Choke, HV Filament are the candidates. Did I miss anything?

In other discussions, the mention of fusing the center tap/ secondary of the HV Plate transformer could have issues??? The desk uses a full wave rectifier circuit, not a bridge. I've had others on the west coast 3870 group that were against insulators and suggested fusing the center tap. I don't quite agree with their logic.

Since the entire electronics are within an enclosure where fingers can't touch the transformer frames, insulators at first glance seems good enough?Huh

Tnx, & 73's
wd8kdg
Craig
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WQ9E
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« Reply #1 on: October 17, 2009, 02:42:58 PM »

Craig,

First, congratulations on making good progress on the Desk and it is worth the effort.  If you haven't already done so, make sure to oil the bearings on all 4 of the fans while they are accessible.  After one of the pedestal fans went up in smoke I replaced both of them with externally mounted Rotron Patriot fans with a filter on the lower one.  Without filtered air, you should check/clean the intake for the lower fan in the RF deck around once a year if you use it a lot; otherwise you may not have sufficient cooling for the base of the 4-250/400 finals.

As to mounting transformers off ground:  I like saving parts and money but if a transformer or choke is defective to the point that it is leaking/shorting to the core or frame it is time for a rewind.  Rest assured that at some point you (or a subsequent owner) are going to be doing some observation and troubleshooting with the works outside of the pedestal.  Like most of us you have a long time in radio being used to big metal transformer cases being at ground potential and all it takes is one mental slip causing you to rest your hand on one of those conveniently placed pieces of iron and you or the next owner will be history.  I value my life as being worth a whole lot more than the cost of a rewound or hypersil type replacement.  

Disclaimer: most of my vintage collection was purchased using proceeds from my consulting work in enterprise risk management.  You can interpret that as either I am well paid because I know a lot about risk management or I am overly concerned about risks because I spend a lot of time in the domain; your call.

The Desk is pretty ruggedly built and if you run it near the current legal limit (only half an S unit down from its full bore rating) it is going to last a long time.  In the "low" power position you can easily load it to over 300 watts out and the power supply is running at the half voltage setting.  Nothing is working very hard and it puts out a big signal.  I added inrush limiters to all supplies on mine and run it through a properly fused source.  Make sure that the modulation transformer gap is set properly and use a properly sequenced T/R relay.  The only failures I have experienced in multiple years of use are one fan and one crimped connector in the power supply distribution that finally gave up after 50 years.

The only modifications I made to mine are the inrush limiters, a pair of SS relays to replace a bad plate transformer relay (these also help to "softly" key the plate transformer), a screen current meter in an external box, and the replacement fans which are much quieter and move more air than the originals.  

Good luck on getting it back on the air soon!
Rodger WQ9E
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Rodger WQ9E
WV Hoopie
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« Reply #2 on: October 17, 2009, 05:00:22 PM »

Rodger,

Tnx's for the input. My desk, serial #100124 has been on the air at this QTH for about three years. Knock on wood, no real problems so far. There were a few bugs when I brought it home in many pieces. Someone had used an ignition points file on the HV relay and not much was left of the contacts. Low power was ok, but it spit arcs n' sparks on the high tap. Hard to believe, I had a new spare in the junk box. Several weeks were needed to go all over the beast before turning on the filaments. Found that someone with little or no soldering experience had been in the meter shielding box. Solder looked like rabbit droppings. The felts for the fan bearings received the 3 in 1 oil treatment. There is a NOS fan motor for either the 500 or the desk in the junk box if the occasion occurs. Relay sequencing hasn't been an issue, KW matchbox works fine with the 75 meter full wave loop. Got an extra delay relay on the R-390A to keep it quiet for a moment longer after the matchbox relay switches to receive.

The risk factor is a real bugger. I don't lean on anything with HV, but I'm getting older and might have a senior moment. Then there is the issue of fewer places to get big iron rewound or to purchase new iron.

What's a poor man to do in a big town?
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WQ9E
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« Reply #3 on: October 17, 2009, 05:50:38 PM »

Craig,

You are welcome and as I understand the serial numbering yours is an early one as is mine.  But unlike the early 500's the Desk really doesn't have a reputation for losing transformers or chokes.  There isn't any common failure factor other than age (or abuse) that has shown up.  If the insulation is breaking down on the HV or modulation transformer you would likely soon get one or more shorted turns rendering it useless anyway.  Since the design intent was for the transformer and choke cases to be at ground potential if you raise it above ground and leakage occurs it is possible that a second failure would occur with the high potential case now breaking down to a low potential point in the winding and rendering the component useless anyway.  I would use and enjoy your Desk as intended and if a failure occurs then "bite the bullet" and fix it the right way.  But creating a safety risk in the process of trying to avoid a failure risk is just not a good path to take.

I have seen other relays where someone attacked the contacts with a file instead of a burnishing tool; the only result was a relay that was somewhat bad turned very bad.  The contacts on my plate relay were burnt and I think a lot of this was due to some dirt buildup on the mechanism that caused excessive contact bounce.  When I got the Desk, the 872 rectifiers had been replaced by the encapsulated cylindrical SS units that plug into the 872A socket.  I removed the rectifier filament transformer and put it in one of the desk drawers and the space and existing holes allowed me to mount the new SS relays on their heat sink with no holes required.  These relays switch on at the zero crossing point and have built in surge suppression and the only issue I had was they needed an RF choke in the AC control lead to prevent RF modulation of the relays (that created some interesting audio!).

I also use a full wave 80 meter loop with mine and tune it with a homebrew reversible L network for all band use.  I use a vacuum relay with delay on break for antenna switching and an additional relay takes care of muting; currently I have an SX-88 paired with my Desk.  I have been tempted several times by the KW matchbox but I don't want to modify one for 160 and I also use the same tuner with a couple of other operating positions that cover 160.

Keep an eye on your 500 and make sure the mechanical delay relay is still providing delay on break; otherwise you will hot switch your antenna with resulting arcing.  I think the mechanical delay tends to go away with age and I added a small electrolytic capacitor in parallel with the relay coil along with an isolating diode which took care of the sudden appearance of arcing at the end of transmissions.

Hopefully we will have a Desk to Desk QSO soon.

Rodger WQ9E
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Rodger WQ9E
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