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Author Topic: Please consider a fuse in the hv supply of your boat anchor.  (Read 1935 times)
N4MQ
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Posts: 353




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« on: October 04, 2018, 04:06:28 AM »

The medium voltage power supply of my Invader 2000 died while I was completing alignment of my restoration.  The choke went up in smoke during testing and smelled really bad.  The windings were charred but still continuous.  Investigating the 5 henry choke had #32 wire in it and not worth the effort to rewind, a new one was purchased for $30.

There was no indication of a failure part wise in the transmitter, so I added the new choke in and added a 1/4 amp fuse to the b+ line.  I wanted a meter to monitor the power drain but did not want the issues with trying to install another meter - somewhere on the power supply.  I mounted a 6 volt 200 ma light bulb and holder on the power supply chassis in series with the fuse so that I could see the current during various operating modes.  This arrangement will allow checking and protection for the cost of a lamp and fuse.  If the power transformer were to die, the costs would be significant.  I now have more protection, and extended it to the high voltage supply of the exciter as well.

For my Johnson Thunderbolt 3 KV supply I have fused the power transformer center tap with an insulated microwave high fuse, available on ebay,

 https://www.ebay.com/itm/Microwave-Oven-High-Voltage-Fuses-Fuse-Holder-5KV-0-7A-0-85A-HU/232762591544?var=&hash=item3631bb4938.

They are delivered to you!! for $1.28, rated at about 3/4 amp at 5 KV, gotta love ebay, baby!

Protection is cheap and worth the effort to add, please consider it.
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N4MQ
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Posts: 353




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« Reply #1 on: October 04, 2018, 04:42:59 AM »

O B T W,  the failure in my rig was a gassy 12BY7 that was over dissipating and eventually drew enough current to fault the power supply.  It glowed blue in my tube tester, but that was not visible with a tube shield in place on the chassis.  

If you need replacement fuses, you can buy 10 replacement fuses for $1.71     DELIVERED to you !

https://www.ebay.com/itm/10PCS-5KV-0-6A-0-65A-Microwave-Oven-High-Voltage-Fuse/192523522687?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D54443%26meid%3Dbf61d0219afd4b41b2809196a958bf1a%26pid%3D100005%26rk%3D1%26rkt%3D2%26sd%3D232762591544%26itm%3D192523522687&_trksid=p2047675.c100005.m1851

Woody
« Last Edit: October 04, 2018, 04:47:37 AM by N4MQ » Logged
KM1H
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Posts: 5555




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« Reply #2 on: October 04, 2018, 01:47:52 PM »

That 12BY7 failure is often indicative of a bias supply problem in various TX such as defective paper and electrolytic caps dragging the voltage down and putting the tube in thermal runaway and destruction.

The schematic I see on BAMA is a PITA to follow but here are a number of suspect caps that are in several circuits and should be replaced. Some are in the bias circuit and others elsewhere. A leaky electrolytic can easily take out a filter choke and sometimes the rectifier tube.

PAPER:
.1 400V  C-86, C-100, C-122
.1 200V  C-123
Those can all be replaced with a 400 or 600V as modern film sizes are much smaller

.22 200V  C-119
.5   200V  C-23
1    200V  C-68
2    200V  C-123

Electrolytic:
12  150V  C-138
8    450V  C-67
100 25V   C-65

There are several more electrolytics in the PS and all are well beyond useful/safe life.


I have not worked on a Invader 200 or 2000 but plenty of the Rangers, VI and II, and Valiants and they share common defective components across all models due to age. Started my Johnsons in 56 with a V-I and bought its replacement about 20 years ago and added a V-II CDC, Ranger, and Valiant which are all used on AM

Carl
Ham since 1955
 
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G3RZP
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Posts: 1323




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« Reply #3 on: October 05, 2018, 03:54:54 AM »

Bear in mind the quality of the fuses - in the UK, our mains plugs are fused. The spec for the fuse is that it has to be able to interrupt 6000 Amps. Over half a million fuses imported from China had to be removed from the market after mains plugs and sockets were blown apart (and in a few cases, blown out of the wall!) because of  cheapened manufacturing practices meaning that they didn't meet the requirements on rupture capacity.
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VR2AX
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« Reply #4 on: October 05, 2018, 04:29:02 AM »

I think China has tightened up a lot.
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G3RZP
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Posts: 1323




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« Reply #5 on: October 05, 2018, 05:25:16 AM »



Quote
I think China has tightened up a lot.

We are still seeing RFI from small switching PSUs, where the holes in the PCB for the filter components are empty!
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N4MQ
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Posts: 353




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« Reply #6 on: October 05, 2018, 05:46:34 AM »

I was suggesting that the high voltage supplies be protected from damaging currents due to faults.  Adding a fuse to the power supply is a simple way of protecting your power supply magnetics from damage.  Old equipment typically failed to consider the addition of protection.  Even old receivers could benefit from fuse protection from shorts or bad caps taking out other parts

For that matter old equipment sometimes grounded the chassis to one side of the power line, without the benefit of a polarized fuse.

My suggestion of the microwave fuse for hv supplies is to allow others to consider a high voltage rated fuse that is insulate and available at a really low cost.  They are in millions of microwave ovens for their protection.  Off the shelf components can be used to help our hobby, if only we know about them.  Woody-the protected one.
« Last Edit: October 05, 2018, 05:48:47 AM by N4MQ » Logged
VR2AX
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« Reply #7 on: October 05, 2018, 07:27:38 AM »



Quote
I think China has tightened up a lot.

We are still seeing RFI from small switching PSUs, where the holes in the PCB for the filter components are empty!

Good one!
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G3RZP
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« Reply #8 on: October 05, 2018, 09:24:41 AM »

I quite agree about using an HV fuse - just make sure that it is of good quality. The one I have seen that are available from reputable suppliers are the equivalent of several dollars over here. Some 40 or so years ago, a number of amateurs building 2m amplifiers with a pair of 4CX250B at 2kV of B+ tried using standard 1.25 inch long glass fuses: an HV arc would shatter the glass with a very loud bang!

My last amplifier had a 6 inch length of 42 gauge wire between two ceramic standoffs with a small spring to tension it. That worked very well......

But if fusing the HV to a tetrode amplifier, do put in some interlocking to prevent the HV going and the screen grid volts staying on. That can ruin a tetrode very rapidly.

A primary fuse for a boat anchor supply is a good idea, and a B+ fuse is an additional protection.
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KM1H
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« Reply #9 on: October 05, 2018, 09:50:33 AM »

It might be better to forget the HV fuse and use a trigger to cause an instantaneous dump of the HV. Commercial stations have been doing this for decades up to hundreds of KW TX.

I use rebuildable HV fuses made from originals that I replace with a tiny wire packed in sand and the original ends reused. Bussman HV* series are often found at hamfests and make good forms irregardless of original ratings.

Carl
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G3RZP
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« Reply #10 on: October 05, 2018, 10:08:34 AM »

Crowbar circuits are useful, but you need to make sure that the parts can withstand a lot of pulse energy.....
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KM1H
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« Reply #11 on: October 05, 2018, 06:10:13 PM »

One good reason to not get anal about too much C in the PS. Joules can bite you in the butt be it be a lot of stored energy or your dog.
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G3RZP
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« Reply #12 on: October 06, 2018, 01:12:43 AM »

Back in 1966 when I was an apprentice, I worked in a group on a 10kW PEP 6GHz Satellite ground station tx, with 2 TWTs going from 1mW to 10kW. The HV supply for the collector of the final was 18kV at 3 amps: the capacitor was 24mFd rated at 30kV. Almost 3900 J.....just a bit less than 1 gram of TNT. The surge limiting resistor was carbon, 10 ohms and about 18 inches long and 3 inches in diameter. After about 300 deliberate trips of the crowbar (life testing), the resistor literally exploded and came out of the presspahn sided cubicle!
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K0IZ
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« Reply #13 on: October 06, 2018, 05:20:06 AM »

A 1 or 2 watt carbon resistor has been used in a number of vintage amps.  My Collins 30L1, for example, as one.  One time I had the cover plate open, forgot to  unground the safety crowbar.  Turned on power.  Resistor virtually disappeared in a flash.  No damage to rest of HV. 
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KM1H
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« Reply #14 on: October 06, 2018, 08:27:57 AM »

The 1963 National NCL-2000 was the first one I saw that used a WW resistor, 10 Ohm, 10W, that tripped the HV current overload relay and survived. Of course the output C was only 10 uF (8 x 80uF) which was considered adequate then.  I have long since upped it to a string of 100uF Snap Ins which fit the original clips and there is no audible hum even on AM.
Dentron used a pair of 1 Ohm 1/2W carbon in parallel as the HV fuse for various tubes and they failed without a lot of angst, just quietly split open. Drake did similar.

Carl
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