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Author Topic: SB-200 High Voltage (low)  (Read 3964 times)
K5DSQ
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Posts: 8




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« on: January 30, 2012, 09:51:41 AM »

I am in the process of bringing my old SB-200 back into service. It was in storage for several years so I decided to replace the rectifier diodes and HV caps as well as the Bias cap prior to firing it up. I tested the RF chokes on the plates as well as the grid resistors and all seemed well. I plan to fully replace the RF chokes on the plates with better chokes if I can get the unit operating. I then followed all the checks that Tom Sowden suggested http://www.w6kan.com/sb200.htm. All seemed well until the final test with the HV connected. Everything powered up correctly (fan, lights, filaments...) however the HV was reading only 1800 volts (no plate current). After about 15 or 20 seconds I noticed a little smoke coming from what seemed to be the bleeder resitstors in the power supply section. I shut the unit down, rechecked all the wiring and changes... checked each bleeder resistor, rechecked everything several more times, went to bed, got up the next morning and check it again. Fired it up and same problem... I can't find any componets that look bad or test bad. Any suggestions deeply appreciated?  Sad
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KA5N
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Posts: 4380




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« Reply #1 on: January 30, 2012, 11:13:28 AM »

Usually the droping resistor string (three 4.7 Meg resistors in series) have gone bad and the
reading of the high voltage is incorrect.  A good substitute is 14 half watt(or one watt) one meg resistors 1% resistors in series  mounted on a piece of perfboard.

What value bleeder resistors did you use and what power rating?  The originals were 30 KOHM
7 watt.   You can parallel higher value resistors of 5 watt rating and come up with 40-60 KOHM
at 10 watt rating.  Of course if you used inferior resistors you many have just burned the
paint off.  If so toss them and get resistors or reasonable quality. 

Or maybe you are just running the amp off resonace.  That means you didn't tune it correctly.
Allen
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K5DSQ
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Posts: 8




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« Reply #2 on: January 30, 2012, 11:22:18 AM »

Thanks Allen… I did not replace the bleeder resistors as they checked ok. I guess that is what I need to do next as well as replace the dropping resistors in the metering circuit. Thanks for your suggestions. Have not had a chance to tune it up yet… All I did was a HV check and then shut it off when the smoke appeared. Smiley

Sandy
K5DSQ
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AD4U
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Posts: 2173




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« Reply #3 on: January 30, 2012, 11:30:06 AM »

If the original bleeder resistors did not smoke before your "upgrade", they should not smoke after the upgrade.  Something is wrong in that area.  If you installed new bleeder resistors (you did not), sometimes they "smoke" just a bit from the initial heat when first used. I call it "the new wearing off".

I have repaired (worked on) many SB 200 amps.  On just about every one, the 3 each 4.7M ohm HV metering resistors had gone way out of tolerance (high), which makes the HV appear to be low.  Do as KA5N suggested.

Dick  AD4U
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K5DSQ
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Posts: 8




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« Reply #4 on: January 30, 2012, 11:39:39 AM »

Hi Dick...

No the resistors are the ones that came with the original kit. I did not fire up the rig prior to replacing the caps and diodes as I was afraid of the age and how long they had been in inactive. I will look at what KA5N suggest. Thanks
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W8JI
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« Reply #5 on: January 30, 2012, 01:31:45 PM »

Hi Dick...

No the resistors are the ones that came with the original kit. I did not fire up the rig prior to replacing the caps and diodes as I was afraid of the age and how long they had been in inactive. I will look at what KA5N suggest. Thanks

I agree with Dick.

If you saw smoke, you have a wiring issue or a bad part. The HV multiplier resistors do not cause smoke.

Low voltage with smoke means you have:

Wrong parts installed

Capacitors backwards

Wrong bleeders

Wrong wiring of the doubler

Whatever it is, smoke is a really very bad sign. I would not power it back up without a total double check of wiring.

73 Tom
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K5DSQ
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Posts: 8




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« Reply #6 on: January 30, 2012, 02:41:40 PM »

Thanks Tom..

I have checked and rechecked the cap installation as well as the diodes. The Bleeder resistors are the same ones that came with the unit. They are (were) well within tolerance. All the new caps and diodes where checked prior to install. I guess new bleeder resistors are next... Thanks for all the suggestions. I will post if that solves the problem.

Sandy
K5DSQ
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K5DSQ
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« Reply #7 on: January 30, 2012, 03:46:27 PM »

Hey guys,

I pulled all the resistors on the power board. The six 30K's read anywhere from 24.9 to 28.2K. Lower than 30K however I would not think those readings as "bleeder resistors" would cause them to smoke or alter the performance. Perhaps I am naive and not up to date where HV circuits are concerned.. All the rest (700, 1 and 3600) were within a few ohms of being right on. I have not replaced the three 4.7 meg ones in the meter circuit. Should I be concerned about these low reading bleeder resistors?

Sandy
K5DSQ
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KA5N
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Posts: 4380




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« Reply #8 on: January 30, 2012, 05:24:52 PM »

    I wonder if you didn't get something on a bleeder resistor (like oil off your, fingers rosin from
solder, mayonaise from your sandwich or who knows what) that smoked when the resistors
heated up.  The original bleeders are wirewound and would only smoke if cracked and the
varnish on the wire inside smoked.
Are you sure you mounted the capacitors with the correct polarity?  Since the originals had
twist lock tabs and most new capacitors are snap lock, you usually have to drill new mounting
holes for the tabs which are pretty small.  Or maybe you used axial capacitors and just got
one or more backwards.
Good Luck
Allen
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K5DSQ
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Posts: 8




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« Reply #9 on: January 30, 2012, 06:11:59 PM »

HAHAHAHAHA... Great Allen. I have been known to carry an excessive amount of food from the table on my tie or shirt, but never my fingers. Going pass that on to the XYL!! She will love it!!! Unfortunately where the bleeders are located makes it very hard for me to put my fat fingers anywhere around them.
The new caps are well marked, the old ones listed the can as being negative (the outer lugs), the schematic is pretty easy to understand, I did need to get creative on how they mounted, but I feel pretty certain I have them correct. Perhaps I just need to put the old resistors back in and let them burn. At this point I doubt it will hurt anything but the caps or the resistors (most probable)... Think I might replace the three 4.7 meg resistors prior to the scheduled burn just to make sure the HV is correct. Any suggestions appreciated. I will keep you posted unless you have other suggestions...

Many thanks,

Sandy
K5DSQ
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KA5N
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Posts: 4380




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« Reply #10 on: January 30, 2012, 07:26:20 PM »

I didn't mean to imply that you were a chow hound, but I have lost sandwiches several times
when I was working on something.

You can't buy the two watt 4.7 Meg composition resistors anymore (unless you can find
NOS ones) and they would eventually go bad anyway.  14  1%  1/2 watts are better and
will last a long time.  I just did mine on a piece of perf board and raised the board a half inch
or so off the chassis.  I have two SB200's one is older and one is newer and the former
owner had put the Harbach power supply board in.    There is no difference in the operation.
The only difference is that the harbach board costs about  $ 100 while new capacitors and
other parts are less than $50.  The old PCB is ok as it is with maybe a few holes drilled for
the caps. 

73  Allen
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VE7RF
Member

Posts: 212




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« Reply #11 on: January 31, 2012, 04:23:58 AM »

Use  100 K- 3 watt  MOF  resistor's...one per HV cap.   Mouser has em..and  in 1% Tol  if you like.  Nobody puts 30 k  wire wounds across hv caps these days.  RL Drake used  100 K-2 watt carbons across all their  450 vdc caps..and that was from 1963  till they stopped making the L4B.
I buy the  100 k- 3 watt MOF's  from Mouser... made by VISHAY. These are 1% Tol  rated.  I bought  3 box's  of em... 100 per box.  All 300 of em  are exactly .2%  on the low side. There is no more than just a few ohms between any of em.   That's the very best resistor you can get for putting across a 450 vdc cap.   No need to play match up either.   The vdc across each cap  will be virtually identical.

As  far as the multiplier resistors for the HV meter.  Mouser carries the ohmite slimox brand and also others.  These are made for  HV.    You can get em up to 25 kv..EACH if you want. ..and up to I think 5 watt.   They also have em in 3 kv.   And you can get all of em in 1% Tol too.

Later.... Jim   VE7RF
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W8JI
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« Reply #12 on: January 31, 2012, 05:08:34 AM »

Hey guys,

I pulled all the resistors on the power board. The six 30K's read anywhere from 24.9 to 28.2K. Lower than 30K however I would not think those readings as "bleeder resistors" would cause them to smoke or alter the performance. Perhaps I am naive and not up to date where HV circuits are concerned.. All the rest (700, 1 and 3600) were within a few ohms of being right on. I have not replaced the three 4.7 meg ones in the meter circuit. Should I be concerned about these low reading bleeder resistors?

Sandy
K5DSQ

Sandy,

That is a problem some others deny, that is a common problem with carbon resistors. DO NOT ever use carbons in high temperature high voltage drop applications.

Replace them all with metal type resistors, either oxide, metal composition, or wire wound.

Carbon is a semiconductor, and as such the common failure mode is decrease in resistance. The carbon resistors in the early AL80 were a source of many infant electrolytic failures. This was within a year of being sold. Looking back at the history of Dentron warranty service, there were abnormally high infant failures of decreased resistance.

No one should ever be using carbons in high-dissipation critical applications where a stable resistance is required, especially voltage dropping, because carbons can and often do fail low in resistance.

The initial value of the resistors is fine, but the material is dumb-headed.

73 Tom

 
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K5DSQ
Member

Posts: 8




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« Reply #13 on: January 31, 2012, 07:23:30 AM »

Thanks guys for all the help. Have the new resistors on order. I will let you know how it goes.

Sandy
K5DSQ
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K5DSQ
Member

Posts: 8




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« Reply #14 on: February 04, 2012, 02:00:19 PM »

The new 100K 3 watts did the job!! All is well in Amplifer land. Thanks for all your help!!

Sandy
K5DSQ
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