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Author Topic: Heath kit SB-200 bleeder resistor temperatures  (Read 8440 times)
WA7PRC
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« Reply #15 on: March 03, 2018, 03:03:06 PM »

Those electrolytics appear to be original. If they're not restuffed with new capacitors, they're almost assuredly dried-up and should be replaced. Then, the bleeder/equalizer resistors can be changed to something close to 100K. If the capacitors have been restuffed w/ new, just change the resistors.
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W1QJ
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« Reply #16 on: March 03, 2018, 08:05:59 PM »

In the 40+ years I’ve been working on SB220 and 200’s I don’t think I’ve seen more than only a couple of bad bleeders.  This still amazes me.
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WA7PRC
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« Reply #17 on: March 04, 2018, 03:35:43 AM »

In the 40+ years I’ve been working on SB220 and 200’s I don’t think I’ve seen more than only a couple of bad bleeders.  This still amazes me.
Mine were still good after 40 years when I replaced them. It didn't make sense to leave them in, with newer/better electrolytic capacitors.
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HAMHOCK75
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« Reply #18 on: March 04, 2018, 02:09:41 PM »

I was just curious how hot these resistors actually were when I read about them being hot here and elsewhere on the Internet. As mentioned by K6AER, there is high voltage down there, so I used my handheld infrared thermometer to scan the capacitors and resistors initially. The capacitors were just above ambient, but when I aimed the laser dot on the resistors, the temperature soared to over 150 degrees C.  As you are aware, the accuracy of infrared thermometers is subject to the emissivity of the device under test. I then used the thermocouple thermometer with a bit of thermal grease to get a more accurate reading which ended up at 173 degrees C.

Have you ever been able to identify who made those resistors? These have no markings of any kind on them.
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KM1H
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« Reply #19 on: March 05, 2018, 06:25:52 PM »

Quote
In the 40+ years I’ve been working on SB220 and 200’s I don’t think I’ve seen more than only a couple of bad bleeders.  This still amazes me.

Yet Ameritron still uses failure prone bleeders, too bad nobody there wants to rock the boat and modify the BOM.

Alpha used A-B carbon resistors that are still well within tolerance.

I have not heard of a bad Amp Supply bleeder but their filter caps are well past the expiration date, especially the 3-500Z amps where the 3100V rating is pushing things.

Carl
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W1QJ
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« Reply #20 on: March 05, 2018, 06:57:35 PM »

Ameritron uses Dale resistors which is a well known brand and not considered junk however they fail miserably in Ameritron amps.  They use 50K ohms and arebrated 7 watts.  Other companies used higher value carbon resistors and they seem to last forever it seems.  I’m always changing Ameritron bleeder resistors.  Our new 10 cap/diode board addresses all issues.
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N2EY
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« Reply #21 on: March 06, 2018, 07:32:39 AM »

The six resistors were carefully inspected. They look like new. None show any signs of cracks in the coating. With the front panel off, I was able to look underneath them since they are spaced off the board but they have no markings of any kind. No manufacturer name, no value. I was hoping to to identify the manufacturer to see what their recommended de-rating for these parts was.

The temperature rise inside that compartment with the cover in place ( but still without the external box ) was 24 degrees C.

Which should be no surprise, considering that each resistor is dissipating a bit more than 5 watts, and the 6 of them about 30 watts. And they are dissipating that much for the entire time the amp is producing HV.

I suspect that years of operation simply roasted off any and all markings.

As WA7PRC notes, in the bad old days the tolerances on electrolytics were greater, so the equalizing resistors had to be fairly low in value to avoid too much unbalance. Not the case today.

Ameritron uses Dale resistors which is a well known brand and not considered junk however they fail miserably in Ameritron amps.  They use 50K ohms and arebrated 7 watts.  Other companies used higher value carbon resistors and they seem to last forever it seems.  I’m always changing Ameritron bleeder resistors.  Our new 10 cap/diode board addresses all issues.

Any resistor will fail if pushed too hard. My rule-of-thumb is that continuous-duty resistors should be derated by 50% - if you need to dissipate 5 watts continuously, use at least a 10 watt resistor.

73 de Jim, N2EY
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W8JX
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« Reply #22 on: March 06, 2018, 07:51:39 AM »


Any resistor will fail if pushed too hard. My rule-of-thumb is that continuous-duty resistors should be derated by 50% - if you need to dissipate 5 watts continuously, use at least a 10 watt resistor.


Very good advise along with earlier mentioned spacing them at least a 1/4 inch or more off of PCB too.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
K6BRN
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« Reply #23 on: March 08, 2018, 08:13:03 PM »

The SB-200 I rebuilt last year had an add-on muffin fan installed in the voltage doubler compartment just to remove all the excess heat.  And it ran HOT.  When I replaced the entire voltage doubler board/capacitors/bleeder resistors with the Harbach kit, I pulled out the fan and now it runs much cooler, even without the extra airflow.
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WA7PRC
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« Reply #24 on: March 09, 2018, 01:15:30 PM »

The SB-200 I rebuilt last year had an add-on muffin fan installed in the voltage doubler compartment just to remove all the excess heat.  And it ran HOT.  When I replaced the entire voltage doubler board/capacitors/bleeder resistors with the Harbach kit, I pulled out the fan and now it runs much cooler, even without the extra airflow.
The H*rbach PM-200 uses 82K resistors or 100K, as shown here:

(click for yuuge image)

Given the same applied voltage, there's a lot less power dissipated in 82K or 100K resistors than 30K resistors. Based on no load
+B = 2400V as shown in the manual...
P = E^2 / R
OE = 2400^2 / 30K x 6 = 32W
H*rbach = 2400^2 / 82K x 6 = 11.7W
or
H*rbach = 2400^2 / 100K x 6 = 9.6W

Whichever you consider the H*rbach setup, as I showed here (link), it costs A LOT more than just restuffing the OE PCB.
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KM1H
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« Reply #25 on: March 09, 2018, 03:08:14 PM »

Quote
Whichever you consider the H*rbach setup, as I showed here (link), it costs A LOT more than just restuffing the OE PCB.

Which should include all resistors and diodes.

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WA7PRC
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« Reply #26 on: March 09, 2018, 03:22:07 PM »

Whichever you consider the H*rbach setup, as I showed here (link), it costs A LOT more than just restuffing the OE PCB.
Quote
Which should include all resistors and diodes.
As I showed, it does:
[snip]
Total = $34.55
OTOH, the Harbach pn PM-200 runs $95.00.
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KM1H
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« Reply #27 on: March 09, 2018, 03:55:50 PM »

Quote
Whichever you consider the H*rbach setup, as I showed here (link), it costs A LOT more than just restuffing the OE PCB.

Quote
Which should include all resistors and diodes.
As I showed, it does:

I was replying to your restuffing the OEM board statement. The Harbach board is quite obviously new.
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K6BRN
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« Reply #28 on: March 14, 2018, 07:56:09 PM »

Bryan:

The ancient phenolic PWB in the SB-200 voltage doubler compartment was disintegrating into annoying small flakes - it needed to be replaced.  At about $90 with all parts, I didn't think it was too expensive.  While I was at it, i replaced the output band-switch wafer (blasted to bits on 10M), added the soft-start kit (not really needed but cool to have), soft-key kit, metering protection diodes, two NOS Cetron 572B's (old ones had slightly low output and envelopes had deformed from heat), new power cord, added the Heathkit service note ALC limiting zener for use with solid-state rigs and re-tuned the input networks, including a change of mica caps on 10M.  Oh, and I installed a rotary operate/standby switch built into the metering sensitivity knob - looks stock except for an OEM-like "STBY" label next to the knob.  I put about $300 into the amp, some elbow grease  and now its in pristine shape and delivers full 600+ watts CW on most bands (10M is slightly less).  It was a fun project.

Brian - K6BRN
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WA7PRC
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« Reply #29 on: March 14, 2018, 08:50:22 PM »

The ancient phenolic PWB in the SB-200 voltage doubler compartment was disintegrating into annoying small flakes - it needed to be replaced.  At about $90 with all parts, I didn't think it was too expensive.
You can buy the H*rbach's bare SB-20x rectifier/filter PCB for $25, and buy the parts I researched earlier in this thread (or similar) for A LOT less than $90. Lou W1QJ likely has good OE PCBs for cheeper. Since the board is very simple, even a piece of perforated board will work for a one-off board. Since cheap/easy solutions abound, there's no need to spend more money.
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