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Author Topic: Heath kit SB-200 bleeder resistor temperatures  (Read 8447 times)
HAMHOCK75
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« on: February 28, 2018, 03:25:58 PM »

I recently had chance to examine a Heath SB-200. This was the biggest surprise. The wirewound bleeder resistors in the power supply get extremely hot reaching 173 degrees C.



The voltages across each resistor and the value of each resistor were all measured. All the 30K resistors were within 10% with the lowest values being 28K.

This seems to be not the best design practice.

http://www.interfacebus.com/Resistor_Derating_Guide-MIL-R-39007.html
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W1QJ
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« Reply #1 on: February 28, 2018, 03:33:05 PM »

Those resistors run VERY VERY hot, nothing new there.  Despite that, I have never seen one go open.  You can leave them or change them if you wish.
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HAMHOCK75
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« Reply #2 on: February 28, 2018, 03:42:05 PM »

I have read some comments about them being hot but no measurements. As you say that is very, very hot!

I do restoration so it is not part of any objective to improve upon anyone's design. Thank you for that feedback. I plan to leave them as they are.

They do nice job of equalizing the voltages across those 450 WVDC capacitors.
« Last Edit: February 28, 2018, 03:51:34 PM by HAMHOCK75 » Logged
N8CBX
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« Reply #3 on: February 28, 2018, 03:59:55 PM »

Placing them close to the PCB is probably the worst place for cooling, making them even hotter.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
KM1H
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« Reply #4 on: February 28, 2018, 05:31:24 PM »

Im sure they elevated the filter cap temperatures quite a bit in the SB-200,and 220.

When I was building them for Heath stores, converting to 6, or just repairing I used 50K, then 75K.

While Heath bought the cheapest caps I never measured any excessive deltas in an amp  that would have required lower value equalizers. There was also sufficient voltage overhead and on the 200 I left maybe a 1/4" of air space beneath them.

When CDE 381LX series Snap In caps came along I used 100K, and some of my own amps and others Ive kept track of had zero problems since they came out in the early 90's or so.

Carl
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AA4HA
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« Reply #5 on: February 28, 2018, 06:09:07 PM »

You can see how a standard cap that is rated at 85C might be leading a hard life being in such close proximity to components that are running almost 90C hotter just a centimeter or so away. The radiant heating coming off of the resistors is going to shorten the life expectancy of those caps.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
N2EY
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« Reply #6 on: March 01, 2018, 06:48:59 AM »

Let's do the math.....

Suppose there's 390 volts across a 30,000 ohm resistor. Current is then 13 mA. Power dissipated is 5.07 watts. That will make a resistor of that rating quite warm.

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WA7PRC
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« Reply #7 on: March 01, 2018, 07:01:06 AM »

When these amplifiers were new, electrolytic capacitors had widely varying characteristics. At THAT time, ≈ 30KΩ bleeder/equalizer resistors were appropriate. Electrolytic capacitors produced in recent decades are MUCH better, and don't need "stiff" equalization.  Smiley

OTOH, low value resistors will dissipate more power, and help keep you warm in Winter.  Wink
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KE7KPB
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« Reply #8 on: March 02, 2018, 05:58:34 AM »

Time to rebuild or replace the power supply. I used the Harbach kit and they work great.
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K8AXW
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« Reply #9 on: March 02, 2018, 09:32:02 AM »

I rebuilt my SB-200 power supply, recapping it and changing the bleeder resistors.  The PCB was not 'charred' but showed evidence of extreme heating.

When I installed my new resistors I elevated them about a 1/4" above the board. CBX described this problem very well.
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A Pessimist is Never Disappointed!
W9PMZ
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« Reply #10 on: March 02, 2018, 01:32:16 PM »

actually that is a mistake I made in assembly.
The assembly manual states to place the resistors off of the PWB.  Maybe 1/4 of an inch but I don't recall.
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WA7PRC
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« Reply #11 on: March 02, 2018, 10:42:11 PM »

Time to rebuild or replace the power supply. I used the Harbach kit and they work great.
If you don't need a PCB, the cost is a LOT more. Digi-Key has:

Capacitors C4-C9: CDE capacitors pn 381LX151M450H032 (150uF/450V) as stk no 338-3698-ND for $3.96 each.
Resistors R5-R11: TE Connectivity pn ROX3SJ100K resistors (100K/3W) as stk no A106004CT-ND for $0.46 each.
Diodes D1-D16 (1N2071/1N4005/1N5397) use 1N5408: Diodes Incorporated pn 1N5408-T (3A/1KV) as stk no 1N5408DICT-ND for $0.39 each.

Added up, it's:
$3.96 x 6 =  $23.76
$0.46 x 6 =  $ 2.76
$0.35 x 16 = $ 6.24

Assuming R14 (3.6K/1%/0.5W) and R15 are needed, those can be added:
Ohmite pn WHE1R0FET, (1 Ohm/1%/5W), stk no WHE1R0FETCT-ND for $1.50 each
Vishay/BC Components pn MBB02070C3601FCT00 (3600 Ohm/1%/0.6W), stk no BC4467CT-ND for $0.29 each.

Total = $34.55
OTOH, the Harbach pn PM-200 runs $95.00.

≥ 3A meter protection diode(s) such as 1N540x or 6A10 should be added between -B & chassis. W8JI s'plains it here: http://www.w8ji.com/metering_amplifier.htm

Better HV meter multiplier resistors (R3 = 3 x 4M7/1W) should be used. Digi-Key has Vishay/BC Components pn VR68000004704JAC00 stk no VR68J4.7MCT-ND for $0.78 each. Unlike OE carbon composition resistors, these are rated to withstand the HV, and won't soar in value.
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AD4U
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« Reply #12 on: March 03, 2018, 05:18:44 AM »

I built my SB 200 in my college dorm room in one "all night session" in 1971. It was a 1970 Christmas gift from my Dad who died a month later.

As I remember in the instructions, you were told to use a square resistor to space the bleeder resistors about 1/4 inch off the PS board. When I turned my SB 200 on for the first time as the sun came up after some 15 hours building the SB 200, I thought something was burning and a little smoke came out of the top of the SB 200 cabinet. I though that I made a wiring error but it was just the bleeder resistors "burning in".

As of 2018 some 47 years later, my SB 200 is still running the original bleeder resistors and the original Cetron 572 tubes and it still gives full output. Granted my SB 200 has not seen a lot of use in the past 30 years, but every now-and-then, I will fire up my SB 102 and my SB 200 and make several contacts.

Dick AD4U
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K6AER
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« Reply #13 on: March 03, 2018, 09:08:37 AM »

I noticed the thermal conductor wire laying on the chassis. Good thing the resistor had no voltage breakdown of the ceramic. Depending on the resistor chosen you might have had a couple thousand volts on the thermal lead.
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HAMHOCK75
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Posts: 617




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« Reply #14 on: March 03, 2018, 02:55:46 PM »

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.
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