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Author Topic: SB220 with Harbaugh mods  (Read 3192 times)
W7VO
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« on: November 02, 2017, 12:42:50 PM »

A ham friend of mine just built a NOS 1978 vintage Heathkit SB-220 kit that was missing the original power supply board and associated parts. He bought the Harbaugh PS board to replace it, and also purchased a softstart and softkey kits at the same time. Then he built the kit from scratch, installing the new stuff as he went along. Probably not the wisest choice for a new kit builder who is not an engineer! 

He brought it to my house for a check out, we plugged it in, turned it on and noticed that the soft start resistors were getting very warm and the HV was very low. I bypassed the softstart 20 ohm resistors, and the HV came up as it should. I did some checking and found out that the 120V supply was only 8 volts though. Not good, and the most likely the reason the soft start is not working. Now I need to troubleshoot this, but it appears that there is considerable mods done to the 120V supply as part of one of these kits. Is there anybody that has or knows of a SB-220 schematic of the amp with the Harbaugh mods installed to make it easier for me to trace this out?

Thanks!

73;
Mike
W7VO
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W1QJ
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« Reply #1 on: November 02, 2017, 01:07:19 PM »

When you install the step start the parts kit includes a new bridge rectifier for the 120vdc supply.  It switches the old single half wave to the bridge.  Perhaps something isn’t right with that wiring.  Make sure that circuit is correct.  You should be reading about 120vdc coming out.
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KM4AH
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« Reply #2 on: November 02, 2017, 08:23:12 PM »

Yep. Make sure the soft start is wired correctly. If 240 it's hard to wire it wrong because both legs are the same. But, on 120 you can get it crossed .

I bought a Clipperton that had the SS installed and it worked fine on 240, but when I changed it to 120 I had to change the leads.
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W7VO
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« Reply #3 on: November 03, 2017, 02:44:20 PM »

No, if you read my initial post, the +120V supply is only something like +8 volts. I'm pretty sure that is where the issue is. Maybe the diode bridge rectifier is miswired, who knows. Lacking a proper schematic of the SB-220 with the Harbaugh softstart mod makes things a bit more difficult....

Time to dive in and trace out the wiring, and compare it to the instructions.  ;-)

73;
Mike
W7VO
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W1QJ
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« Reply #4 on: November 03, 2017, 03:00:09 PM »

No, if you read my initial post, the +120V supply is only something like +8 volts. I'm pretty sure that is where the issue is. Maybe the diode bridge rectifier is miswired, who knows. Lacking a proper schematic of the SB-220 with the Harbaugh softstart mod makes things a bit more difficult....

Time to dive in and trace out the wiring, and compare it to the instructions.  ;-)

73;
Mike
W7VO

The instructions for the addition of the soft start assume that the amplifier is already built to the original design.  If it has been buit already then adding the soft start is very easy if you follow the instructions with the soft start kit.  Suffice to say that 8vdc coming out of the 120vdc supply isn't going to make the step start relays kick in and the resistors will heat up quickly and get very hot.  The circuit is quite simple.
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W7VO
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« Reply #5 on: November 03, 2017, 04:28:00 PM »

Well, it turns out that he had the red wire from the +120V supply to the T/R relay connected to Pin 6 of the relay, not Pin 3. Oops.... Moved the wire and the +120V supply is now +122V! The soft start now works, as does the soft key (the other place the +120V supply is used). That issue is history.

All that is left now is to find out if the 40 year old NOS Eimac 3-500Zs from the kit are still good.... That might be a bit more exciting!  Wink

73;
Mike
W7VO
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K1ZJH
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« Reply #6 on: November 03, 2017, 04:47:26 PM »

Start it off in CW, and get the plates red!!!
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W7VO
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« Reply #7 on: November 03, 2017, 05:17:01 PM »

Good idea. I wonder if it might be prudent to fuse the HV lead with a temporary 1A fuse in case the tubes are gassy? 
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W1QJ
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« Reply #8 on: November 03, 2017, 05:25:36 PM »

Use a variac bring up slow, look for gas in tube and negative grid current.
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KM1H
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« Reply #9 on: November 04, 2017, 06:02:01 AM »

You dont want to bring the filaments on that way more than once or so.
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WB4SPT
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« Reply #10 on: November 04, 2017, 07:45:07 AM »

Good idea. I wonder if it might be prudent to fuse the HV lead with a temporary 1A fuse in case the tubes are gassy?  

On my 220, i installed the ww R that is in the drake linear.  Or, simply use a 3kV 1A fuse that we all have laying in our junkbox.  But seriously, a tube arc is trouble for various items including the plate meter. Far more useful than the softstart mods are the protection mods. Plate meter diodes, elininate the silly 120v bias arrangement, and some type of HV fusable link.
Especially important if you firmly ground the grids and loose those inductors as fuses!
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WA7PRC
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« Reply #11 on: November 04, 2017, 08:48:04 AM »

Good idea. I wonder if it might be prudent to fuse the HV lead with a temporary 1A fuse in case the tubes are gassy?  

On my 220, i installed the ww R that is in the drake linear.  Or, simply use a 3kV 1A fuse that we all have laying in our junkbox.  But seriously, a tube arc is trouble for various items including the plate meter. Far more useful than the softstart mods are the protection mods. Plate meter diodes, elininate the silly 120v bias arrangement, and some type of HV fusable link.
Especially important if you firmly ground the grids and loose those inductors as fuses!
The Harbach RM-220 board includes 1N5408 rectifiers that protect the meters (D3/D4 across R1 and D5/D6 across R3). Nothing else is needed.

The problem with using an ordinary fuse at 3KV is, when it blows open, it can arc until the PS is turned off. THAT's why fuses have a voltage rating. Better to install a "glitch" resistor in series w/ HV... a wirewound vitreous enamel type that can withstand the yuuge current spike 'til the mains breakers blow. I replaced RFC2 (8.5μH) w/ a 10Ω/20W wirewound (Ohmite pn B20J10RE):


If you want to hard-ground the 3-500Z grids, strips of soft copper can be cut and installed like this:

The socket terminals are bent upward, and the copper strips are fitted. Hardware is #4-40.
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KM1H
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« Reply #12 on: November 04, 2017, 10:06:11 AM »

Quote
If you want to hard-ground the 3-500Z grids, strips of soft copper can be cut and installed like this:

A single point ground might be better at 10/12M as direct grounding raises the tube gain and is more apt to lead to instability than the factory method.
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WA7PRC
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« Reply #13 on: November 04, 2017, 12:06:42 PM »

If you want to hard-ground the 3-500Z grids, strips of soft copper can be cut and installed like this:

The socket terminals are bent upward, and the copper strips are fitted. Hardware is #4-40.
A single point ground might be better at 10/12M as direct grounding raises the tube gain and is more apt to lead to instability than the factory method.
The method I showed (each terminal to chassis) is as close to single-point as is practicable. So far for me, even on 10m, it has proven to be stable.
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W7VO
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« Reply #14 on: November 04, 2017, 01:05:57 PM »

This is not my amplifier, so I'm not going to do any other mods beyond what the owner has already done. This is essentially a brand new SB-220!

Question though.... I don't have a 220 Vac variac, but I do have a 5A 0-150 Vac one.  Can I just bring the amp (still wired with for 220) up on the 150 Vac variac to full voltage? The HV should go up to about half (1500 Vdc or so on SSB, about 1200 Vdc on CW/TUNE) when the variac is on full. (Of course I'll have to short out the soft start resistors first.)

Will that be enough voltage to see if the tubes are gassy before trying the full 240VAC jolt?

Thanks for all the help on this guys!

73;
Mike
W7VO
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