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Author Topic: SB-220 refurb time, check list??  (Read 3821 times)
K1ZJH
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Posts: 1090




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« on: December 12, 2011, 10:36:48 AM »

I'm looking for suggestions on what should be done to refurb my old SB-220.

Filter caps were replaced a few years back, and the amp is seldom used...

I'm planning on replacing the parasitic suppressor resistors, adding glitch resistors,
directly grounding the grids, negative rail protection diodes, and installing a standby
switch.

Any other suggestions? Amp bandswitch was "hotswitched" once, does that usually
destroy a set of contacts? Amp seems to work??  Sources for new or used parts?

PA tune cap arced due a high SWR condition on my tuner... can I simply file the
damage areas smooth?

I'm almost tempted to install a new grid input tuning board, since the input
SWR sucks and the Heathkit arrangement is equally bad... and impossible to
tune. Worth doing? 

Any other suggestions that don't involve nichrome would be appreciated. I only
want to do this once.

Pete
 
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W1QJ
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Posts: 1460




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« Reply #1 on: December 12, 2011, 01:10:36 PM »

Replace suppressors with a clone of the original design.  Negative rail diode and glitch resistor can't hurt either.  Soft key is a must.  You can try retuning the inputs, I think some other devices could be difficult to employ.  Most SB-220 amps have pretty good input tuning but sometimes the caps drift or go bad.  I can rebuild your tune capacitor if it has removable plates.  SOme do some don't. 
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K1ZJH
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Posts: 1090




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« Reply #2 on: December 13, 2011, 07:07:25 AM »

Quote
You can try retuning the inputs, I think some other devices could be difficult to employ.

Thanks Lou. I was thinking of a WD7S TU6 board for the input matching; that is if I have problems
adjusting the slugs or getting into other binds on the original input circuit.  Hopefully it will go
off as planned and work as intended.

Pete
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K1ZJH
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Posts: 1090




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« Reply #3 on: December 13, 2011, 12:08:26 PM »

Is the Zener "protected" when the grids are directly grounded in case the tubes
flash over?

Pete
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KA5N
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Posts: 4380




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« Reply #4 on: December 13, 2011, 12:49:01 PM »

Of course it is your amplifier and you can do as you wish.  However I become a bit
nervous when someone is going to make wholesale changes to a piece of gear and
asks advice (which usually means that the questioner is unsure).
First off the input impedance of a 3-500Z is 115 Ohms and two in parallel will be about
56 Ohms.  So a simple pi network with 50 Ohms in and out should work pretty well
(such as the WD7S circuit)  You could modify the circuit a bit by adding a padder (or
trimmer) to the output side so to fine tune a bit.  With the WD7S you have all those relays which might be a source of trouble and a simple band switch is less complicated.
My advice is don't make a lot a changes at one time, because if you have problems you
will have difficulty tracing the root cause to the correct "mod".
Good Luck Allen
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W8JI
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« Reply #5 on: December 13, 2011, 12:52:17 PM »

Is the Zener "protected" when the grids are directly grounded in case the tubes
flash over?

Pete

The zener is better protected, as is everything else.

I would do minimal changes. The SB220 was a pretty good amp as delivered.

You cannot fix caps properly by filing. Replace the plates. They are readily available from many places for most of the cap vendors that were used by Heath.
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K1ZJH
Member

Posts: 1090




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« Reply #6 on: December 13, 2011, 06:25:00 PM »

Of course it is your amplifier and you can do as you wish.  However I become a bit
nervous when someone is going to make wholesale changes to a piece of gear and
asks advice (which usually means that the questioner is unsure).
Good Luck Allen

Hi Allen... I've been building gear since I was a teen,  about 50 years ago.
I may be confident about what I am doing, but it doesn't hurt to ask regardless.
Sometimes we overlook the obvious; so playing dumb is often the safest
route Smiley

Pete
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AH6RR
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Posts: 803




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« Reply #7 on: December 14, 2011, 10:27:51 AM »

Well since no one mentioned replacement parts then I will. Harbach Electronics has a few replacement parts for the 220 including the soft-key mod that Lou mentioned and is well worth it. I would also recommend the relay even if you bought it for a spare (they do wear out). As for your Tune cap RF parts offers a direct replacement for this cap a little pricey RFP part number 73-180-35/26-131 @ $59.95 ea. I used one from Ameritron on mine but had to cut the front mounting tab down to make it fit right and it was part # 282-2112-1 @$36.52 otherwise it works just fine.

Good Luck and have fun.

Roland AH6RR
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K1ZJH
Member

Posts: 1090




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« Reply #8 on: December 14, 2011, 03:36:56 PM »

Thanks Roland

The interface for the key line is high on the list. I have to open the amp and see how bad the
tuning cap is. If I can remove the plates and polish them smooth, I may go that route. There
a few on eBay from parted out SB-220s that I have been watching.
This amp has never seen heavy use by myself or the previous owner; but I suspect the bandswitch
and tuning cap will need some intervention due to my carelessness while  chasing a few
DX stations.

Pete
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AH6RR
Member

Posts: 803




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« Reply #9 on: December 14, 2011, 07:49:33 PM »

Thanks Roland

The interface for the key line is high on the list. I have to open the amp and see how bad the
tuning cap is. If I can remove the plates and polish them smooth, I may go that route. There
a few on eBay from parted out SB-220s that I have been watching.
This amp has never seen heavy use by myself or the previous owner; but I suspect the bandswitch
and tuning cap will need some intervention due to my carelessness while  chasing a few
DX stations.

Pete

Pete if the band switch is bad Harbach has a replacement. The 220 I have was a freebie that was sitting outside under a carport with a blanket over it in the Hilo Rain Forest so you can imagine what it looked like. I did a complete rebuild on it. Stripping it down to a bare chassis and starting over. It already had the Harbach filter caps, soft start, and fan it also had a Peter Dahl filament transformer. But the Tune cap was pitted beyond repair and the Plate choke was bad so I home brewed one. I added the Harbach Rectifier/Metering board, grounded the grids directly added a glitch resistor and did a complete paint job and made decals for the face plate I painted it a Stainless Steel silver. I also added a stand-by switch and a Transmit LED it already had a home brew 12V keying circuit that works very well. It still has the original Eimac 3-500z's and they still have full output in fact it gained 200-300 Watts out with the grids directly grounded. I also made new Parasitic Suppressors just like the original ones. I took me a couple of months to complete but I am very happy with the results it has been a workhorse since.

Roland AH6RR     
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