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Author Topic: SB-220 refurb time, check list??  (Read 7150 times)
K1ZJH
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Posts: 2931




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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: 2196




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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: 2931




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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: 2931




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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
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Posts: 2931




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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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« 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
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Posts: 2931




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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
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Posts: 846




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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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N4UE
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Posts: 431




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« Reply #10 on: August 31, 2016, 03:22:46 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. 

Old thread, I know, but.............. Like Pete, K1ZJH, I recently purchased a pretty nice SB-220 without tubes. I have some spare 3-400Zs and some 3-500ZGs.
My first thought was to just clean it up a bit and fix what 'might' be wrong. I rewired it for 110V (to check it out) and brought it up with a Variac. Seemed to be ok. It will be run on 220V.

However, the more I looked, the more issues I found, so since I enjoy (no, LOVE) building stuff, I decided to 'unbuild' it. The previous builder made a mess of the wiring, but that's OK. This is not my first amp. Retired, and have time.

I just wanted to thank the contributors of this topic! I've read and reread about everything amp related and  learn something every day.
W1QJ, I read your comment about checking the parasitic anode resistors. Sure enough, one was open.
I've purchased some 47 ohm, 2 watt, Ohmite 'OY' resistors as suggested by W8JI.

I've already purchased and built the Harbach:
Cap board (had my own caps)
Rectifier board (does away with the Zener)
Soft Start
Soft key  (I had already made my own, but it was not really what I wanted)

I'm not going to do a bunch of 'mods' to it, other than grounding the grids, maybe a glitch resistor.

I'm also building a LK-500, dedicated 6 Meter amp in concert with the Heathkit. I have a large supply of parts I've been acquiring for 20 years for the 6 Meter amp. I also have a machine shop, so I can make about anything. I already have a LARGE legal limit amp for 6M, but I cannot use it in my home, due to sheer physical size!
I'm open to suggestions, and will be here asking questions of the 'brain trust'.

Thanks again!

ron
N4UE
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WA7PRC
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« Reply #11 on: August 31, 2016, 04:47:27 PM »

NOS Allen-Bradly 2W carbon composition resistors are available from ebay seller 'bigsmythe74' (link) cheaper than you can buy Ohmite OX/OY resistors (when factoring shipping cost).

You'll want to take a close look at the tube sockets to verify the contacts are in
good shape. If overheated, the tensioners can relax. Note this overheated socket
with a good grid contact on the left, and an overheated filament contact on the right:

Note that the overheated contacts don't close completely.

When hard-grounding grids, you want short & wide conductors for lowest XL.
This is how I did it in my SB-220:

Hardware is #4-40.

In the PA compartment sheetmetal, I had some screw holes with pulled-out threads.
I installed self-clinching (PEM™) nuts and replaced all the #6 sheetmetal screws w/ #6-32 machine screws:

Since they're all near the edge of the panel, I used locking pliers to squeeze them into place.
A couple of scraps of steel (or flat washers) prevents marring the aluminum panels.

For a 'glitch' resistor, I used a 10Ω/20W wirewound (Ohmite B20J10RE) in place of RFC2:


David K5DBX found that adding a shroud to the fan reduces noise and increases airflow:

A couple of 1-1/2" wide strips of soft aluminum (top AND bottom) gets it done.
They're secured at the ends by the existing PA compartment hardware.
I added a bead of silicone adhesive in the middle of the top piece.
This is worthwhile, especially if you use the aftermarket fan motor (Dayton #4M070) from
Grainger. A well-known reseller offers them for twice the cost.

The OEM open-frame relay is S-L-O-W and unreliable. It's best replaced with FAST vacuum & reed relays
that have hermetically sealed contacts. Jim W7RY devised a driver circuit that also gives you
electronically regulated and switched bias. He sells on ebay as user 'radioamplifiers'. More info
is on his QRZ bio: http://www.qrz.com/db/w7ry. When you make your purchase, he emails
complete documentation and a link to the BOM on Mouser's website.
He suggests a source for the vacuum relay (Max Gain Systems) and can supply a suitable reed relay.

Bryan WA7PRC
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N4UE
Member

Posts: 431




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« Reply #12 on: September 01, 2016, 01:52:00 PM »

Hi Bryan, thank you for the reply. I've already acquired all the parts I'll need for the SB-220 and the LK-500 6 Meter amp.
One question....... Why did you form the tube sockets up, to strap them to ground? Never seen that before. Seen some strange wiring on YouTube, by 11 Meter 'builders' of the SB-220.

A buddy of mine had a copper roof put over his poolside outdoor kitchen. I've got several lifetimes worth of copper flashing and also brass in my stockpile.
I'm never going to do QSK with the Heathkit, so I just ordered a stock replacement. The original builder made a mess around the filament choke/relay area. I spent a couple of hours today 'unbuilding' all the sub assemblies. luckily, the builder used the 'stick a wire in a hole and solder' method. No wrapping, anywhere.

ron
N4UE
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WA7PRC
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« Reply #13 on: September 01, 2016, 03:13:07 PM »

Hi Bryan, thank you for the reply. I've already acquired all the parts I'll need for the SB-220 and the LK-500 6 Meter amp.
One question....... Why did you form the tube sockets up, to strap them to ground? Never seen that before. Seen some strange wiring on YouTube, by 11 Meter 'builders' of the SB-220.

A buddy of mine had a copper roof put over his poolside outdoor kitchen. I've got several lifetimes worth of copper flashing and also brass in my stockpile.
I'm never going to do QSK with the Heathkit, so I just ordered a stock replacement. The original builder made a mess around the filament choke/relay area. I spent a couple of hours today 'unbuilding' all the sub assemblies. luckily, the builder used the 'stick a wire in a hole and solder' method. No wrapping, anywhere.

ron
N4UE
Hi Ron,

The reason for bending the tube socket terminals up is to minimize the distance between the tube's grid pins and chassis, and thereby minimize the series reactance. I used soft copper strip that was left over from another project.

My main reasons for changing to FAST & sealed vacuum & reed relays is not QSK. It's mostly about reliability and not hot-switching. Sealed relay contacts don't oxidize. Faster switching means there's zero chance of the amplifier switching lagging RF coming from the rig.  QSK, dead-quiet operation, and electronic bias regulation & switching are only bonuses. LOTS of hams have converted to vacuum relays. Some newer amplifiers come with it. My total one-time cost was about $100. Rebuilding your SB220 would be a great opportunity to do the same. Smiley

Bryan WA7PRC
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K1ZJH
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Posts: 2931




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« Reply #14 on: September 03, 2016, 08:40:47 AM »

Rich, I ended up doing a full blown rebuild which included several major changes.  It is no longer in anyway related to a SB-220.  Added full WARC and 160 meter coverage, full QSK switching, etc.  My amp was abused for many years, and it did not bother me to tear into her and give her a new chance at a new life... the original 3-500Z tubes are still clunking along at rated power.

Pete
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