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Author Topic: SB-200 switch wafer...again!  (Read 3332 times)
K8AXW
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Posts: 3296




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« on: October 09, 2010, 01:07:16 PM »

Greetings all:

If you are a follower of the eHam "Amplifier Forum" you might recall my previous posts about destroying my SB-200 bandswitch wafer; trials and tribulations of finding and installing one I found.

At any rate, I received a great deal of help from the experts on this forum dealing with the correct way of loading this amplifier.  I followed each suggestion as well as I could up to including loading the amp on each band and marking down the settings.

Well, after marking the settings for the 80-40 & 20m bands using the original Centron 572B tubes, I installed a new set of matched Taylor 572Bs from RF Parts.  I started with the 20m band, which was where I left off and first thing out of the bag I heard a "zap" but the amp continued to work OK for the next 5 minutes then I started to get wild readings on the Grid Current meter... sometimes deflecting DOWN!  Not a good sign!  Cut to the work bench and found the 80m additional capacitance wafer contacts GONE!

There is nothing worse than destroying something that is very difficult to replace and not having a clue on what caused it.

I cannot believe that these amplifers are so touchy that one little tuning mistake will wipe out components!  I am NOT heavy handed when it comes to something like this.  Each adjustment I make is quick and pre-planned.  If a value exceeds the specs it is only for a second, or less. 

But what I believe and $2.00 will get me a cup of coffee most places.

I also cannot understand why the wafer that simply adds a fixed capacitor for 80m operations would arc when in the 20m position!  But it did.

Now, all that to ask this.  I'm sure someone out there has ceramic switch wafters that will fit in this amp.  How can I provide detailed information to anyone who is kind enough to go through their junk boxes and try to find one that I can buy?

The second question I have is this.  Unless I am the only clutz in the country with a blown up switch wafer in thier SB-200 (twice) I'd like to hear suggestions on how to modify this amp to switch in the additional capacitance for 80m.  It doesn't have to be "automatically switched" when the band switch is in the 80m position.... but just be available when needed. 

I really am smart enough to do this modification but I would prefer to minimize the butchering of the front panel. 

Overall the amplifier is a pizza sheet cosmetically so I'm not in the mood for historical preservation or concerned about resale value. 

Thank you for reading this long drawn out post, if for nothing else.  Oh yes, one other thing... please, no flames.  I'm in no mood for that either!

73

Al - K8AXW
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KA5N
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Posts: 4380




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« Reply #1 on: October 09, 2010, 04:45:51 PM »


Sounds like your new tubes arced (at least one of them).  Have you installed a glitch resistor (and maybe a series fuse) in the hv line?  This is covered in most of the "mod" writeups. 
Tuning a properly working SB200 is a snap and takes a few seconds.  If you have read much in this
forum, the reports on Chinese made 572B's is mixed.  I had a pair of them in a SB200 and after 2.5 years
one of them had a filament burn out.  I am so-so with them (they were from RF Parts).  I have since bought NOS Cetrons and they work great.  I have a life time supply (either me or the amps).  I have two
SB200's and use one and have the other ready to go.  The only real mods (not including new caps and bigger diodes etc.) was to change the input circuits for 15 and 10 so that 17 and 12 meters tune as well
as 15 and 10.
Anyway if you can't find another wafer you could use toggle switches to switch in the caps.  You'd
have to make sure that the switches could take voltage and current.  Or you could use a separate
ceramic rotary switch.  Neither would look all that great and you would ruin the risk of not switching
when needed.  (a rare dx station calling will make you forget all kinds of stuff).
Last, sometimes it takes a new pair of eyes to spot something you've overlooked.  Know anybody
who can help?

Good Luck Allen
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K8AXW
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« Reply #2 on: October 09, 2010, 08:06:06 PM »

Hi Allen,

Thank you for your reply.  Last first.... no, nobody around here that can help me.  There are over 100 "hams" in my county and as far as I know, there are none who can help me.

As for installing a toggle switch, I really don't know if this kind of switch would handle the voltages.  Apparently, the RF voltage on these contacts is very high to do the damage I see.

I am leaning toward a homebrew knife switch of some kind.

As for getting in a hurry for "a DX station," that will never happen.  I am so afraid of screwing up this amp (again) that I don't allow myself to get in a hurry.  However, I am also so inclined to forget things!  That is the major problem.


As for a tube being shorted.  I considered this but have just two options.  One is to test them with my old Hickok 752A tube tester which has only the 811A tube listed.... no 572B.  Since the pinout is the same as the 811A I think it should also show any "shorts."  Option 2 is to return the tubes to RF Parts so they can test them.  I kinda figure that this will lead to a mess.... but will see.

I do have a HV series fuse resistor but so far it has not blown.  I also use 33 ohm grid resistors.  I also installed new parasitic suppressors on the plate caps.  The only other "mods" I have made are new filter caps. 

Keep in touch.

73

Al - K8AXW
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W8JI
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« Reply #3 on: October 10, 2010, 01:47:20 PM »

Al,

If you blew the wafer A contact that adds 80 meter capacitance, you had the amp mistuned or had a dummy load or antenna system problem. That much is certain!

http://www.w8ji.com/loading_amplifier.htm


As for the grid meter and popping, that sounds like a bad tube.

http://www.w8ji.com/572b_problems.htm

http://www.w8ji.com/fault_protection.htm

I'm pretty sure I have some wafers that are superior replacements, but you have to be very careful installing a bandswitch. Voltages are so high any small sharp points can cause corona and premature switch failure by increasing voltage gradient between contact posts.

http://www.w8ji.com/bandswitch_failures.htm

The mods didn't help you because they don't address the real problem. For a glitch resistor, the resistor has to be pretty big and it has to have a high voltage rating. An RCD 175P resistor style would work. If you thing a ting metal film does anything to help, thing again. It fails only AFTER other more sensitive parts are gone. The same is true for a fuse. A good HV rated fuse is very expensive, and hardly helps anyway. It takes resistors and fuses far too long to blow to be useful for much other than protecting transformers and big rectifier diodes.

A tiny standard low power glitch resistor fails after the protection is needed.

Also, I dislike the grid resistors and grid mica caps in that amplifier. It was a bad idea.

73 Tom









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K8AXW
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« Reply #4 on: October 10, 2010, 07:00:35 PM »

Tom,

Thank you for your reply.  It is indeed possible that I might have mistuned the linear.... again.  However, as I mentioned before, I have a difficult time believing that these things are so unforgiving!  I also repeat, when I tune, each move is for only a second or less until I see the grid/plate currents are in specs.

The wafer that I installed to replace the other one that "zapped" was much heavier than the original.  However, it wasn't like a snow white new one.  So wafer contamination could also be a factor.

The contact alignment was spot on.  I checked that over and over before closing it up.

Wiring was done per the Heath assembly manual.  Actually, Heath had two ways of wiring up that 80m padder cap and I have both manuals.  The first wafer was wired per one manual and the replacment was wired according to the second manual I own.

The big resistor in the HV lead is a 10 ohm WW "HV fuse resistor" and it looks like it's about 10W minimum. It is marked, "VL10  MEMCOR 9613."

OK on "pretty sure you have some wafers." This I am interested in!!  Please email me details... cost, shipping, etc.  Also do you have more than one?

Last question Tom.... please comment on my testing the 572Bs with a tube tester for SHORTS.

73

Al

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W8JI
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« Reply #5 on: October 11, 2010, 07:46:09 AM »

Tom,

Thank you for your reply.  It is indeed possible that I might have mistuned the linear.... again.  However, as I mentioned before, I have a difficult time believing that these things are so unforgiving!  I also repeat, when I tune, each move is for only a second or less until I see the grid/plate currents are in specs.

The wafer that I installed to replace the other one that "zapped" was much heavier than the original.  However, it wasn't like a snow white new one.  So wafer contamination could also be a factor.

The contact alignment was spot on.  I checked that over and over before closing it up.

Many subtle things affect bandswitch voltage rating, and for that wafer VOLTAGE is the concern. If I could see a good detailed picture of the wafer I could tell you if it is any good or a poor design. Size matters less than how it is constructed in tiny details that most people miss.

Also, an amplifier can have grid and plate currents within safe limits and still be mistuned.

Quote
Wiring was done per the Heath assembly manual.  Actually, Heath had two ways of wiring up that 80m padder cap and I have both manuals.  The first wafer was wired per one manual and the replacement was wired according to the second manual I own.

I'm not sure what those ways are, but there is only one overall way to do it right.

Quote
The big resistor in the HV lead is a 10 ohm WW "HV fuse resistor" and it looks like it's about 10W minimum. It is marked, "VL10  MEMCOR 9613."

That's a standard LV resistor. I'm unsure how it would behave with a few thousand volts surge across the leads.

Quote
OK on "pretty sure you have some wafers." This I am interested in!!  Please email me details... cost, shipping, etc.  Also do you have more than one?

OK

Quote
Last question Tom.... please comment on my testing the 572Bs with a tube tester for SHORTS.

You tester might check for filament-grid shorts (especially if you tap on the tube while testing), but there is no way it could test for anode to grid faults from gas or other contamination.

73 Tom
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K8AXW
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« Reply #6 on: October 11, 2010, 09:31:30 AM »

Tom,

Please email me via:  k8axw@arrl.net.

Al - K8AXW
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K8AXW
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« Reply #7 on: October 11, 2010, 10:52:58 AM »

Tom,

Forgot to comment.  There is only one correct way to "wire up" the 80m padder.  However there are two ways to physically install the capacitor, each requiring its own wiring path to ground.  One method is mounting the cap on the front bulkhead behind the front panel and the other is to mount it on the end of the TUNE cap.

I have no idea which of these is the "latest" version.  The bulkhead mount is in a yellow assembly manual.... the TUNE mounted cap is in a gray assembly manual)

Al
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W8JI
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« Reply #8 on: October 11, 2010, 12:42:02 PM »

Tom,

Please email me via:  k8axw@arrl.net.

Al - K8AXW

I tried the email on QRZ but I guess it is dead?
I'll try that one.
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W8JI
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« Reply #9 on: October 11, 2010, 12:44:44 PM »

Tom,

Forgot to comment.  There is only one correct way to "wire up" the 80m padder.  However there are two ways to physically install the capacitor, each requiring its own wiring path to ground.  One method is mounting the cap on the front bulkhead behind the front panel and the other is to mount it on the end of the TUNE cap.


On the chassis or bulkhead would place the bandswitch at high voltage all the time. Bad.

On the tune cap would place the bandswitch rotor at ground and only one contact hot. Good.
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K4DPK
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« Reply #10 on: October 14, 2010, 10:05:41 PM »

Here's how I sometimes do it on HB amps.....

Take a common wiper contact from a large relay, and solder the non-contact end of it to a brass 1/4" collar.  Slip the collar onto the end of the bandswitch shaft.

Mount the pad capacitor to the capacitor stator, and position it and the relay wiper so they come into positive contact and ground the open end of the capacitor in, and only in, the required band position. 

A flexible strap from the collar to ground is a good idea.

This way, HV relay contacts aren't needed on a separate wafer.

Phil C. Sr.
k4dpk
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W8JI
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« Reply #11 on: October 14, 2010, 10:37:54 PM »

This way, HV relay contacts aren't needed on a separate wafer.

Phil C. Sr.
k4dpk

Heath made an interesting mistake Phil, and the pictures of the wafer he sent me showed the replacement wafer was not as good as the original in design. With two errors, it is no wonder he had troubles.

Heath has the cap mounted on the chassis in his wiring and the swicth interrupts the line to the hot tank end. This method places full tank voltage on the contact right next to the screws going through the wafers when the switch is on 80 meters.

While your suggestion is good, I'm sure his troubles will go away when he changes the wiring and uses a wider contact spacing wafer.

73 Tom

 

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K2QPN
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« Reply #12 on: October 15, 2010, 05:57:36 AM »

I had the occasion to replace a switch wafer in my SB-220. My own fault - I grabbed the wrong knob when tuning the amp. I got the wafer from harbachelectronics.com. Good quality wafer.

I am surprised that you are having so much trouble. I have a an SB-200 that I built in 1968 and it is still going strong with the original tubes.

73, Bob K2QPN
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K4DPK
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« Reply #13 on: October 15, 2010, 09:13:50 AM »

For Tom, W8JI:

"Heath has the cap mounted on the chassis in his wiring and the swicth interrupts the line to the hot tank end. This method places full tank voltage on the contact right next to the screws going through the wafers when the switch is on 80 meters."

Holy Cow!!!  No wonder there's a problem.  

Phil C. Sr.
k4dpk

« Last Edit: October 15, 2010, 09:42:37 AM by Phil Chambley, Sr. » Logged
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