Globe Scout 680

[N8IE]

Hello all,
I'm in the process of bringing an old Scout back to life. I won't say "restore" because I'm not worried about it being pretty. 

I bought it last year off e-Bay (one of those "estate sales") and it was "in great condition" and "worked when last used". Other than a broken fuse holder, broken accessory socket, broken crystal socket, and a  few bad solder joints, it's in mint condition. 

Anyway, I have it up and running and smoke free, and for the most part stable. I need to replace the filter caps (12uF, 700VDC) with replacements. I have a few 40uF, 450VDC I can use in series and they will require resisters to balance the voltage.

My question is, what formula do you use to arrive at the proper ohm and wattage for these resisters?

Thanks es 73!
Dan, N8IE

[Carl WA1KPD]

I don't bother with them
 
It is my understanding that the balancing resistors are not as critical as they used to be.
I have built or rebuilt a number of supplies putting caps in series with no failures to date.
But maybe I have been lucky...

Carl
/KPD

[W9GT]

Hi Dan,

Its interesting to note, if you tear one of those 700 V electrolytics apart there may just be two 350 volt units in series inside.  I have seen several that were constructed that way. (no resistors)  You may not really need resistors.  Newer 'lytics are better quality and as long as the values of capacitance are close, they will be fine.  If you do still want to use balancing resistors, use fairly high values of resistance >100K because you really don't want to draw a lot of current through an additional "bleeder" resistance and just waste power and generate heat.  If you have wirewound resistors available, it would be good to use 5 watters. 

73,  Jack, W9GT

[N8IE]

Thanks for the tips guys.
I'll swap them out tonight and see what happens.

I'm really surprised the old gal is in as good a condition as it is.

I also noticed in this rig that the tube sockets and the accessory socket were all installed with rivets. Over time they have become loose and sloppy. I guess when WRL shut down, they became MFJ? 

73!
Dan, N8IE

[W3NP]

I did up a few Scouts last year - a 66 and a 65A.  I used 2x 40uf @ 450 in series to replace that HV cap. Didn't use any equalizing resistors and they are doing fine - no problems.

[K8WBL]

Maybe you got yours from where I got mine..hi hi ...just acquired my 680A, dont know what the differences are on the "A" version.  I posted a question a couple days ago on this forum.  Mine lost most of the modulation after a couple of weeks, now I am searching for that problem.

73, Tim K8WBL

[W7XXX]

In many circuits where you see the resistors used it is for bleeder purposes. The caps will bleed off more evenly with a resistor around each one. I would remove the old bleeder resistor and replace it with 2 resistors of half the value and wattage each around each of the caps.

[N8IE]

I replaced the caps last night without the resisters and all went well.
Power out is within specs and audio sounds good as well.

I did notice the oscillator seems a tad bit more stable now, just a slight drift and minimal chirp. Sounds like a Globe Scout should.

Only a couple of cosmetic things to fix and it'll be put in line.
I have 80M AM crystals on order so maybe next week I can get it on the DX-60 net.

Thanks for the tips guys es 73!

Dan, N8IE

[w1vtp]

I replaced the caps last night without the resisters and all went well.

<snip>

Dan, N8IE

Dan -- even though the newer 'lytics' may have lower leakage across them, I'd vote for some sort of nominal equalizing resistors.  At some time one of the caps might develop some leakage -- what may happen after that is the lower leakage cap would start to take up more of the voltage division between the two.  At some point it's conceivable that you might have a catastrophic failure.  I've seen the ends of caps blow off like a canon!  Fortunately, in that case the technician in my cal and repair lab was not looking down the "barrel" of the cap -- it deposited the innards of the cap into the ceiling!! 

I strongly suggest you put the equalizing resistors in there now - and use wirewounds not film or comp types.

Stay safe, Al VTP

[w1vtp]

<snip>
...I posted a question a couple days ago on this forum.  Mine lost most of the modulation after a couple of weeks, now I am searching for that problem.

73, Tim K8WBL

Tim - I sent you an email regarding your situation and never got a response.  I'm still willing to follow your modulation situation.

73, Al

[N8IE]

I replaced the caps last night without the resisters and all went well.

<snip>

Dan, N8IE

Dan -- even though the newer 'lytics' may have lower leakage across them, I'd vote for some sort of nominal equalizing resistors.  At some time one of the caps might develop some leakage -- what may happen after that is the lower leakage cap would start to take up more of the voltage division between the two.  At some point it's conceivable that you might have a catastrophic failure.  I've seen the ends of caps blow off like a canon!  Fortunately, in that case the technician in my cal and repair lab was not looking down the "barrel" of the cap -- it deposited the innards of the cap into the ceiling!! 

I strongly suggest you put the equalizing resistors in there now - and use wirewounds not film or comp types.

Stay safe, Al VTP

Hello Al,
Okay, then back to the original question.
What is the proper guideline for choosing resistors?
Do I need one across each cap or across the pair?

Thanks again es 73
Dan, N8IE

[w1vtp]

I replaced the caps last night without the resisters and all went well.

<snip>

Dan, N8IE

Dan -- even though the newer 'lytics' may have lower leakage across them, I'd vote for some sort of nominal equalizing resistors.  At some time one of the caps might develop some leakage -- what may happen after that is the lower leakage cap would start to take up more of the voltage division between the two.  At some point it's conceivable that you might have a catastrophic failure.  I've seen the ends of caps blow off like a canon!  Fortunately, in that case the technician in my cal and repair lab was not looking down the "barrel" of the cap -- it deposited the innards of the cap into the ceiling!! 

I strongly suggest you put the equalizing resistors in there now - and use wirewounds not film or comp types.

Stay safe, Al VTP

Hello Al,
Okay, then back to the original question.
What is the proper guideline for choosing resistors?
Do I need one across each cap or across the pair?

Thanks again es 73
Dan, N8IE

About 220 K ohms @ 1/2 watt will do.  The actual value comes out to be 183 K ohms but since a typical 'lytic'  has about 20 M ohm leakage this value will do perfectly.

Here's the supporting data to back this up.  According to the manual the PS output voltage is 440 Volts.  That being the case - the rule of thumb for a bleeder resistor is 100 ohms per volt.  This makes the ideal bleeder resistance for this voltage (440 volts) @ 44K ohms.  The Globe Scout actually has 50 K in there as a bleeder so we have a little fudge room to work with.  I've uploaded a PDF of the Excel spreadsheet that I used to calculate the resistance required plus the wattage needed.  A rule of thumb for the power is use X2 the power dissipated across a given resistor.

I can supply a link for the actual calculations done on the Excel spreadsheet if anyone wants it.  I can't upload it here as the forum does not allow those kinds of files.

Regards, Al

PS:  I've also uploaded parts of the manual thanks to Bama, of course.

[K1ZJH]

I don't bother with them
 
It is my understanding that the balancing resistors are not as critical as they used to be.
I have built or rebuilt a number of supplies putting caps in series with no failures to date.
But maybe I have been lucky...

Carl
/KPD

I agree, it was my understanding that they would self equalize on their own. But, old habits and practices die hard.

Pete

[w1vtp]

I don't bother with them
 
It is my understanding that the balancing resistors are not as critical as they used to be.
I have built or rebuilt a number of supplies putting caps in series with no failures to date.
But maybe I have been lucky...

Carl
/KPD

I agree, it was my understanding that they would self equalize on their own. But, old habits and practices die hard.

Pete

Pete -- only if the 'lytic' opens up.  If one of the caps starts to leak (internal leakage resistance drops) then the distributed voltage across the two starts to divide up -- lower across the leaking cap and higher across the "good" one.  At some point there can be way too much voltage across the good one which may lead to catastrophic failure.

I repeat -- I've heard and seen those beasties go off in failure -- they go off like a gun.

Al

[K1ZJH]

That can happen with or without the balancing resistors.  In any event, here is a link to a post by Alan Douglas regarding the use of balancing resistors and the discussion that ensued...

http://antiqueradios.com/forums/viewtopic.php?t=67720&view=next&sid=8a3041a073a912aa4025a183e1861a8e

Pete

[KD6VXI]

I replaced the caps last night without the resisters and all went well.

<snip>

Dan, N8IE

Dan -- even though the newer 'lytics' may have lower leakage across them, I'd vote for some sort of nominal equalizing resistors.  At some time one of the caps might develop some leakage -- what may happen after that is the lower leakage cap would start to take up more of the voltage division between the two.  At some point it's conceivable that you might have a catastrophic failure.  I've seen the ends of caps blow off like a canon!  Fortunately, in that case the technician in my cal and repair lab was not looking down the "barrel" of the cap -- it deposited the innards of the cap into the ceiling!! 

I strongly suggest you put the equalizing resistors in there now - and use wirewounds not film or comp types.

Stay safe, Al VTP

Hello Al,
Okay, then back to the original question.
What is the proper guideline for choosing resistors?
Do I need one across each cap or across the pair?

Thanks again es 73
Dan, N8IE

Building between 4 and 6 HV supplies a year (from 1800 VDC to 8+ KV, up to 12A), I've found that the equalizing resistors aren't really necessary, but I do the following.  (I use strings of electrolytics, haven't had a real HV cap here in a few years...  Lectrolytics are just too cheap and lotsa C available)

I install them, but use the value for the bleeder R, divided by the amount of caps in the supply.  Use LONG leads on the resistors (leads help to heat sink as well), as well as splitting the value between two resistors helps with keeping the R's cool)...  ie, if you need 100K of bleeder resistor, and have 4 caps, 25K across each cap means 12.5K on each resistor, two resistors per cap.

This serves to help equalize voltage across the cap stack, as well as gives you the amount of bleed R you need.  Adding Mo Rbleed just ensures your making a nice heater in the shack.

--Shane

[W7XXX]

As I said before the easiest and safest way to resolve the issue is to REMOVE the old 50K bleeder and put a 25k 10 or more watt wirewound around each of those new series caps. These series resistors are now the bleeder and bleed the capacitors off more uniformly.

[w1vtp]

As I said before the easiest and safest way to resolve the issue is to REMOVE the old 50K bleeder and put a 25k 10 or more watt wirewound around each of those new series caps. These series resistors are now the bleeder and bleed the capacitors off more uniformly.

Absolutely - Sam.  That certainly is a good option as long as the 25 K ohm wirewound resistors are of known integrity.  My EF Johnson Courier and all the Ameritron amps use your method.

My effort was a combination of personal curiosity and to share a couple of good PS design approaches - the rule of thumb for bleeder size and how to figger out changing a resistor network to a lower one + using the power formula.

So, there you have it Tim - a couple of good ways to go when putting a couple of PS filter caps in a series string.

73, es GL  AL

[N8IE]

Thanks for all the information guys, and the link to the Orr book (great reading).
I'll get the resistors in place and button this project down.

Next up,
Figure out just what the hell mod was done to a DX-35 I just got.
But that's for another post.........

73
Dan, N8IE