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Thread: Gibson Ranger GA20-T Rebuild

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    Supporting Member TomCarlos's Avatar
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    Gibson Ranger GA20-T Rebuild

    Hello Friends...

    A friend handed me an old Gibson Ranger GA20-T amp for restoration. He recently bought this chassis and besides being old, it is missing the Power Transformer (and who knows what else).

    We purchased a new Mercury Magnetics PT GA20-T-P. It's supposed to be an exact replacement. I wrote to Mercury to get a spec sheet because the wiring colors do not seem to be an exact match to the amp. So while I wait for that...

    I'll be taking my time on this one... lots of cleaning, cap replacements, will check everything from top to bottom.

    Step 1 - We will be installing a 3 prong electrical plug. I'll ground the line right at the screw holding the transformer.

    On the schematic, I see a .022uf cap on the switch side of the transformer. I don't think we'll be needing this cap, will we? I thought this is an old wiring scheme for providing some type of ground to the chassis- true?

    Step 2 - The two big caps you see in the photo will be replaced. One cap is the 10/20 cap that appears on the left side of the schematic pic shown here. The other cap is a 10/10 in parallel that serves as the 20uf shown on the right side schematic (connecting to the 10K). I guess I will buy two 20uf and a single 10uf to replace these.

    Thanks, Tom
    Attached Thumbnails Attached Thumbnails filter-caps.jpg   ga20-t-power-supply.jpg  

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    Quote Originally Posted by TomCarlos View Post
    On the schematic, I see a .022uf cap on the switch side of the transformer. I don't think we'll be needing this cap, will we?
    Yes, that is a ground cap or "death cap". With the 3-wire ac cord, you will not need it.

    Quote Originally Posted by TomCarlos View Post
    I guess I will buy two 20uf and a single 10uf to replace these.
    Yes, that will work fine and will take up less space as well.

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    Supporting Member TomCarlos's Avatar
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    Thanks Bill...

    I found another schematic for this amp and the cap is not there. So on to the rest of Step 2 and mounting the new PT. The Mercury transformer mounting clamps do not fit exactly over the existing holes in the chassis so I'll need to drill a couple holes. A good stopping point would be to get the incoming AC taken care of and connecting the 6.3 volt taps. Always nice to have a power light working! I'll hook up the secondary connections once I get to Step 2 and make sure I properly map the windings to the existing circuit.

    Tom

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    Supporting Member Chuck H's Avatar
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    What Bill said, and...

    If you find it's easier to get 22uf (a more standard value) that should work just the same as any 20uf of the period. Modern caps have much better tolerance than older and the 22uf value is well within the +/- 20% of the day. 10uf should be easy enough to acquire.

    WRT old Gibsons... The only one I've ever worked on had a leaky coupling cap. I've read MANY threads here where an old Gibson had a leaky coupling cap. I'm not suggesting that you change all the coupling caps, but since you'll be doing extensive work anyway I would suggest that you test the caps that are there to avoid problems.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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    Supporting Member TomCarlos's Avatar
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    Gents...

    I am starting to compile my shopping list and will come back to ask some questions about the caps. But first...

    I am trying to map the wiring for the new transformer. The old PT was not on the chassis when purchased. I see 7 original wires that went to the old PT. It looks like it went something like this...

    2 wires (appear to be Red) to the 5Y3GT Anodes, (Pins 4 and 6)
    1 Wire (appears to be Yellow) to the 5Y3GT Heaters (Pin 2)
    1 Wire (appears to be Yellow) to the 5Y3GT Cathode / Heater (Pin 8)
    2 wires go to ground (hard to tell the colors) - The empty pin 1 of the 6V6 tube was used as a ground point.
    1 Wire (looks like a Green/Yellow) goes to the 6V6s Pin 8 (Cathode / Beam Plates)
    Total = 7 wires

    The new Mercury Magnetics PT GA20-T-P has the following:

    2 Green wires on the Primary side that are for the filaments - Measured on all tube sockets!

    On the Secondary side, there are also 7 wires.

    I am guessing the Reds will go to the 5Y3GT Anodes, (Pins 4 and 6)
    Looks like the Yellows will go to the Cathode / Heaters (Pin 2 and 8)
    That leaves us a (1) White, (2) Red with Yellow, and a (3) Green with Yellow.

    You'll notice the Green with Yellow has a red sticker- that says "If this Center tap is not shown on the schematic, do not use it." PERFECT, I believe I have identified the Center tap that is shown next to the tube name 5Y3GT. That one should go to Ground.

    So that leaves me with the (1) White and (2) Red with Yellow.

    One of these must go to Ground, the other will go to the 6V6s.

    I wrote to Mercury to ask for a Spec sheet but I'm not sure that will answer my question.

    Any "intelligent" guesses out there?
    Attached Thumbnails Attached Thumbnails harness.jpg   transformer.jpg   old-xformer-wiring.jpg  
    Last edited by TomCarlos; 04-20-2015 at 01:36 AM.

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    Standard American transformer color codes must be out there on the web.

    Yellow is always for the 5 volt rectifier filaments, Green for the 6 volt filaments with a Green/Yellow center tap, Red for the high voltage with red/yellow for the center tap. I will assume the white to be the internal electrostatic shield.

    The center taps can be checked with an ohm meter, but use caution, because when you connect and then disconnect the meter leads you can induce a pulse in the windings that will cause a high voltage to appear on the other windings.

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    Supporting Member TomCarlos's Avatar
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    Yup... I think you have it Bill... thanks!!

    I am attaching a drawing showing what the wiring should be... NOT UNLESS anyone weighs in otherwise.

    But it makes sense now.. you have Red Wires, the Red/Yellow is the center tap. You have Green wires, the Green/Yellow is the center tap. It's the Green/Yellow that goes to the 6V6s.

    For the Greens, I know they are typically the Heaters. But I didn't realize the 6.3 volts could have a center tap- lesson learned.

    And the White wire has not connectivity to the others, so it must be the electrostatic shield.

    I guess I am still learning how all this works. But I wanted to make sure I understood the wiring before I connected anything!!
    Attached Files Attached Files
    Last edited by TomCarlos; 04-20-2015 at 05:19 AM.

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    Quote Originally Posted by TomCarlos View Post
    But it makes sense now.. you have Red Wires, the Red/Yellow is the center tap. You have Green wires, the Green/Yellow is the center tap. It's the Green/Yellow that goes to the 6V6s.

    For the Greens, I know they are typically the Heaters. But I didn't realize the 6.3 volts could have a center tap- lesson learned.
    With the filament center tap connecting to the output tubes cathodes, the filament voltage will be raised by the dc bias voltage of the output tubes. This will aid in lowering the hum from the tube filaments.

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    Supporting Member TomCarlos's Avatar
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    Ok... next thing I need to look at carefully are the ground connections....

    I am attaching the schematic for the GA-20T amp. Keeping in mind that I now have a 3 prong electrical wire and the chassis is grounded... Is this where I would use what I learned about Star Grounding and bring all ground connections back to a grounding block?

    I'm sure there is probably a strategy of where and how to tie the grounds together.... High Signal vs Low Signal vs Input grounds... etc etc etc. The Input jacks are grounded through the chassis.... I get that. But I want to look at all the other ground connections running through this amp.

    Any advice would be appreciated.
    Attached Thumbnails Attached Thumbnails ga20t.jpg  

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    Senior Member Enzo's Avatar
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    Changing from a two wire power cord to a three wire grounded cord doesn't change anything in the amp circuitry. I would not recommend wholesale changing of the grounding unless you have a good reason to. Don't do it just because it "ought to be better this way". You often will run into the old unintended consequences.
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    Supporting Member Chuck H's Avatar
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    Enzo is right about unintended consequences. You could just see to it that no preamp grounds are daisy chained with power supply grounds. They probably aren't now, but who knows. Beyond that, why fix what's not broken? Ok, now to go against what I just said...

    I might try a redesign on that trem circuit intensity control. I don't know what sort of pots Gibson used, but I wouldn't trust a pot that old or a modern built pot to carry HV. It's probably possible to hardwire the plate circuit and variably couple the cathodes instead.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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    Supporting Member TomCarlos's Avatar
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    Thanks Enzo and Chuck....

    All ground connections from the transformer go to the unused Pin 1 of the 6V6- including the electrostatic shield (I guess the old transformers had one too). So I will probably leave that as-is. I am debating on whether or not to splice the new PT to existing wires or to remove all wires from that connection point and resolder. With such a tight squeeze, I am hesitant to do that.

    I think my bud would prefer that I not alter this amp (if we can avoid it, safety issues an exception). So I'll look to see if there are examples of how to redo the wiring Chuck mentions.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by TomCarlos View Post
    ... I am debating on whether or not to splice the new PT to existing wires or to remove all wires from that connection point and resolder...
    My vote is to remove the old wires and connect the new transformer leads directly to the intended original terminals. The splice method is a sure sign of a quick hack job and I'm sure you don't want your careful work to look like that.

    I recommend that you retain the original ground scheme to start. Get the amp working and then decide if improvements are necessary. Best not to flit with unintended consequences as Enzo pointed out.

    Cheers,
    Tom
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    I rebuilt one of these amps for my uncle about 10 years ago. My brother has the amp now. All coupling caps leaked DC badly and had to be replaced except for the flat mica cap. It had bumblebees and Astrons in it originally. All electrolytics were replaced in the amp as they had physically leaked inside the chassis and obviously didn't work, and really those should be replaced anyway. A 3 wire cord was installed and the noisy carbon comp plate resistors were replaced with new ones. There were a couple other resistors that were replaced too because they had badly drifted but I forget at the moment which ones. The Gibson board is annoying as they have the caps on the bottom and the less often changed resistors on top. The whole board has to be pulled to work on the amp properly. The grounding is to a terminal on the tube sockets as you mentioned. It isn't technically correct but works well in that amp. The preamp sockets are also shock mounted which is a nice touch. I still have info around here somewhere about that amp, and it is across town if you get stuck and need some info, though I am very busy with last term of an EE program in school at the moment. Hope the info I gave you helps!

    Greg

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    Supporting Member TomCarlos's Avatar
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    Thanks Greg... Yes, that is helpful.

    I'm going to creat my shopping list soon. I guess the questions I have center around the types of caps I should get. I see the bumble bees and the ones I call "the bananas." Based on the experience of others, i am wondering if Orange drops would work ok in some spots but axials in others.

    Coming off the power supply, we have the two 10k resistors. One measures at 10.5k and the other at 9.5k. So I will strat there and continue on.

    Tom

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    Thanks Greg... Yes, that is helpful.

    I'm going to creat my shopping list soon. I guess the questions I have center around the types of caps I should get. I see the bumble bees and the ones I call "the bananas." Based on the experience of others, i am wondering if Orange drops would work ok in some spots but axials in others.

    Coming off the power supply, we have the two 10k resistors. One measures at 10.5k and the other at 9.5k. So I will strat there and continue on.

    Tom
    OD's will work fine for all applications, but my preference is Mallory 150. Antique has some even cheaper film caps which are OK. I always opt for 600 or 630V ratings. No point in buying bumble bees. New ones are merely film caps encased in plastic and painted with stripes and an absurd price tag. Old ones, who knows whether they work or not no matter what the seller claims. Either way, a lot of $$$. Your 10K resistors are close enough for rock'n'roll. If you choose to replace, you can get spendy carbon comps, but I use 2 watt metal oxide resistors, cheap & I never had a failure.

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    Quote Originally Posted by TomCarlos View Post
    I'm going to creat my shopping list soon.
    Tom, are you planning on a shotgun replacement of everything in the amp or are you going to power up the amp first and see where you are at before making repairs?

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    Supporting Member TomCarlos's Avatar
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    I will change out the electrolytics and any resistors that appear to be way out of whack. The initial power on will be with a variac and bulb limiter. I'd hope to see a signal at the output. From there, it will probably be a step by step. But I think my friend would be ok if we replaced all the caps.

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    I would say that due the difficulty of replacing the caps on this amp without having to take the whole board out, then you should connect each signal cap to a DC voltage like 300V and see how much they leak DC. Most likely all of them are bad except perhaps the domino mica cap, and should be replaced along with the electrolytics. I used M150's in the one my brother has. They fit well and sound great. I didn't use any orange drops....the usual orange drops that people tend to get are the 715P which are polypropylene and are not going to sound the same as the polyester and paper or paper-in-oil caps that were in that amp when new. Polyester caps like the M150's, Sozos, the AES cheap yellow polyesters, etc will sound much closer to original than anything else you might get. The PS series Orange Drops are polyester but they are film/foil type of construction so they are huge, and they are also radial caps. The amp was made to use axial caps so they will fit better.

    For resistors, the usual ones that need replacement are the ones in the power supply and any plate resistors in the preamp as they tend to get noisy and crackle.

    Greg

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    Supporting Member TomCarlos's Avatar
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    Thanks Greg....

    I appreciate the advice and that from Leo. I'll have some time this weekend and will order some parts for the amp. I'm looking forward to the first slow power up and seeing this amp come alive!

    I'll also post some photos and notes of my work along the way. One thing I want to point out are exceptions to the belief of "this is an exact replacement...." For example, the Power Transformer windings (and voltages) might be an exact replacement but to mount the unit, you need to drill a couple holes in the chassis. Otherwise, the PT brackets do not align with existing chassis holes- stuff like that.

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    Quote Originally Posted by TomCarlos View Post
    One thing I want to point out are exceptions to the belief of "this is an exact replacement...." For example, the Power Transformer windings (and voltages) might be an exact replacement but to mount the unit, you need to drill a couple holes in the chassis. Otherwise, the PT brackets do not align with existing chassis holes- stuff like that.
    I had the same experience with one their output transformers. A friend asked me to look at his mid 60s Dual Showman. The OT was shorted, etc. He wanted a MM so he ordered one and it didn't physically match up either.

    I can never find any real information on their transformers, like dimensions or elec. specs. I guess that's part of the mystery or smoke and mirrors.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    I'll also post some photos and notes of my work along the way. One thing I want to point out are exceptions to the belief of "this is an exact replacement...." For example, the Power Transformer windings (and voltages) might be an exact replacement but to mount the unit, you need to drill a couple holes in the chassis. Otherwise, the PT brackets do not align with existing chassis holes- stuff like that.
    Gibson's caps stashed "behind the board" - I've managed to fish them out when necessary without having to pry the whole board out. A good flashlight & dental mirror help find where they are, then snip leads & unsolder the stub leads from solder tags. What a hassle, but a little easier than prying the board out, especially when board mounting post screws are hidden under transformers. I put the replacement caps on the board side, no more hidden surprises.

    I've always been able to source replacement transformers from Antique/CE or Magic Parts (Ruby) or Mojo. If they don't fit exactly, gotta drill holes, at least I'm not paying a super-premium price like Mercury. Some seat of the pants guesswork may be involved. Mostly I look at whatever Fender amp resembles the amp that needs the iron, and get the Fender part. Failing that, theres' a good selection from Hammond. Another good spot to hunt for iron is Hoffman. So far I haven't dealt directly with Heyboer but they do have an excellent reputation and prices are reasonable. Weber carries some Heyboers, and I've built kit amps with transformers sourced from them, they work and sound just fine.

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    Senior Member Enzo's Avatar
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    If a film cap is on the back side, I usually snip the leads where they curl around the board edge, then fish the cap carcass out from under. I just munt the new cap on top, stacked on a resistor if need be.
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    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    If a film cap is on the back side, I usually snip the leads where they curl around the board edge, then fish the cap carcass out from under. I just munt the new cap on top, stacked on a resistor if need be.
    I catch some flak here whenever I suggest anything like this. When Enzo does it, it's "clever".
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    Senior Member Enzo's Avatar
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    It's because of my good looks...
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    Thanks mates... I am building up my shopping list and looking at the various caps in this circuit.

    I see the bananas, the bumble bees, and then there is that big white CD .25uf cap (400v) that hangs off a grid from the 5879 tube (the second input circuit). Anyone have any ideas why this is a "different" type of cap? Is it some type of electrolytic with a polarity and that is the difference? Is that why I see a black band around one end of the cap (which happens to tie to ground)?
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    I see the bananas, the bumble bees, and then there is that big white CD .25uf cap (400v) that hangs off a grid from the 5879 tube (the second input circuit). Anyone have any ideas why this is a "different" type of cap? Is it some type of electrolytic with a polarity and that is the difference? Is that why I see a black band around one end of the cap (which happens to tie to ground)?
    Looks like a film cap, not electrolytic. I'll bet it goes to one of the 5879 grids, likely g2, not the control grid. Black band probably indicates "outside foil."

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    Supporting Member TomCarlos's Avatar
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    Thanks Leo.... I'll take a careful look at the caps when I get this. This one from tubesandmore doesn't have a band around it.

  29. #29
    Senior Member Enzo's Avatar
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    Look at the other caps in the amp. They are 0.02, 0.005, 0.01, 0.05uf. Now look at your 0.25, it is larger because it has 10 or 20 times the value or more. it is bigger because it is a larger cap value, that is all. The band means outside foil, and really, though people will debate it forever on the internet, it won't matter which end is which when you replace it. it is just another film cap.

    In the circuit, this cap is a screen bypass cap for that 5879. it is enough, it doesn't have to be a larger electrolytic.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by TomCarlos View Post
    Thanks Leo.... I'll take a careful look at the caps when I get this. This one from tubesandmore doesn't have a band around it.
    As Enzo said, no worries about outside foil. Plus "close enough" value will work just fine, 0.22 or 0.33 uF film cap 400V. That cap's not going to see more than 300V max in its application. (Sunday morning back of the envelope guesstimate.)

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    Senior Member guitician's Avatar
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    One thing about that "Death Cap", it may be needed, if later you notice that there is a static electricity type of pops when touching the strings. I have an amp that uses a power transformer that does not have a electrostatc shield around the core and it will do this without a 250V ac cap on the primary. I dont know if that MM replacement uses a copper wrap around their cores.

    Edit: just noticed post #7 so your ok without the cap.
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    Supporting Member TomCarlos's Avatar
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    Gents... Parts have arrived and I'll work on the amp this weekend.

    A question on the Cap across the 200 ohm Cathode resistor for the 6V6 tubes. The schematic shows a 20uf cap. The current part is an electrolytic, looks like a 25 watt rating. Is this a place where you can use a Non-Polarized cap? I've got a 22uf, 100v Non-Polarized cap that I was going to use here. Just curious if this would work here (or cause some odd problem).
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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by TomCarlos View Post
    ...Is this a place where you can use a Non-Polarized cap? I've got a 22uf, 100v Non-Polarized cap that I was going to use here. Just curious if this would work here (or cause some odd problem...
    That will work just fine if you can fit it in physically.


    Quote Originally Posted by TomCarlos View Post
    ...The current part is an electrolytic, looks like a 25 watt rating.
    Note: That's a 25 "Volt" rating. WVDC stands for "Working Volts Direct Current."

  34. #34
    Supporting Member TomCarlos's Avatar
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    Thanks TP !!

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    Senior Member guitician's Avatar
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    That looks like one of those fusible resistors covered with sand. I guess so that if it were to be overloaded it wouldn't catch fire, just burn open.

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