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Thread: First Act amp repair

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    First Act amp repair

    I'd been happily using this amp for years. It developed some scratchiness in the main On/Off volume pot so I gave it a squirt of alcohol. In the process of turning the pot to clear it, I either went past the stop or the stop adhesive let go. The pot doesn't work now, it spins past the On/Off click point and goes from full volume to ground hum. I'm going to be taking it apart today and was hoping someone could help me make sure I don't make things worse with my repair attempt. I bought a solder station yesterday and have a cheap meter but am otherwise clueless.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by jomama View Post
    I'd been happily using this amp for years. It developed some scratchiness in the main On/Off volume pot so I gave it a squirt of alcohol. In the process of turning the pot to clear it, I either went past the stop or the stop adhesive let go. The pot doesn't work now, it spins past the On/Off click point and goes from full volume to ground hum. I'm going to be taking it apart today and was hoping someone could help me make sure I don't make things worse with my repair attempt. I bought a solder station yesterday and have a cheap meter but am otherwise clueless.
    You may find it needs to be replace, check this link and see if it's within your scope.
    https://www.ifixit.com/Guide/How+to+...r+or+pot/67387
    Good luck and welcome to the group
    nosaj

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    What a great link, thanks! I've decided to try and repair this. If I screw up, well, that's life. I'm taking the thing apart and will report back on what I find.

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    The innards of the pot are probably soft plastic, and the alcohol softened or dissolved it. I wouldn't bother trying to repair the pot, just replace it if possible.

    Justin

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jomama View Post
    What a great link, thanks! I've decided to try and repair this. If I screw up, well, that's life. I'm taking the thing apart and will report back on what I find.
    Donīt waste time.

    1) plain replace it and get your amp working.

    2) if you are curious, fine with me, open the old pot and explore it as long as you wish, with a microscope if necessary.
    Remember itīs physically *broken* and you lack the tools necessary to repair it.

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    I got the pot off the board and opened the case. It looks like I crumpled the wiper. It's a B50K pot so I guessed a linear 50 kilohm pot. I ordered 5 and should get them in a few days. I'll keep you guys posted. Thanks for the help

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    Don't forget the joker g1's Avatar
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    Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by g1 View Post
    Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.
    Good call, went completely past me on the safety part.
    nosaj

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    Quote Originally Posted by g1
    Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.
    What's this now, did I order the wrong pots? It's a 10W amp so what's the right pot? When the pot was turned down, even before the off position, I could get no sound from it so the 50K value must reduce the voltage to an insignificant level. Please explain the potential danger.

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    Most guitar amps have an AC power switch and a separate volume control pot.
    Yours uses a volume pot with a built-in in power switch, which is more common in consumer products like radios.
    Some folks here missed the fact that yours is a switched pot.
    For example, that "how to repair a pot" link was for a "regular" pot without a power switch.
    The potential danger is that a compromised pot with AC power switch would be more dangerous than a compromised pot that doesn't get anywhere near AC power.

    Free associating: I have a little (supposedly) 10W amp that runs on (6)AA batteries or a 12VDC adaptor. Never mind; I see it has a separate power switch.
    So anyways, you need to be sure to order a potentiometer with on/off switch.

    -rb

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    Last edited by rjb; 10-23-2018 at 02:00 AM.

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    I think I understand. If it's an on/off pot and it fails short, the full power feeds through the amp. A pot not rated for the power used is more likely to fail. Does a separate power switch on the amp free me from this safety concern?

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    Yes. A separate power switch would allow you to use a standard type volume pot (no switch).
    It may even reduce the hum due to being further away from the volume control.
    But safety is a big concern here. If you do not know exactly what you are doing, it is best left to a technician.

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    I did admit I was new at this stuff but I'm a pretty fast learner. This amp has low voltage caps and a separate power on switch. I'm only replacing a pot on a disconnected circuit board so can you be a little more specific about where the danger is?

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    The safety concern was that if you have a bad AC power switch (which we assumed was built into the volume pot) you might get arcin' and sparkin' and udder bad tings when you plug the amp into a wall socket.

    In the first post, you said you have an "on/off volume pot" with a "click point". If it has a "click point", I assume that it contains a switch. If the switch isn't a power switch, then what are the switch terminals connected to? Maybe the manufacturer used a switched pot just because they had them in stock for other products? Or when you say the pot goes past the "click point" do you mean it turns beyond the original stop point?

    I tried to find a manual (or just a picture) online, but there are several 10W models & I haven't found a manual for any of them.
    If the pot does have a switch, what did the switch do before you sprayed the innards with alcohol?

    -rb

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    Old Timer J M Fahey's Avatar
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    It most certainly is a Power On/Off control ..... what else could it be?
    So it carries mains voltage on 2 terminals, and internal 2 switch contacts, one of which rotates under pressure and touches or not the other, it also has a small spring so it snaps one way or the other (that's why it clicks mechanically):
    All of those functional parts are small bits of metal pressed into holes punched in a phenolic paper disk, not exactly the most strong or stable assembly technology available.

    The point is that switch parts, which carry mains voltage, may stay in their place for a long time if not messed with, but if pot was dissassembled (it was) they may very well become loose and touch pot case, which is connected to chassis, which is connected to end user (you).

    So the sensible thing is to replace switched pot with a new one or even better a regular pot plus separate mains switch (safely mounted of course).

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    Quote Originally Posted by J M Fahey View Post
    It most certainly is a Power On/Off control ..... what else could it be?
    I dunno. In post #13, jomama says the amp has "a separate power on switch". That's where the confusion lies.

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    Ah yes. That is confusing.
    The power switch is either separate, or the on/off volume switch. It can't be both.
    Pictures of the front and rear panels would be a big help.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jomama View Post
    I did admit I was new at this stuff but I'm a pretty fast learner. This amp has low voltage caps and a separate power on switch. I'm only replacing a pot on a disconnected circuit board so can you be a little more specific about where the danger is?
    it spins past the On/Off click point
    Ok, then,
    * how do you turn that amplifier on?
    * What happens when the volume control clicks?

    Please answer those two questions.

    Make those three:
    * what is connected to the pot switch contacts?
    Show a picture.

    I am also intrigued by:
    Does a separate power switch on the amp free me from ..... ?
    notice the conditional/interrogative tense which strongly implies *today* there is not such a switch present.

    And:
    In post #13, jomama says the amp has "a separate power on switch"
    4 posts, alternatively opposing each other.

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    Last edited by J M Fahey; 10-25-2018 at 03:35 AM.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post

    Please answer those two questions.

    Make those three:
    Hell, let's make it four:
    Does the amp have a 3-prong plug power cord, or does it run off an AC/DC adaptor (AKA wall wart) and/or batteries?

    -rb

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    I have seen some small tube amps with trem, and the trem pot has a click switch to turn trem off. There would be a separate power switch. I can see a novice calling the trem with switch a "volume control".

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    Quote Originally Posted by rjb
    The safety concern was that if you have a bad AC power switch (which we assumed was built into the volume pot) you might get arcin' and sparkin' and udder bad tings when you plug the amp into a wall socket.
    Let's clear some stuff up:
    1. There is an on/off mains switch on the back of the amp.
    2. There is a three pronged, IEC grounded plug in addition to that switch.
    3. There was an on/off, B50K linear taper volume pot on the front which failed and I removed.
    4. I ordered this pot value without an on/off feature and will be installing it today unless advised otherwise.

    I hope I provided enough information now to eliminate any confusion. Thanks again to all helping.

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    Quote Originally Posted by jomama View Post
    Let's clear some stuff up
    There's one last thing to clear up: The volume pot w/switch has three terminals for the pot and at least two for the switch. What, if anything, were those switch terminals connected to?

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    Quote Originally Posted by tboy View Post
    There's one last thing to clear up:
    We're gettin' there. I can feel it.

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    Both the volume pot that was removed and the pots I ordered are three leg, single output. I have a bigger problem now, the pot leg spacing on the replacements ordered are too small (not wide enough)

    Does anyone have a stash of cheap linear 50K pots with the right lead spacing to swap?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jomama View Post
    Both the volume pot that was removed and the pots I ordered are three leg, single output. I have a bigger problem now, the pot leg spacing on the replacements ordered are too small (not wide enough)

    Does anyone have a stash of cheap linear 50K pots with the right lead spacing to swap?
    We *definitely* need pictures here, since we are on post#24 and still do not know what kind of pot you are talking about.
    * 3 leg? ... check .
    * "single output" ? ... what does that mean???
    * you talked about ON-OFF and clicking. Standard 3 leg pots do not "click" nor turn anything ON-OFF
    * "the right lead spacing" .... which would be? ....
    * "the pots I ordered" .... I have no clue about what you ordered.

    We need pictures showing old and new pots and pcb where it sits.
    Also measure lead spacing on old and new.
    And pot case diameter(s).

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    we are on post#24 and still do not know what kind of pot you are talking about.
    Oh, that's nothing. Another thread is on post #262 and we still do not know what "immediacy" means.
    -rb

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    Old Timer J M Fahey's Avatar
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    Well, "immediacy" can not be photographed, measured with a ruler or ordered at Mouser , while a pot can

    Cīmon jomama, ease our anxiety with a couple pictures

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    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by jomama View Post
    Does anyone have a stash of cheap linear 50K pots with the right lead spacing to swap?
    I got a huge bag of old 50k pots. Right lead spacing??? Follow Juan Fahey's recommendation and take some pictures/measurements.

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    I'm sorry for the late response. I went out and got a replacement at a pawn shop so repair became low priority. Pictures are coming. It's always good to have a spare

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    So, what did you get as a replacement?

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by jomama View Post
    Both the volume pot that was removed and the pots I ordered are three leg, single output. I have a bigger problem now, the pot leg spacing on the replacements ordered are too small (not wide enough)

    Does anyone have a stash of cheap linear 50K pots with the right lead spacing to swap?
    You could always add some wire to the pot legs and solder into the board. Use the mounting nut to hold it into the chassis.

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    Quote Originally Posted by The Dude
    You could always add some wire to the pot legs and solder into the board. Use the mounting nut to hold it into the chassis.
    Do you mean soldering wires to the pot legs and then making the connection to the board? I see two issues with that:

    1. The board might flop around a bit because the wires won't be as tight.
    2. I don't know how thick the wire should be and doesn't wire length change things?

    Quote Originally Posted by rjb
    So, what did you get as a replacement?
    I picked up a Johnson 25B. It has much nicer cleans but has no distortion. A week later I picked up a DS-1 for $17. My total was $27 for the lot. I'm still learning how to get the same distortion tone with the DS-1. It seems much noisier than the distortion on the First Act but maybe it's just nostalgia. I guess the longer you play something, the more you get used to it so I don't think I'm imagining the difference. Maybe I am. I PM-ed you my address to send the 50K pot. I can return the smaller one(s) in swap if you include a return address. You'll let me know what you consider fair and we'll work it out.

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    Quote Originally Posted by jomama View Post
    I PM-ed you my address to send the 50K pot. I can return the smaller one(s) in swap if you include a return address. You'll let me know what you consider fair and we'll work it out.
    DrGonz78 is the guy you want (see post #28).
    -rb

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    Corrected, thanks.

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    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by J M Fahey View Post
    We *definitely* need pictures here, since we are on post#24 and still do not know what kind of pot you are talking about.
    * 3 leg? ... check .
    * "single output" ? ... what does that mean???
    * you talked about ON-OFF and clicking. Standard 3 leg pots do not "click" nor turn anything ON-OFF
    * "the right lead spacing" .... which would be? ....
    * "the pots I ordered" .... I have no clue about what you ordered.

    We need pictures showing old and new pots and pcb where it sits.
    Also measure lead spacing on old and new.
    And pot case diameter(s).
    So we still need pictures that were asked about. I have a bunch of the 50k value pots but that does not help determine if any of them are good candidates for your amp. We need pictures to help determine what you need. In fact, we might see a picture and hunt down the exact replacement part for that amp.

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