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Thread: New Project, Finger's Crossed

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    New Project, Finger's Crossed

    I actually don't have much hope in this working the way a good friend of mine wants.

    It's a MASCO MA 25P

    He wants me to try and make it work as it used to so he can play some 78 RPM records and just have it around as a working conversation piece. Chances of the tone arm and needle working are probably slim to none.

    Two questions:
    1: The schematic I found shows the 2 main filter caps at 8Uf each, this one has 2 - 16Uf 600V installed. I will be replacing them but just wondering if I should stay with 16's or go for the 8's?
    2: Would this be considered a hot chassis unit? If so what is the best fix? Hot chassis is new to me.

    Needs every resistor and every cap replaced. It's had few things replaced a long time ago but they will also be replaced.
    Only good thing is all the tube are OK.
    The wires going to the turntable motor were petrified but good to go now.
    Still have to cover a few spots on some of the transformer wires and then start replacing the rest of the parts.

    If it doesn't work maybe it will become a harp amp.

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    Since you have the schematic, can you please post it here? You'll get a lot more action that way...

    Justin

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    Justin,

    Here is the schematic I currently have.


    Click image for larger version. 

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by J Luth View Post
    ...
    Two questions:
    1: The schematic I found shows the 2 main filter caps at 8Uf each, this one has 2 - 16Uf 600V installed. I will be replacing them but just wondering if I should stay with 16's or go for the 8's?
    2: Would this be considered a hot chassis unit? If so what is the best fix? Hot chassis is new to me...
    My opinions are:
    1. It would be OK and an improvement to increase the capacitance of the first two filter caps. If it were mine I'd just use 22uF since that is a standard easy to source value.
    2. Since the amp has a power transformer it is not a "hot chassis" unit.
    To bring it up to modern safety standards you should:
    a) Remove the 0.1 cap currently connecting one side of the line input to ground.
    b) Upgrade to a 3 wire grounded power cord. There is a good discussion about how to do a correct conversion at http://music-electronics-forum.com/t35916/

    Cheers,
    Tom

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    Last edited by Tom Phillips; 03-02-2018 at 03:35 AM.

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    Old Timer olddawg's Avatar
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    Concerning the turntable. My guess is that it's a big old ceramic cartridge. If the needle is shot and unavailable any ceramic cartridge would probably work fine. You just have to figure out how to mount it. Sometimes just a dab of 5 minute epoxy works fine. You will also need to adjust the tone arm balance. Those old TT with the giant cartridges had a lot more weight on the needle. If you want to restore it to original, eBay is your friend. There's lots of NOS and mildly used parts for these old clunkers out there left over from grandpop's small town radio shop, etc.

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    Thanks Tom.

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    Thanks olddawg

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    Tom, the link comes up as page not found.

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    Tom, on the first two filter caps the ones that are installed are rated for 600v. Would that be the correct voltage or could it be less? I generally order from Mouser but the highest voltage 22uf they have listed is 500v.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by J Luth View Post
    Tom, the link comes up as page not found.
    It worked on my home computer but doesn't work on the computer I'm using at the moment. I'll troubleshoot that.
    Edit: I re-did the link.

    Quote Originally Posted by J Luth View Post
    Tom, on the first two filter caps the ones that are installed are rated for 600v. Would that be the correct voltage or could it be less? I generally order from Mouser but the highest voltage 22uf they have listed is 500v.
    Your schematic doesn't list voltage readings. However, the old equipment like yours usually had B+ voltages in the range of 375 to 425V even after taking into account today's higher line voltages. Therefore, I think that 500V or even 450V filter caps will be fine.

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    Last edited by Tom Phillips; 03-02-2018 at 03:36 AM.

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    Thanks Tom

    The link works now.

    The schematic doesn't list any voltages, it's the only one I found in a Google search. I haven't done much to the unit yet so I was not sure what the voltage should be. Going to check voltages once I start working on it. The first two filter caps in the unit appear to be replacements for whatever was in there originally. I don't see them on any of pictures of the unit I have found while searching for info about this. I kind of thought 600v was a bit high but always better to ask those that might know before I attempt it.

    Here's a picture of the unit with the turntable removed. You can see the first two caps at the back. In between them there is a hole in the chassis that I would say shouldn't be there if those were original or original style for the unit. One of the caps partially covers it for no reason. That's why I figure they are replacements for whatever was in there originally.


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    Last edited by J Luth; 03-02-2018 at 04:36 AM.

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    Old Timer Tom Phillips's Avatar
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    I don't see any clues in the photo to help us know if there was anything originally mounted in the extra hole near the two large filter caps. Can you post some photos of the underside? It would also be interesting to know the rating of the other 8uF electrolytic caps.

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    Tom,

    Maybe there was a single or dual can cap installed where the hole is and the cap held in place with the retaining nut like the current 8uf's have. The hole looks to be the correct size for that type of cap. The hole is close to the transformer but there is enough space to get a nut on. There is slight bit of paint missing on the underside of the hole.

    The other 8uf caps are rated for 450v the other caps in it are rated 400v and 50v. The outside 8uf are the only ones rated for 600v. 450v on all will probably be fine.

    This is the only picture I have of the inside right now.
    The caps missing the paper covering are 2 - .1uf and 2 - .01uf, the one with the red on it is 25uf 50v., the lone blue one is the third 8uf.

    Click image for larger version. 

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    Last edited by J Luth; 03-03-2018 at 10:16 PM.

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    Doing a few things on the Masco in between other projects.

    Looking at the schematic it looks like it calls for 1 - 10 ohm 200watt and 1 - 5k 10watt located by the 6l6's. The unit has the 2 wire wound resistors installed but both need to be replaced.

    Do they need to be those values or should they be something else?

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    Quote Originally Posted by J Luth View Post
    Doing a few things on the Masco in between other projects.

    Looking at the schematic it looks like it calls for 1 - 10 ohm 200watt and 1 - 5k 10watt located by the 6l6's. The unit has the 2 wire wound resistors installed but both need to be replaced.

    Do they need to be those values or should they be something else?
    I think there's a typo on the schematic. the cathode resistor would be 200 Ohm 10W, and not 10 Ohm 200W
    or "10W 200W" as it actually says

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    Thanks eschertron.

    10 watt seams kind of large but I'm still a rookie.

    I should have looked closer at the schematic to see the 10W 200W. DOH! The old brain saw it a 10 ohm.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by J Luth View Post
    Thanks eschertron.

    10 watt seams kind of large but I'm still a rookie.

    I should have looked closer at the schematic to see the 10W 200W. DOH! The old brain saw it a 10 ohm.
    Golden age engineering. Circuit analysis would probably reveal that 5W is plenty for a normally-operating stage. We'd need the expected (or actual) voltages to know.

    edit: why do those two wire-wound resistors need to be replaced? Of all the components, they'd be least likely to suffer from age.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

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    The voltage at the 5k resistor is 500v with only the rectifier installed, it drops to 300 with all tubes installed and warmed up.

    I will recheck the resistors again now that I know what the correct value.

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    What to do on a heavy snow day.
    Got the MASCO somewhat working, new resistors and capacitors. Fired it up and it hums a bit so I start looking for things, found a few bad solder joints and fixed them. Hum is there but not too bad. Moved the output transformer to a different location which helped. This is not a question about the hum, which may come later, it’s about what I am experiencing.

    Here’s what is weird, well at least weird to me.
    All tubes removed and the output transformer disconnected from the tube sockets and the resistor but the 4ohm speaker tap or any or all of the speaker taps connected and common connected it still has hum through the speaker. The farther I move the output transformer away from the chassis and the power transformer the less it hums just about down to none. If the output transformer is completely disconnected no hum which stands to reason, it only does it with the taps connected. It also does the same with all the tubes installed.

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    Try turning the OT 90 degrees. Look at atypical old Fender. The power transformer sits in a chassis hole so the laminations are flat - parallel to the chassis surface. The Output Transformer is mounted standing up, so the laminations are standing perpendicular to the chassis. The two transformer lamination stacks are perpendicular. If your OT makes hum out the speaker even when the power tubes are disconnected, that is a common reason.

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    Enzo,

    Yup, I tried that also. The transformers are the same as Fender amp. Power transformer laminations lay flat, output transformer vertical. The output transformer was right beside the power transformer with the laminations facing each other so I figured it would be better to move it and turn it. What I did was to move the output as far away from the power transformer as possible and turned it 90 so the the bell was facing the power transformer not the laminations. It was better like that. The thing is I get the hum through the speaker with any or all of the output taps connected but the primary leads are disconnected and no tubes. That seems strange to me, how can I get output if the primary leads are disconnected?

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by J Luth View Post
    ......how can I get output if the primary is disconnected?
    It's being radiated into the OT from another source. Most likely the PT. It's the same principal as how the transformer itself works. Primary and secondary do not connect directly to each other, yet signal still gets from one to the other.

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    Last edited by The Dude; 03-14-2018 at 04:00 AM.
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    Lifetime Member Enzo's Avatar
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    Oh one tiny remote possibility. I am really stretching to make this. If your PI tube is installed, there is some current flowing through the tail, and is COULD somehow send a tiny current through the feedback line to the OT secondary. I sure wouldn;t bet my dessert money on it though.

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    Well that appears to be the case since everything is disconnected. Anyway to stop it or cut it down? Although it's not that bad.The amp actually sound pretty good connected to 2 10" speakers and playing a guitar through it.

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    No tubes installed.
    With the tubes installed it is no louder but it does vary a little with the volume and tone pots and gets noticeably louder with the volume turned way up, but I would expect that.

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    Lifetime Member Enzo's Avatar
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    The hum you hear with no tubes is probably not same hum as the hum that turns up with the controls. Hum is not generic, it has multiple sources, and each source needs its own solution.

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    Pretty much all better now. Moved the output transformer and turned it 90 degrees to the power transformer. Moved a few wires around, re-soldered a few connections and it's fairly quite.

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    The Masco is almost done.
    All re-capped, replaced all the resistors, new input jacks, moved and rotated the output transformer, nice and quite and sounds good with a guitar. The turntable now works, new mounting grommets, new power wires, a bit of oil on the motor bearings and a piece of shrink tubing on the spindle that drives the wheel. The plater turned real slow and would barely turn with a record on it and the tone arm on the record. The shrink tubing cured that. The crystal cartridge was all dried out, I cut up a piezo element and stuck it in there and it works even sounds like it is turning at the correct speed. A bit low volume but it works, the owner can send the cartridge out and have it rebuilt if he wants. New tubes might be a good thing he can also re-tube if he wants. A couple of brackets and it should be done. I have to raise the turntable part about 3/4" higher do to the new output transformer location, the turntable motor hits it.

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    Old Timer olddawg's Avatar
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    That heat shrink trick on the spindle won't work very long. There used to be a product kind of like nail polish with fine sand in it that was used to paint spindles. Also "Rubber Restore" for the idler surface. If the rubber is hard as a rock I think there are still places that will rebuild the idler.

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    Lifetime Member Enzo's Avatar
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    SOunds like you need a new idler wheel or a new idler tire.

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    Rubber on the idler is not all dried out but I'll look into something to put on it anyway. The spindle was just slipping on the rubber of the idler wheel, the shrink tubing was to test it out. The spindle has nothing on it and is almost looks like it polished from rubbing on the rubber. It works for now but I'll look into what can be done. It's going to be more of a conversation piece at his house maybe play a record or two occasionally. He just wanted me to get it working. It may end up as a guitar or harp amp at some point.

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    That's possible but it's up to the owner, as stated below probably just going to be a conversation piece at his house.

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    Lifetime Member Enzo's Avatar
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    Is there a spring to hold the idler wheel in contact with the spindle and platter?

    The rubber should feel, well, rubbery, not like hard plastic.

    Mineral spirits is a great rubber rejuvenator. We use that in the pinball world - where every machine is full of rubber rings. Wipe it on, it usually rejuves the rubber. Don't soak the rubber or it will swell up like those growing dinosaurs for kids - soak them in water.

    Actually gasoline works too, but is way to dangerous around electronics to use.

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    Yes there is a tension spring.
    The rubber on the idler wheel is soft enough that I can push a fingernail in which leaves a slight mark but the mark goes away after a bit. I'll check it again and maybe give a bit Mineral spirits, probably can't hurt. I have some here.

    Gasoline works great for a lot of things, I use a lot in ways I probably shouldn't.

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    Lifetime Member Enzo's Avatar
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    You replaced the motor grommets. Did you use real turntable motor grommets? Or just grommets?


    Oh, if you give the platter a spin by hand, does it go around and around? Or does it stop real quick? I usually have to pull the platter and remove all the old lube. And relube

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