New Project, Finger's Crossed

[J Luth]

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.

[Justin Thomas]

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

Justin

[J Luth]

Justin,

Here is the schematic I currently have.

[Tom Phillips]

...
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

[olddawg]

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.

[J Luth]

Thanks Tom.

[J Luth]

Thanks olddawg

[J Luth]

Tom, the link comes up as page not found.

[J Luth]

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.

[Tom Phillips]

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.

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.

[J Luth]

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.

[Tom Phillips]

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.

[J Luth]

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.

[J Luth]

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?

[eschertron]

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