|J Luth||2/27/2018 7:42 PM|
|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.
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||2/27/2018 9:53 PM|
|Since you have the schematic, can you please post it here? You'll get a lot more action that way...|
|J Luth||2/28/2018 2:18 PM|
Here is the schematic I currently have.
|Tom Phillips||3/1/2018 10:08 AM|
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/
|olddawg||3/1/2018 12:56 PM|
|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||3/1/2018 1:07 PM|
|J Luth||3/1/2018 1:07 PM|
|J Luth||3/1/2018 1:08 PM|
|Tom, the link comes up as page not found.|
|J Luth||3/1/2018 1:22 PM|
|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||3/1/2018 4:07 PM|
Edit: I re-did the link.
|J Luth||3/1/2018 9:16 PM|
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||3/1/2018 10:22 PM|
|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||3/3/2018 2:59 PM|
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||3/6/2018 4:11 PM|
|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||3/6/2018 4:18 PM|
or "10W 200W" as it actually says
|J Luth||3/6/2018 4:52 PM|
|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.
|eschertron||3/6/2018 5:34 PM|
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.
|J Luth||3/6/2018 6:36 PM|
|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.
|J Luth||3/13/2018 5:05 PM|
|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.
|Enzo||3/13/2018 6:55 PM|
|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.|
|J Luth||3/13/2018 8:00 PM|
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?
|The Dude||3/13/2018 8:05 PM|
|Enzo||3/13/2018 9:01 PM|
|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.|
|J Luth||3/13/2018 9:36 PM|
|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.|
|J Luth||3/13/2018 9:41 PM|
|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.
|Enzo||3/14/2018 4:20 PM|
|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.|
|J Luth||3/15/2018 4:47 PM|
|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.|
|J Luth||3/17/2018 6:51 PM|
|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.