Magnatone 280 on the Bench

[JustMike]

I've never seen one of these before. It works and the stereo Vibrato is pretty cool. The owner says he wants it "tuned up". Tubes check out good, some (Maybe all) have been replaced. Some cap work has been done inside. The filter caps look original, but the values don't match what I see on the schematic I downloaded here. I see 4 25u caps. Resistors look suspect too. Have a look;

[sluckey]

I see 4 25u caps. Resistors look suspect too. Have a look;

I don't see any 25µF caps. Which resistors look suspicious to you??? They all look brand new to me.

[JustMike]

Thats what I meant, they've been replaced. And I poorly articulated my attempt to describe the resistors across the  .25u caps.
 The amp does have a little hum which Im guessing is the old filter caps.

[sluckey]

Thats what I meant, they've been replaced. And I poorly articulated my attempt to describe the resistors across the  .25u caps.
 The amp does have a little hum which Im guessing is the old filter caps.

Those caps are not filter caps. They are part of the Vibrato Oscillator and Control V6-A and V7-A,B. The .25µF caps and 22K resistors are original. I highly recommend to let them be. Look for your little hum in the power supply or output tubes.

I've attached a schematic with those four caps/resistors circled. A block diagram of the amp is also included. Should be helpful to understand this amp.

[JustMike]

Upon further examination, the original can caps have been left in place but taken out of circuit and replaced with these little brown caps. Some of the tube sockets are loose and cause noise when I move the tubes. Any tricks to retensioning tube sockets? Not much else to be done.

[JustMike]

 We want the hum to go away. The tech that previously serviced this amp just bypassed the original can caps and put in the little brown electrolytics. I don't think those original Astron caps are made anymore. Antique electronics has some 20/10/10/10's which gives me one more 10u than I need which is Ok. BUT the cap values that are in the amp don't match the values on the schematic. See my snippet. I'm guessing this was done on purpose to tighten the amp up a little? 5u4 is rated for 40u max so there's no issue there. The beauty of the 20/10/10/10 is I could always start out with the proper 10/20/10 values and I'll have an extra 10u I can parallel to make the 20u. The downside is the new caps are larger in diameter and I'll have to drill the chassis. Is this a no no on a 1958 collector's item or do you think it's just a service necessity?
 

[thetragichero]

Is this a no no on a 1958 collector's item or do you think it's just a service necessity?

there's the rub. the "collector"-types want everything original, even if the thing sounds horrible (sometimes a baggy with the original components is sufficient). the rest of us want the daggone thing to work reliably and don't care if components are changed
that being said, a slightly bigger hole to accommodate a vintage-style can cap (i'm assuming the ce manufacturing ones but the jj can caps are like a quarter of the price) is probably not the end of the world resale value-wise (just hold onto anything you remove)

[HotBluePlates]

We want the hum to go away. The tech that previously serviced this amp just bypassed the original can caps and put in the little brown electrolytics. ... BUT the cap values that are in the amp don't match the values on the schematic. ...

The cap values used are fine, and if anything would reduce hum more than the original values.  I suspect the etch used the parts he had on hand, and that is why some were changed away from 10µF to 22µF.  No problem there.

We want the hum to go away. ...

How do you know the hum is caused by the filter caps?  How do we know they need replacing?

Try this:  measure the AC Volts at each filter cap, and mark it on your schematic snippet.  We expect 5-10vac on the cap at the rectifier output, southing less on each filter cap afterward.  What do you get?

[JustMike]

Thanks HBP. I went ahead and replaced the caps before I saw your message, and it solved the problem. The amp is dead quiet now. But I'll remember to check for ACV on the filter caps on the next patient.
Funny story; When the owner came over to pick it up today, we were playing thru it and every once in a while we would hear some random buzzing and clicking sounds. I pulled the chassis and started chopsticking and it was rock solid. Nothing. I'm thinking "Hmm, I've heard this before on old Fenders and it turned out to be the tremolo caps". So I start looking at the schematic and it dawned on me-CELLPHONES IN OUR POCKETS. He put his out in his car and I turned mine off. Problem solved. 

[sluckey]

Should Hoffman be getting a percentage of your business?

[JustMike]

I don't know. What does that mean? 1 or 2 amps a month for friends and references. I do it because it's a hobby that pays for itself. Mostly I buy dead or broken amps and fix 'em and flip them. It's fun for me.

[sluckey]

I don't know. What does that mean?

It means exactly what I said.

Just an observation. Seems to me the only time you post is when you want the forum to help you fix a "friends" amp. You never try to help someone else. All take. No give. Seems like there should be a price on that.

[JustMike]

Well,  the only thing I can offer is my thanks. There are a lot of folks here that know a lot more than me, including you Sluckey. And I have and do thank you for the knowledge and guidance that you've given me. Next time you're in Orlando HMU and Ill buy the first round!