magnatone 280 help

[toneknob]

Oh knowledgable ones:

I'm working on a Magnatone 280. channel 2 is working fine. Channel 1 is a belligerent $%&*@$.

Schematic says i want 120V on pin 1 and 130V on pin 6 (V1). i have 170 on pin 1 and 88 on pin 6.

I have 258V in front of the plate resistor (on pin 6) and I swapped the plate resistor for a new one-- no effect. Then I used my decade box and found that it made no difference what value plate resistor was in place. That made me think it was something wrong with the cathode (not bypassed, no resistor), but giving it a better path to ground had no effect (0V on the cathode).

It wants a 10mfd/450V filter cap for that section. I found that the original had drifted to 21mfd. I don't have a 10mfd laying around, but I disconnected it from the circuit and alligator clipped a 16/450 in its place. No effect.

The age old 'pop test' has isolated the problem to (V1). And thank goodness for that because it is a 12AX7 and not one of the other wacky tubes in that amp (you know, the ones you don't know the pins by heart....).

Ideas please.

Pretty please.

[Reeltarded]

Hey! Thanks for a decent maiden voyage here! You didn't say anything about actually swapping V1 for a known good tube though.



A couple good pictures of that area of the amp for reference will help. It doesn't seem to be anything major.

Try a tube swap first, but still post pics. We like those amps.

[Blackburn]

Yeah, Magnatone is the shit! My woman just bought me a Magnatone DVD player.

I hope it has that great pitch shift vibrato!

[martin manning]

I think it would be odd for a filter capacitor to drift up in value, but in any case that shouldn't prevent the tube from operating. This sounds like a bad connection somewhere, perhaps between the cathode pin and the socket. I would re-flow the solder joints from the B+ to ground, and clean the socket and pins. You have tried a different tube, right?

[Structo]

Is this a 280, 280A or 280B?

What is channel 1 doing audibly?

Did you try different 12ax7 tubes there?
Different tubes, even the same brand can draw different amount of current affecting the voltage reading.

Check the dropping resistors in the power supply for drift.

They usually drift up in value so that should reduce voltages.

[toneknob]

Come on guys!

Swapping a tube is ALWAYS the 4th thing I do! After I take 3 unnecessary steps, then remember to swap the tube. (Remembering to plug it in comes in somewhere around step

Anyway, I have a good tube in there.

As for maiden voyage-- I didn't realize it was my first post here. Wow. I have gotten through a Rocket and an Express and I never had any occasion to post a stupid question. That, folks, is a tribute to the vastness of the information on this site!

I'll work on those connections as per Martin's suggestion and I'll get some pics. But you won't like them. We all like the board mounted components, right? We just seem to know the shapes of those sections on the board: PI, bias section, B+, etc.... On this %$%# Maggie you have to strain what is left of your eyesight to follow those wires around. I feel like the kid in The Shining, trying to get through that hedge maze...

[Blackburn]

That, folks, is a tribute to the vastness of the information on this site!

Amen!

If only everyone knew that!

[toneknob]

Still working...

Here are some pics. Careful what you ask for!

[toneknob]

As far as the model number-- I don't really know for sure. It is an Estey-made Titano Model 383. I was told it was a 280, so that's all I know. It seems to be pretty consistent with the 280B schematic that I found:

I'm also including a hand drawn layout that I found on the web somewhere (it is easier to read than the schematics and easier to look at than my pics):

[Blackburn]

I think I see some tropicaps in there. I have some pulls that I hope to use eventually. Some tone stack values and some huge .47ufs that would probably make good cathode bypass caps. I guess that would make yer Mag an early sixties model?

[Structo]

Forgive us, we can't assume anything when you had only one post to your name.

When measuring plate voltage are you measuring at pin 1 & 6 to ground?

This is after the plate resistor and you must have a tube in the socket to measure plate voltage.

Clean (DeOxit D) and tension socket pin holders.

[ampgeek]

I don't think that I saw what you are measuring on the cathode of V-1A (pin 3). If that looks significantly different than 0.6 V, look closely at the cathode resistor and the 220K feedback resistor.

Good luck,
Dave O.

[toneknob]

So... I get around .6 on the cathode (pin 3).

I experimented with cathode resistors until I got the voltages that I wanted. Now my plates are very much in the ballpark, but the volume is still very weak from that channel.

I sprayed buckets of zero residue cleaner in and around the socket (I heard horror stories about de-oxit leaving lubricating residue behind and causing tubes to arc and take down PTs... true or not, that image has stayed with me).

One of the big dropping resistors coming off the rectifier has drifted to 4.4 meg (I think it's supposed to be 3.3 meg), but I can't imagine that my trouble is coming from that. Second channel has the same voltage, but is loud and proud.

So... my tube is conducting the way it should. 'Amplification', therefore, is presumably taking place. Yet... not even half the output of channel 2.

Stumped.

Oh-- and I hope I didn't come of snarky... I love the site and have spent tons of time here, but-- as this 'repair' will attest-- I haven't felt I've had much to add to the tech-talk here. So... no prior posts. Sorry... definitely no snark intended.

[Reeltarded]

Where does the signal get dropped? Crack both channels equally and do a probe hunt from where the channels mix to the inputs. When you compare them you will find a place where channel 1 has much less tic or pop than chan 2. The next component toward the input is now where you focus.

Make sense?

[ampgeek]

Build and use the Listening Amp tool described near the end of the page here for that probe hunt:

http://www.el34world.com/Hoffman/tools.htm


Truly invaluable tool and really easy to build. I highly recommend adding a volume pot.

Your troubleshooting time will be cut by 10X or more...really!

Good luck!
Dave O.