Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Watto on April 22, 2015, 10:19:52 pm
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Hi folks,
I recently bought a 1960 Magnatone 260 , all original, after new filter caps she fired up beautifully,
The pitch vibrato circuit is working , but the speed control isn't , I'm not sure where to start looking , I have metered the pot , and it still works well on resistance, so I'm guessing it's not the pot, any suggestions of where to start trouble shooting would be appreciated.
I've attached the schematic for reference
regards in advance
Watto
(http://i20.photobucket.com/albums/b210/amwatts/magnatone_260a_zpsvup53ke2.jpg) (http://s20.photobucket.com/user/amwatts/media/magnatone_260a_zpsvup53ke2.jpg.html)
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I couldn't get the schematic zoomed in enough to cite part nos, but the problem is almost certainly in the phase-shift oscillator triode which is in the lower left. Note the three caps in series, from the plate to the grid of that tube. That's the tell. Phase shift oscillators are not reliable starters, usually they have to build up a tad to get going. The flipside is, if any of the 3 series-caps are leaky, you can have trouble.
First, see if you get a small, 1/2 - 2 volt reading on the cathode. That shows the tube is conducting. If it is, the usual culprits are those series caps. You say the e-caps were replaced, but probably the various .01's and .02's scattered about the amp may not have been. If those caps are the originals, they could be leaky and that WILL cause stating problems. Still, before you change them out, if the shared 12AX7 triode is weak, that could also be another cause. If you have a known really fresh new one, swap that one in. Of course.....it could be BOTH the tube and the caps. Obviously you want to try the easy stuff first.
Phase shift oscillators are weak starters and weak caps or weak tubes can prevent the thing from starting up. Just for fun, I have built PS oscs on breadboards and changing the 12AX7 (highest gain in the series) to a 12AT7 (less gain) and it won't start or it might take 5 seconds to start. A 12AU7 (lowest gain) and it will not oscillate at all. They need to have everything going for them.
You are actually lucky if this is your problem because it is a pretty easy thing to isolate and troubleshoot.
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I couldn't get the schematic zoomed in enough to cite part nos
This is the best image I was able to find
http://www.blueguitar.org/new/schem/misc_amp/magnatone_m260a.gif (http://www.blueguitar.org/new/schem/misc_amp/magnatone_m260a.gif)
K
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Thanks for the reply eleventeen.
I was hoping it was just that bunch of caps, I don't think the tube is the issue, though I have others I can swap out if it is, the rate is set at halfway, so it does oscilate, it's just that by moving the speed pot, nothing changes.
I'll put in some new coupling caps and see how she goes, thanks again
here's a tighter shot of the section of the schem we are talking about
(http://i20.photobucket.com/albums/b210/amwatts/magnatone_260a%201_zps1vfpy989.jpg) (http://s20.photobucket.com/user/amwatts/media/magnatone_260a%201_zps1vfpy989.jpg.html)
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Since the vibrato works but you cannot change the speed, the pot would be suspect. Or it's wired wrong. I know you said the pot is good but try this...
Test the pot in circuit. Connect your ohm meter to chassis ground and to the top side of the pot (as viewed on the schematic). Does the resistance change from 500K to zero as you turn the pot?
BTW, do you have a 260 or 260A? There are differences between the two models.
EDIT... I've attached a better scan of the 260-A schematic.
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Sluckey you were spot on.
The pot metered out fine , yet wouldn't work in circuit . A new pot and it's back to life ... Thanks for your help , amp sounds great now