MA230 Issue ???

CANT

CANT

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I'm not sure if this question/line of questioning would be better suited in here or the general tube section but I have a MA230 that I am having issues with...

When I purchased this amp about a year ago it was in rough shape but appeared to be a good deal. It was sold as working, which was just barely true... I started out with cleaning the controls and replacing all of the power supply capacitors (all bad). Then I found out both of the output transformers were bad so I had to get them rewound (took a few months). After reinstalling the OPT's it still wasn't behaving correctly and appeared to feedback in the left channel. So I went back through it and found that the volume and balance pots had been worked on and incorrectly wired... I also found quite a few resistors that had drifted so I replaced them. This cleaned up the signal and helped the feedback issue but didn't get rid of it? At this point I was ready to pass it off to a local tech I occasionally take things but he was booked out for a few months so I decided to stare at it a little bit longer.

I am currently leaning toward the issue being in the power amp or tube section of this device but I can not figure our what could be causing this??? At it's worst the left channel sounded like a fog horn, now it sounds kind of like a building charge and then a pop slowly repeating, I would describe both as feedback. While it appears to behave at lower voltages, increasing the signal will activate the feedback but ultimately the problem exists with no input signal at full line voltage? I have tried switching tubes L to R and replacing all of them individually. Some times the right channel will sound/behave perfect and other times will not work at all, which is weird with all known good tubes, the left always retains the same issue. I'm starting to wonder if it's one or all of the preamp sockets but I can not see anything wrong with any of them? How does a socket really go bad??? I have also disconnected the SS pre to rule it out/test it separately and bypassed pretty much if not all of the selectable switches on the face.

Does any of this sound familiar to anyone? Any ideas what is going on here?

Thanks in advance to any one who might be able to shed some light on the issue.
 
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Never worked on a MA230 but I restored a MAC1500 and the preamp and power amp sections are very close in design. First, verify ALL voltages from the PS are correct. The +10vdc output is set by D4, a 1N714 zener diode and must be +10vdc or the SS preamp section will not behave. Second, will the output tubes bias properly at .7vdc and have you replaced the coupling caps C31, 32, 33, 34, all .1Uf @ 600v? Third, have you cleaned all tube sockets? They do loose their grip and they do get dirty, to answer you question "How does a socket really go bad???" Fourth, do you have a scope and a signal generator? You need to be able to signal trace if steps one through three don't correct or change significantly the symptoms.
 
You are correct the 230 and 1500 are nearly identical +/- the tuner...

Normally verifying voltage would be step one but I actually have yet to do that because of whatever is going on in the left channel. I essentially haven't really gotten past 85V or so on the variac. What I originally thought was feedback I am now thinking might be arcing in some form? I have not worked on much tube equipment so arcing is new to me. I have not been able to properly bias the tube set since I have not been able to run the unit at full power for any real length of time but when the popping occurs I have seen the voltage at the test points essentially double and the tubes do also flicker in the dark.

It is good to know of the importance of the +10VDC line but I believe the preamp section to be behaving at this point. I have modified the LF switch to bypass the rumble filter feature and act as a power amp direct input switch. I did this to earlier on to rule out noise/weirdness from the SS pre.

Yes, I have replaced the coupling caps (all .1uF 630V). At this point, I have basically replaced all of the caps and resistors in the power amp section. I think the only things original left are the .25uF's because that was an odd value and they tested fine? I might have a couple .27uF's if I really needed to replace them?

Yes, I have cleaned and re-tensioned all of the tube socket. I actually just abraded, re-cleaned and re-tensioned the 12A_7 sockets since my previous post which cleaned up the intermittent right channel, it is now crystal clear.

Unfortunately, I do not have a scope at this time and I do realized that the path forward likely requires more tools than I currently have... this is really just an attempt to draw out any possible information that might just happen to help
 
Ok. Remove all of the tubes and bring it up again on the variac. And yes it could be arcing. Get the power supply good before moving to listening. The .27Uf caps are fine to replace the .25s as long as their voltage rating is the same or higher.
 
Without tubes…

480, 480, 430, 245, 60 & 4 on the PS caps

-34 on bias w/ -20 @ ea G1

also verified 480 @ ea P & G2 on the 7591’s
 
At this point I would remove the modification you did to the LF switch; a direct input to the grid of the 12AX7 is not a good idea (hum and noise). I would concentrate on the left channel by making sure the tube sockets for the 12AU7 and 7591s are tight and clean, install the tubes and bring it up slowly on the variac. The voltages for the power tubes should settle at or near specs once the tubes are at bias. The 60v and 4V instead of 85 v and 10v aren't right, but might be once the voltage divider network is under load.
 
Still no luck... can't get past 75V on the variac before the left channel starts popping?

Only thing I can add is that if I pull the 12AX7 the popping stops immediately. I can even increase the voltage with it out and everything stays quite. As soon as I throw the AX back in and it warms up, it starts popping again???
 
Are you referring to the LF and HF just before the AX circuit or something farther up stream?

I do not believe I have tested these filter switches when the issue is present but can do so on lunch... I have tested the mode selector, balance, loudness and volume. They all have no affect other than what I have previously stated. All of the switches and pots have been cleaned with either Deoxit or Fader lube at some point during this repair.
 
As a side note, even when the left channel is not popping there does appear to be a constant minor distortion present when compared to the right channel.
 
Pio1980 said:
I'm wondering abt the problem being previous to the power stage input.
Click to expand...

ramjet suggested I undo a modification I did early on and I will need to to fully test the LF and HF switches but I did perform this mod specifically to isolate the power stage from the pre and all it did was permanently defeat the LF circuit. It was very easy to preform and can be easily undone... maybe this information helps?
 

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Hello

Then I found out both of the output transformers were bad so I had to get them rewound (took a few months)

Really difficult to believe ! two output transformers ?
rewound them ? I can't believe this is possible to do the way it should be done
I think you have a problem with your unit but you are not looking at the right area
 
clinic-audio said:
Really difficult to believe ! two output transformers ?
rewound them ? I can't believe this is possible to do the way it should be done
Click to expand...

Essentially one half of the primary winding was dead on both OPTs. One measured open and the other measured OL. Both were rewound by a reputable shop... I will stop short of throwing out any names only because I do not think they want to work on any more of these, I was definitely given the impression these OPTs were quite complicated.

You have the right to be skeptical but trust the work done on the transformers so I would prefer to focus on other areas for troubleshooting the issue at hand.
 
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