The "ringing" in the 2Khz curves, are they there at source too?
Otherwise You have an oscillation-problem to deal with.
Hard to determine what to do actually.
Often my sollutions come to me when having my head deep into such amps.
Try alter the component values in the zobel network (if present).
Also check the caps around the drivers and VAS. Any millercaps there?
Otherwise You have an oscillation-problem to deal with.
Hard to determine what to do actually.
Often my sollutions come to me when having my head deep into such amps.
Try alter the component values in the zobel network (if present).
Also check the caps around the drivers and VAS. Any millercaps there?
You had strange effects when running with signal in to only one channel, and then pressed the mono button?
I think You have to look closer to the cirquits around the tone controls.
That is where the monoswitch is.
I would have thought that there is DC present where it shouldnt be.
Or are You using the Tape for inputsignal?
So far the biggest suggestion of a frequency imblance is in what comes out the speakersYou can see 5x greater woofer excursion on one side, and if you shift the balance all the way to one side or the other, one side sounds like its missing bass. Not completely, i.e. the woofer is getting some signal. Flipping which preamp goes to which power amp flips which speaker has greater bass output.
I'll spend more time with it tonight and try to get some better documentation and confirmation. Thanks very much Mooly.
This points at a possible fault either in the Bass-pot for the channel either having all that bass, or the channel missing bass. (Depends on wich one You are refeering to as "normal".)
Possibly there is some trouble in the tonecontrol cirquit aswell.
Source was much the same at 2kHz as 200Hz.
All the caps are new, good Nichicons and Panasonics mostly or all audio grade.
I will try to figure out what a Zobel network is and if I have one
I'm a sculptor and learning this as I go. Would that be associated with the output section? I do know that the problem is upstream of the power amps in any case.Using Aux for input. It may be that it was not good to have one of the two Aux rca's open when running mono. I'll have to recheck this. The problem I had with mono was trying to get the oscilloscope to stabilize. Which could also be a scope problem, it's an old beater that I just picked up for $30. and my first scope.
I do suspect, too, that the problem is around the tone controls, but I haven't yet developed a sufficiently regimented procedure to be sure, as I'm spending a lot of time fiddling with the scope...
Yeah it's hard to say which is normal as frankly I feel like it's a bit exagerated in one and deficient in the other. But if one's correct it's probably the one with greater bass.
I'm thinking of perhaps just going ahead and getting a new bass pot in there, maybe refresh some caps on the tone board at the same time.
BTW what are "millercaps"? Google just showed me a bunch of beer bottle caps.
Source was much the same at 2kHz as 200Hz.
All the caps are new, good Nichicons and Panasonics mostly or all audio grade.
I will try to figure out what a Zobel network is and if I have one
Disregard the zobelnetwork.
Not present in this amp.
Lets look at the tone controls.
I also think thats the place to go.
Millercaps aka Mille capasitance is the small capacitors you see between B-C on TR508 / TR608.
They are there to remove a problem, but if they "crack" they rather produce one.
100pF I see they are, ceramics. I would have them replaced anyway. SilMic is good.
Is also a very plausible explanation to Your bassloss.
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I will order some replacements for those caps. I've replaced all the electrolytics only. Thanks for all your help!
Those are within the power amplifiers, yes? I've already determined it is before that point in the circuit. If I send the right preamp into the left power amp, and vice versa, the problem stays with the preamp not with the power amp. So I think I should go directly into the tone board and controls...
Just to go back on all this can you confirm that you can see a difference in the scope measurements between left and right channels when fed from the same 200hz source ?
Measure at two points (four results in all) by checking left and right preamp outputs and left and right power amp outputs (with these connected to the dummy load).
can you confirm that you can see a difference in the scope measurements between left and right channels when fed from the same 200hz source ?Measure at two points (four results in all) by checking left and right preamp outputs and left and right power amp outputs (with these connected to the dummy load).
Will definitely go back to this. I feel like my work so far has been sloppy, the scope is funky, and my logs are inconsistent.
I think the starting point is I'll compare the L & R outputs of the phone and make sure the levels are matched.
But to go back to the very beginning, I kind of ruled out the power amps early on by jumpering (from the back panel) the right preamp to the left power amp and vice versa. The difference in channels concurrently switched speakers, which to me says it's before the power amps, no? Of course I tested switching the source channels as well and that had no effect.
I think the starting point is I'll compare the L & R outputs of the phone and make sure the levels are matched.
But to go back to the very beginning, I kind of ruled out the power amps early on by jumpering (from the back panel) the right preamp to the left power amp and vice versa. The difference in channels concurrently switched speakers, which to me says it's before the power amps, no? Of course I tested switching the source channels as well and that had no effect.
Took a break from this to recapacitate a friend's Fisher 440T. Just getting back to the 330A. I think I've traced the imbalance to the volume pot. Checked back and forth with the scope at the collectors and bases of T501/601, 502/603 and 503/603. Clearly was going bad somewhere between 502 and 503 ie. similar levels at T502/602 and different levels at 503/603. I stuck both probes on the same point at the same time before each measurement to make sure they were calibrated to each other at that level before moving one probe to the other side. Checked the volume pot, good going in and bad coming out. Can I consider this definitive for the volume pot? I guess I could resolder the ground connections on the pot to make sure both sides are connected well. But I'm thinking this is it.
Is it unusual for a volume pot to be way off like this? I can't imagine it was sold like this, could it have drifted a bunch over the years? If so, what's the mechanism behind such drifting?
Is it unusual for a volume pot to be way off like this? I can't imagine it was sold like this, could it have drifted a bunch over the years? If so, what's the mechanism behind such drifting?
Poor tracking of volume controls is quite common. Is it definitive ? 95% I would say, the only other reason (highly unlikely) is that there was some variation in input impedance between the the power amp inputs.
If it does have a tapping for loudness compensation then you either have to source something similar, or, if you can live without the loudness function, fit a normal 3 terminal pot. Should be log law for audio but linears are better matched. You can fit a "law faking" resistor to a linear if you want.
ESP - A Better Volume Control
If it does have a tapping for loudness compensation then you either have to source something similar, or, if you can live without the loudness function, fit a normal 3 terminal pot. Should be log law for audio but linears are better matched. You can fit a "law faking" resistor to a linear if you want.
ESP - A Better Volume Control
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