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In Reply to: RE: Thank you! And cap questions? posted by immatthewj on July 26, 2020 at 17:44:30
Well built amp!
If you can't get close to the pot, see if you can follow the colored wires to a spot more accessible. Agree on not removing the pot. You'll want one DVM probe to go to the center pin wire. The other probe to either of the outside pins. This is the quick test method when you turn and measure a full swing at each end. The "turned up" position should have "less" resistance.
I should have mentioned to get the "in circuit" capacitor checker. Most dedicated cap checkers are. Then you don't need to unsolder anything. That one Jensen cap in particular.
My run away red plate bias was the result of a bad PCB tube socket pin solder joint. Very rare with point to point wiring.
Still wouldn't hurt to check the integrity of each connection. Finding something loose would actually be great as it is easily fixed and you're up and running.
Cheers!
Jonesy
P.S. Glad to hear your wife is helping you. My wife is also following this adventure.
"I know just enough to get into trouble. But not enough to get out of it."
Follow Ups:
"Adventure"? & an audience? Oh my.
Anyway, I think than that I essentially did have 1 probe on a center pin. Since the results of everything I did yesterday were so unsubstantial & inconclusive, I used the notes I took while I was making the checks to start my fire for the grill. Seriously. But I do believe that the center pin on both pots was the accessible one. As I typed previously (going back & looking at my post, because I no longer have my notes) my meter showed 0.6 (& that was with both pots backed out where they should be allowing no current). I can get back into it, though, & at the same time check tube socket wiring. The in circuit cap checker is on top of my list, & right now it's a pretty short list.
I emailed Cary on Thursday, & I'd love to hear back from them, but I am afraid that if I was holding my breath I would suffocate. I have a feeling they are going to advise me to send them the amp, but that means find the box & that thing weighs a ton. But it might come to that.
I wouldn't exactly call what I am getting from my spouse "help", but anyway, I appreciate your support. (& thank you for that link to the article about bias. I am going to get into that today.)
Unlike a potentiometer (volume control), a trim pot will not shut off completely. They are meant to finely tune a small widow of the existing inline flow only, +/- a small percentage. So if you are getting .6 backed out (highest resistance), backed in (lowest resistance) should give you .5 or lower.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Since my bias starts running away, almost instantly, on the right side, with the bias pot on that side backed all the way off,
and the resistance measures 0.6 from the same terminals on BOTH pots,
wouldn't this indicate that it is NOT the bias pot that is malfunctioning?
Another thought... if the screen pin is not making good contact with the socket, red plating can occur.
So something you can try without having to go to a tech is cleaning all the socket contacts, AND tightening them. Checking and cleaning all the tube pins wouldn't hurt either.
By the way, I noticed an AA member tubes should be discarded after red plating. That is good advice if you want to be on the safeside. But I have also found tubes don't skip a beat after redplating and can be fine for years. A lot depends on the age of the current tube. And the duration of redplating. Sounds like you caught yours right away. So they are likely fine. But it's up to you.
I have a tube tester and though not always the final word on the tube's health, it gives you a pretty good idea. Best investment I ever made.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Is red plating always a product of excessive bias current?
I will get the amp apart again & inspect the tube sockets.
nt
"I know just enough to get into trouble. But not enough to get out of it."
Your previous explanation was great. thanks!
As far as input signal changing the bias, before I bought the Cary I had a pair of ARCs & they just had me spooked. For example, it wasn't uncommon for a grid resistor to go "POP!" shortly after I hit the power switch. I used to cross my fingers whenever I turned them on. & yes, I did notice that turning on the preamp made the bias do crazy things.I didn't see that in the Cary. It never seemed to matter what I did with the preamp, the bias always stayed put.
Edits: 07/28/20 07/28/20
ARC was "infamous" for using those resistors as a protection circuit. That's how my old ARC D70 is designed. I've veer had any resisters blow over the years. Though I have had tubes go bad with destroying anything around them. Lucky I guess.
That's interesting about your bias not changing relative to the input signal. So I found an article that says if a certain impedance load is in place, bias will remain as set. Furthermore, if the impedance was too low (bad resistor), red plating can occur. I'm sure there's more to it than than just a resistor regarding the impedance circuit design. Either way, we're learning as we go!
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
My ARCs were the VTM 120s & I used to sincerely wish that a fuse would have blown instead of a resistor.Anyway, I am not ignoring your previous suggestions, but could I beat you up for one more favor? Could you paste a link to the cap tester that you bought from Amazon? I want to make sure that I get the one that will do what I need it to do. When I get back into it to check the caps, I'll inspect the tube sockets as you suggested.
Edits: 07/29/20
Yes, I remember you mentioning the ARC 120's. You have/had some nice amps.
Here's a link to the "in-circuit" capacitor tester I use. Looks like a newer model and shipping direct from China.
They also have an ESR meter that checks the health of a capacitor. I use them in tandem, but if I could only choose one, it would be that shown in the link.
Sometimes you can get both as a set. Much cheaper that way. You may wish to check eBay or other sites as well.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Jonesy, I have said it before & I will say it again: you are the best!
Thanks!
You're welcome.
I'm just re-reading the online description. I don't see it saying "in circuit" anywhere, so check the manual that comes with it. Either way it's still the one to get.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
It has been ordered . . . .
Thank you, Jonesy, for all the time & effort on & offline helping me solve this!
Excessive current.
Here's my non-technical explanation as it is the only one I have. AA members may chime in if my "gist" of it is way off.
To provide current to a tube there is a circuit. Practically the whole amp really. In this circuit you have a power supply, capacitors, resistors, diodes, etc, including the tube itself. A circuit by definition means everything is connected forming a proverbial circle.
Once the circuit is set up, current will flow steadily within the designed theoretical range. The job of the trim pot is to finely tune the current to run at an ideal value or bias point for everything to work as intended.
In the scheme of things, the trim pot has no idea what it is adjusting. It's job is just to add more or less resistance.
If something goes "amiss" in the circuit, the trim pot can only control so much. Remember it is for fine tuning only.
Unlike a fuse that blows under duress, the "amiss" circuit will still operate at less than ideal conditions. A bad resistor will sill operate, but not at it's intended value. A bad cap may be allowing DC to pass through. A broken connection in one part of the circuit will cause the other parts to either shut down or now behave differently.
The tube unfortunately suffers because it keeps doing what it supposed to, but now sendly over double the electrons it should be to the anode plate, causing it to get extremely hot or red plate.
I may have gotten carried away with this answer. Hope it explains what you asked. Otherwise, no worries, ask again.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Ah, yes, you are correct. I misunderstood that you were checking both sides, left and right channels.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
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