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In Reply to: RE: my bias is running away on one side even with the bias backed all the way off on that side! posted by Jonesy on July 23, 2020 at 13:10:22
Thanks, Jonesy.
Whewwww, that was a lot of resistors!
All the resistors on the tube sockets were reading nearly identical, left & right. There were some other resistors (not soldered to the tube sockets) that were nearly identical, also, left to right. Then there are a pair of blue resistors with 2 gold & 2 red stripes & they took a long time to get there, but on the good side one reads 196.6 & on the bad side 207.3.
Then there were two black resistors with a silver stripe & "540" on them that, not understanding electricity, seemed totally bizarre to me. The first time I checked them, they were 237.6 & 317.1; I checked them a second time & they both counted down from 11 to under 1!?!?
So anyway, thanks for the info on the caps. I guess that would explain why I cannot get consistent readings on the yellow Kimber kaps. But I cannot get any consistent readings on the large black caps with the terminals that I unscrewed the leads from when I checked them, either. I wonder why I am getting consistent readings on 3 of the 4 Jensen caps? & no, I definitely would NOT be comfortable working on this while it was powered up.
I have sent Cary an email describing the scenario & what I measured on the caps, but I am not holding my breath for a reply. Their customer service used to be so good back in the '90s when I first started buying Cray products. I guess with a subdued economy, no telephone tech support is part of necessary cost savings.
I still have the very first tube amp I ever bought (back in '93 or '94), a Cary SLA70, a little stereo amp that runs with 4 6550s & a pair 6SL7s & a pair of rectifier tubes. The last time I had it running was forever ago, back in '99 after I caught one of my ARC VTM 120s on fire & had to send it to Mn to repair my handiwork.
Oh well, I'll go on hold on the V12 & start getting the little Cary set up. hey, thanks again for your input, Jonesy!
Follow Ups:
"Their customer service used to be so good back in the '90s when I first started buying Cray products"
Yup, that was back in the Dennis Had era. Dennis was not just a businessman but he truly enjoyed audio and did everything he could to help customers, personally.
I remember there used to be a gentleman, at Cary Audio, named Kirk Owens that they would always put on the phone whenever I had a problem/question. He was always pleasant & extremely helpful. Today, I did get an email from Cary Tech Support; Tech Support basically told me that they would get back to me. Oh well, probably better than I'd get if I was dealing with JVC or the like. I'll keep on waiting.
The black 540 with silver stripe are "diodes". You're DVM should have a diode check option so you can recheck those with it. Negative probe goes to the silver stripe side. You should get a reading. Reversing the DVM leads should read infinite. Do this to both diodes and see how they compare.
If the Jensen cap had the wires unscrewed when tested, then indeed it is bad. BUT, replace it with a cheap cap first to see if that was the problem. So if you order a replacement Jensen, also order another cap or two of something less exotic to try first. Don't want to ruin a new Jensen.
Electrolytic caps should last about 20 years. Depending on age, you may wish to replace them all.
I've provided a link for an on-line resistor calculator as a quick was to determine values when only color bands are present on the resistor. I couldn't find the one I normally use, but this one looks ok.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
thanks a million, Jonesy!
I just got back to this thread; I put the V12 back together & spent the rest of the night trying to get the little Cary up & running. Sometimes I swear that I should write a book: "What happens When The Handicapped try To Trouble Shoot."
I'll check the diodes tomorrow. You are the best!
Anyway: no, the Jensens were still soldered into the system when I checked them & got the 3 readings I previously listed.
I did unscrew the leads from the 4 large black caps which, as I understand, provide oomph under peak load & for some reason still could not get a reading on them.
Thanks again! You have been a wealth of information!
I couldn't find a service manual online, but looking at the user manual I see that besides the 6 EL84 output tubes per channel, you also have a 6922 signal + EL 84 current source tube. Either of those 2 tubes may impact your bias if defective. In particular the EL84 used as a current source regulator.
Also, if the large black caps are not discharged before testing, they can ruin a DVM. I've had that happen to me. The DVM will no longer work on caps in that high value range. Check to see if your DVM has more than 1 fuse and if either is blown.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Wow, thanks for your time & effort, Jonesy!
I actually do have extra EL84s & 6922s & I knew that the EL84s were referred to as "power source" for the 6922s, but since the 6922s had been referred to by someone I had talked to as "input drivers" I just assumed that they would not effect the current running through the output tubes.
The amp had been sitting overnight before I took it down & opened it up, & I did use a shunt on the 4 large caps, but I'll get inside the multimeter & check.
Anyway, thanks a lot!
Ah, that's how those input and current source tubes work. If so then I may have surmised incorrectly. Worth a try though.
Good luck!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
I appreciate your help Jonesy; as it is probably way more than obvious by now, I don't have a very good grasp on electrical theory.
I did find in the manual this: "[. . . .] Next is an additional dual channel bias supply for the EL84 current source tubes. This is factory preset & will not need any adjustment."
Also, those diodes you identified for me which I thought were resistors, I think might be these: "The rectification is accomplished utilizing multiple solid state 3A diodes in a center tap full wave rectification."
Anyway, thanks for all your help!
You mentioned earlier on if fault in trim pot a possibility?
If you are able to get your DVM probes onto the trim pot leads, and check the resistance turned one way then all the way to the other end. Just to see if it is working. Can compare it to ones you know are good.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Jonesy, first of all I want to express my admiration for your knowledge & my appreciation for your assistance. I took a couple of days off from the Big Cary, but tomorrow, I am taking the bottom back off & checking the 540 diode & the resistance of the bias pots as you suggested.
& then, after buttering you up, I was wanting to ask you a couple of more questions regarding caps.
But first: after the Bigg Cary went down I got the Little Cary out of mothballs. Right off the bat a rectifier tube went "fizz-bang!" & after I changed a tube I wound up with about a channel & a half (verified with pink noise & sound pressure meter), & after that I did a few sweaty hours of some completely retarded trouble shooting (to no avail), at which point I came back to it with a logical approach & fixed the 1.5 channel issue with a couple of spare 6SL7s I had in my "spare tube box."
Now the thing is, back in '99, when I had to send one of my ARCs back to Mn. because I set it on fire, I brought the little Cary out of retirement to fill in. At that time, I bought (from Cary) a DIY outboard (4 each 560 uf 400V caps) "cap bank" & umbilical cord to give the Cary some more pop. A couple of years later, I replaced my ARCs with my present day Big Cary, but, I got instructions & bought parts to hook "the cap bank" up into the Big Cary.
I am leery about everything right now, as as much as I'd like to add the dynamic extra capacitance back to the Little Cary, I am afraid that if one of those large caps in "the cap bank" went bad & was what brought the big guy down, I don't want to smoke the little one the same way. I realize that I could set the Big Cary back up WITHOUT the cap bank to see if that resolves everything (& I may do that), but it seems like it would be easier if I knew how to check those large caps.
Referring back to a previous reply you gave me, I am relatively sure that the caps were discharged when I put my multimeter on them, & regardless, the multimeter seems to have survived.
My first question is: is there something I need to check large caps with that I could pick up at Lowes?
Secondly: earlier you mentioned that electrolytic caps have a finite life span (which I wasn't aware of) of around 20 years. But does that include just sitting a circuit being unused?
Thaks!
Matt
While "film" capacitor life is practically infinite, "electrolytic" caps have an average lifespan of 20 years. But like "best before dates on food products", it really is just a recommended guideline. Some caps will age while in use. Others can actually age faster because they are not in use.
Firstly, with your DVM in "resistance" mode, your cap should not show any resistance value but climb to infinity. If it holds steady at a resistance value, the cap is "shorted", or bad.
In capacitance mode, your DVM may have an auto range value detector, or has to be selected manually. With the capacitors you have, you want it to be set to/or allow auto range upto 1000uf.
If your current DVM does not go that high, look at what Lowes has online. Capacitance range might be in the online description, or you might have to Google and download the manual to check. (Reminder, if your DVM in hand is supposed to go that high, check the DVM fuses. Often there are two different values and one may have blown.)
If you don't have any luck at Lowes, there is a capacitor value meter and capacitor ESR meter sold as a set on Amazon. Cost next to nothing, (and it shows in the way they are built and the probes are practically useless.) But for as often as I use them, they are fine.
Do try swapping out the EL84 current source and 6922 on the big Cary. Even though biasing is not required, a bad tube may cause the problem in that channel.
Also, if/when you get a chance, can you upload some pics of the underside of the big Cary with the bottom off? I cannot find anything decent on the net. Would be great to see what's going on in there.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Jonesy, I am not tech savvy at all. As a matter of fact, I am probably categorized as tech retarded. But this MIGHT work.
Edits: 07/26/20
Sorry for the quality of that pic. I think I maight be able to find another one I took.
Anyway: both fuses in multimeter 1 ohmed out good with multimeter 2.
"540 diode" checked at 0.56 on both sides.
I attempted to check resistance on bias pots for comparison. That was a little bit of a bltch to get at. There is a black wire, a yellow wire & a purple wire coming off each pot. I did check, on both sides, yellow to purple with bias pot backed off all the way. 0.6. To really check out from each point to each point with the bias either fullin & then full out, I think I would have to remove the pot. That doesn't look like a cake walk.
As far as checking the large caps, I think I found the cap meter on amazon you were referring to for $20. I may have to go that way. I did use my multimeter to do the resistance check you taught me on the large caps, & what I got were readings that fluctuated a bit & then, on all 4, slowwwwwly started to rise. I assume they were headed for infinity?
I put the bottom back on & hit the power switches (this would be to see what happened WITHOUT the cap bank I previously alluded to. No change.
Then I swapped EL84s. No change.
Then I swapped 6922s. No change.
Let me see if my wife just found me a better pic.
Thanks for your help.
Here's another bad pic where she chopped off the front of the amp. Anyway, I am standing on the bad side.
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."
"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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