After many years of sitting around I finally powered up my Peavey VXT Classic. I acquired it along with a 1966 KAY Truetone Guitar for the amazing price of $25.00. Anyway after God knows how many years of not being used I fired up the amp and she sounded Great!!! I do not have the foot pedal so I used the dirty channel but really kept it as clean as I could. Just at the point if I rolled my volume down on my guitar she would be fairly clean & and if I cranked the guitar up to 11 the amp would break up. I was really impressed with the tone and it had a mile of headroom.
Well that didn't last very long. Maybe a couple of hours over several days. Two days ago I noticed that the amp started to sound like it had some type of effect on the lows like a octave shift or something that lasted about as long as took me to walk over to the amp and really give it a listen. Then it developed a static-Distortion my guitar was still audible but it had a hell of a lot of noise along with it. I plugged the guitar in to the return and it does the same thing.
So I spent most of yesterday reading the old inter web and came to several conclusions: I downloaded the Schematic. I ordered 2 100 uF 350V Axial Electrolytic-Illinois Capacitors, Several Audio driver transistors (MJE 15030) for Q7 & Q8, 2n3904 transistors for Q4 & Q5, and a new set of tubes. After looking at the board it looks original and for an amp that was made in the early 80's I thought that was good. The large Caps on the power side have slight splits in the cover of the cap. Other than that it looked okay.
I really don't have a lot of experience with electronics other than a couple of soildering jobs with cold joints and an upgrade I did on my Epiphone Les Pual. Upgrade included: SD Humbuckers, CTS pots, Orange Drop Caps, Switch Craft switch, and 50's style wiring.
My question does anyone know of a procedure I can do with your help that would or could possibly pinpoint the problem????
Like I stated earlier. I don't have a lot of experience but I'm no dummy!!! LOL... I have a Fluke 26III True RMS Multimeter & Fluke 12 Meter & quality Wells soldering gun. So I'm pretty dangerous with all that equipment...... LOL
Please Help if you have knowledge and Patience????
Quote from: Bquick on December 21, 2017, 11:20:10 AM
The large Caps on the power side have slight splits in the cover of the cap. Other than that it looked okay.
if by "splits" you mean something that looks like this:
https://upload.wikimedia.org/wikipedia/commons/6/6d/Electrolytic_capacitor_pressure_release_valve_blown.jpg
Then congratulations, we found ONE issue! Those capacitors are definitely blown.
Now, we need to figure out what CAUSED them to blow, IF that's the case.
Pictures help greatly, if you could snap a couple and post 'em here. Both sides of the board, if possible.
Not sure why??? I'm following direction but I can't get the pics I just took, saved, added as an attachment, to post...
FYI Both boards are still in the chassis. I thought it may be a good idea to keep them in until I receive the parts I ordered and if any testing or probing needed to be done...
Well I keep getting an error message when I try and post the PIC's????? But after looking at the picture of the capacitor with the split ends. No the large caps on this board are okay on the ends but the cover or wrapper has a split running long ways. Not sure what your opinion is but after almost 40 years couldn't it go bad because of age? Or because I see the split some other component caused that to happen? If you send me your Email address I'll send you pics thru email...
I'm at: BrantQuick@gmail.com
This reminds me of a chap who replaced the radiator in his car (at substantial cost) only to find out he only needed to replace the cap (minimal cost) :duh
The idea is to find the fault,,THEN you replace parts otherwise you may just add more confusion making it even harder to find the initial fault. 8|
First check all the voltages are within spec.
Having just checked a schematic (Which is below) the PSU board is separate so should be easy to check but check power pins on the opamps are getting power.
This thing has a fancy output section, SS driven power valves so I'm out of my depth with that circuit idea but others here will know more.
Re posting pics,, keep in mind,,Max file size is 5120KB.
some hi res shots need to be resized.
Or;
I'm not sure but new members may have to make several postings before you can up load pics,, Joe or others here will know why :tu:
Phil
Oh yeah,,,Regards split cap wrappers; they are a type of shrink wrap plastic and if subjected to heat over a long time the plastic will eventually split. The cap may still be working fine.
Phil.
Bquick, post your pics on IMGUR and paste the links to them here.
I didn't notice you were talking about the plastic covers on the caps having splits, my mistake! If it's just that, then yes, they are probably okay.
I'd still like to take a good look at the board myself.
https://imgur.com/a/Ou4mB
I hope this works.
Yea!!!! Thanks blackcorvo that looks like it worked. As far as the parts I've ordered I think I've spent less than $20.00. I think just having the peace of mind knowing that parts that are close to forty years old are new know. I'll feel a lot better when I go to a GIG and plug the Amp in... LOL
Correction: The (2) new JJ 6L6 tubes cost $35.00 plus the $20.00 for transistors and Caps.
Let me know what I can do to (or how to) check components??? like checking voltage at specific points on the board. AC or DC ??? or resistance = Ohms Continuity = Beep.....
See I know a little... AC/DC I love that band!!!!! Just kidding around.....
I hope someone can give some help and guide me thru the technical stuff????
Merry Christmas
Pics...
Well if one of the DC supply voltages has blown then it won't work.
So obviously all the DC voltages need to be checked.
The PSU board should be obvious if you follow the schematic.
The in coming transformer wires are all the ACV so set DMM to ACV.
Then switch DMM to DCV and check all the DCV.
Make sure those voltages are present at the pins on the opamps and also the HT at the Valve sockets. The Red,Yellow and Black wires that run to the power valves will be the high voltage. you can check those by probing directly into the back of the white sockets (safer)
BUT Before you go pulling stuff apart.
Pull the power Valves and reinsert them,
Reason;
You mentioned the Amp has been sitting for a long time and valve sockets corrode and just by reinserting scrapes the corrosion and amp comes back to life.
I had a messa rig come to me with horrible noise issues,, had me scratching my head for a while but it was just dirty pins on the valve sockets.
If you live in a humid climate or near the sea you are prone to corrosion.
Safety;
Regards checking voltages,, First establish where the Mains wires run and cover them with plastic if they look like you could touch them. Be careful not to short any terminals.
I use an alligator clip on the neg probe this allows you to probe freely and keep your left hand behind your back.
Don't rush things.
Phil.
Cool deal Phatt Phil!!! I'm away from the house for the Holidays but will be back home in a couple of days. I really want to say Thank you for taking the time to give me a hand. I cant say thanks enough!!! That goes to everyone that is willing to give me a hand. Thanks also galaxiex for getting the Pic's posted on this thread. It's really a cool thing when others with much more experience and wisdom are willing to share valuable information.. It must be the holiday magic everyone is experiencing. LOL
I'll post my results and/or questions as soon as I get home.
Okay I used some deoxit on the terminals of the Tube or Valve. I worked the tube in and out of the socket and NO difference is the sound. It still sounds like s*#t= crap !!! So I put two brand new tubes in the Amps and same thing I sat and watched about 30 YouTube videos on reading schematics today and really none turned the light bulb on??? for me... I know that the transformer takes AC power and turns it into DC. Power = Voltage. How do I verify that the amp is getting the -15 and the +15 volts that I see on the schematic? I think I should attach the black lead to ground or the chassis and what terminals or pins do I need to touch? on the tube socket? to find the tubes are getting the proper power?????????
If you think you can help me please let me know??? IF YOU THINK THIS IS A LOST Cause THEN PLEASE LET ME KNOW.
As far as the terminology goes No I really don't know as much as you... If anyone really wants to help and see what I know and what I don't know you can call me at 321-234-9745. If I do not answer that's because I drive a tour bus and sometimes it's tuff. Just like the music world today. If your really in to music you'll know what I'm talking about. I work just about all year long. Artist include: Jackson Brown, Joan Baez, W. Nelson, TRHCP, Neal young, and many, many, more.... I can't really bother other tour members because we all have a job to do. SO LEAVE A MESSAGE AND WE CAN FIX THIS GOD DAMN AMP!!! I'm NOT A ROBOOT AND WOULD REALLY LIKE TO LEARN!!!! MAYBE I COULD HELP YOU I??????
Ok, so for checking the DC voltages:
If you look at the schematic, you will see that each op amp has +15 and -15V connections, which connect to the pins specified on the schematic (like U1 has them on pins 4 and 8). Keep in mind that U1A and U1B are the same chip, just different sections. When you locate the op-amp power supply pins, you connect the black probe of your voltmeter to ground (or chassis) and check those pins with the other probe. The voltmeter must be set to DCV, with the range set accordingly.
You check the power supply board and tubes the same way. Basically, if it says there should be DC voltage on the schematic, you should locate that point on the layout and measure it. If it differs from whats written on the schematic a lot, you might have found the problem.
Wow, seems like a cool job you have there. Are you trying to make us jealous? :)
Probing around on the tube sockets I can't locate a negative -15 volts DC. Pin #5 has +15.35v on both tube sockets. Should the tube sockets have a negative voltage?
I cleaned out the tube sockets with contact cleaner. sprayed a little deoxit in each socket and pulled tubes in and out several times.
Now when I turn the amp on I hear a Humm that goes up in volume then back down after a couple seconds when I turn the amp on. The amp is quiet after that. When I strum the strings on the guitar I can hear the guitar but it also has a crackling kind of distortion along with it.
The LED's are working fine the red one comes on with power switch and the green status LED comes on when I switch the stand by.
To be honest when I look at and study the Schematic I have a hard time comprehending it??? I have been able to locate both negative and positive DC voltage on the pre amp side. I did notice that the positive is around 15.60 vDC and the negative is a little higher -16.36 vDC.
Maybe information will help someone with more experience than me???
Look at the schematic. DO you see -15v there on the power tubes anywhere? Pin 5 of each is wired to the +15v supply through a couple resistors. it is normal to find +15 there.
Read the schematic notes for what Vk should be, the cathode voltage on those tubes. Ther is no negative voltage anywhere on the power tubes.
Start with the power amp, is your Vk in range? Also the associated Vi reading, is yours in range?
Plug your guitar into the POWER AMP IN jack, next to the PREAMP OUT jack. How does that sound?
A quick check, see the pairs of diodes to +/-15v near the input jack, and also by the power amp in and preamp out jacks? Make sure none are shorted, easiest way is to check for 15v on the jacks or IC pins they are wired to.
Just wanted to let everyone know that after watching countless hours of youtube videos like D-Lab I was able to figure out how to test capacitors for leakage. I thought that was what this site was about??? Experts trying to help folks with less knowledge. I guess I was wrong about that.. Because the only thing that anyone said to me was read the schematic. Just letting you know if you don't have any experience doing that it is a lot easier said then done. I originally suspected the caps being bad and ordered new ones and someone on this site said that's like replacing you radiator when it only needs a new cap. Well that might be true but for about $6.00 It sure is nice to have the piece of mind knowing that I replaced parts that can and usually do go bad after forty years.. Even if it's not bad. Big deal I spent $6.00.
Maybe some of you experts will realize that not everyone has as much experience as you do and if you really read the post you could take just a little more time to help someone out. Don't take it personal but if your going to respond to someone's post it only takes a little more effort to really try and help them instead of ridiculing them and/or trying to make yourself sound so smart... Think about it??? Thank you to everyone that tried to help me.
Oh yea, after replacing the one capacitor that I originally suspected the amp worked. I went ahead and replaced all of them and it has been working fine all day. I hope that was the only problem. Because getting any real help is next to impossible...
I am sorry you approach this with such bitterness. I can talk to you from the other side of this discussion. I have been training technicians for over 40 years.
When I tell someone to look at the schematic, it is in hopes they learn more about reading schematics, because all that information is there, even if you don;t see it now. it may not speak to you now, but as we go through it, you might pick up something about relating the diagram to the circuit in the amp.
For example you were expecting a negative voltage on the grid. I pointed out that the schematic shows only positive voltages there. Hopefully if it comes up again you might remember that. We all build our experience by accumulating information.
I don't believe any ridicule was involved. I mentioned the notes in the corner. I very often do that in any schematic. You did not seem to know what voltages to expect in the circuit. The schematic shows Vk at the cathode of the power tube, pin 8, and the note tells you what range of voltage is normal there. Likewise nearby the schematic notes Vi, and abbreviation for the voltage indicating current through the tubes. it is across a resistor for each tube. The notes tell you the normal range of voltages to find there.
I could have taken the time to make up a chart from that same information, but I chose to tell YOU to read that information, as it will better make you understand what you are working with than me just rattling off some voltages, voltages you would then have no idea where I came up with them.
Troubleshooting and repair is a systematic approach to circuits with problems. If I can impart that approach to you, you are then better armed in the future when problems arise. When I/we tell you not to just wholesale replace a bunch of parts it is because that assumes a part is always the problem. I am glad replacing your caps solved your problem. That series of amps tends to have trouble with the connectors between the power supply board and the tube board. What if you replaced every cap in the thing and still had the problem because the ribbon cable had a bad contact in the connector? That is why we try to lead you through a systematic approach.
I am sorry you see no value in any of that. if you thought we could just say "replace this and replace that" and all your problems would be solved, well it just doesn;t work that way. There are people here like myself who are industry professionals, and we have given countless hours - hours I could be doing repairs and charging for - in free assistance. SO it rings a bit hollow to be told I should take the time...
In any case, I wish you well.
Right On, Enzo.
Like I said don't take it personal.
What I expected was some help. If you really were concerned with helping someone you could take just a little bit more time and explain things. Believe me I looked over the schematic for hours after you said "Read the Schematic" I even posted the voltage readings that I found... Yes you called me out because I asked if I should have found a negative voltage on the tube socket...
Look I'm not here to argue or point fingers. The point I was making is: You have a tremendous amount of knowledge and I don't!!! Why are you so aggressive and angry? If you really wanted to help me you know you could have??? But you choose to take a different approach... Maybe you had a bad day? Maybe your sick of dumb questions? I'm not sure what it is but I am sure that you could have made this experience a better one.
If your going to take the time to respond to a post, try and help someone not make them feel like a dummy.
We all live here on this place called earth. We should be kind and compassionate. Not rude and condescending.
I could sit here all day and give you examples of the point I'm trying to make. Maybe you can think about what I'm saying and leave it at that. I said Thank you for the help that I did get. Out of all the people I really didn't want to argue with you because your probably the most knowledgeable person on this site and a couple others that I looked at and read over the last couple of weeks Enzo. It's not the first time I've seen your name. Believe me I read a lot of your post and sometimes you try and help and sometimes your a DICK. LOL
I learned a lot about reading a schematic, Capacitors, Circuits, the components that make up an amp. I don't understand all of it. I probably got Lucky with my fix. But like I stated in the earlier post the amp sounded great a couple weeks ago. Then in a matter of seconds it sounded like crap. In my mind that probably means something failed. That's why I assumed it was a component....
Have a Good day and maybe, just maybe when you make your next post you will think about what I'm trying to say. I'm not saying because it makes me feel better I'm saying it because maybe the next kid that ask a question, could possibly get a little help instead of being made fun of or feeling like a dumb *s!!t* for asking...
No one here has been angry or aggressive toward you.
That you misunderstood and took it that way does not make it so.
This is a technical discussion and may seem short and to the point, but that is often how technical discussions go.
I'd suggest you re-read what was said without taking the attitude that anyone was talking down to you, rather just presenting stripped down facts.
Sorry I had to post something to just laugh at this thread... Good thing Juan didn't chime in yet... :dbtu:
@ Bquick,
Yes you got lucky and I'm happy for you but when the amp does have a really hard problem to solve you won't get much help if you take offense. This is not face book where everyone slags off at anything that has the slightest whiff of insult.
I was not trying to de-mean you in anyway,, just imparting the fact that you could go round in circles replacing parts shotgun style. More often than not it just complicates the problem.
As we say in my world,, shut up and play your guitar and don't take life so seriously.
So get over it as there are far bigger things in life to worry about.
And Don't forget to have fun. :tu:
ps,
Now that you understand (hopefully?) lets get back to the amp.
As you have the amp working again did you happen to note which cap was dud?
I'd be interested to know so we have record of the suss component for future reference.
That way you can help for the common good.
Thanks, Phil.