Fixed a MKII Boogie Yesterday - Bad Coupling Cap

chilipeppermaniac said:
Ampmad, thanks for that breakdown and procedure. Yes, I do have Temp controllable Soldering Station(s). Where I fall short is in about 3-6 aspects I can improve on.

1. Access to the correct replacement wires/ ( I believe Laney used some Solid wire in these amps) , correct caps, resistors etc. (Knowing Laney's schematics are hard to read, I might need help here)

2. Practice doing the things you listed in your advice especially the heat management soldering procedures. I am a big fan of not screwing up the good stuff when I can practice on sacrificial stuff.

3. Having all parts and supplies on hand, a clean and organized work space and TIME to finish from start to end with no delays or distractions.

4. The good luck needed to walk through the process in my head before starting, and then having the actual work go smoothly as my mental walk through envisioned.

Thoughts to be continued.

5. Oh yeah, and the bravery needed to totally remove the board and corrolary parts from the chassis. I have a feeling some repairs will be needed between the board and the underside of the chassis.
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Seems like you understand it pretty well.
 
One of my Boogie MKIII heads had the exact same problem but the orange drops were 600v so I just replaced them, luckly the original tubes were fine, and at that I was pretty happy. You know ya gettin old when an amp you brought new starts needing repairs due to aging teething pains. To be fair the head did see 100's of gigs. Mostly I used the JMI Vox's in pairs but there was always that boogie behind them w/2-12's, a closed back Thiel under an open back 12" cab, with the amp on standby. Just had to step on a stage switch, on the fly, if an issue developed with one of the Vox's. If you used matched pairs or quads old Mullard EL84's with a low current draw I could run them as hot as I wanted as they shine when they get hot! After 1 1/2 sets I'd just yank the effects and go direct into them since by that time I didn't need anything else in line. What I found interesting is they never sounded better with reverb. I checked the MK IV head and it seems to be happy. When you live on a beach the salt in the air is a bit of a struggle unless you do regular maintenance all is fine but when you have 40 or so it seems like every weekend I have something to do! These Boogie heads can easily stand up to a 100v Marshall stack w/o any issues.
 
WOW @


"The funny thing is:
When I started doing this, about 90% of all people who worked on tube amps did not know:
that there is negative voltage (bias voltage) on pin 5 of the power tube.
They were completely unaware of this negative voltage, or what the negative voltage accomplished.

They didn't realize that this negative voltage is the most important voltage in the entire amplifier.

Even The "Guitologist" Who is a self proclaimed "expert," had no clue that this voltage existed --- never mentioned testing it---- not even once. :pound-hand:
and the whole time he was "fixing" amplifiers, he was fighting this unseen mystery.
Which is fking funny to me."
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See the stuff I learn from the forum, plus all because I once had an intermittent issue in my 1972 JMP 1987 head that a Tech supposedly "fixed" then with encouragement and confidence in me that he knew I " could do it?"

I owe all I know about electronics to that guy, who along with DonP that are former members here, Neikeel, IvanH, Bastarddon, 67plexi, Sysco, Smitty and Ampmad, plus all the countless hours on youtube and in books. To have learned the basics of electronics, the schematics, color codes for resistors, multi meters, Solder stations, solder suckers, Chop sticks, eyes and ears, etc has been invaluable to me the past 2-3 years.
 
chilipeppermaniac said:
How old are these MK II amps? 80's 90's?
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gball said:
'80's. If its a Mk IIB it'll be early '80's if it's a IIC/IIC+ it's from around '83/'84.

IIC+ is early Metallica sound in one box
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The first Mark IIs were made in '78 and '79 when Randy was still building 'em at home in his shed. I have one of the early ones.
FX loop return is behind the power tubes, not on the back panel. These were simply the Mk II, no letter designation.
That may correspond with the hand-drawn schematic pictured in the OP.

When factory production began and the Mk IIBs came out (early 1980?), the earlier version came to be known as the Mk IIA.
 
Amp Mad Scientist said:
Do you have temperature controlled soldering station?
I would not work on a board like that without one.

1. Cut the leads of the part. Leave some length on the cut lead.
2. Straighten the lead out w/ needle nose pliers.
3. Heat the solder connection and gently push the straight lead through the hole.
4. Do not try to heat both legs and remove the whole part without cutting the leads first.
5. Do not keep heating one connection over and over. Let the board cool down.
6. After the lead is out of the hole, use solder wick to suck up the excess solder.
7. Clean the flux off the board with 91% isopropyl alcohol. Not rubbing alcohol.
The board trace should be all clean and free of flux and dirt when you are done.
8. The leads of the NEW part must be shiny clean and free of oxide.
9. Clean the leads of the new part with Scotch brite before you try to solder it.
10. The tip of the soldering iron must be shiny clean too. Apply fresh solder to the tip before you use the iron, each time you solder a connection.

The key to soldering is to make sure everything is clean before applying the solder. If you try to solder dirty or oxidized metal, that is why it fails.

Many times, leads are folded down against the board and then soldered.
DO NOT heat the lead and try to UN-bend it.
Cut the lead instead as above, and push it straight out of the hole.
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Excellent! Thanks, AMS... :cheers:
 
Amp Mad Scientist said:
When I started doing this, about 90% of all people who worked on tube amps did not know:
that there is negative voltage (bias voltage) on pin 5 of the power tube.
They were completely unaware of this negative voltage, or what the negative voltage accomplished.

They didn't realize that this negative voltage is the most important voltage in the entire amplifier.

Even The "Guitologist" Who is a self proclaimed "expert," had no clue that this voltage existed --- never mentioned testing it---- not even once. :pound-hand:
and the whole time he was "fixing" amplifiers, he was fighting this unseen mystery.
Which is fking funny to me.
Click to expand...
I meant to comment on this yesterday, but ran out of time.
This totally blows my mind. I just can't wrap my head around a "tech" (or anyone who supposedly know valves) not knowing this most basic aspect of "fixed bias" valves.
I first started to learn about valves over 45 yrs ago (about half way through secondary school) when a new teacher started a radio/audio electronics group. (I didn't "seriously" start learning them 'til a couple of yrs later when I left school & got into electric guitar & amps). Before learning anything at all about passive circuit components & their application etc etc, we had to learn "thermionic valve" principals & it went like this;
1) Current flows in a valve in the form of electrons & electrons carry a negative charge (this is important to understanding valve operation).
2) Thermionic Emission.
3) The Diode & half wave rectification, the dual diode & full wave rectification. Of the many aspects of this lesson we first learnt that valves operate on the principal that "likes repell & opposites attract" (again, important to understanding valve operation).
4) The Triode & controlling the flow of current (negatively charged electrons) through the valve (& this is why it's called a "valve") by;
4a) Making the control grid "negative with respect to the cathode", as a result some portion of the electrons liberated from the cathode are repelled, so that not all electrons make it to the positively charged anode. Just how negative the control grid is with respect to the cathode controls how much current flows.
4b) Setting a "no signal" (quiescent) current flow condition, aka "bias" point by;
"Cathode bias", where a resistor slows the replenishment of electrons to the cathode (from ground), leaving it with a positive charge with respect to the control grid. The valve sees this as the control grid being negative with respect to the cathode.
"Fixed bias", where the cathode is grounded & a negative voltage (B-) is applied to the control grid.

I'll leave off the lesson here as we've covered the all important aspect of "the control grid being negative with respect to the cathode" to control current flow through the valve (sorry for the long rant folks), but how even a valve "tinkerer", let alone a "technician" could not have a thorough knowledge of this most basic aspect of valve operating principles is beyond me & is totally mind boggling. I mean come on, this was a FUNDAMENTAL ASPECT of the VERY FIRST LESSON on audio valve electronics. A tech not knowing this, Good Lord, surely there ought to be a law,,,,

Coupling capacitors going "leaky" with age is just something we have to live with. We very often see this, as has been mentioned, in the capacitors coupling the PI to the output tubes & the reason it is most common in this position is because here the caps have the highest "potential difference" across them, the high positive voltage of the PI anodes (plates) on one side & the output tubes negative bias voltage on the other side. Will higher voltage rated caps alleviate the problem,,, possibly, definitely can't hurt anyway, but as stated by @Vox AC30, his Boogie had the 630VDC rated cap & still suffered the problem. The Sprague 715P orange drops used in these amps are tested to twice their DC voltage withstand rating, so a 400V cap is tested to 800VDC. Capacitors leaking DC voltage is age related. Again, apologies for the long winded rant. Cheers
 
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What IVAN posted, is essentially what I learned ( certainly could not write the description how he did word for word), But in my study on my own to understand Valves, Circuits and why all the components in an amp are there and the reasons they do what they do,,,,,,, is basically like he said, a push pull repel attract type basic principle of electron( ics) and the construction of a valve/tube and why it does what it does.

Maybe I can find the lesson/ explanation source I found that made this very easy to understand even for a greenhorn like me. Ivan also somehow speaks in a way that naturally I understand. I have told Ivan this many times in our student learning/ Ivan Teacher teaching sessions where he and DonP and others helped me learn amps and terms and testing/ specs/ etc
 
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ivan H said:
I meant to comment on this yesterday, but ran out of time.
This totally blows my mind. I just can't wrap my head around a "tech" (or anyone who supposedly know valves) not knowing this most basic aspect of "fixed bias" valves.
I first started to learn about valves over 45 yrs ago (about half way through secondary school) when a new teacher started a radio/audio electronics group. (I didn't "seriously" start learning them 'til a couple of yrs later when I left school & got into electric guitar & amps). Before learning anything at all about passive circuit components & their application etc etc, we had to learn "thermionic valve" principals & it went like this;
1) Current flows in a valve in the form of electrons & electrons carry a negative charge (this is important to understanding valve operation).
2) Thermionic Emission.
3) The Diode & half wave rectification, the dual diode & full wave rectification. Of the many aspects of this lesson we first learnt that valves operate on the principal that "likes repell & opposites attract" (again, important to understanding valve operation).
4) The Triode & controlling the flow of current (negatively charged electrons) through the valve (& this is why it's called a "valve") by;
4a) Making the control grid "negative with respect to the cathode", as a result some portion of the electrons liberated from the cathode are repelled, so that not all electrons make it to the positively charged anode. Just how negative the control grid is with respect to the cathode controls how much current flows.
4b) Setting a "no signal" (quiescent) current flow condition, aka "bias" point by;
"Cathode bias", where a resistor slows the replenishment of electrons to the cathode (from ground), leaving it with a positive charge with respect to the control grid. The valve sees this as the control grid being negative with respect to the cathode.
"Fixed bias", where the cathode is grounded & a negative voltage (B-) is applied to the control grid.

I'll leave off the lesson here as we've covered the all important aspect of "the control grid being negative with respect to the cathode" to control current flow through the valve (sorry for the long rant folks), but how even a valve "tinkerer", let alone a "technician" could not have a thorough knowledge of this most basic aspect of valve operating principles is beyond me & is totally mind boggling. I mean come on, this was a FUNDAMENTAL ASPECT of the VERY FIRST LESSON on audio valve electronics. A tech not knowing this, Good Lord, surely there ought to be a law,,,,

Coupling capacitors going "leaky" with age is just something we have to live with. We very often see this, as has been mentioned, in the capacitors coupling the PI to the output tubes & the reason it is most common in this position is because here the caps have the highest "potential difference" across them, the high positive voltage of the PI anodes (plates) on one side & the output tubes negative bias voltage on the other side. Will higher voltage rated caps alleviate the problem,,, possibly, definitely can't hurt anyway, but as stated by @Vox AC30, his Boogie had the 630VDC rated cap & still suffered the problem. The Sprague 715P orange drops used in these amps are tested to twice their DC voltage withstand rating, so a 400V cap is tested to 800VDC. Capacitors leaking DC voltage is age related. Again, apologies for the long winded rant. Cheers
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his Boogie had the 630VDC rated cap & still suffered the problem.

Maybe we should use 1000V caps.
I thought 600 would be enough but maybe not.

Well you have the plate voltage, the bias voltage...
and you have the audio AC voltage as well. Probably makes it break down faster....
 
eSGEe said:
awh :poo: these have tubes????????????????????? :(
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Adrian, one may think non tube Amps are superior and in some cases some facets are. BUT even they rely on the basic ELECTRONICS principles Tube amps are derived from. AND when a Solid State unit goes bad. I hear they are reallllllllly a job to figure out what part is actually the culprit of the failure.
 
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my brains full......Ill skip desert :)


Stop trying to make me LEARN THINGS CHILI!!!!
 
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