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 Post subject: Redplating problem
PostPosted: Sun Feb 12, 2017 7:27 pm 
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Any technological minded folks around? I've been fighting a redplating EL-34 in the outer socket of my Mark IVb. It began only redplating when I would switch modes (either pentode/ triode, or Class A/ Simulclass), but now it redplates any time the Amp is on. I have tried swapping the outside EL-34s, and the same socket/ position red plated, and also tried putting in all 6L6's, and again the same position red plated, so it isn't a single faulty tube. I've checked voltages in several places in the amp, and nothing is reading abnormal. Pin 3 (the anode/ plate pin) on the redplating socket was at 440V in both pentode and triode, while the same pin on the other socket varied from 450V to 454V in triode/ pentode modes. Inside, no components are obviously broken/fried/burnt, but I'm not trusting any piece.

So my question is: where should I look for faulty pieces? I have a copy of the schematic and a DMM, and have checked voltages safely thus far (using a wooden skewer taped to the test probe to prevent any shocks, and the ground probe clipped to the amp's neutral). Reading a few posts on this site I found by searching, redplating is caused by an overvoltage in the bias supply (?), but I haven't found a source of the overvoltage. Additionally, the tube does not redplate in Tweed power mode. V on pin 3 for that mode is 410 for both EL34 sockets.

Thanks for any help offered. If I can't fix it DIY, I will probably have a local tech look at it, but I feel this repair should be well within my capabilities.

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 Post subject: Re: Redplating problem
PostPosted: Mon Feb 13, 2017 2:47 pm 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
With all 4 power tubes removed take readings for pin(s) 3,4 & 5...

That mismatched plate voltage on pin 3 is cause for concern. Is that measurement with the tubes in or out?

It could be several things that voltage checks will reveal. Bad 2.7k screen grid resistor on the outer sockets is not uncommon if you have been running it hard and close to redplating without knowing it.


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 Post subject: Re: Redplating problem
PostPosted: Mon Feb 13, 2017 3:52 pm 
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Thanks for the response! Those numbers were taken with the tubes in. I will remove the tubes this evening and take the readings you mentioned. I haven't been running it hard. It's pretty much always at louder house volumes. The resistor on the tube socket itself looks fine (a Dale wire wound resistor based on my internet search last night). I will take a resistance measurement of it as well.

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Gone: Quad MIDI Pre, TriAxis V1 & V2, Formula Pre, 2:90, 20/20
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 Post subject: Re: Redplating problem
PostPosted: Mon Feb 13, 2017 10:43 pm 
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I just performed the voltage checks without power tubes installed and discovered an interesting mismatch. The EL34 sockets are mirrors of each other voltage wise. Pin 3 is at 506V, pin 4 at 504V, and pin 5 hovering between -38 and -39 V. I also checked the 6L6 sockets, and all is well there, too. Both sockets read the same, at 507, 0, and -45. The 0V reading I'm assuming is because the 6L6's are removed, and the 220K resistor headed to pin 4 prevents an accumulation of charge to sustain a readable voltage, but the voltage on the other side of that resistor is also 505 for both 6L6 tubes. All resistances are the same between each socket vs its opposite.

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Great deals with: Prestige20, DMTransmutation, PaxMan, Forty6and2, jdurso


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 Post subject: Re: Redplating problem
PostPosted: Tue Feb 14, 2017 9:55 am 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
Those pin 5 readings are too low....has the amp previously been worked on or has anyone tweaked the bias supply on this amp.

Make sure you are doing the checks in high power mode, pentode, simul-class...

There is a methodical way to work back through the bias supply using voltage checks to see where it is getting thrown off....


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 Post subject: Re: Redplating problem
PostPosted: Tue Feb 14, 2017 2:00 pm 
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Location: Canberra, Australia
JMMP - while not worked on a Mk4 for ages, redplating is almost exclusively a lack of bias.
Lovetoboogie is steering U towards that and I reckon you'll find the culprit easily.

Tubes out, track the bias supply through each component and see where it's failing.
Something is dropping the voltage (possibly to ground) or gone high resistance..
A dry joint, a failing resistor.. tons of possible fun..!!

Don't put toobs in until yr nearer the correct voltage - they won't thank U for the redplating..

Hope this helps..
db

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 Post subject: Re: Redplating problem
PostPosted: Tue Feb 14, 2017 2:47 pm 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
McBarry wrote:
JMMP - while not worked on a Mk4 for ages, redplating is almost exclusively a lack of bias.
Lovetoboogie is steering U towards that and I reckon you'll find the culprit easily.

Tubes out, track the bias supply through each component and see where it's failing.
Something is dropping the voltage (possibly to ground) or gone high resistance..
A dry joint, a failing resistor.. tons of possible fun..!!

Don't put toobs in until yr nearer the correct voltage - they won't thank U for the redplating..

Hope this helps..
db


Excellent reminder McB... I always recommend pulling tubes and getting those static voltages where you need them to be.

You should be seeing -50v to -60v on pin 5...

Since you are not you will want to follow the bias rail starting at the point where it is rectified from AC>DC...

I have seen a number of Mark IV's, absent any previous bias mods, to have a simultaneous problem on the EQ low voltage rail which shares the winding out of the transformer. If you are comfortable reading a schematic then you can troubleshoot this without alot of time wasted. Let us know what you find and BE CAREFUL poking around please! ;)


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 Post subject: Re: Redplating problem
PostPosted: Wed Feb 15, 2017 1:06 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1455
Location: NYC
That 0 voltage at pin 4 is the problem.Those 220k resistors wont block all the voltage,or as you "prevents an accumulation of charge to sustain a readable voltage",they are "open" and need to be changed.


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 Post subject: Re: Redplating problem
PostPosted: Wed Mar 01, 2017 4:23 pm 
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Joined: Wed Dec 22, 2010 2:57 pm
Posts: 294
Thank you all for replies. I have finally had some time to sit down with the amp and schem and try to sort it out.

For starters, I took voltages again with no tubes in place, in simulclass/ pentode setting (on the clean channel, if that affects anything). The EL34 sockets read 495, 493, and -38 on pins 3, 4, and 5. The 6L6 sockets read 495, 0, -44 on pins 3, 4, and 5. If I go just upstream of the resistor on pin 4 of the 6L6 sockets, I get a voltage of 494.

After reading lovetoboogie's response that my bias supply (pin 5) should be between -50 and -60, I checked voltages from the diode to the distribution strip for the sockets. I found a schematic for the power supply in post 4 at this link http://music-electronics-forum.com/t41204/

I began checking after diode D15 (-69.2V), R105 (-67.1V), and R233 (-59V). The last V should read -57V, according to the schem, but I assume those voltages were measured with the tubes in. From here, I jumped over to point E in the output section schematic which precedes a 220K resistor to each side of the output section. These 220K resistors are dropping the bias from -59V to -44.9V. R105 and R233 have the same amount of current going through them (about 2.5 mA), but using ohms law on those 220k resistors give .06mA per side, so .12mA total. Is this disparity showing where my current is going?

After checking these voltages and thinking, I had another thought. Since I have been careful to not mix up tubes except to test if it was the one EL34 or the socket, and every voltage was the same between each socket/ side, I decided to test the 6L6 partnered with the bad socket. After changing it to the other 6L6 socket, the opposite EL34 socket began to display the red plating issue, while the previous offender played nice.

I switched 6L6s to a known good pair, and the problem took longer to occur, but happened after turning up the volume some and playing with the SC/ClassA switch. Should I be concerned about the bias voltages I am getting? With new tubes all around, I am getting 485V, 481V, and -38.3V on the EL34 sockets, 485V, 0V, and -44.9V on the 6L6 sockets, and 486-487V on the other side of the resistor on pin 4 of the 6L6. That 0 voltage reading on pin 4 is puzzling me, as the schematic makes it seem like the voltages on pin 4 of every power tube socket should be in the same ballpark. Perhaps that is the rock I need to uncover some more to solve this issue?

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Gone: Quad MIDI Pre, TriAxis V1 & V2, Formula Pre, 2:90, 20/20
Great deals with: Prestige20, DMTransmutation, PaxMan, Forty6and2, jdurso


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 Post subject: Re: Redplating problem
PostPosted: Wed Mar 01, 2017 7:37 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1455
Location: NYC
Like I said in my previous post,that 0 volts on your screens is a problem.I suspect the screen resistors are "open" and not letting the voltage thru to your screens.


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 Post subject: Re: Redplating problem
PostPosted: Thu Mar 02, 2017 12:36 am 
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Mark II

Joined: Tue Mar 25, 2014 10:14 pm
Posts: 71
Location: New York City
...as Stokes has pointed out, and you have realized yourself, there has to be voltage at pin 4 of those inside sockets. With the amp powered down and no power tubes present, measure the 470 ohm resistors tied to pin 4....while you are at it measure the 2.7k resistors tied to pin 4 of the outer sockets.

The pentode/triode switch is tied to this matrix and in rare circumstances can go south...

But there is a secondary problem in the bias supply irregardless if the screen resistors are open...

Start measuring all resistors in the bias feed from the 220k splitters on ,including the additional 220k and 2.2m resistors tied to pin 5 on the outer sockets.

Make sure your PI tube is good/strong...

Another problem that can come up that will wreck havoc in the bias voltage at that stage after point E is dc getting by the output caps(C243, 244)...

Be methodical. Its there somewhere. Takes a bit of poking!


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 Post subject: Re: Redplating problem
PostPosted: Mon May 29, 2017 4:50 pm 
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Posts: 294
I finally got to mess around inside the amp again, and here is what I found:

My bias supply at point E is -59 volts (Mesa schem says -57, but my value seems fine, and all the resistors up to that point measure well).
After point E, so inside of the power amp schematic, the first set of 220k's measure 201K each. My amp technically has 221Ks installed, but I figure that additional 1K is meaningless in this discussion. The additional 2K2 to each 6L6 is fine, and measures 2K21. The additional 220K to each EL34 socket is actually a 150K in my amp (as labelled, color codes) but each measures 140K. I do not know why these are different. Finally, my 2M2 to ground from pin 5 on the EL34 sockets measures 330K on each side, however the resistors are color coded to 2M2. I'm wondering if there is a parallel resistance that is bringing down the value here. Quick math indicates it would be a resistor of ~388K in parallel. I'm still not sure where my bias supply is messing up. Attached is a schematic with my amp's voltages filled in to the appropriate locations.

Stokes and lovetoboogie nailed it. The 470 Ohm resistors on pin 4 are indeed open for the 6L6 sockets, and the EL34 sockets that ought to be 2K7 according to schematic are reading ~995 and ~990, so I guess my amp has 1K installed there.

Since my amp seems to work fine in tweed mode (slight reduction in voltages), I think it's safe to assume my it isn't red plating by a huge amount, IE the system is only a little out of whack, not a lot. Obviously I need to change those wire wound resistors, but should I go replace the 2M2 resistors as well? What about the 2K7/1K? Should I change those?

Thanks for all of the help,
JMMP


https://www.dropbox.com/s/5n7q04bcz8ukb ... 3.JPG?dl=0

https://www.dropbox.com/s/ol0x7f5jwz219 ... 4.JPG?dl=0

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Gone: Quad MIDI Pre, TriAxis V1 & V2, Formula Pre, 2:90, 20/20
Great deals with: Prestige20, DMTransmutation, PaxMan, Forty6and2, jdurso


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 Post subject: Re: Redplating problem
PostPosted: Mon May 29, 2017 5:26 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1455
Location: NYC
I would just replace the "open" 470 ohm resistors and pay no attention to the resistors whose values dont seem "right" unless you have other problems after you get those inner sockets to stop red plating.


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 Post subject: Re: Redplating problem
PostPosted: Mon May 29, 2017 6:17 pm 
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Joined: Wed Dec 22, 2010 2:57 pm
Posts: 294
The problem is it's the outer sockets which are redplating. Is the lack of current going to pin 4 on the inner sockets able to cause the outer sockets to redplate?

Also, while looking inside the amp, I noticed the 470 ohm resistors are not in series with the "2k7" resistors as shown on the schematic. Both of those sets of resistors are branches from the "B" input on the schematic. Otherwise those open 470s would keep current from reaching all 4 of the tubes.

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Roadster, Mark IV, Traditional/ Stiletto 412
Gone: Quad MIDI Pre, TriAxis V1 & V2, Formula Pre, 2:90, 20/20
Great deals with: Prestige20, DMTransmutation, PaxMan, Forty6and2, jdurso


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 Post subject: Re: Redplating problem
PostPosted: Mon May 29, 2017 6:30 pm 
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Single Recto

Joined: Sun Feb 19, 2006 11:43 am
Posts: 1455
Location: NYC
Did somebody alter things in there before you got it?


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