Plate Resistor burning up on DC-3 - help!

[Jimmy74]

Here's my problem, the plate resistor (62k) on pin 6 of V1 burnt up yesterday while I was testing voltages. It happened a short while after I turned off the standby. Also there are no tubes in at all so voltages are higher all over the board. I replaced the burnt resistor with a 100k 2 watt, and turned the amp on, the voltage going into the resistor is 350v. I tested for continuity between pin 6 and the other 100k resistors but all looks fine, and there don't seem to be any shorts anywhere.

The strange thing is that it's the only resistor that heats up , the other 100k resistors on the same path are fine.

what could be the problem?

thanks

 

[Monsta-Tone]

You're going to have to test the voltages with the tubes in. Much like resistors, they will draw current and thus lower the voltages present in the power supply.

Let me know if you need a drawing, I'll be happy to email one to you.

Raising the 62K resistor to a 100K will give you more gain, but may push the amp into buzzy territory.


With no tubes in the amp, the resistor should have no current flowing through it. So...in theory, it shouldn't burn up.
I would say, check the coupling capacitor (.02 mf) that is attached to it. If the cap is shorted and allows DC, then there is a potential of bleeding some voltage to ground, but it would be very little since there is about 620K ohms from the output side of this cap to ground.

I would also check for resistance (not continuity) to ground from Pin 6. It may be that there is a small glob of solder or some dirt that is providing a slow path to ground. It can't be that much or the resistor would have burned up imediately.

Another thing I would check is the power supply.
Are you getting any AC voltage at Pin 6?

With the tubes installed, if your voltages are more than + or - 10%, I would check the dropping resistors between points A & B and Points B & C in the power supply.

Also, if you have new tubes and are not comfortable using a new tube in V1, use a known good old tube in this spot. That way, you're not out a new tube if anything fails.


Hope this helps!

 

[Jimmy74]

Hi thanks very much and happy easter to all.... I have gone further and resolved that problem.... there actually was a small solder glob shorting out on the 3vDC heater wire that is very close to one end of that resistor. I know have another problem which is hard to fix and to explain.... anyways after testing all the voltages which were all correct and in the right range, I proceded to fitting in the tubes and Iìm getting strange hum and the clean channel doesn't work at all... DC heater voltage to V1 is correct and heater AC voltage to the other tubes is correct too. The hum seems to be something in the power supply maybe a filter cap.... but I'm not sure. I moved all the HV wires going to the power tube circuit away from the signal wires which go to the output circuit board but that didn't make any difference. I'm suspecting one of the LDR's or a short under the V1 tube socket, once I figure that out I will procede to trying the tubes and see if the hum disappears.

Does anyone have a blue sheet of the DC-23C circuit board?

 

[Monsta-Tone]

If you have a strange hum and the clean channel doesn't work, try this:

1. Replace V1. 1st Gain stages for both channels. A bad tube here will cause a channel not to work, or hum, or lots of other weird problems. It can also affect both channels differently.

2. Replace V4. This is the 2nd & 3rd gain stages for channel 1.

3. If your clean channel comes back, but you still have the hum, replace each of the remaining tubes 1 at a time to see if it goes away.


Filter caps will definitely help the amp. I replaced the ones in my DC-5 a long time ago. It wasn't humming and had plenty of power, but the difference was still night & day!

 

[Jimmy74]

Well from V1 to V4 the tubes were brand new mesa's so I don't think the tube is the problem, this is another strange thing that I noticed.... If I pull V1 out and leave all the other tubes in... the amp doesn't let through HV to the power tubes... that is if I hit the standby there's no difference what so ever and also I noticed that bias voltage reaches -66vDC as if there were no tubes in at all.
When I put V1 in the amp the standby works and bias is correct to -11vDC. Another thing I must add is that I did replace the 2 LDR's sitting next to eachother, and also I pulled out the V1 tube socket and found that there was a solder glob probably shorting out under the socket... so now I'll just rewire that mystery LDR solder point, redo the soldering points under the socket and put the socket back in. I will then forward to following your advice in seeing if the rectifier bridge for V1 is letting through any AC voltage to the heaters and go from there.
I have a good question for you, why on earth did mesa draw out the wrong rectifier scheme for the switching matrix?? I did find out the hard way (I had to re-build the rectifer bridge for the switching matrix and followed exactly what is drawn in the schematic) and when I flipped the clean channel it started making a rattling noise. I had to swap out one of the diodes.... just wondering if you think anything else could have been damaged?

Also take a look at this resistor:
http://www.flickr.com/photos/73835524@N00/3356870910/sizes/l/

what do you make of it? It was inserted between the input jack and the 1.5mohm resistor. It was measuring 2 ohms on my MM, but I doubt that that's what it was supposed to measure. I've put in a 100k resistor for now since that mystery resistor has a snapped leg. Let me know what you think.
thanks

 

[Monsta-Tone]

I couldn't get the picture to load.

You can email it to me if you want. I may not be able to look at it until tomorrow morning though, I'm about to head off to work.

My site is advertised above with the email. I would type it here, but I'm a bit paranoid about spam. I just got rid of a bunch of it.


As for the Rectifier circuit and the mystery resistor:
There are 2 official versions of the DC series. 'A' & 'B' versions. There may well be a few oddballs in between the versions where maybe 1/2 of the 'B' modifications were done but they were still working on the remainder of them.

 

[Jimmy74]

I couldn't find your email address, you can try this link:

http://farm4.static.flickr.com/3580/3356870910_6871f941c7_b.jpg

or just PM me with your email address.

Just another couple of questions, what voltages are supposed to be seen on the LDR's? Specialy the opto side of them.

What would you make of this: I'm getting -14vDC on most pins of the NE5532 op-amp chip. I can send you a voltage chart of the various voltage points drawn out in the schematic.

 

[Jimmy74]

well after mounting the amp up I turned it on and I am getting some sort of strange DC voltage all round, with the preamp tubes in I was getting like -46vDC on pins 3 & 7 also on the on the NE5532 opamp and also on those 2 zener diodes so it looks like it could be a bad cap up in the low voltage supply.

what do you think?

thanks

 

[Monsta-Tone]

I think you need to take it to a tech or send it to me.
You've gone way past the easy to fix over the internet stuff.

If you have errant voltages present in multiple areas of the amp, it needs some serious help before something else fries.

I've got an add at the top of this site if you need contact info, or references.

Sorry man, this is just way too hard to do over a keyboard.

 

[Jimmy74]

Well thanks for the help, but the problem is that I live in Italy so shipping it to the US would be really expensive that's why I'm writing everything down here. I'll get that voltage chart done and post it up.

thanks

 

[Monsta-Tone]

Well, shoot.
You might try emailing Mesa and asking them if they have a Tech in your area.
They usually respond within 48 hours.
You might also list your problems and what you have done to fix it so far.
Usually George Mueller is the guy to talk to about the DC's. They might give you his direct email, or you can ask that they forward your email to him.

 

[Jimmy74]

Thanks again for all the help, I did try contacting Mesa US before posting my problems here, but they passed me onto Mesa Italy, who wasn't able to help me.

Anyways getting back to the amp, I may have pin pointed the place where the short is, but if possible, I would need you to do some continuity tests at the following points:

1) from the positive leg of the 2200uf 35v electrolytic cap to 330k resistor that's found just above that cap
2) again from the same positive leg of the 2200uf 35v electrolytic cap to the first of the zener diodes
3) from the negative leg of the 220uf 63v bias cap to the second of the zener diodes
4) if you can please send me a photo of the under side of the mesa board in the area where the 2200uf 35v cap sits

thanks very much

 

[Monsta-Tone]

****!

I don't have a DC-3 anymore, sorry. Maybe someone else here can chime in.

You might have to break out the ohm meter and just start measuring everything.
Sometimes, you have to lift a leg of a resistor to measure it properly.
Also, you could check for AC coming out of capacitors. They are supposed to block AC, so, if it were present and the cap were good, you would only have AC on one side.


Otherwise, the parts are cheap, you could simply replace whatever the parts are in the problem area.

 

[Jimmy74]

well I had already changed all the electrolytics except for the filter caps so I suppose they're alll alright, I had to solder the 2200uf 35v under the baord because the solder points above the board had been damaged and I had to bridge the connections with wires and I just wanted to be sure that I hadn't bridged the cap to the wrong parts

thanks

 

[Monsta-Tone]

I'll look through my files this afternoon and see if I have any pictures of the boards.

I would be willing to bet that you may have accidentally connected it to the wrong place or maybe a trace is broken and the voltages are not going where they should.


If I find any pics, I'll forward them to you. I have an add at the top of the page with my email address on it.
I don't want to type it out here because I don't want to get spammed.

 

[Jimmy74]

PM sent..... just one question, I'm wondering where that 330k resistor is in the schematic.... can't seem to find it anywhere.

 

[Monsta-Tone]

All I had were some DC-5 pics. I just emailed them to you.
It should be relatively similar with the exception of the Bias power supply.

Not sure what else I can do for you.

 

[Jimmy74]

Ok I've done some testing and I'm posting up the results here. What I tried to do is find out where the negative voltage is coming from. I tested the bias supply and the low voltage supply and a components.

here are the links to the charts and schematics:

1) bias supply voltages:
http://farm4.static.flickr.com/3372/3479262359_6cc2b2773b_o.jpg

2) Low voltage supply readings:
http://farm4.static.flickr.com/3602/3479262367_3b33e63913_o.jpg

3) pre amp tube voltage chart (no HV applied):
http://farm4.static.flickr.com/3653/3479284311_786512abb9_o.jpg

4) pre amp tube voltage chart (HV applied):
http://farm4.static.flickr.com/3237/3479284319_d85c4f0e75_o.jpg

Other strange things I noticed:

1) Bias voltage supply.... there is AC voltage coming off the bias diode around 130vAC
2) Low voltage supply.... on one of the diodes I'm getting around 20vAC through
3) No ground to chassis on positive leg of the 10uf 63v electrolytic cap
4) I'm getting negative voltage reading when I touch the top of the 47uf electrolytic cap

any help at all will be appreciated

thanks

 

[Monsta-Tone]

If you are missing a ground on a capacitor, that may cause errant voltages.

I would go through the whole amp and test each ground. It may be that simple. Start with the power supply caps and work your way to the other side of the amp chassis.

You should also check every component in the high voltage supply. It could be that a failed capacitor or diode is feeding back into the rest of the circuit.


I know this may be a stupid question, but are you sure that you have the proper transformer for your amp?
If you are using a power converter, maybe try another one, just in case.

 

[Jimmy74]

No I don't think it's a tranny problem, they're all original and haven't been giving me troubles. I will follow ur advice and start at the HV but I really think the problem is in the LV supply, and I suspect those 2 zener diodes too!