fix blown 50/50 amp

[tnd2gen]

My Mesa 50/50 Rack amp just blew a few resistors (see photo). This amps been sitting for 10+ years and was used 10+ years prior to that.

Could you please let me know exactly what these 2 blown parts are (circled in white)? and why they blew?

Also, since the amps sitting for so long:
- MUST I also replace the filter caps (circled in RED)?
- Replace the preamp tubes?
- Replace the 4 main 6L6 tubes?

Really appreciate your help with this. If there's anything you can recommend to get this back to 100%, I will be able to try my new Mesa 2x12 rectifier cab.
https://ibb.co/2K12yy7
https://ibb.co/j6GvX7Q

 

[woodbutcher65]

I see no photos or even broken links. My browser usually sees those things just fine.

Burned resistors are always signs of other things that have failed and drawn too much current.

If the amp has sat for 10 years, yes, it should be recapped. Letting them sit with no charge on them is literally the worst thing you can do to an electrolytic capacitor. The best thing is keep it at full rated voltage ALL THE TIME.

Whomever does the recap should hopefully be qualified to replace the resistors and ensure that whatever caused them to burn is taken care of as well. Which might just be the capacitors. Or it could be bad tubes as well.

 

[tnd2gen]

Good info - fixed images - please have look and see if this is a common or known issue. Amp was used pretty hard for 10 years on the road...time to finally treat it to some new parts!

 

[woodbutcher65]

Well, I know to never use THAT image hosting service! The images still haven't fully loaded and it's been several minutes!

What are they on, a 300 baud dial-up connection? That's just absurdly bad data transfer rates right there.

Those caps are so old, it'd be a crime not to change them. And if any of them has shorted and failed, it could be responsible for the burned resistors.

I'm saying that you need to take that amp to a qualified technician. If you don't have one in your local area, seriously consider sending it to Mesa or a Mesa authorized service center for repair.

 

[Henz]

Greetings WoodButcher, I'm glad I caught you here. This is off topic but I need your advice. I repaired my 26 yr old 2:90. First,let me itemize what I did. Hard wired PT leads directly to PCB,replaced two Orange drops (C16, C20),replaced cracked 2.7k 2 watt carbon comp resistor with a wirewound (resistor between two 30uf 500v caps),replaced 6.8 ohm carbon comp (R 102) which measured 13 ohm with a new 6.8 carbon film ,repaired arcing on board and repaired B+ trace w/ jumper wire, re attached feedback lead to Ch. B ,replaced two LDR's, recapped every electrolytic... filter, bias etc, new pilot light and power switch and retubed. Question: after completion, I have to keep levels very low, say at 9 o'clock where I normally would have them around 12 o'clock. Even when amp was new I had levels at noon. I can't open her up and get things pushing like I used too. It just sounds different. Most ppl would probably love the extra head room but I think it's not letting me open her up. With your expert experience, what do you think gave me this extra head room? Thanks for your time, Jim. http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/2901.gif http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/290pwr.gif

 

[tnd2gen]

Ended up replacing with 2 metal film resistors, 4 filter caps and 4 new tubes. Fuse was also blown, and waiting for that to come in before I power up.

 

[woodbutcher65]

You might try installing Tung-Sol 7581a power tubes. They have increased power handling and headroom capacity, 35 watts per tube rather than 25 per tube for a 6L6. If your amp has the power delivery capacity then you're going to reach 70 watts per channel before tube clipping.


Be careful when changing resistor TYPES. There are some resistors that are chosen for their type so that they DO fail and open the circuit in the event of a serious fault. But carbon comps are not that kind of safety resistor. Quite the opposite, an overloaded carbon comp resistor is a fire hazard.

Check the schematic to be sure of the application of the resistor before you go changing it to something else.

 

[tnd2gen]

fuse came in. One tube socket is creating red plating-not the tube, its the tube socket closest to the fan (50/50 rack amp). If anyone knows the best easy-to-hard procedure for testing components that are creating the red plating, i'd be interested to fix this while I have time working from home. PS, I don't have a schematic.

 

[Henz]

Try to locate and measure bias resistor . Pin 5 . 220k (red,red,yellow bands). Screen grid resistor as well 470 ohm (yellow,purple,brown) . If it's just one socket, that's what I would do. Maybe .1 coupling cap to that socket (Orange drop).It also would have been good to put two new bias caps in (47uf 100v). If you don't feel comfortable inside amp, have an experienced tech do it. Good luck. http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/50501.gif. http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/50502.gif

 

[tnd2gen]

Henz - thanks for the tips. I've since cleaned out all the contact sockets and tightened them up, also tested with known good tube with the same result - red plating only in that socket.

I was hoping you could point out where these specific bias and screen grid resistors are?

https://flic.kr/s/aHsmMKxxWT

 

[Henz]

Sure. I'll try. Locate socket inside amp. The screen grid resistor will be very close to the lead wire from pin 3 on socket. 470 ohm. It will, or most likely be, a large ,brown carbon composition resistor 2 watt.( yellow, purple, brown color bands). From there, nearby, off of pin 5, there will be a much smaller resistor, ( red, red, yellow bands). 220k 1/2 watt. In general, measure all of those resistors in that area. The amp does NOT have to be on when measuring resistor readings in most cases. The readings usually won't be exact due to the tolerance. Let's say you measure the 470 and get a reading of 465 or 473, just saying, that resistor is good. But if they are way off or not reading anything, it's bad or open. Replace it. Let me know if it helps.

 

[Henz]

Another quick way to determine if it is a bias issue ( resistor hopefully) is to check the socket while amp is on stand by. But please, there is dangerous voltages inside. If you choose this, be very careful and take your time. Get your volt meter, put it on DC . Take the red probe and attach it to ground chassis. Usually the threaded hole where cover goes. Secure it . Use one hand. Put your other hand in pocket. Take the black probe and locate pin 5. On those white ceramic sockets they are usually numbered if you look carefully. Carefully take black probe and touch pin 5 and see what meter reads. It should be around 50vdc. Tube can be in or out. Be careful! I was measuring a socket in my 2:90 and the probe slipped and there was a fireworks display ! Let me know. Then we'll move on.

 

[woodbutcher65]

As for the question about headroom changes after working on that 50/50 amp, I'd bet long odds that the change is due to having replaced worn out filter caps. Those are your power reserve for high demands, and when the caps are done they will not store the energy they used to.


I picked up a Simul-Class 2:90 for a ridiculously low price recently. It needed a set of new front panel pots and new filter caps. It's ready for me to use it or sell it. I won't sell it until the economy gets back on track because its resale value is currently tanked because of this media driven virus paranoia that's killing the economy.