no reading at bias points?

[musicdr]

Longtime lurker here.Finally registered.I started with an rm80 over a year ago and now have 2 rm100s.I'm no stranger to biasing these amps.I just swapped tubes and went to bias and got a reading of 00.0.I changed the batteries in my multimeter and bought another multimeter just in case.Still no reading.When I put the red test lead in I start to get a reading then it ends up on zero.All tubes are lit up.Is there a fuse for the bias points.Anyone ever have this happen?For the record I did about 70 gigs with these amps and other than a tube blowing have had no problems.Any help is appreciated.Great forum!

 

[kc2eeb]

First, check the obvious. Are your test leads sound. Then look at the test point sockets. Slightly angle the tips, or rock them a little to see if you get contact. Make sure the meter is set to read DC. You don't mention if all the test points are all the same. Then, look at the HV fuses. Check them with the meter even if they look good. Is your standby switch on? etc. Next, I believe the connections from the cathodes to the resistor you are reading the voltage drop across is through a multi-pin block connector. See if they are properly seated. You will have to go into the amp for this so.... if you are not experienced with high voltage, don't go in there. If you are, then by all means. Also, what tubes did you install. There seems to be an issue with JJ tubes changing their pin diameters. I use JJ myself, they are excellent. Hope this helps! All the best! Pat L.

 

[musicdr]

Test leads are good.Fuses are good.Same reading on all test points.Tried rocking the leads, no help.Installed eh el34's.Had 2 6l6 in outside slots and 2 el34's(eh) in inside slots.I only swapped the two outside tubes.Before going in the amp I will put the 6l6 tubes back in and see if I get a reading.Strange problem! keep you posted on my progress.

 

[musicdr]

put original tubes back in,still no reading.Will check connectors.

 

[musicdr]

And yes the standby switch is on.

 

[kc2eeb]

Real interesting... Hopefully, loose connectors. If the heaters are lit, next thing I'd start looking for voltages. Do you have HV on both sides of the standby switch in operate position. There are some internal fuses to check including for the HV winding and one, I believe, for the bias supply. Keep us posted. Pat L.

 

[musicdr]

I found 7 fuses inside.I removed all and checked with continuity tester,all are good.Two were under the midi board.The wiring is very tidy in there.All connectors are good.Don't think there are any more fuses? Unless they are under the board.Any other ideas?

 

[musicdr]

Put it back together and an internal fuse blew.Back apart,replaced fuse and now no lights,fan and the 3 12ax7 tubes are not lit.Power tubes are lit and thats it.Frustrating.Fortunately its under warranty.Anything else I should check? I always had the impedence set correctly.I noticed no problems until I switched the tubes and tried to bias.Maybe a bad coincidence.Thanks for the responses.

 

[musicdr]

Hey rich,how do I go about getting the amp fixed under warranty.Any authorized repair centers in the mpls.,mn. area?

 

[kc2eeb]

You may want to check for a damaged or cracked tube socket. I'd suspect either a shorted pin socket to ground, or possibly, a carbonizing of the socket material. I believe the preamp tube heaters run DC and the power tube heaters don't.(They are 180 degrees out of phase, so the 60 cycle hum cancels.)So the heater supply is separate to the output tubes. Carbonizing is caused, other than by excessive heat, by dirt of some kind on the pins or sockets. Voltage going through the resistance created draws current. That makes heat. Anyway worth looking at. All the best! Pat L.

 

[bruce egnater]

Does the amp make any sound?

 

[musicdr]

No sound.

 

[Bantam Menace]

Hey rich,how do I go about getting the amp fixed under warranty.Any authorized repair centers in the mpls.,mn. area?

Who do you play with? I also play in the area and would be interested to see an MTS on stage...

Sorry...I'm no help.

 

[musicdr]

You just want my mr.scary module! Anyways guys,I noticed nothing new except when I tried some other tubes they either fit extremely loose in the socket or too tight(not seating all the way).The heaters still lit and I could here a slight humming through the cabinet, transformer? Since I have come to the limits of my technical knowledge and don't want to delve further and void the warranty its time to bring it to a tech.Thanks everyone,especially bruce for taking the time to reply,you are the man! I did pickup an rm20 head on the way home to console myself.

 

[Bantam Menace]

Mike?!?

 

[musicdr]

Lucky guess.

 

[Bantam Menace]

Come on! Sell me the grail!!!

 

[bruce egnater]

Sorry I didn't notice the "fuse blew, no lights, no fan etc. Sounds like the low voltage (12 VDC) supply has failed. Needs to be serviced by a competent repair person.

 

[RD/Rich]

PM Sent

 

[musicdr]

Email sent, thanks rich