Looking for ideas for my ailing RM100

[stpetecubby]

Hey guys,

I've got a RM100 that has the power output severely limited. It made a loud pop a couple weeks ago, and I didn't think much of it at the time, but later realized that for where the levels were set and where the master was set, it should have been screaming loud. I did a sanity check with the RM20, which (sadly) blows the RM100 away right now.

I did some reading here about problems that people have had, and I'm starting to worry. I checked the 4 external fuses, I found 7 fuses inside (are there more hidden?). All of the elements looked intact. I put all of the tubes on a tester, and found them all to test strong without any shorts. I also swapped the 12AX7s for some vintage tubes. Everything glows. I haven't checked the bias yet to see if something in the bias circuit was damaged. When I was inside the amp, I didn't see any vaporized semiconductors, no capacitor explosions, no burned traces, nothing out of the ordinary. Doesn't matter which channel its on.

Any ideas on things to check? Or should I proceed to panic mode and prep to send this thing into Randall? My biggest fear is having cooked the primary of the OPT.

Thanks for your help guys.

Jeff

 

[kc2eeb]

Are you competent with high voltage circuits?

 

[Mattfig]

Are you competent with high voltage circuits?

Probably the best opener you could have received....Seriously!

 

[stpetecubby]

I can hold my own. I've designed and built a little tube amp before, and despite my best efforts at times, I haven't stopped my heart yet. I'm an EE, but I work in the utility business. (You know, shattered dreams...)

 

[kc2eeb]

First thing, with the amp AC disconnected, is connect a multimeter to the
bias ground connection (black lead to common/ground), set the meter to a low voltage range and touch the tip of the red probe (ohms/volts) to the exposed metal tops of the power supply caps. The amps have bleeder resistors to drain them when the power is off.
Then check each fuse with the meter set to low ohms. A fuse could look
perfect and still be open.
Things to check:
Give a light pull to the wires going to the impedance switch. Check each for a sound solder joint.
Connect a speaker cable to the "use first" speaker jack. Set the ohms range to the lowest setting. Measure resistance from the tip to sleeve.
16,8,and 4 should read fractions of an ohm with 16 highest to 4 lowest.
To check the O.T. primary you have to disconnect the 3 insulated spade
connectors from the board. They are usually blue, red, brown with red as the center tap. Read the resistance between red to blue, red to brown, and brown to blue. Brown to blue should show the highest resistance
(tens of ohms). Brown to red and blue to red should be about half of the
reading between blue and brown but NOT equal. What are equal are the turns ratio, NOT the resistance. If you read open or shorted, then it's the
O.T. A true test requires an AC (NOT LINE) signal source.
With a magnifing glass or eye loupe, look for any carbon traces around the output tube sockets or the HV connections on the boards.
Use a strong light.

 

[stpetecubby]

Awesome, thank you for you help! I've got some work to do.