Music Man Amps

Hi Craig, An "Amp-Head Dual Bias Tester" is not required for the job. Measure the 25mV voltage drop across each of the two 3.9 ohm resistors (R39 and R40) with a DMM. Follow the procedure mm210 pointed you to on the other thread. http://pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bullet... The bias procedure is at the top of the page. You will actually be setting the bias of the power transistors that feed the cathodes of the tubes. That's why the Dual Bias Tester is not the right tool for the job. Here is the schematic in case you need it. http://pacair.com/mmamps3/sites/default/files/docs/2165-rd_and_2100-rd.pdf The section you want is right in the middle. Be safe. There is 700+Volts on the hot side of the tubes. Stay away from there if you're unsure. Don't get hurt. Good Luck.

Hi Craig, Question? Are those the best values you can get when adjusting the bias pot? Can you get the 21.5mV to come up to 25mV? When the bias circuit is working well, the trim pot should be closer the the middle spred of its rotational range give or take. If it's 31mV and 21.5 mV is the best you can get it adjusted to and you like the sound, it's probably good enough. -mgriffin

Find the 8 pin IC4 (LM1458) that drives and sums the reverb tank. It should be the one closest to the reverb tank jacks. Pull the IC out then clean the leads and the socket too. Use contact cleaner like Deoxit. Re-install the IC and see if that made a difference. The IC cleaning procedure is a good and simple thing to do whenever servicing an MM amp. Over the years, dust and oxidation gets into the all of the nooks and crannies. Cleaning doesn't cost much and quite often yields good surprises. -mgriffin

I would suggest you get the foot switch out of the mix. Unplug it and set it aside. All that switch does is short out the reverb output. The other switch selects the clean and dirty channels. Clean is the default. With the foot switch disconnected, crank the reverb knob to 10 and retest. If that doesn't get you a bunch of reverb, let's do a poor man's test of the ICs. Try swapping IC-1 with IC-4. If it is bad, the front end input will stop working and the amp will not amplify. If it does amplify, the IC is good and we'll need to look elsewhere. The other thing to do is measure the input and output resistances of the reverb tank. Also, verify good continuity of the reverb in and out cables. Any break in the signal path will make it not work. -mgriffin

Well it looks like the IC's are fine so let's quit looking there. I measured 180 ohms DC resistance at the input and 175 ohms DC resistance at the output jacks on my spare reverb tank. Even though different from your values, your numbers are not out of whack. We just want to verify that the values aren't short circuit or open circuit and somewhere in the ball park. The black cable connector goes to the Input of the tank and the Red cable connects to the Output of the tank. That Red/Black ordering is shown on the schematic. The IN and OUT jacks should be clearly marked on the tank. Also, make sure your cables are in perfect condition. My RD112 uses an 8FB2A1B reverb tank. It has around but not exactly 200 ohms DC resistance both sides. Here is a link to buy replacement, but I'm not convinced your tank is bad. http://amprepairparts.com/reverb.htm#8 Also, here is how to decode the numbers on your tank...http://amprepairparts.com/reverb_numbering.htm. Maybe try cleaning the tank RCA jacks and cable plugs, double check your connections and try again. Why do I have a spare tank? Because I thought mine was bad as well when I resurrected my RD112. It didn't work because I connected it backwards. When you look at the Numbering chart, scroll down to the table that shows many tank examples and how to decode both AC Impedance / DC Resistance and how they relate to each other. Good Luck. -mgriffin