16 posts / 0 new
Last post
craig

low bias voltage 112RD 65

I replaced all the electrolytic caps and the 6CA7 tubes. With the bias adjustment pot turned completely clockwise I am only reading 15.0mV and 10.2mV. What can I replace or check to get 25.0mV read? The circuit board is a GD-2.

mgriffin155

DOWNLOADS AND RESOURCES

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.

craig

low bias voltage

okay I measured across the 3.9 ohm resistors and got 31mv and 21.5mv. Is that close enough or should I replace the power transistors? thx

mgriffin155

Is that close enough?

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

craig

reverb not working

after recap and new tubes I assembled the amp and the reverb is not working. It was working before the recap job and new tubes. I reversed the leads going to the reverb spring with no change. When the reverb foot switch is turned on the reverb noise can be heard when the amp is moved or tapped with hand. Any ideas on what can be done to fix the reverb? thx craig

mgriffin155

Clean things first.

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

craig

removed and cleaned the IC4

removed and cleaned the IC4 (LM1458) and installed, still no reverb.

craig

no change after cleaning and

no change after cleaning and re-installing the IC4

mgriffin155

Things to try.......

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

craig

removed the foot switch.

removed the foot switch. Cranked reverb to 10, heard no reverb. Swapped IC-1 with IC-4 and the amp worked. I believe that IC-1 is located in the rear left corner on the printed circuit board, looking at the front of the amp. Is that IC-1? I believe it is a GD-2 board ( there is A1` 12-11-79 inked on the board) it does not have a 12AX7 tube. Resistance on the spring is RED .412K ohm and BLACK 292.1 ohm. thx craig

mgriffin155

Reverb Tanks

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

craig

is there a voltage check on

is there a voltage check on the reverb in and out jacks on the chase that I can check?

mgriffin155

Sig Gen and O'Scope

I don't know of any unless you have a Signal Generator and O'Scope. It would be hard with only a DMM. What are the part numbers on your reverb tank? Here's another wild guess. You said you hear the tank when you bang the amp. That means the output transducer is probably working. Try reversing the cables and do the bang test again. If you hear the tank warble with the cables swapped, that means both input and output transducers are working. Try it both ways and report back. -mgriffin

craig

reversed the leads going to

reversed the leads going to the reverb tank and the noise sounds the same to me. thunder/rumble

mgriffin155

Not sure

It sounds to me that both transducers of the tank are working. What you're hearing is the springs yanking on the transducers and that's a good thing. You've also verified IC4 is good. A new tank is not overly expensive if you want to go that route. FYI: The short tanks like those in the 112's is not radical surfer reverb. They are much more subtle due to the short decay time. Do you hear any changes when you crank the Reverb control back and forth with a signal running through the amp? Strike a chord and quickly rotate the Reverb knob back and forth and hear what happens. Other than that, I'm out of ideas for the time being. Keep banging away. -mgriffin

craig

okay I will try rotating the

okay I will try rotating the reverb pot while playing a cord and will get back to you. I might just forget the reverb for now. The amp sounds good with a lot of head room and I can always use a reverb pedal. thx craig

Log in or register to post comments