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112 RD50 bass control has no effect

Hi all,
I have a black grill 112 RD50 (1984 I think) that I bought used a couple of years ago. At the time I purchased it, I was told it had a recent re-cap and all was good to go for a while. After using it a bit, I noticed a slight distortion that was hard to define. It seemed to ride the signal, as the amp was quiet for the most part when idling, but occasionally would exhibit a static noise even with no signal. Last week I opened it up and found the electrolytic caps all replaced as described with Sprague Atom caps. I found the JE1692 transistors were also replaced with a NTE196 type, so I ordered some 2N6488's and replaced them. I replaced all the tubes with JJ's, and replaced the original ceramic speaker with a new Jensen Alnico 5.
Pretty sure the distortion is gone now and the amp is super quiet, but it seems very crisp and heavy mid range. The treble control operates as expected, but the bass control does nothing. Either channel has the same behavior. I haven't taken it out of the cabinet yet, it seems odd two separate circuits would be affected the same way.
Can anyone steer me in the right direction?
Any and all suggestions and comments are appreciated.
Thanks in advance for your help.


Check this thread

Hi, Check this thread for similar problem with RD50. http://pacair.com/mmamps3/node/2327


Thanks for the link Mike

The symptoms in that thread do sound very similar to this one. And in that case he said both yellow caps were shorted. Looks like mine will go on the bench this weekend.
Oddly enough, I'm pretty sure the original poster of that thread is the same person I bought this amp from.


Couple of Suggestions

I've updated that thread that Mike provided the link to. The beat up amp referred to had some serious electrical/overheating problems earlier in its life, not sure what else may have caused the tone caps to go bad. It had been worked on (and not in a good way) prior to my receiving it.

If you don't have replacement opamps on hand, you can just try moving them around between sockets(IC-1, IC-3, IC-4) to see if you have a bad one.

Check your zener diode, R62, and the value of the resistors on the power tube sockets.

Did you measure your replacement driver transistors to see how closely they were matched? If not, that might cause a problem. See this thread for more info:


Good luck, and feel free to message me to discuss further.


Match them up

Hi Folks, I agree wholeheartedly with Inertian. The better matched the power transistors are, the more balanced the push-pull circuit will be. If you're a nut like me, go a step further and match the 6ea. 680 ohm resistors that form the bias network for the transistors and the 2ea. 6.8 Ohm resistors that feed the emitters. Although resistor values are different, I did this same operation on my 112-RD65. After balancing, the bias voltage on each tube was within <1mV of each other. -mgriffin


How Best to Match Up Power Transistors

Mike - how do you prefer to measure/match the power transistors?


Balancing Act

You're gonna hate this. I don't have the luxury of a transistor gain tester, but I'm real good with a DMM. My secret is to buy replacements that come physically taped together from a tape reel. I then look to see if both have the same date code. If they do, I make the assumption that they were "baked" from the same batch of cookie dough in the same oven by the same baker. There is a very good chance that they will perform very much the same. The same holds true for resistors. For matched sets, they should be just like their next door neighbors on the tape reel. Use your DMM to verify accuracy. I know this method may not work if scrounging from parts left in a drawer. If you order parts from Mouser.com, they usually are cut from tape reels. Good Luck and it's OK to yell and laugh. Oh, for measuring transistors, I can only perform functional tests. I use my DMM to "Diode" check base to emitter and base to collector. I also measure the Base Emitter voltage drop in the energized circuit. -mgriffin

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