Yamaha G50 112II reverb unit repair

[phantombox]

I have a Yamaha G50 112II amp with a broken reverb unit. This unit is made by Accutronics in Geneva, Ill. (it even says MADE IN USA), and has 2 springs inside.

After I checked everything obvious, I took the reverb tank out of the amp and opened it. On the output transducer, one spring had come out of the small hole where the hook engages but I managed to put it back in place. However, on the input transducer there are no holes to hook up the springs. Instead, the ends are kind of soldered in place. Also, this end of each spring has a thicker part just before the thinner end pin.

I took a couple of pics so you can see what I'm talking about.

It is not clearly visible but on the opposite of where each spring enters the transducer, there is a receiving pipe sticking out with solder inside. That is where I would melt the tin, put the spring back inside the hole and hold it in place until it cooled down enough to hold the tension of the spring.

My question is if these springs can simply be soldered back in place the way they came out or would I have to compensate for any missing piece that might have broken off the end of the spring?

Or would it be a good idea to replace the pin with a small hook and try to solder a piece of wire into the receiving end? This wire would then be fixed to the spring and allow it to vibrate. It probably would have to be able to generate a magnetic field that then can be picked up by the transducer.

[Enzo]

There should be a number stamped on that reverb pan, something like 4EB2C1B or 4AB2C1B. That is the type pan you have. And the various brands now making them use the same sort of code. For $20-30 I think I would simply get a new pan.

On the photo of the ling spring end, see the dark "bead" around the thin end wire? And note the transducer looks like a little transformer? That bead needs to sit in the middle of the transducer laminations. Without that, it won't "transduce". That means the thin end wire has to be long enough to leave the bead centered in the transducer and also come out the end of the brass tube. I suspect your end wire snapped off inside the brass tube, and is no longer long enough.

You also need to check to see if either transducer is open electrically.

[phantombox]

reply to "Yamaha G50 112II reverb unit repair"

There is no such number stamped on the unit. Maybe its because this Yamaha is a 30+ year amp and was manufactured before Accutronics was sold to Beltron / Korea...
Anyways, I already fixed it.
After a little more analyzing the reverb, I bent the short wire that was left after the “bead” into a sharp hook (hoping it wouldn’t break off again – … it didn’t), did the same thing to a single strand of “Belden” audio wire (this time I took a long strand, bent it in the middle and twisted both halves together), hooked both ends up and fixed them with a small amount of solder tin. Now I had a long wire end.
Next step was to get a small hole into the brass tube at the transducer. Turns out the tube has an end cap on it that came off. That made the job a lot easier.
After I got most of the old solder out of the end cap and a hole inside, I introduced the new wire end into the tube and fixed it at dead center inside the transducer with some new solder at the end of the cap.
Now it is working again. I had no idea how the reverb is supposed to sound, but compared to a Fender Twin Reverb, its kind of sublte. But hey, at least its working again…

[nickb]

Glad you got it working.

This amp uses a small power amp chip to drive the reverb so my guess is a low impedance type is used, perhaps an 8AB2C1B. Anyway, you can find out for sure since it is currently working. Simply measure the DC resistance of the two coils and from that we can look up the type. Do it now while you still can once the coil goes open you will never know.

[Enzo]

Yes, if it works, it works.

Accutronics is the one who came up with the numbering scheme. That number system was in use 30 years ago. it may not be stamped on ther, but your amp was from the era when that number sequence was in use.

[J M Fahey]

Just checked TA7220 datasheet, it´s a "6W power amp" which means it drives the tank hard and also that it should be low impedance.
In any case, please post your coil resistance measurements.
Output end will be "high" , meaning , say, 100 ohms or nearby; the doubt is on the drive end; if low impedance (nominally 8 ohms) it would have around 2 ohms resistance; if high impedance will also be around 100 ohms.
Based on those values, you will get a modern equivalent.
I suggest you buy a new one, price is low and yours has been quite damaged.
I see no springs at all on one end, the other shows just one, and with broken wire.
The pencil lead sized magnets must float in the middle of iron cores.
Yours may work again but weakly ("subtle") , but for a very low cost you can fully restore that amplifier.
Nothing sounds like a full tank tube driven Fender reverb, but Yamaha had a very reasonable one anyway.