Author Topic: Restoring '71 Fender Rhodes 73 Suitcase - Fuse Issue  (Read 179 times)

Offline ginobeano

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Restoring '71 Fender Rhodes 73 Suitcase - Fuse Issue
« on: July 29, 2018, 08:22:57 AM »
Hey guys, this is my first post so let me know if this is in the wrong spot.

I am restoring a '71 Fender Rhodes Suitcase (4 pin) and I ran into an issue with the fuse blowing. Basically, any time I turned on the amp it would immediately pop the fuse. The first time was an original fuse so I chalked it up to just being old. But when I replaced it with a new fuse, it blew immediately again when I turned it on and I knew something deeper was going on.

I bought a few more packs of fuses and was able to narrow the issue down. The fuse would blow if the preamp was disconnected so I am ruling out anything preamp/ keyboard related. Investigating the modules inside the Peterson Amp case, it appears that (following the service manual at http://www.fenderrhodes.com/service/manual.html) the channel 1 power module was causing the fuses to pop. I disconnected the channel one power module, leaving the cannel two power module connected, and the suitcase was able to remain on without blowing the fuse. Replacing the channel one power module into the channel two module plug shows that it is still operational. So, in my opinion, this narrows it down to either an issue with the channel one power module, or it could be an issue that only occurs when both are plugged in.

Investigating the channel one power module, nothing seems to have visibly malfunctioned (no smoke or burn marks), all physical and solder connections seem to be intact, and both of the internal 3 amp fuses on the Power Supply Regulator Assembly are still functional (thank god because they are soldered in).

Looking at the power supply regulator assembly, it is absent of any signs of a burned out components either. The first thing past the fuse is the transformer which, according to the maintenance manual linked above, is the T1 transformer which is the heavy, hulking box at the bottom of the power supply regulator. There is a chance that this could be malfunctioning, however I was unable to test it. Poking around on the pins for each of the plugs for the channel one and two power modules I got some consistent readings from the 25v pins but not the 35v pins (positive or negative) although I am not sure if that indicates something is actually wrong.

Has anyone fixed a similar issue/ would have some insight on where I can start investigating? I won't be able to just replace an entire module due to current cost constraints, and also I want to keep as many original parts as possible for this build, but I do have a multimeter and a soldering iron, and would be willing to try and document any tests that might lead to this issue being fixed. Also, please feel free to request any supplemental material from me such as pics and I will do my best to deliver.

Thanks!


Offline Ben Bove

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Re: Restoring '71 Fender Rhodes 73 Suitcase - Fuse Issue
« Reply #1 on: October 05, 2018, 12:27:45 PM »
This sounds like a power supply regulator assembly issue.  With your multimeter, I would unplug both amp modules from the supply, and test the pins inside the connectors.  Referring to the schematic, you should be getting readings around +35, +25, and -35 for both. 

Does the Rhodes make sound on channel 2, no matter which amp module is plugged into it?  That will help you rule out a bad amp module - that you can swap either out on a good channel.
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