TFM-35 Input Board Oddities

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TFM-35 Input Board Oddities

Post by wrf » Tue Dec 28, 2010 10:08 am

I am deep into an attempt to fix a TFM-35 and have encountered some oddities on the input board. My testing lead me to suspect problems on the input board, so I pulled it out and started testing its components. Getting an accurate schematic of the TFM-35 input board seems to be impossible, so I was using the TFM-35x and TFM-25 schematic. Judging from the board components, the TFM-35 input board closely resembles the TFM-25 input board. Of course the TFM-35 has an additional connection for the input level controls.

Now the weird stuff. Neither R163 or R165 are installed on my input board. There is a place for them, but no resistors. Judging from the appearance of the board, they have NEVER been installed. Looking at the schematic, you can see that they provide a feedback path from a couple of op amps. The missing resistors mean that this feedback signal is NEVER used. Very odd, since the op amps are installed on the board. There are also a few resistors that are significantly different from the schematic. Specifically, R121, R123, R115 and their counterparts in the right channel. R121 and R123 show as 8.2K but measure at 6.4K. R115 shows as 10K but measures at 909 ohms. Same measurements in the right channel. Far too symmetric to be a component failure, this board was assembled this way on purpose. Even more interesting, the schematic for the TFM-35x Input Board omits the feedback op amp components. That schematic shows R121 and R123 at 8.2K (same as the TFM-25) but R115 shows as 3.6K. Still not the measured (and color coded) 909 ohms on my board.

I'm puzzled as to the purpose of these "changes". They are not the source of my problems (high DC component in the left channel forcing the amp into protection mode). Anybody have any idea?
TFM-25 Input Board.jpg
TFM-25 Input Board.jpg (40.64 KiB) Viewed 879 times

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Re: TFM-35 Input Board Oddities

Post by F1nut » Tue Dec 28, 2010 7:51 pm

Carver always liked to tweak things, still does for that matter. Go with what you have on the boards, not the schematic.
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