Ampex 351 retrofit

[Brian Roth]

This is a branch from here:

https://groupdiy.com/threads/studios-with-ampex-350-351-–-u-s-europe.86819/post-1137660

@marklacas

Interesting work in progress there! Hope you catch this message. It's been eons since I even looked into a 351 electronics chassis.

I looked into my files and found a "rusty/crusty" page scan from the Ampex 351 manual and a very clear schematic which I've attached.

I do find it interesting that your PCB for what was once the 351 reproduce amp board is Labelled as "Meter and Tape Saturation". Changing that to a solid state design seems like a logical change.

The original Ampex record/bias oscillator PCB is now called "Ampex 351 Preamp" on your new board silkscreen. Makes sense in year 2024!

Glad you came up with a modern solution for those horrible "slide on clips" that Ampex used for the connections to the OEM PC boards!

Bri

 

[Brian Roth]

@ruffrecords As I mentioned earlier it's been MANY years since I had to snoop around the guts of a 351.

I went back through the schematic I posted above and was scratching my head about the component numbering. It slowly came back to me in my dotage.

The prefix for a parts designator indicates where that component is physically located. IE:

1R6, 1R7 1R6 etc. were on the input/record/bias PCB.

2R25, 2R26, 2R27 etc. were on the repro PCB.

3R55, 3R56, 3R57 etc. were on the power supply PCB.

4R9, 4R36, 4S3 etc. were on the front panel.

Designators like 5J1, 5J3, etc ere on the rear chassis panel.

And 6T1, 6T4 etc. on the side panel.

In the schematic, the R1/R2 and C1/C2 etc numbers follow a logical pattern if you ignore the prefix. A crazy numbering scheme until I realized it was a VERY early jump for Ampex as they moved from point-point to PC boards. And the schemo is no help when seeking which connection points hooked to WHAT ins/outs of the PC boards. And Ampex used a GOOFY choice for wire connects onto the boards. Bless their hearts! And these dinosaurs still pass signal with some 2024 TLC.

I also recall what I ran across as a typo many years ago. Almost certain that the 12AU7 marked as 1V5a and 1V5b (line output driver) should be 2V5a and 2V5b because of their (probable?) physical location on the repro PCB.

Damn.....too many brain cells burning here....LOL!!!!!

Bri