Ampex PR-10 (354) not working reliably; I'm going in

[rmburrow]

So yesterday was the day. It takes me a bit, especially when there's all of this [gestures vaguely and erratically all around with both hands] BS going on, to get to things sometimes; I have to get mad about it almost.

Put the MF on the bench, IEC cord into the lightbulb current limiter, phone plugged into the inputs as a signal source, and fired her up.... what, she works fine (gains of the 2 channels don't track very well but sounds great). Huh.

View attachment 132400

Notice the Output Selector switches though. Apparently, these sliders have gotten dirty (open frame on the rear) and paradoxically at the same time, loose, so that the upper position (the position I need) is succumbing to gravity and just disconnects, randomly either channel while looking like it is still set to the correct option.

So that's good, but too easy; I know this beast and it plays games, only working when it wants to. I know it's playing nice because it got powered up for the first time in years and is worried it would be tossed in the bin if it didn't pull it's socks up. In any case the channel gains don't match with the gain knobs in identical positions so I have more to do here anyways.

I knew where I had to start: ripping out all the unnecessary tape playback electronics to make space and be able to see what's up in there. This was never gonna go back into service as a recorder (besides the PR-10 tape transports sucked balls apparently - were also built to be "portable").

So here's comparison of my PR-10 to a 354:
View attachment 132401

I had already removed the rec and playback EQ modules when the above pic was taken, or they would be blocking the entire view and prevent me from getting a finger or tool in there, vs the 354 where you could actually change out the tubes without removing those EQ modules. Note also that relay/solenoid on the side with the XLR jacks on my PR-10 vs the octal plug in input accessory sockets on a bracket (with black dummy plugs fitted) on the 354 - all other PR-10-2s I've seen have those sockets.

...

I feel for you, I owned a PR-10-2 a long time ago and got rid of it. Head alignment never stayed put. The electronics was so cramped accessing the components was a real pain, and heat from the tubes cooked the parts. I acquired a 354 and that was much easier to work on. I still have the machine, and it runs well. My PR-10-2 and the 354 both used the Ampex input transformers. I recall the Ampex plug in mic preamps; there was a 40 db version with a 6CW4 nuvistor, and a 60 db version using two 6CW4 nuvistors, both very rare. I have not heard any recording made using the Ampex plug in mic preamps. All of my work has been with outboard mic preamps or console output delivering standard 600 ohm line level to the recorder. The 6CW4 nuvistor was used as a RF amplifier front end in RCA television sets and these were noisy at audio. I like the 7586 nuvistor for mic preamps, is a good condenser mic tube, very high input impedance, and was quiet. You are probably aware the condenser mic tube discussion is never ending...

 

[GrannyGremlin]

I feel for you, I owned a PR-10-2 a long time ago and got rid of it. Head alignment never stayed put. The electronics was so cramped accessing the components was a real pain, and heat from the tubes cooked the parts. I acquired a 354 and that was much easier to work on. I still have the machine, and it runs well. My PR-10-2 and the 354 both used the Ampex input transformers. I recall the Ampex plug in mic preamps; there was a 40 db version with a 6CW4 nuvistor, and a 60 db version using two 6CW4 nuvistors, both very rare. I have not heard any recording made using the Ampex plug in mic preamps. All of my work has been with outboard mic preamps or console output delivering standard 600 ohm line level to the recorder. The 6CW4 nuvistor was used as a RF amplifier front end in RCA television sets and these were noisy at audio. I like the 7586 nuvistor for mic preamps, is a good condenser mic tube, very high input impedance, and was quiet. You are probably aware the condenser mic tube discussion is never ending...

I never even bothered trying to use these as tape machines. I had heard all about how horrible the transports were. Problem is I love the amps. The second unit (with the accessory sockets) I bought later and I never had the transport for that one.

As for the input accessories, they all use the 7586 according to the schems but you do see other nuvistors in there sometimes; mine have 6DS4s in them now - they were somewhat interchangeable. There were actually 4 models:

96440-01 - 40db premap
96440-02 - 60db premap
and their successor models:
96440-03 - 45db preamp (what I have)
96440-04 - 45/60db switchable preamp

The 01 and 02 use more cathode degenerative-feedback than the 03 and 04; fidelity/gain trade off.

After that they changed the pinout when they switched to solid state (part# a few digits longer and starts with a 4, also the Beyer input transformers lost the metal can - white plastic; looked like Mallory caps). That was around the time of the AG series. Same with the plug in line input transformers (diff pinout).

editted to add:

So any of the 96440-xx accessories will work for the PR-10, 354, and MX-10 mixer (Not to be confused with the similar but solid state AM-10 mixer, which requires the solid state accessories).

 

[Disco Volante]

Great work! Those ancient paper-wrapped caps would all be way past retirement by now...
Cheers, V!

 

[GrannyGremlin]

Those filament supply filter caps have been changed out. Made sure they're not touching the PT case - I will also be zip tying them down once I find my adhesive anchors. Cleaned the PT case before installing the new caps so they will stick.



Then all the same deletions/replacements as the first unit, as well as the filter caps on the input accessory sockets.

Before:



After:



In my experiments with replacing the grid input resistor on the other unit I noticed it was acting like a pad. On the other unit (no input accessories) I need all that gain so left them permanently jumpered. On this unit I am not so pressed for gain so I replaced them with modern (and more closely matching) resistors and repurposed the output select switches (located conveniently close by; cleaned them out real good first) to bypass them so as to have the option.