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.

 

[GrannyGremlin]

Put the gain pot grounds back (lead to pin on board as stock) and they work now but this unit has very little gain - 20db down from the other one and we can't have that. Checked plate voltages (all easily over what the schem says) and tried changing out the tubes. I even put cathode bypass caps on the 6AW8 triodes like the other unit has (helped but still not all the way there yet). I swear it wasn't this quiet before but can't figure out what's up. I went through and can't find anything amiss. Next step is a part by part comparison of the 2 units and tracing the signal path with an audio probe.

 

[GrannyGremlin]

Audio probing was underwhelming - I have signal everywhere, it doesn't do anything particularly weird other than not get very much louder - but in the process I noticed that the output pin for one of the 6AW8s was loose, so reflowed that and things got better... started to test poke all the other pins (fine) and noticed that when the board moves (it's on a bit of a suspension) the signal drops a few db and then if you jiggle it again it comes back. Also now I can't turn down the signal to nothing; coulda swore that was working yesterday after I put back the grounds on the gain pots. Turned off and flipped but couldn't see any more bad solder joints or cracked traces

Went through and finished the cleaning of all the pins and slip on connectors, and plugged in to retest and the current limiter bulb comes on. Shit. readjusted things (back down and front panel unmounted) to be sure that nothing is touching something it shouldn't and tried again just to see the magic smoke. Looks like it came from the biggest fatty of a resistor in there (1R27) but it could have also been the tiny one next to it (1R51) or the multi-section cap can (1C16).

That's all power supply (though I am having a hard time finding R27 on the schem). Anyway, looks like I'm going shopping again after I figure out what all the above (and a few other things around them) are and if I need all the cap sections etc. Really glad for the large printouts now.

 

[GrannyGremlin]

OK so

Deletions:
1R27 (and 28) - 2W screen resistors on 6AW8A pentode
1C23 (a, b, c and d) - filter caps feeding B+ to the 12AX7s in the reproduce amp
1C16a - filter cap feeding the record oscillator ( 12BH7 via it's OT, T4, used as a choke)

Replace:
1R51 - V drop for B+ before it is split to both 6AW8 triodes, V1 and V2 (schem says 4.7k; measured and confirmed colour code to be 470R - considering the next item, I am going with the latter)
1R9 (and 10) - further V drop for B+ (immediately after R51) splitting to V1 and V2 (again, schem says 68k; colour code and measurement says 47k; sticking with that)
1C16c (450V) and b (250v) - first B+ filter; the the lower V section is part of the rectifier
1C35 - the other cap in the rectifier
1C31 and 3C30 - output caps off the 6DJ8 cathode follower feeding the OT (diff prefix is not a typo - 1 is on the PCB and the other on the front panel; 1 radial and the other axial; the layout of this thing is bloody insane. See also R27/28 - 28 is right near the relevant tubes, 27 is all the bloody way on the other side of the chassis by the power supply and is connected to the tube socket via a long offboard wire).

So as for the current limiter and the smoke - apparently when I was cleaning the connectors earlier, I accidentally put one of the gain pot grounds onto the wrong pin when I put it back. Corrected that and it powers on without the current limiter lighting up, but the volume still can't go down to silence despite working - gets louder vs before without the ground did nothing. That's really stumping me, because there are no other components between the input (via the dummy plugs in the accessory sockets, which I have checked) and V1/V2. I guess we'll see after I do all the above deletions/replacements unless someone has a bright idea that just eludes me right now.

 

[GrannyGremlin]

OMG sorted! It was a compound problem: the original Cannon (pre XLR but compatible) input jacks were dirty as snot and had an intermittent short between hot and ground - that's why moving/jiggling things made it come back momentarily (cleaned and redid the solder connections). Then the RCA to XLR adapters I was using for source input from my fone (pin 2 tied to 3 from factory - opened up the XLR connectors and snipped that). That took me way too long to diagnose and I ain't proud. Figured that out after I decided to, because those adapters came to me in a bag of random old cables, bypass that variable and directly inject the signal with alligator clips off the RCA jack tips to the hot lead on the back of the Cannons and gained over 20 db.

The other unit has TRS IO added so I wasn't using these adapters to plug my fone in to it.

Anyway, now we're boomin. Gain knobs working smooth and give me silence if I want it.

Turns out the pad I put in isn't worth it (barely noticeable) so I will remove it - 10k to 0R is a negligible difference when the original value was 1M/47k (as found in my units/as per the schem). It was worth a try; really just wanted to reuse those switches for something and fill a few holes on the front panel.

Will still do the aforementioned replacements/deletions (some of those caps are the original cardboard ones still).

 

[GrannyGremlin]

Been a bit too busy for a few weeks but got back at it today.

Finished changing out all the electros and deleted a bunch of resistors and a whole multi-section can while I was at it. This also enabled me to delete some of the offboard wiring on the underside of the PCB.

I brain farted and bought electros for the 4uF output caps (the originals were electros) instead of upgrading to films (I checked - they were available and there was an option that was not more expensive than the EPCOS electros I got), but that may be for the best because the one mounted to the faceplace needed the long leads to reach the terminal strips, and the one on the board has really tight hole spacing so they wouldn't have fit. Due to the tightness of components on the faceplate, it was a right pain to get the soldering iron in to first remove and then attach the negative lead of that output cap.



Sounding great and the channels are still tracking perfect despite totally unmatched tubes. Now I can get to troubleshooting the preamp modules without worry that I'ma blow more nuvistors. ... and then finish this same round of deletions and replacements on the other unit. ... and then I am thinking I will build one from scratch, cuz I've put in the time and understand the circuit now.



Oh, the reason I have the channels maxed out here is because the channel B meter gets stuck on lower gain settings. I will be low key looking for a replacement but it's not urgent (which is good because it may be next to impossible to find one). I don't suppose anyone knows if a VU meter can be cleaned or refurbbed somehow?

 

[GrannyGremlin]

So I plugged in a mic and the 'good' nuvistor module to confirm that still works (it does). Then I removed the nuvistor form that working module and plugged it in to the 'bad' module and that also worked. Great - so my only problem now is the tube for the second module.

While I was at it I plugged in the 2 other nuvistors I had on hand but as I suspected those were the ones that had blown previously - off to order me some more nuvistors.

 

[rmburrow]

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).

This clarifies a lot. The PR-10 transport was junk. Ampex built good electronics for their equipment during the 1960's...

 

[GrannyGremlin]

Success! The 2nd unit with the nuvistor input modules is all done and working perfectly (aside from the CH 2 VU meter as mentioned previously).

The problem with the non-functioning input module was just the tube in the end. Now with the cap job, putting back the V5 12AX7 to complete the DC fillament supply centre ground and the part deletions (better airflow - not as hot in there) I am confident they won't continue to blow, but just in case I ordered 3 spare pair of 7586es (the original tube in these modules) because they were stupid cheap ($9ea) in addition to a single 6DS4 (more expensive) to match the one in the other module that works. I was a bit worried when it came in the mail (the 6DS4) because the metal can was a bit rusty, but plugged it in and fine.



Now I just gotta finish the deletions/replacements on the first unit, but I did notice as I was switching them out on the bench that the unfinished unit does not make the current limiter bulb glow quite as much (momentary on power on with both units; not nearly danger level bright, but enough to see that it is lit) as the fully recapped unit does. I am taking this as a sign that the cap job is well warranted, and will compare again when I am finished with the other unit in case that is really something else that I should probably worry about.

 

[GrannyGremlin]

All done and back inna rack.




And just to see how far this has come:


It's kinda hard to stop working on these - so much potential for things to do. Like I considered adding output attenuators (I love to overdrive them, but that's too hot for the converters). I have a bank of 8 passive attenuators on the patchbay - 6 are just high quality 10k dual gang pots, Sifam I think, but 2 are stepped attenuators which may be wired as variable H pads for exactly that purpose (you can see them in the pic above - the knobs below the TRS patchbay, and on the patchbay labels). It's probably best to get the Ampexes back in service ASAP (I need to finish an album). ... Also I don't know what would be best for output trim - I'm assuming a 600 Ohm variable HPad like on a Gates STA-level would be safest but I know I've used my 10k stepped attenuators before with no issues and the outputs aren't quite as finicky about load as similar era gear due to the cathode follower capacative output stage.

... So I may come back to that after some thought or advice, because there are those convenient pot shaft sized holes on the right side where they could go after all (and then less patching and more attenuators for other mic preamps to use).

I am also thinking i wanna try building one from scratch, but so far just thinking about transformer selection (would want to use something off the shelf; repeatable) is giving me a headache. I would also hope to figure out a nice way to utilise the otherwise unused 6AW8 pentode half to replace the nuvistor modules because there's no way I am recreating that, and the amps don't have quite enough gain for all situations without them.

-fin-

(or is it?)