Ampex 351-2 restoration project

pitdogg2

vmaxer wrote: »

KennethSwauger wrote: »

Thanks, Mark, I'll post some more when I get the roll developed (just kidding).

At least use this:



https://m.youtube.com/watch?v=qrRRhoS3KFk

You need a better camera or that film sucks for pictures. lol

vmaxer

Better than what??

pitdogg2

vmaxer wrote: »

Better than what??

On my phone it just looks like a double exposed blured picture. I was teasing Trey
I didn't realize it was a video as I'm no Paul Simon fan

mhardy6647

vmaxer wrote: »

Analog/digital

Film/digital

Could be another "cable discussion"

Nothin' to discuss, from my perspective.
Film (ahem) trumps digital, any time.



Sort of like high-performance analog tape equipment vs. digital audio.

(see how good I am at keeping this thread on topic for Ken -- what a brown-nose I am!)

heiney9

Ken,

Spectacular work and restoration and documentation.

My hats off to you for being so meticulous. It takes the right kind of person to be so patient and thorough.

Thanks for sharing and keep the updates coming.

H9

[Deleted User]

Thanks, H9!

[Deleted User]

Here are some photos of the complete chassis. The originals were in pretty bad shape the coating that Ampex had used had become scratched and heavily stained. My original plan was to remove the coating and lettering on just the rear panel and clean it up with Flitz and a re-spray with a tinted aerosol lacquer. I found two sets of dry transfer lettering that was the exact copy of the original. Then as I took more and more of the internal components out of the chassis I became dissatisfied with that plan. So, I brought everything to the shop that has done all of my powder coating and they suggested another approach.
They offered to photograph the lettering on the back panel and then use a computer controlled router to etch through the powder coating and create the original lettering. It really turned out great and now I can see how it looked when it was built.

Northamusi

Hello Ken:
Beautiful work on the Ampex tape machine! I'm about to embark on rebuilding one channel of Ampex 351 electronics and would like your opinion: given the opportunity to do it again, would you still rebuild the original power supply card, record and playback cards, or would you go with the GAS Recording Equipment replacement PCBs?
thanks and best regards
Steve La Cerra

[Deleted User]

Hello Steve,
Welcome to Polk's forum! Thank you for your comments. I also have the GAS replacement boards and wouldn't hesitate to recommend them to anyone doing a rebuild. The original boards use Faston type connectors which, as I'm sure you know, slide onto the edges of the boards. The replacement GAS boards use the more secure bare wire screw type connectors. For me, I wanted to understand a bit more about the differences between the 351 controller and the 351 slave versions before I committed to the replacement boards.
I've been trying to find NOS replacement for the selenium rectifier and the back panel trim pot, but haven't had much success. I get fixated with getting things "just right" and anything can derail my progress.
It's great to hear from someone else interested in the Ampex machines!
Best regards, Ken

Northamusi

Hi Ken:
From what I understand the selenium rectifiers are tough to find but hopefully not impossible(!). I'm leaning toward replacing the power supply PCB with the GAS PCB, and then rebuilding the original record and playback PCBs similar to the manner in which you had shown in the photos. What do you think of that idea?
best regards
Steve

txcoastal1

Interesting thread, please post pics if possible??
I love oldskool

[Deleted User]

Hi Steve,
I think that would be a great idea, it makes power supply filter cap placement easier since GAS has designed them with what's available today and includes a bill of materials list with part numbers. Couldn't be easier.
Is this amp being used as part of a tape system, or recording studio? That's where a great deal of the 350/351 electronics end up, being used as microphone preamps in recording studios.
Cheers, Ken

Northamusi

Hi Ken:
Thank you for the information. I plan to use the amp in the studio as a microphone preamp. Before everyone yells at me for hacking up a tape machine -- this was given to me by a friend/mentor. He did not have the transport, just a channel or two of electronics.

This is how it looks after I cleaned up the front panel a bit:




Since txcoastal1 has asked I'm posting some photos and some more information.

The first order of business was to see if the unit powered up but I did not have the AC power cable. I rigged an AC cable directly to the rear panel connector. The connector is show here:



The unit did power up(!) so I brought it into the studio to see if it passed audio (it did) and if it sounded any good (ditto, even without any work). Knowing that it was alive, I was ready to jump in.

It was easy to track down the correct mating AC plug, a Hubbell 7464 about $15:



My idea for this project is to restore the audio path as close to original as possible but if needed, I have no issues with modifying the power supply. When I started searching for modifications to Ampex 350/351 I came across Ken's beautiful work. It was easy to find schematics (though there are several versions so make sure you have the correct one. This is a 30960-01).

Along the way I found a paper regarding selenium rectifiers (the 351 uses one in the power supply). Very informative. Search "Replacing Selenium Rectifiers." I learned that selenium rectifiers don't age well and can fail. When they fail they can take out other components as well. I'd like to avoid this. Here is the original selenium rectifier, mounted to the inside of the chassis. The mounting bolt has been removed.




Here is another photo showing the rectifier moved away from the chassis:



These rectifiers are easily replaced with modern diodes, in this case two 1N4004 diodes wired "common cathode." I had some of these in a spare parts drawer. They sell for pennies. Here is the replacement rectifier, mounted on a terminal strip:



The next step will probably be to rebuild the remainder of the power supply. Rather than replacing the components on the original power supply PCB, I will probably purchase a replacement PCB from GAS.

[Deleted User]

Very nice work, Steve. The selenium rectifier is being used to provide the filament voltage (12.6VDC) for tubes V1, V2 and V3 the other tubes are AC powered for their filaments. As I'm sure you've read you'll need to change resistor 3R-60 since the diodes will have a lower loss than the original rectifier. Tubes can tolerate a lower filament voltage, but not a higher one and without increasing the value of 3R-60 it will be too high.

Northamusi

Hi Ken, thanks. That's exactly what happened. I was anticipating it, so I was ready with a meter when I powered it. If I recall correctly, the filament voltage went up to 13.2. I think my next step is to get the GAS power supply PCB and when I order the components I will order a few variations of the value for 3R-60 so I can 'tune' the filament voltage closer to 12.6 VDC.

[Deleted User]

Hi Steve,
One approach would be to measure the value of your particular 3R-60 it should be around 1.5 ohms. Then with the unit powered measure the voltage drop across this resistor. Take that measurement and divide it by the resistance you measured, this will give the current flow through the resistor. The goal is to increase the voltage drop by 0.6 volts. If you take the previously measured voltage drop and add 0.6 volts and divide this number by the current flow you should get the necessary resistance to reduce the filament voltage.
I hope this is helpful information.
Cheers, Ken

Northamusi

Hi Ken.
My resistor is indeed 1.5 ohms. I'll try to describe my process accurately...
I measure voltage between anode of the rectifier and one side of the resistor. I get reading of 14.75 V. I measure the voltage between the anode and the other side of the resistor and get a reading of 14.05. If I understand correctly, my voltage drop is 0.7V.
If my calculations are correct, the current flow is 0.46A, and I believe that I need a 4.6 ohm resistor to drop the voltage to the required 12.6 VDC (or as close to that value as possible).
Does that make sense to you?
Please let me know what you think. I appreciate your help!
Steve

[Deleted User]

Hi Steve,
The three tube filaments (2 12AX7 and 1 12AT7) have a filament current draw of 0.15 amperes each, so the 0.46 amperes seems correct. The target voltage drop should be 1.3 volts (0.7 original plus 0.6 desired) and 1.3 divided by 0.46 is 2.82 ohms. Multiply the 1.3 volts by 0.46 ampere and the power dissipation will be 0.6 watts, so a 3 watt resistor should be fine. Here is a link to a 2.7 ohm 3 watt Panasonic resistor that is in stock:
https://www.digikey.com/product-detail/en/panasonic-electronic-components/ERX-3SJ2R7A/P2.7W-3TR-ND/77762
I've used this type of resistor for lots of projects with fine results, no value change with heat and long lasting.
Cheers, Ken

Northamusi

Hi Ken:
Looks like I've run into a bit of a snag.., Recording With Gas seems to be MIA. Te web site is down and I didn't get any response from emails or a message I sent via Gearslutz. Do you know if anyone else is making replacement PCBs for the 350?
thanks, best regards
Steve

[Deleted User]

Hi Steve,
That was the only source I am aware of, sorry. Maybe they just have to restock and they'll be back? Wishful thinking is my default setting.
Regards, Ken

Northamusi

That's what I am hoping, otherwise I'll do a rebuild of the stock power supply PCB. I was hoping not to have to deal with the multi-section capacitors(!).
best regards
Steve

Northamusi

Hi Ken:
I hope you are well.
Looks like GAS is up and running again, and I just ordered the replacement PCB for the power supply. I plan to start rounding up the components and do the rebuild. Looking forward to it (wish I had more time off...) and will take some photos along the way.
best regards
Steve

txcoastal1

We're looking forward to the pics

[Deleted User]

That's great Steve, good luck with the project.

Northamusi

Here are photos of the power supply PCB purchased from Recording With GAS
Component side: the upper left shows the footprint for the four discrete capacitors that replace the old-style multi-section capacitor (C16a/b/c/d)



Solder Side:

Northamusi

Hi Guys: Finally have some spare time to start working on the power supply board. I'm starting with the resistors since they have the lowest profile. Here is the first resistor ready to be soldered...



This next photo shows the backside of the PCB. I have bent the leads so that the resistor doesn't fall out when I turn the PCB upside down to solder it:





And the leads after having been soldered:




Here is the component side showing most of the resistors soldered in place. The solder has flowed through the circuit board holes, which is why you see solder filling the holes on the component side



This next photo is a zoom out from the one above, showing the terminal connectors that will accept the existing wiring:

Northamusi

Hi Ken:
Happy New Year.
I thought you might find this interesting... I removed 3R-60 from the power supply board. The color code indicates 1.5 Ohms (brown, green, gold, silver) but it actually measures 2.54 Ohms. I replaced it with 3.3 Ohm (3 watt/5%) and the filament voltage dropped to 12.87 VDC. I'm thinking that a 3.6 or 3.9 Ohm resistor should get me to the requisite 12.6 VDC. Any thoughts?

txcoastal1

My question is, how much is the load? With a load will it drop below spec?

Northamusi

That measurement was taken with the power supply in-circuit, full load.

txcoastal1

Cool, just was wondering...keep us posted

TFS