Ampex 351-2 restoration project

[Deleted User]

I thought I would post some updates on this restoration's progress. The two 351 electronic chassis are just about finished and have been tested for playback performance. I've decided I want to have the ability to switch between NAB and IEC playback equalizations from the front panel. I still want to maintain the overall integrity of the original Ampex design but make it more flexible for me. A great deal of my playback use will be using NAB EQ, but there are some prerecorded tape companies (The Tape Project and Chad Kassem's Acoustic Sounds) that are recorded using the European IEC EQ.
The 351's playback EQ design uses the negative feedback between the two gain stages to shape the frequency response. So, I'll have to create two different feedback circuits each one using different component values to tune the circuit. I wanted to use Electro-Switch rotary switches since they would have been the same vintage and ultra-high reliability contacts as the original design. However all I could source was a three pole variety and I only needed a two pole switch. So I had to remove one of the pole sections. You can see the results.

[Deleted User]

These photos show the interior and rear panel of the two 351 electronic chassis. The original chassis were fairly heavily marred and pitted. I had them sandblasted and powder coated with replacement lettering etched onto the rear panel. I tried to find NOS replacement input and output connectors, fuse holders chassis screws and whatever I couldn't replace I would clean and polish. All of the internal components were removed and either upgraded or cleaned up as best as possible. The connecting wires use Fastons for circuit board connectors. I found that using a small wire brush accessory on my Dremel allowed very fine cleaning of the center of each connector for improved contact. I was able to clean the wiring harness in an ultrasound cleaner and get all the age old grime trapped in the wiring bundles. Lot's of work.

dromunds

Outstanding!

[Deleted User]

Thank you!

[Deleted User]

I've also been able to make some improvements to the 351's transport, one cosmetic and the other improving the way the transport handles the tape. For the first I was able to find a new stainless steel faceplate on eBay. After getting it and a couple of polishes with Flitz and some auto wax treatment I was able to exchange the old faceplate for the new one. This involves removing everything from the main front of the machine, reels and their motors, head assembly, control buttons including on/off switch and speed selector and the reel size toggle switch. Then, of course, replacing everything and getting all the adjustments back to their proper setting.

[Deleted User]

The second improvement involved adding a Tentrol 400 tension control system to the transport. A fellow member on the Ampex user group posted that he had a 400 available for sale, but that the unit might not work. The 400 was introduced by the Inovonics Company in the middle '70's allowing owners of the Ampex decks to add a servo controlled, fully adjustable tape tensioning control either to the supply or take up reels (or both). Previously the tape's tension is adjusted by placing the two reels in equal quantities by fast forwarding and rewinding until there's the same amount of tape on both reels. Then large variable resistors are adjusted until a certain amount of torque is measured, thereby setting the amount of tension. The problem is that when there's a full platter of tape on one or the other reels (beginning or end of tape) the tension is going to be different.
What the Tentrol does is use an optical tachometer (you can see it on the right hand motor) placed on either supply or take up reel flywheel which couples to a feedback circuit that measures the rotational speed of the motor and either increases or decreases the voltage going to that motor. So no matter where in the reel's playing the tape tension is kept perfectly controlled. However, I first had to make a control cable that would go from the Ampex control box (seen in the middle of the rear of the chassis) and attach to the 400's circuit. That proved easier when another Ampex member sold me his old control box with a 400 cable already attached. After this I mounted the tach and started to troubleshoot the circuit. I ended up replacing all of the older capacitors and removed and tested every single component until I found the open resistor. All back together and now the tape tension stays exactly the same throughout the tape play (you use a Tentelometer to do this, interesting device).

audioluvr

Nice work! My uncle was a DJ at some radio station in Kenosha, WI back in the 60's and 70's and would use his reel to reel for all his music. He had a wall full of them tapes and when he had parties there was always endless music.

mhardy6647

makin' good use of that new camera, @KennethSwauger, I see!
Well done (on all accounts)!

pitdogg2

Simply outstanding to watch and read the progress.

[Deleted User]

I received the necessary .0015uF film capacitor today so I could finish the IEC EQ adjustment on the rotary switch. This capacitor is in series with a 22k ohm carbon film resistor to provide the playback EQ. I then was able to measure the frequency response for both the NAB and IEC selections and compare them to the published specifications found in the 351 service manual.

[Deleted User]

The Ampex playback EQ has a series variable potentiometer that allows for adjusting the high frequency response using a calibrated tape. However, I wanted to have the ability to switch between NAB and IEC EQ settings and didn't want to include a potentiometer. So, after I'd built the selector with correct EQ components I carefully adjusted the frequency response using an NAB tape then measured what the potentiometer was reading at this setting. Then I did the same thing using an IEC calibration tape, going back and forth making an adjustment then playing the tape and seeing what had changed. The tapes I was using had over two dozen frequencies to it so I was able to fine tune the response. After doing this and again measuring what the potentiometer was adding to the resistive components on the selectors I was able to replace the complete resistor with a new value resistor that added together the original value and the needed measured value. This not only gave me accurate switchable selecting but eliminated a signal path component.

[Deleted User]

pitdogg2

very nice skills there Ken.

[Deleted User]

Companies like Ampex and Revox/Studer produced such thorough service manuals and service bulletins a person can fairly easily follow along with the original designs and figure out how things were done. Very similar to what has been done with Polk's classic speakers and this forum.

[Deleted User]

After figuring out a buzzing sound which turned out to be XLR cable problem I'm able to give a first listen to the deck. I've placed it on my test cart and am playing an UltraAnalogue tape of violin and piano.

dromunds

Its been fascinating to me following this. Thanks again.

dromunds

Nice vinyl collection also by the way.

[Deleted User]

I've ordered 12AX7, 12AU7 and 6X4W tubes from Brent Jessee and replacement output transformers from Sowter Company in the UK. I'm using Telefunken for the dual triodes and a JAN for the rectifier. I've asked Sowter about using silver clad wire for transformers, but not sure if that's even possible. I've made up a pair of cables with 1/4" phone plugs on one end and female RCA on the other to be able to compare the two different outputs (one is called "Monitor" and the other is the standard XLR balanced). These are fed from different secondary windings of the output transformers.

[Deleted User]

Thank you "dromunds"! I appreciate everyone's comments.

[Deleted User]

After cleaning the tube pins with Citranox cleaner I like to verify their performance on my Navy issue TV-10.

pitdogg2

Nice may look into that cleaner for my pins

[Deleted User]

The metal that is used for the pins is called Kovar and it is used because it has a similar expansion and contraction characteristic as the borosilicate glass (similar to Pyrex glass) which the bottle is made from. This is important to prevent heating and cooling differences causing small vacuum leaks to form around the areas where the pins pass through the glass. If needed Korvar can be cleaned with a solution of Citranox.

F1nut

KennethSwauger wrote: »

Companies like Ampex and Revox/Studer produced such thorough service manuals and service bulletins a person can fairly easily follow along with the original designs and figure out how things were done. Very similar to what has been done with Polk's classic speakers and this forum.

I don't know Ken, what you're doing is much further up the mod ladder.

westmassguy

Incredible! Wonderful work Ken.

skrol

Interesting about the pins and the glass. As glass is really a liquid in an extremely viscus state, I wonder if old tubes have issues with the glass relieving over time. I saw such issues when I worked at NASA's parts analysis lab with Unitrode glass diodes that did not have a hollow cavity and no S spring to the die. The leads were connected to the die with slugs. The glass would relieve and cause the slugs crack away from the die. There was also intermetalic growth between the die and the slugs that caused a resistance. The diode measured as a diode with a resistor in series.



Work that magic Ken.

BlueBirdMusic

Ken, you take great pictures too.

[Deleted User]

Thank you! I enjoy sharing them with a like-minded group of audio and music enthusiasts.

Hermitism

I think it's very cool that KS participates on the forum after retiring as moderator. Class act for sure!

[Deleted User]

I've replaced the assorted tubes in one of the electronics sections and am looking at the harmonic distortion with an h/p 334A distortion meter and an ancient h/p oscilloscope.

[Deleted User]

I've received the two output transformers from the Sowter Transformer company.