DIY help for the Ampex 600 series

Ampex 601 manual (EF86 schematic)

I often get questions from people who want to work on the 600 series recorders, both the electronics and the transport. I don't believe the transports are worth spending much money on. I used to repair the transports, but now I just advise people working on them as DIY projects.

Which tape to use
My favorite tape for the 600 series is Quantegy / Ampex 632. You can also use Scotch 111, Ampex 631, and others. 632 is not as common as 631, but it is out there at good prices. The 600 series recorders won't erase or bias tapes like 456.

The transport
The 600 and 601 transports are essentially identical. The heads, head shield, mounting plate, and perhaps other parts from the 602 transports are not interchangeable with the 600 and 601.

I strongly advise that only transports in very good condition be restored. If the heads are worn out, the pinch roller has a permanent dent (from being left in "play") or if any of the internal rubber parts or belts are bad; look for a better one. They are often free or very cheap. You should be able to buy several transports for the cost of having a pinch roller rubber or heads replaced. If a transport has been rigged in any way - run away. Unskilled people add shims, bend springs, and all manor of what the manual refers to as "tampering" when trying to get one of these working. The 600 series transports seem to inspire such tampering because their functioning is not very intuitive. "Tampering" will often render a transport unrepairable or needing replacement parts.

The manuals offer detail on assembly and disassembly, as well as calibration instructions. There is one section of the manual I have problems with. The functioning of the Play Clutch is not described well. I believe the instructions for adjusting this very important aspect are flawed. Perhaps a bad communication between the engineers and the technical writers was at fault. In the years when I was repairing these transports, not once did I ever get a 600 series in my shop with a working play clutch. I suspect that most repair techs that worked on these, misunderstood the design and went by the manual. If this is not carefully set, the machine will default into an alternate operating mode which eventually wears out the clutch parts and increases wow and flutter.




(Old Play Clutch video) - This video shows the proper operation of the play clutch, the holdback springs, and the tire brake. Please download the file to play the video.

In the first part of the video, you will see the play clutch which includes the round rubber belt, the alloy pulley, the plastic disc, and the felt covered alloy plate. As you can see, the plastic disc is supposed to slip against the felt. It is very common for the drive belt to slip instead of the felt. If you follow the instructions in the manual for setting this you will almost certainly end up with this alternate operating mode. As you can see in the video, this unit operated like that long enough to wear the belt flat. I have seen units where the plastic clutch was worn completely through (cut into two pieces.) As the camera pans down you can see where the belt is running on the plastic disc. You can also see a darkened band to the right of where the belt is now. That is where it was when I received the unit. It is where the manual would have you set it with the feeler gauge technique. This is where I believe the manual is wrong, or at least confusing.

The belt must not slip against the plastic disc. In the alternate operating mode, the unit will function but the wow and flutter goes up. I find myself wondering whether they came from the factory that way. They can run for years in this alternate mode before the clutch parts wear out, but the clutch is not controlling the pull tension. The slipping belt is! This makes the wow vary unpredictably.

As the camera pulls back and moves to the other side, you will see the wheel, tire, alloy plate w/felt, and brake of the holdback tension system. Once again, it is supposed to slip on the felt, but often does not. As the rubber of the tire ages and hardens, the tire will start slipping on the brake instead. Often they will both be slipping. This "stick-slip" action causes severe variation in the holdback tension. This same system handles the FF and RW clutch action. With a heavy reel of tape on the machine, the wheel will slip against the felt as it comes up to speed. The little question mark shaped springs with felt on them provide light holdback braking for the FF and RW functions. They only contribute a small amount to the holdback tension. People often try to make them do things for which they were not designed.

Sanding the rubber:
The right way to repair the aged rubber parts is to replace them with new ones or have them "recapped" (new rubber applied.) This is usually impossible or WAY too expensive for these machines, so sanding the hard rubber off is the next best option. There is plenty of "take-up" in the design to allow for removing a small amount from the rubber parts. Here's how I do it:
1) Remove the wheels/tires being careful to observe the locations of the fiber washers.
2) Mount a 1/4" drill bit in a variable speed drill.
3) Mount the wheel on the drill bit.
4) Use 320 to 600 grit wet/dry sandpaper and do the sanding wet. You can wet and rinse the paper as you sand.
5) Use a flat sanding block and run the drill at a low to moderate speed. Keep the rubber cool by rinsing as you sand. You will know you are done when you dry the tire and can see an even matte finish all the way around. No shiny spots.
6) The pinch roller can be cleaned in place. Use a flat block with the sandpaper (a paint stirring stick works.) Be careful not to use much water. Just rinse the paper a few times. Put the deck in play and sand the pinch roller. Do NOT sand the capstan. Control any water. Don't let water get into the capstan bearings. The deck needs to be vertical, BTW. Clean the roller and capstan when finished.

The brakes should be cleaned too. They are the little metal wheels with the serrated edges. Clean out the grooves and clean the surface with solvent (like goof-off) and a Q-tip.

Felt: The felt parts can be cleaned with soap and water if they are packed with oily residue. If the felt was severly compressed, I used to pick at it to loosen it some with a stiff wire brush. Replacing the felt isn't recommended. If it is that bad, the rest of the transport is probably worn out too.

I am not going to offer instructions on the more advanced mods I do to these circuits. These instructions offer help with the most important repairs required to get good sound from the 600 series electronics. If your DIY skills are up to things like converting the tube heater power to DC current, then great, go for it.

As a preamp only: There are 3 tubes (not including the 5y3 rectifier) which are used when a 600 or 601 electronics is used as a mic preamp: V101 (5879 or 6267,) V102 (12ay7,) and V106 (12au7.) There are 5 capacitors in the audio path which should be replaced: C102 - .1/400V, C104 - .047/400V, C106 - .1/400V, C122 - .01/400V, and C123 - 4mf/150V. Film caps should be used. The "rec cal" pot should be replaced with a pot which will accept a knob. All the electrolytic filter and cathode caps should be replaced also. It is easier but more expensive to replace the big can caps. Most techs (including me) use individual caps under the chassis. This is not a particularly easy thing to do in a neat way - plan ahead. The cables hanging off can be removed. Ground the grid of the 6F5 to keep it from oscillating. Good luck.

Do not remove tubes unless you are going to adjust the power supply circuits.

For use with a transport: The rest of the audio caps in the record amplifier and the playback amplifier should be replaced. The mica caps (domino) are fine and can be left. The plate resistors should be replaced with low noise metal film. The EQ networks and the whole unit will benefit from 1% metal film R's.

Changing the 5879 tube to 6267: When changing the tube socket wiring to use the 6267, be sure to cut away the ground wire connecting the socket's center pin to pins 2 & 6.

The right input transformer for the 601 is going to have either 17331 or 580022 on it (often w/prefix and/or suffix. I.E. 4580022-1D) Not 580200! Not 580116!

The one on the right is not from Ampex.

This chart shows the typical frequency response curves of a 9er-ized 601. Please notice that I didn't use the common marketing trick of setting the dB scale to make this look flat. The vertical divisions are only 4 dB, so you can actually see the variations. I've pushed the frequency response of these preamps down to about 10 Hz in an attempt to minimize phase distortions in the audible low end. I'm very happy with the bass sound I get using this preamp.