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  • Gates Level Devil Restoration 'Blog'

    I found this gigantic Level-Devil 1-channel compressor/expander from 1959 in my studio bone yard. I thought I'd share my restoration efforts in this thread



    So far:

    - I bought and installed new tubes for it



    The thing has about 12 tubes in it... So even if it doesn't sound great when I'm done, I can use it to heat the studio . I got the tubes from a place in Memphis called TubeDepot.com -- they seemed to have really great prices (like 1/3 what I saw in other sites), sell the same brand of NOS tubes as the others, and they test each one before shipping.



    - Am redrawing the schematic from a scanned .pdf file I found on the web.



    The scanned schematic was pretty raggy resolution - - I had to guess or derive certain values. But I got it about 2/3 done yesterday. The control circuits (to sample the audio and control compression/expansion) are pretty bizarre looking. It may take a bit of time to sort out just how they work.

  • #2
    This looks interesting...it will be interesting to see how you solve some of the inevitable problems that crop up.
    _____________________________________________
    There are now 14 music videos posted on my YouTube channel, including four songs by Mark Longworth. Watch the music video playlist, subscribe, and spread the links! Check back often, because there's more to come...

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    • #3
      I have the schematic redrawing finished, but don't know how to upload images to the forum. I can find no clues on how to do it, which leads me to believe it's pretty simple...



      Anyway, it appears the control circuitry for the expander and limiter were deliberately drawn to confuse anybody who was trying to sort out how it works. The redrawn version is a lot better; though it took four passes through to sort out what was inputs, what was outputs, and how the tubes were all biased. I'm glad that's over! It might seem like overkill to do a complete redrawing, but it's amazing how much a clearly drawn schematic can help when things don't work the first time. Especially when working with lethal voltages...



      I'm going to power it up, after cleaning it up and repairing a few broken wires, and see what happens. I expect I'll need to replace all the electrolytic caps, since they're about 50 years old, but a brief power up shouldn't hurt anything unless those old caps are shorted.



      Ah, I figured it out:





      Comment


      • #4






        Quote Originally Posted by philbo
        View Post

        I have the schematic redrawing finished




        Lovely. Can I get you to draw all of my schematics? You must have used a program. What did you use? I've been looking for a good program for making block or signal flow diagrams for years and I still can't find what I'm dreaming about.



        If it's not too late, folks on the Ampex list, where they deal with 50 year old electronics all the time, recommend the first power-up after a long vacation using a Variac. By raising the voltage slowly, taking half an hour or so to go between 50 and 120 volts, the electrolyte has a better chance of re-forming than just hitting it will full voltage.
        --
        "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
        Drop by http://mikeriversaudio.wordpress.com now and then

        Comment


        • barant123
          barant123 commented
          Editing a comment

          MikeRivers wrote:
           
          Originally Posted by philbo
          I have the schematic redrawing finished
          Lovely. Can I get you to draw all of my schematics? You must have used a program. What did you use? I've been looking for a good program for making block or signal flow diagrams for years and I still can't find what I'm dreaming about.

          If it's not too late, folks on the Ampex list, where they deal with 50 year old electronics all the time, recommend the first power-up after a long vacation using a Variac. By raising the voltage slowly, taking half an hour or so to go between 50 and 120 volts, the electrolyte has a better chance of re-forming than just hitting it will full voltage.


          If you don't have an electrical engineering program, there is a brush set for photoshop that helps make the schematics a lot easier.


      • #5






        Quote Originally Posted by MikeRivers
        View Post

        If it's not too late, folks on the Ampex list, where they deal with 50 year old electronics all the time, recommend the first power-up after a long vacation using a Variac. By raising the voltage slowly, taking half an hour or so to go between 50 and 120 volts, the electrolyte has a better chance of re-forming than just hitting it will full voltage.




        That is a very cool tip. I was about to post something about using an ohmmeter to check for shorts with the caps, but I like the idea of raising the voltage and seeing if it can rejuvenate the electrolytics.
        _____________________________________________
        There are now 14 music videos posted on my YouTube channel, including four songs by Mark Longworth. Watch the music video playlist, subscribe, and spread the links! Check back often, because there's more to come...

        Comment


        • #6
          Beautiful drawings, philbo.

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          • #7
            Thanks for the kudos guys. I used Visio to do the drawing. It's a program that is great at nothing, but can be forced, with effort, to do almost anything. I would certainly consider doing drawing work for anybody who wanted, for a stipend...



            I've heard of the variac trick too; it's a lot kinder and gentler on the aged 'lytic caps. Unfortunately I don't have one. What I probably will do, though, is to put a 25W 110V incandescent bulb in series with the line cord when I power it the first time. That way, if anything goes horribly wrong, I'll just get a bit of light instead of a lot of smoke...

            Comment


            • #8
              I've heard about the light bulb trick, too. If nothing else, it provides a soft start since the resistance of the bulb filament is higher when it's cold than when it gets hot.
              --
              "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
              Drop by http://mikeriversaudio.wordpress.com now and then

              Comment


              • #9






                Quote Originally Posted by philbo
                View Post

                It's a program that is great at nothing, but can be forced, with effort, to do almost anything.




                This sounds like a few of my co-workers.
                Ken Lee on 500px / Ken's Photo Store / Ken Lee Photography Facebook Website / Blueberry Buddha Studios / Ajanta Palace Houseboat - Kashmir / Hotel Green View - Kashmir / Eleven Shadows website / Ken Lee Photography Blog / Akai 12-track tape transfers / MY NEW ALBUM! The Mercury Seven

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                • #10
                  I have been waiting for some stuff from Parts Express before the test powerup.



                  I want to change the line cord socket to a standard 'IEC' power socket, so I don't have to dredge up a power cord from a 1960's TV set. Plus, it would be good for safety to have the chassis grounded. This will detract a little from the authenticity of the restoration but will pay off in lower chances of electrocution.



                  The other part is a fuse holder for the front panel. Somebody stole the cap off the one that's there now, so it's easier just to replace the whole holder.



                  Delays... delays...

                  I guess you can't rush quality.

                  Comment


                  • #11
                    That's a fascinating design. I'd nearly forgotten that there was a time before silicon diodes... ;-)
                    <div class="signaturecontainer">&quot;There is no best in music.&quot;<br />
                    -- Neil Young, 1987<br />
                    <br />
                    <a href="http://lamkins-guitar.com/" target="_blank">My music pages</a></div>

                    Comment


                    • philbo
                      philbo commented
                      Editing a comment

                      I made an insert cable for the Level Devil to test it out.  The input & output on the unit is a terminal strip with 3 flat head screws each (2 balanced lines & ground), so the double-wire end of the insert cable needed to be stripped wires.  

                      I converted it to an unbalanced In & Out by jumpering the low side of each balanced line to ground.  This would be a dicey thing to do if you didn't know the circuit topology, but I know both input and output are transformer isolated, with no ground reference on either (the ground screw simply hooks to the chassis).  So this worked okay.

                      Since this is a dynamics processor, it would make no sense to just feed it a constant amplitude sine wave.  I wanted to see how it would react to different input levels.  So instead of using an audio generator as a signal source, I used a waveform editor to create a test signal.  This consisted of 2 second bursts of 400Hz at various levels from -40 to 0 dBFS, with 5 seconds of silence between each burst.

                      I fed the test signal through my console and recorded it back into the PC through the channel strip direct output.  I set the channel strip attenuator and direct out level to feed the 0dBFS test signal back into the DAW at -6dBFS, so there would be no clipping if the unit had up to 6 dB of gain.

                      When I tried using the insert, I saw no signal coming out of the unit.  I verified my insert cable wasn't wired wrong, then started signal tracing through the unit with an oscilloscope.  The first amp stage was fine, but the second amp (the 12AU7 tube) had input signal on its grids, but no output signal on the plate.  I checked the DC bias on the tube, and the plate voltage was way too high, and there was 0V on the cathode.  This told me there was no current flow through the tube.  I touched the tube and it was cold, so I suspected the heater filaments weren't getting hot.  

                      I checked the heater voltage, and it looked fine.  I pulled out the tube and checked the continuity on the filaments and it checked fine too.  I swapped the tube with the other 12AU7 in the unit, and it didn't light in that location either.  For a while I sat there and thought about it, then took a close look at the socket, and saw the contact for pin 9 (the filament center tap) was really sloppy.  I took my smallest miniature screwdriver and pried the contacts back together a bit.  This fixed it; when I plugged the tube back in and powered it up, the filaments lit, and the unit started passing signal.

                      I put my fingertips together like Montgomery Burns and said 'Excellent!' and had a celebratory beer.

                      After going through the calibration and balancing procedure again, I was ready to test the unit with some audio.

                      I used Reaper to test and compare the raw and processed audio.  I played back my test signal on the wave editor program and fed the signal through the channel strip.  First I pulled out the insert cable and recorded a dry signal for reference.  Then I put in the insert cable into the channel strip, and recorded the wet (processed) signal into another track.

                      Here are the gains at various input levels:

                      Note:
                      Input levels are dBFS, with 0dBFS = +4dBu
                      Gain levels are dB differences between the Wet track and Dry track.

                      input  gain (dB)
                      -40    +15.2
                      -30    +13.9
                      -20    +15.6
                      -15    +14.5
                      -10    +9.9
                      -7.5   +7.7
                      -5     +5.4
                      -2.5   +3.2
                      0      +0.9

                      I didn't drive the unit hard enough to put it into compression, but the test shows the unit definitely expands low level signals, and the gain reduces as the signal level increases.   So it appears to work as it should.  

                      Next Steps:
                      - I plan to bypass the 20 dB attenuator on the input side, both to be able to test out the compression function and to reduce the amount of 60 Hz hum in the output. In the next post below, you can see a thickened line in the 'Wet' wave plots for where the test signal is silent - - that's hum.  Bypassing that 20dB pad should improve the signal-to-noise ratio by 20 dB (the hum will be reduced when I turn down the output level control by 20 dB to get a neutral gain through the unit). This thing was designed to handle some pretty large input levels!  

                      - I also plan to burn it in for a few days just to make sure the remaining power supply capacitors are not going to die.

                      - I have one more little 'machining' job left.  The new 110VAC jack I put on is larger than the original part, so the slot in the back cover doesn't quite fit over it any more.  I need to get out my files again and put some time into expanding the slot so to fix that.  A couple hours should do the trick.

                      If you have followed this thread this far, I hope you found it interesting and entertaining.  Please feel free to comment or ask questions.


                  • #12
                    Yeah, the real craziness appears in the control section, where a couple of the triodes are used as diodes, and there is no plate bias. As far as I've been able to deduce, these conduct when there is sufficient audio signal (the blue numbers on the schematic) to bias them on.

                    Comment


                    • #13
                      Today I took the whole thing to the garage and blew out all the dirt & dust, then spent 3 or 4 hours filing out the opening for the new 3-prong power cord jack. This thing has some good quality steel; I'm guessing it's 16 or 14 gauge - - so it was quite a job! I'll post some photos of it tomorrow.

                      Comment


                      • #14

                        Here are some photos:

                        100_2985: Front View
                        100_2986: Front Open, showing decades of dirt
                        100_3004: Back View
                        100_3002: Side removed for power jack installation
                        100_3007: New & Old Power Jacks

                        *edit* - It appears these photos disappeared when the forum software was upgraded.  If anyone wants to see them, send me a PM.

                        New power jack fitted - just need to drill one more hole..

                        Comment


                        • #15
                          Update:



                          I got the new fuseholder installed, and rigged up a light bulb socket in series with a line cord to try the unit with a 'soft start'. The bulb (60W, which would be about 240 Ohms if fully lit) glowed pretty dimly. The off/on toggle switch was a little flakey, it wanted to stay off, so I switched it a couple hundred times to clear any oxide from the contacts, until it worked reliably. The pilot light came on and some of the tube filaments started glowing a bit, so that was a good sign.



                          I checked the B+ (high voltage output of the power supply). It is normally +420 VDC, but with the bulb in series with the line cord, it slowly crept up from 163 and settled at about 193 V. All good so far! I let the unit 'cook' a while this way, about 20 minutes, to see if that would recondition the electrolytic capacitors in the power supply.



                          After a bit of time, I tried plugging it in directly. There was a small "snap" sound, and the transformer started humming pretty loudly. Since this can indicate an overload, I went back to the series light bulb set up, and noticed the bulb was quite a bit brighter than before (indicating the unit was acting more like a short circuit than before). I checked the B+ voltage again, and it was something like 1 or 2 volts. Bad news!



                          I powered down the unit and did an ohmmeter check from some B+ points to the chassis ground. It read zero ohms from the rectifier tube (5V4) output on the cathode, but was several ohms on the output side of L1 & L2 (gigantic iron core filter coils). I isolated the problem to C25 (which I had forgotten to label on the schematic), a 1 uF 600V part connected from the rectifier cathode to ground. I lifted one lead of that part, and found the cap was shorted (had zero ohms). With the cap out of the circuit the B+ measured a couple megohms to ground, indicating there were no other shorts.



                          I went ahead and powered it up again, first with the series bulb, then directly, and measured about +340V on the B+ point. This was a really good sign. The expected voltage of +420 would not be attainable with the cap missing, since the cap acts as a 'peak hold' device, and without it one would expect a much lower voltage. This tells me that there are no additional shorts (at the moment) and that the other electrolytic filter caps are tolerating the voltage without dying like C25 did.



                          Time will tell if I lose any other caps; I hope not. The others are multsection caps in aluminum cans, about 5 or 6 inches tall. One has an 8-pin tube socket, the others are riveted into the chassis. So it'd be a real pain to remove the old ones, and would be very tough to find a replacement that would fit in the cubic space provided. They haven't made caps like these since the mid-70's, as far as I know...



                          So, at this point, I need to go shopping again, this time for a 1uF 600V cap.... I'll check in again when I get one installed. Wish me luck!

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