01-21-2013 12:31 AM - edited 01-21-2013 01:13 AM
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:
Input levels are dBFS, with 0dBFS = +4dBu
Gain levels are dB differences between the Wet track and Dry track.
input gain (dB)
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.
- 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.
01-21-2013 12:43 AM - edited 01-21-2013 01:06 AM
I found bypassing the 20 dB input pad was a bit too much, so I bypassed the 10 dB input pad instead. The unit seems to work well that way with a +4dBu output.
Here is a Reaper screenshot. The top track shows the channel strip output with the insert unplugged, so it is basically a duplicate of the test tones I created with the wave editor. The second track is with the insert plugged in and indicates the units response to the tones in track 1.
This does show the expansion and compression of the unit, but I wanted to see the compression curve too. So I wrote down level readings from each tone burst and put them into an Excel sheet. The compression curve is shown in the next graphic.
Overall it looks pretty good, but the output level control could be decreased by 5 dB or so to give the unit a 'flat gain' at 0 dB.
The only thing left now is to chase down the cause of a 4 Hz oscillation that occurs when the unit is hit by a transient or impulse. I suspect the cause is another bad capacitor on the B+ (+400v) lines. Besides reducing the 120 Hz ripple from the power supply, another function of these caps is to make sure no audio signal exists on these lines. When that happens, audio can be sent from a later (higher audio voltage) stage to an earlier (low level) stage, and cause really low frequency oscillations like I'm seeing here. This symptom used to be called 'motorboating' because thats what it sounds like. So I need to hunt through the circuit and look for open capacitors.
I plugged a bass guitar into the strip and played it a while. The reputation of these units is well deserved. It sounded really smooth. I played as loudly and as softly as I could, and the output level would return to 0 dB within a few hundred milliseconds after a level change.
Now my quandry: Do I sell the unit or keep it? They've been going for $2200-2500 on Ebay recently, so it's a pretty healthy chunk of change. I could use that to pay for a new, shorter mortgage... On the other hand, I'll likely never see another one of these, and when this one is gone, it's likely gone for good. What do you guys think?
01-24-2013 08:47 PM
So... This pretty much ends this thread. I had an interesting time working on this thing. It was fairly easy, relative to the stuff I do during my day job. It was actually sort of relaxing.
To those who didn't follow the electronics geekery in my post, I hope you at least got a sense of what kind of thinking and care goes into working on something like this.
I think, for now, I'm going to hang onto this unit for a while and use it (sparingly) on my new album. I'll probably sell it some time, but no hurry... They're not making any more of them.
02-01-2013 09:11 PM
02-02-2013 05:02 AM - edited 02-02-2013 06:29 AM
It's the same as a Sta-Level except it also has expansion, where the Sta-Level was compression only.
It's nice to hear somebody else is out rescuing one of these jewels... Here is a link to a article discussing the unit if you're interested. I couldn't find a manual for free downloading, but you can get one here for $5.
First of all, it's extremely important to assume that any point you touch while the unit is powered will kill you. Just like in Mission Impossible, if you get yourself killed I'll have to disavow any knowledge of your actions...
So, when measuring voltage, always use only one hand to hold the positive meter probe. Get into the habit of putting your other hand into your back pocket before taking a reading. Wear a long sleeve shirt; heavy flannel is good. Don't use a metal workbench. Wear rubber tennis shoes, too - - none of this 'barefoot, standing in water' type stuff, please! The idea is, if you do accidentally touch a high voltage and allow current to enter your body, you want to limit how much current will exit your body. Doing these things can make the difference between a death-shock and a "damn, that really tingled!" shock.
Also find a good secure point on the chassis to clamp the negative meter lead to so you don't have to hold it in your hand (an alligator clip on an unpainted part of the chassis would work; it would be even better to find a screw hole and put a crimp lug on the screw to connect to - - you don't want the lead to fall off).
Be careful not to accidentally touch two points together with your test probe, which could kill parts and make exciting sparks. I like to put a piece of heat shrink tubing or electrical tape over the probe point so only 1/16" of the tip is exposed.
Now, with the safety stuff handled:
Here is a schematic for the Sta-Level. Print out a copy so you have it for reference at your workbench. Even if it doesn't make a lot of sense to you right now, it does tell what is connected to what, which can help you sort out where to find test points to measure.
Start with the power supply. See if there is any high DC voltage on the cathode of the rectifier tube (Pin 8 of the 5Y3). It's the big tube next to the power transformer.
Look for pin numbers on the socket. When working from the back, the pin numbers increase as you go clockwise. It should have more than +300V; the schematic says +332V. Get back to me on what you read there, so this post doesn't get too long.
02-03-2013 05:14 PM
02-03-2013 09:19 PM
02-05-2013 02:05 AM - edited 02-05-2013 02:25 AM
Yes, the 420V is a little high. This usually indicates there is not enough load on the power supply (not enough current being drawn from it). As you draw more & more current from a power source, the voltage at the power source decreases. Your next post shows the reason: 2 tubes are not conducting because their heater filaments are not powered. So on to that:
This diode (1N2069) is not easy to find. However, a perfectly functional replacement can be found. The datasheet for the part lists the important specifications for a rectifier: Forward voltage = 1 V at 0.5 A, Forward current = 750 mA, Peak Reverse Voltage (PRV) = 200V
The first one, Forward Voltage, is important in this circuit. The circuit is a bridge rectifier. What matters here is that the tube filament current flows through 2 diodes at a time, which drops 2V off the voltage available to the filaments. So if you replaced it with a more modern part that drops only 0.2V each, the filaments would get 1.6V too much which would shorten the life of the tubes.
The forward current of 750 mA and PRV of 200V can be considered minimums to find a replacement (though the reverse voltage in this circuit is less than 20V, so 200V is a bit of overkill).
Anyway, to cut to the chase, this part will do the job fine. And the price is good (10 for 50 cents). I recommend you change out all four. It's part number is 1N4003. It can handle up to 1 Amp, has the same PRV of 200V, and has a forward voltage of 1.1V at 1 amp.
Before starting, take a closeup photo of the old parts in-circuit. Then replace them one at a time, making sure the band on each one is facing the same way it did in the photo. (That should avoid the problem you had of putting one of them in backwards.)
Apparently, I didn't get the right schematic diagram for you - - the one I linked to above doesn't have a DC supply for the tube filaments... Hopefully you can find the right one out there somewhere.
If you want a little techie background info:
(let me know if this is Too Much Information and I'll skip the lecture in the future...)
These parts are used to convert the 60Hz AC voltage from the power transformer into DC. This is done for the preamp tubes because a little hum in those early amplifier stages gets boosted by all the following stages, so they use DC voltage to operate the tube filaments instead. (DC has no frequency, so it has no hum.)
The diodes act as one-way valves for current, so the schematic symbol for them is sort of an arrow with a band or bar at the point of the arrow. That bar is the same bar that is printed on the part. The end with the bar is called the cathode, which comes from some German word meaning negative (I think it is kathode). Anyway, that is why the specs for the part talk about forward voltage, forward current, reverse voltage; etc. The part acts differently for different directions of current or voltage.
The point of all this is that the part will conduct if the arrow (or bar end) is pointing toward a negative voltage source. If you hook one with the cathode pointing to an AC voltage source (like the power transformer in your unit), it will only conduct current when the AC source is negative, and block conduction when the source is positive.
The bridge rectifier uses diodes to provide paths for allowing current when the AC is either positive or negative. The way the parts are arranged allows current through the tube filaments in only one direction (which makes it DC).
02-05-2013 02:31 AM
02-06-2013 01:55 PM
so I let the unit run over night and I followed the instructions to balance it but either im doing it incorrectly or somethings different because of the caps I put in... anyways it says to put a volt meter on the amplifer output. I assume that means on the output of the unit, on the positive and negitive leads on the back. so thats what I did, but the meter always reads zero or very very close to zero no matter what I do. sometimes it changes a (very) little when I turn some of the knobs or R67 but it always goes back to zero or very close... I also tried putting the meter between the positive contact or negitive contact and the ground and still either zero or very close to it. The weird part is that the unit sounds really good, I tested it out at work (I run live sound) and everything seemed fine. I ran it through a good 5 hours of use and everything seems to be in order. Could it be because I put in non polar caps? (mostly mallory 150's and a couple orange drops) also I replaced the 5749's in V1 and V2 with a matched pair of sylvania 5749W's I'm not sure what the W indicates but maybe it has something to do with it. oh and here are a couple of pics
02-08-2013 03:16 AM - edited 02-08-2013 03:23 AM
You do have the meter set to read AC volts, right? No, the non-polarized caps are fine; in general they are a step up from the polarized ones.
The photos look great! Nice job.
For some reason I got the impression you were working on a Sta-Level. This unit looks identical to mine (except for the paint job). Looking back at your original post, I see you did mention it's a Level Devil. Brain decay, I guess..
02-08-2013 04:01 AM
05-03-2013 02:11 AM - edited 05-03-2013 02:14 AM
MikeRivers wrote: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.Originally Posted by philboI have the schematic redrawing finished
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.
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