From what I understand, a Darlington transistor is basically 2 transistors in one, where the second one amplifies the first. So even though this particular part, the D16P1, is long gone (can't even find them on ebay), there should be a way to recreate its functionality with two readily available transistors based on the specs, right?

Here's the best datasheet I could find:
http://www.datasheetarchive.com/pdf/getfile.php?dir=Databooks-3&file=Book517-24805.pdf&scan=

How would I go about finding which two transistors I could use for this? Or is there a single-package Darlington with similar specs still being made that could substitute? I looked at the MPSA12 and similar ones, which are pretty commonly used, but most of the specs weren't anything alike, and I don't know which specs are important and which aren't.

I've made 3 or 4 posts here about my Lab Series L5 amp since last year... I think I finally fixed it yesterday. This year-long process has taken me from clueless beginner to somewhat competent near-intermediate, but I'd still like to learn all I can from this amp before I put it back together.

I have the service notes for the amp which give details on how to set the bias, the offset, the distortion trim and the compressor level. I'd like to learn to do all these things but I don't know where to start. I think the concepts will be pretty easy to explain, but I am lacking a basic educational foundation for most of this and so I'm finding it hard to know exactly what I need to do when faced with these instructions. I've been scouring Google for a couple of days trying to find info on this so I could teach myself, but all search results are flooded with tube amp adjustment information that's not really the same thing.

So anyway, here are the four adjustments from the service notes, with each one followed by my questions for clarification.

BIAS ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 bias adjustment is necessary if Q309, 311, 312, 313, 314,
 output transistors or driver transistors are replaced i.e.:
       Q307 & pre driver
       CR310, 311 bias diodes
       Q308 pre driver
       CR 305 3-layer diode
   1.  Remove fuse F21 inside power supply and insert a D.C. Ammeter, set on
       the highest scale and set the input to meter the edge of power supply.
   2.  Turn on the unit with no signal input applied.
   3.  Allow the amplifier to stabilize for a minimum of 3 minutes, but ideally
       at 10 minutes.
   4.  Set R311 for a reading of 200 ma.
       Note: Unit must be at room temperature before beginning the bias adjustment.

I think I understand this one: I should remove the fuse and clip one probe from my DMM to each of the fuse mounts in its place. Is that correct? Does it matter which probe I use for which side?


OFFSET ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 (best done after bias adjustment is completed)
   1.  Insert an 8 ohm dummy load capable of handling full power of amplifier and
       with the master volume CCW, adjust R306 for 0 volts +/- 10 mv. (across load
       yellow wire on power amp)

I don't have a dummy load but was thinking of building one. But what purpose would a dummy load serve if this adjustment should be done with the volume at 0?

Second, where do I put the two probes to measure the voltage? It says "across load yellow wire on power amp" which is not the clearest description (this document was pretty hastily typed in the 70's and I've noticed a number of other mistakes as well). Does this mean to measure the voltage from the power amp's yellow wire to the load itself?

DISTORTION TRIM ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 (which simulates pseudo-tube distortion which flat tops the
                      waveform)
   1. Insert 30 mv RMS (-29.6 dB) 1 KHz sine wave on pin 2 of A110, 3094 I.C. and
      adjust R171 for 4.4 V (+15 dB) on pin 6 of A110.

First I downloaded a sine-wave generator program, then I configured it to generate a 1KHz sine wave. I turned up the volume until I measured 30 mV across the tip and the sleeve of the cable coming from the sound card. Is this the correct place to measure it? The sound level in the program was very near -29.6 dB so I think I was doing this right, but I wanted to make sure.

Next, it says to "insert" the sine wave on pin 2 of A110 and measure the voltage on pin 6. What I did was clip the red probe to pin 6 and the black probe to the chassis, then I turned on the amp, played the sine wave on the computer and touched the tip of the cable coming from the computer to pin 2 of the op amp. But I only measured a few mV on the DMM, nowhere near 4.4V, and I was getting a lot of ground signal out of the speakers with only a very slight amount of the sine wave signal. What was I doing wrong? What's the correct way to "insert" the signal to pin 2?

COMPRESSOR ADJUSTMENT
 L2, 4, 5, 7, 9 & 11
   1.  Connect a dummy load capable of dissipating full amplifier power (8 ohm)
   2.  Tone controls set flat, the master CW, and the compressor turned off
       increase the channel volume until full power is reached:
           100W @ 8 ohm  28.3V RMS, 80V pp., +29 dB or
           200W @ 8 ohm  40V RMS, 113V pp., + 32 dB as applicable
   3.  Insert a 1KHz sine wave  100 mv  -20dB into the input jack of either channel
       ("Lo" jack).
   4.  Turn compressor switch on and set at "0".
   5.  Adjust R186, the compressor trim, until the compression LED just turns on.

When it says "tone controls set flat", does it mean 0, 5 or 10? Except for the midrange control (which goes from -5 to +5) all the other tone controls are passive. I assume this means 10 (on all but the midrange) but I wanted to be sure.

Anyway, thanks in advance for any help you can offer... This will go a long way in helping me to understand some of these concepts that are eluding me right now. I really appreciate it!

So I found out the problem while working on another unrelated project:

Conductive flux.

I had just switched brands after running out of my Radio Shack water-soluble flux. I was at Home Depot that day and decided to pick some up from there. After I modded a Boss SD-1, some crazy stuff started happening that made me doubt my skill with a soldering iron. It took me a couple of days to track down the problem, but I learned the magic of an audio probe along the way. Turns out I had some leftover flux that was passing signal between unrelated solder points, and when I cleaned up the flux all the problems went away.

So I took a Q-tip to the power supply board on the Lab amp, then flipped it on to test it out, and it was back to normal. (Compressor's still broken of course...) I didn't do a sloppy job with the flux but I guess there was just enough left over to cause the crazy problems I was having.

Anyway, tuck that one away: Home Depot's water-soluble flux is conductive. I'd love to meet the guy who came up with that formula.

OK. I've posted on here a couple of times before about a Lab Series L5 that's been giving me problems. I worked on it off and on for about six months without really making any progress, and finally about a year ago I gave up and put down my soldering iron, and just used the amp as an external cabinet.

But it's not in my nature to just let broken things sit around, and a week ago I was doing some unrelated soldering when I decided to have another go at it.

The good news: I fixed one of the problems, an inexplicable constant crackling sound that was unaffected by the master volume control. I replaced CR101 and CR102 (1N4004 diodes) and it went away. Very cool.

But there was still another problem, a bad compressor that made a loud popping sound every time it kicked on (i.e. when the signal went over the compressor's set threshold). That was detailed in this thread from last year. (I said at the end that it was fixed, but the problem came back about a day after I posted that, which was right about the time I gave up on it.)

All symptoms pointed to the power supply, since I replaced just about every part in the compressor's circuit on the preamp board and nothing changed, plus the LED flickers off when the compressor comes on, like there's insufficient power to supply both the LED and the compressor itself. So I replaced all the 1N4004 diodes on the power supply (couldn't hurt, and I had a bunch of spares), and I strengthened a torn trace coming out of one of the TIP30 transistors. (I had fixed it before, but the two components were only connected by solder, so I put a copper wire over the trace between the parts.)

That's all I did. But I somehow created more problems. The amp makes a loud crack when it first turns on, then it makes a steady, rhythmic popping sound all the time that it's on. And when I turn it off, it makes a very loud pop. I've only turned it on and off three times since messing with the power supply board, but it seems to me that this pop has gotten louder each time. I'm now pretty concerned about turning it back on again at all.

Anyway, I made a recording of it this last time I turned it on, which I've attached to this post as a zip file. This was recorded using an iPod, and it was a bit too close to the speaker, so the crack when it turns on and the crack when it turns off are heavily clipped, but the steady pop is clear.

Isolation details: The cracks when the amp turns on and off are unaffected by the master volume. Volume was at 0 when the amp turned on, then I raised it to listen to the rhythmic popping, then I turned it back to 0 before shutting it off. The rhythmic popping sound is affected by the master volume, but not by the channel volume of either side. Both of these were at 0, and I just raised and lowered the master volume for the sound recording. The loudest I turned it was about 3.

WHAT IN THE WORLD IS WRONG WITH THIS AMP? I am pretty good with a soldering iron so I don't think it's due to a bad soldering job anywhere. I very meticulously researched replacement parts and only used the exact part number, never even an equivalent, and I was very careful that every replacement went in the same way its predecessor came out. I took plenty of pictures before I removed any parts to ensure this. This is just baffling, as well as frustrating, because I feel like every time I do something, something else goes wrong.

For reference: board layout and schematic can be found here -
http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

Fairly new. My experience is limited to a couple of less complex amp repairs as well as a few effects pedal mods. Nothing too extensive...

Unfortunately I don't have access to much in the way of old electronic components... I found these capacitors on Small Bear, but they aren't the same type as the ones I'm replacing, so I'm wondering if they would work for this?

.0022 mf capacitor - This is .0022 instead of .002, and it's a Mallory 150 instead of the box style.

.015 mF capacitor - This is the right value, but it's a Sprague orange drop and is not the box-style one on the board...

sorry for the confusion — what I meant was whether the "Mod" on the chart (red/light blue) was simulated using a 50k pot or a 250k? You had mentioned changing C3, R6 and C13 to new values, but I wasn't sure whether you had started with the 250k pot or the 50k pot when you experimented with those mods.

Where's a good place to get those 3 components? I'm looking on Small Bear Electronics but I can't find the correct component values, and there are so many different types of capacitors that I'm worried I'll get the wrong kind... Resistors are easy enough, but the capacitor search is not going too well!

Awesome! That mod looks like it might be worth trying. Easily reversable, at least. Was the mod simulated using the 250k pot or the 50k?

Quote from: phatt on December 18, 2009, 09:49:29 AM
BTW,50 pot looks like it will work better,, less boost but more refined tone.

One more request... If you saved a copy of the test file you used to generate the simulation for the 50k pot, is there any way you could run it again using the 250k (which is what I have now) so I can see the difference on the graph?

Thank you so much for your help so far.

I was also against active circuitry until I played this guitar! It may not work as well with all amps, but with mine on the right settings I can play clean with the guitar's volume on 6, have a very sweet overdrive on 8 and a heavy overdrive on 10. I'm not too picky about having vintage or classic tone so it's perfect for me.

You guys are the best. I'm still trying to process all the info from phatt's analysis, but I do have one question: What is it about the Elite's circuit that likes the 50k mid-boost pot, and what is different about the Clapton circuit that it needs a 250k? It sounds like if I were to only switch the pot from 250k to 50k with no other mods, it wouldn't make the EC sound more like the Elite, since there is some other stuff going on...

Speculation... What I think happened was this: the EC preamp was based off of the Elite's design. But the EC guitar used low-noice Lace pickups, eliminating the need for a hum-canceling single coil, so the dummy coil and the buffer were eliminated from the new EC circuit. Then in the late 90's Fender developed the Powerhouse based on the EC... but since it used American Standard pickups instead of Lace or one of the Noiseless models, it needed the dummy coil again in order for the active electronics to be usable. Rather than redesign the EC preamp to add a buffer, they just wired the dummy coil after the pickup selector and before the preamp. But the lack of an independently-buffered dummy coil changes the tonal characteristics of the pickups, because the signal goes into the preamp with a much higher impedance than if it was just one pickup. So it's not that the Elite and EC boards themselves are voiced different, but that the EC preamp board was not designed with a dummy coil in mind like the Powerhouse has. (Does any of this sound correct?)

So, short of buying an Elite preamp from eBay for the buffer, my other option is to install some Lace or Noiseless pickups and remove the dummy coil... but then, low-noise or noiseless pickups have a much higher impedance (usually 10-12k), so it probably wouldn't be much different than using a dummy coil...

I am coming to realize how ridiculous this all must sound! but I am fascinated by the sound of an Elite, and am very interested in reverse-engineering exactly where the tone comes from, so I really do appreciate all the help.

I don't have any pics of my own guitar, but here's a picture I found online awhile ago showing the underside of the pickguard. In this pic the dummy coil isn't mounted at all, but normally it is mounted between the pickup selector and the pots. For mine, though, I moved it right above the bridge pickup so it wouldn't be so cramped. I also attached the wiring diagram for the Powerhouse which shows the dummy coil in place.

Also, I should mention that my guitar is a Powerhouse Strat, which is essentially a MIM Clapton. It uses the exact same preamp board except that its output has been reduced to 12dB like the Elite. (I couldn't find any schematics specific to the Powerhouse which is why I referenced the Clapton.)

It has American Standard pickups in it rather than Clapton's Lace Sensors. It also has the hum-canceling dummy coil just like the Elite, except it's wired in parallel right after the pickup selector rather than having a separate wire coming out of the preamp board like the Elite (thus no need for a buffer stage to drive it)

phatt, would any of those components you listed affect the actual mid frequency that is boosted? To my ears the Elite has a bit more "nasal" tone to it... as you and JMF said, it's probably in the pickups, but I just wanted to make sure.

Attached the following:

clapton_layout.gif - parts layout of the Clapton preamp

Elite_PCboard_schematic.jpg - parts layout of the Elite preamp

elite_pcb_closeup.jpg - a very detailed photo of the Elite preamp, front and back

elite_wiring_diagram.jpg - the Elite's guitar wiring diagram showing which wires from the PC board go where - this may not be useful

ec_wiring_diagram.jpg - Clapton wiring diagram - again, may not be useful

Thanks for your willingness to help!

Elite schematic:

http://www.eledar.net/Music/walnelite/photos/elite%20pre%20schem2.jpg

(sorry, won't let me attach it for some reason)

Attachments are being weird... here's the Clapton schematic, Elite to follow.

OK... as a preface, this isn't actually a guitar amp, it's an active preamp inside of a guitar. But the principles are all the same! I need help trying to figure out why two preamps with the same specs sound differently. I have attached schematics for both of them.

One is from a Fender Elite Stratocaster from the early 80's, and the other is the Eric Clapton mid-boost from the late 80's continuing to today. The Elite came first and then the Clapton boost was based on it. The two guitars sound very different, and though this could be caused by the body wood or the pickups, I have a sneaking suspicion that the preamps are actually voiced a bit differently.

I am not well-versed in schematics, so this is like requesting a translation, but I was mainly wondering if someone could compare these two schematics and tell me if there's anything obvious that might cause the two boards to sound different. It seems like different frequencies are being boosted, and it may just be a matter of switching out a couple of components for different values to get the change in sound. (The full story is that I have the newer Clapton preamp and I want it to sound more like the Elite, and was wondering if I could do it by swapping a couple of twelve-cent components.)

A couple of notes on the obvious differences I found:

- The Elite diagram has a whole section for the dummy coil which cancels out hum in all positions, as well as a buffer to drive it. The Clapton model does not use a dummy coil as it's fitted with noiseless pickups, so this difference can probably be ignored.

- The Elite uses a 50k linear taper pot for the mid-boost tone control, whereas the Clapton uses a 250k audio. What effect would this have on the sound? May account for some or all of the difference.

- The Clapton ends up with a 25dB boost while the Elite only has 12dB. From what I've read this is just a difference of a couple of resistors. The total output is not an issue, the idea is just to compare the sound at the same boosted dB level.

I have a number of other documents I can upload if it would help - a parts layout for both boards and a detailed closeup of the Elite circuit board, front and back.

Thanks for any help you can provide! As I said, I'm fairly inexperienced in figuring this stuff out for myself, but I am very curious to see whether the boards are voiced differently.