Need help engineering a replacement for an NLA part in an EML 101 Synth. - Page 1
Forum section: electronics / repair

author: mkiijam
author_rank: Regular Contributor
timestamp: April 18, 2023, 12:47:30 am
content: This is an EML 101 synthesizer.Exponential Amplifier 3 has failed. The uA726 is no longer available in any form that I could find or feel comfortable with.I don't see why I need the matched pair aspect of the original uA726, nor the heated aspect(?)Could it be just as simple as popping in an off-the-shelf exp. amp?

author: Swainster
author_rank: Frequent Contributor
timestamp: April 18, 2023, 07:54:07 am
content: Disclaimer: No relevant experience in this area - just interested in old semiconductors.OK, that out of the way, hopefully, this will at least help to bump your thread...Obviously the matching is not required for this design, however the heating is critical for stable and repeatable performance of the exponential amplifier. This is probably quite important for a musical instrument. I would think that you could test the surrounding circuit by replacing the uA726 with a single transistor, something with high gain like BC109C. If the synthesizer comes back to life then it confirms that uA726 is the culprit. In order to get the stability back then you could then consider adding a crystal heater to the BC109C.Actually, come to think of it, if you are adding a heater then perhaps you could choose a transistor with a bigger TO-220 package, then tape the crystal heater to the heatsink tab (with suitable electrical and thermal insulation)

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 18, 2023, 09:37:34 am
content: It is also worth considering that the original circuit with u726 may have distortion etc characteristics that are highly desirable, sometimes what makes "vintage" synths sound, well, vintage.So you could change/modify the VCA using any modern circuits/IC's, but then you might not have a EML 101 any more, maybe a modern modular instead.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 18, 2023, 11:44:25 am
content: I seem to remember that exponential amps have like two transistors in them and more stuff?

author: mkiijam
author_rank: Regular Contributor
timestamp: April 18, 2023, 08:02:06 pm
content: "...the pA726 works in circuits where the original µA726 has pins 3 and 10 (the transistors emitters) connected only, which is the most widely used design.It won’t work in a few rare exceptions such as the EML 101or Moog Sonic Six"

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2023, 08:07:22 pm
content: Are both halves of the UA726 faulty? If not, since only one half is used here you could bodge it so the other half is used instead.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 18, 2023, 08:23:35 pm
content: The CA3046 has often been cited as an alternative workaround for this device.However, if you buy them on Ebay, or otherwise from China, they are most likely not going to work as most are fake.As I found out.You want the proper Harris ones.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 18, 2023, 09:31:47 pm
content: Quote from: james_s on April 18, 2023, 08:07:22 pmBoth sides read "Diode junction" with my atlas DCA55 transistor checker. So, to be honest I'm not sure. It for sure isn't heating.

author: fzabkar
author_rank: Super Contributor
timestamp: April 18, 2023, 11:22:32 pm
content: https://tonatelierlauter.de/ua726-replace/Quote

author: mkiijam
author_rank: Regular Contributor
timestamp: April 18, 2023, 11:38:01 pm
content: https://reverb.com/shop/tonatelier-lauterNothing in his store.

author: fzabkar
author_rank: Super Contributor
timestamp: April 19, 2023, 12:06:23 am
content: Maybe it's because he wants 100 euros for the real (?) thing?https://reverb.com/item/67392578-ua726hcStill, it's not quite as rich as this guy:https://www.aliexpress.com/item/32789581800.html?spm=a2g0o.productlist.main.1.48924b409WKQQy

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 02:16:20 am
content: Quote from: mkiijam on April 18, 2023, 09:31:47 pmWell if it's not heating then you're pretty much screwed. I think I would try replacing it with discrete transistors just as a proof of concept.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 19, 2023, 03:20:50 am
content: Haha, d'oh! Of course it is worth trying the unused transistor of the pair first.If it isn't heating but the transistors are ok then it wont have a stable gain, however it should still work as an exponential amp so you should see some output. If you are getting nothing then there is probably another problem. Can you share some voltage measurements from around the circuit - the voltage at the IC pins shown in your schematic excerpt would be useful to know.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 04:24:08 am
content: It doesn't seem like the circuit is doing anything *that* special. Surely there's a modern amplifier that could be configured to operate similarly?

author: Swainster
author_rank: Frequent Contributor
timestamp: April 19, 2023, 05:37:04 am
content: I guess that the modern way would be doing everything in software, making use of a high dynamic range ADC and DSP techniques - no temperature dependencies to worry about.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 05:41:00 am
content: Is an analog exponential amplifier not a thing anymore? It's not something I've ever encountered that I can recall. Isn't the UA726 just a pair of matched NPN transistors with a temperature controller? What am I missing?

author: Swainster
author_rank: Frequent Contributor
timestamp: April 19, 2023, 09:08:45 am
content: No, it's still a 'thing' - I was considering using one in a design just recently, but in the end decided that the temperature dependance and general parameter variation was not worth the hassle in a production environment. Adding compensation didn't make sense for the low cost device that I was working on - luckily in my case a linear response was a reasonable compromise in cost vs functionality. As it happens, after the first prototypes came out, the product manager came back and cut so much functionality that the whole question became moot.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 19, 2023, 04:20:14 pm
content: I'll take some measurements and post more in a bit, but wanted to say I tried a NPN transistor and it didn't seem to work or solve anything. Looking at the internal diagram in the spec sheet of the uA726 seems like a lot more going on, but is all that stuff just for the heater?

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2023, 05:13:11 pm
content: I looked at the datasheet yesterday and it looked like it was just two transistors, and then an analog circuit to control the temperature.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 20, 2023, 12:45:45 am
content: Agreed, exactly that. A monolithic matched transistor pair with temperature stabilisation. In this particular circuit it is just used as a single temperature stabilised transistor. If you ignore the matched part, the only thing that I noticed is that the transistor has fairly high gain.

author: fzabkar
author_rank: Super Contributor
timestamp: April 20, 2023, 04:14:03 am
content: AISI, either the transistor is faulty, or the temperature regulation is faulty. The two components appear to be isolated, so if the circuit isn't working at all, this suggests that the transistor must be the culprit. If this is the case, then switching to the unused transistor should have restored the function of the circuit. Or am I missing something?BTW, this is the datasheet:http://www.synthdiy.com/files/2004/ua726.pdf

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2023, 04:31:00 am
content: It's entirely possible for both parts to be faulty, even though they're isolated I think it's very possible, probably likely that they are all part of the same die.

author: fzabkar
author_rank: Super Contributor
timestamp: April 20, 2023, 04:56:34 am
content: What about disconnecting the power to the heater circuit, ie open pins 5 and 8?

author: mkiijam
author_rank: Regular Contributor
timestamp: April 20, 2023, 02:04:59 pm
content: Both sides read "Diode junction" with my atlas DCA55 transistor checker. So, to be honest I'm not sure. It for sure isn't heating.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2023, 05:50:19 pm
content: Don't you have a multimeter? You can check a transistor with the diode test feature, B-E and B-C junctions should appear as diodes. I suspect it is faulty though, have you tried swapping in an ordinary transistor as a test? Assuming the gain is high enough it should work at least to some degree. If you can find a transistor that works then you ought to be able to build an oven to stabilize it, using a circuit similar to that of the original, or one that has been reverse engineered from an ovenized oscillator.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 20, 2023, 06:20:36 pm
content: Just for interest, the Moog Prodigy for example uses a 5 transistor array (CA3046) in its oscillators, where one transistor is a heater, and another a temperature sensor.Two further transistors are used in the oscillator itself, and a fifth one unused.https://dl.lojinx.com/analoghell/MoogProdigy-TechnicalServiceManual.pdfVCO circuit is page 2.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2023, 07:49:39 pm
content: Quote from: Audiorepair on April 20, 2023, 06:20:36 pmThat's quite clever, I enjoy hacks like that.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 28, 2023, 03:28:57 am
content: Any recommendations on a good transistor to try for a sub?

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2023, 11:14:20 pm
content: 2N2925 might work. It's surprisingly hard to find a parametric search that includes gain. uA726 looks like is 200-600 depending on current. You could try just about anything though, 2n3904 might work at least to a point, those have a gain of around 100 IIRC.

author: mkiijam
author_rank: Regular Contributor
timestamp: April 29, 2023, 12:28:59 am
content: Here is a work up from THAT / an idea for replacing uA726. Although I'm not sure where to hook this up exactly!

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 29, 2023, 08:48:12 am
content: Thats a similar circuit to the Moog Prodigy I linked to, which is stable enough to keep a VCO in tune.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 29, 2023, 05:30:55 pm
content: Quote from: mkiijam on April 28, 2023, 03:28:57 am50C02CH-TL-E(SMD, SOT-23) with hFEof 300-800 and a surprising linearity(for a switcher) vs Ic.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 29, 2023, 09:30:26 pm
content: Quote from: james_s on April 28, 2023, 11:14:20 pmManufacturers includes hFEin parametric search; examples:https://www.onsemi.com/products/discrete-power-modules/general-purpose-and-low-vcesat-transistorshttps://www.diodes.com/products/discrete/bipolar-transistors/transistor-bjt-master-table/https://www.nexperia.com/products/bipolar-transistors/general-purpose-and-low-vcesat-bipolar-transistors/single-bipolar-transistors/#/p=1,s=0,f=,c=,rpp=,fs=0,sc=,so=,es=https://www.infineon.com/cms/en/product/small-signal-transistors-diodes/bipolar-transistor/general-purpose-transistor/https://toshiba.semicon-storage.com/parametric?region=ncsa&lang=en_us&code=param_306&p=50&i=1&sort=0,asc&cc=0d,1d,40d,3h,31h,32h,4d,5d,6d,22d,7d,8d,23d,24d,25d,26d,9d,10d,27d,28d,29d,30d,11d,15d,12d,13d,14d,17d,41d,16d,33h,34h,35h,36h,37h,38h,39hAs side note, Nexperia and Diodes offers matched pairs.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 29, 2023, 09:58:31 pm
content: Quote from: james_s on April 28, 2023, 11:14:20 pmProbably the best advice you have heard so far.

author: mkiijam
author_rank: Regular Contributor
timestamp: May 16, 2023, 12:40:16 am
content: Okay, making some progress on this. I put in a 2N3925 and the circuit appears to be functional, or at least a massive difference from being "dead" before. My issue now is finding the best replacement for the specs of the uA726? Again, I only need the transistor part and not the matched pair or heated anything.

author: james_s
author_rank: Super Contributor
timestamp: May 16, 2023, 01:14:36 am
content: You might need the heated part in order for the circuit to be stable, but you can accomplish that by building a little oven for the transistor. Look at ovenized crystal oscillators for ideas, or copying the circuit form the old IC's datasheet might work. I don't have a suggestion for a specific transistor but maybe somebody familiar with more BJTs can offer something.

author: Swainster
author_rank: Frequent Contributor
timestamp: May 16, 2023, 01:36:06 am
content: +1 on what James_S says.When you substitute this transistor then you will need to re-tune the synthesiser to match the new transistor curve. The replacement transistor curve should be something like the old one otherwise you may not have enough tuning range. I'd guess that tuning is accomplished by a trimmer pot or pots.The problem with not using a heated transistor is that you will probably find that you need to retune the instrument every time you turn on, and maybe even during a session as it warms up. With a heated transistor, the pitch should stabilise after maybe 5 minutes or something like that, and to the same point every time you turn it on. At least, that's the theory.BTW 2n3925 is really obscure. Is that a typo?edit: stupid autocorrect

author: mkiijam
author_rank: Regular Contributor
timestamp: May 16, 2023, 07:54:54 pm
content: Quote from: Swainster on May 16, 2023, 01:36:06 amYes, sorry... 2N2925BTW, this transistor feeds EXP 3 which is a VCF. So tuning may be of less concern.

author: james_s
author_rank: Super Contributor
timestamp: May 17, 2023, 12:55:33 am
content: Well play around with it and see. You might try heating the transistor with a hair dryer or something to see how much affect it has on the circuit. I'm just assuming that if they could have gotten adequate performance out of an ordinary transistor without a heater they would have used one. I'm sure transistors have improved a bit since then but I thought the temperature dependence was down to physics.