So I just bought an Alesis Micron off Craigslist for $65. When I purchased it, I knew that the audio outputs including the headphone jack did not produce any sound. I read on the internet that this is a common issue where the JFETs need to be replaced. I just finished opening the keyboard and looked at part number Q2 and Q3. Both transistors were missing. What is strange is that it seems as though this was a manufacturing or service center repair flaw because most all of the ribbons were hot glued in place and the mainboard did not appear to have ever been removed for a repair. I would love to hear your thoughts. I have attached a picture of the places where these transistors should be.
Is there any signal at Q2/Q3 drain (leftmost) pins?
Check U7 (TL084) pins 1,14 with audioprobe, also pins 7,8
Than U10 (AL1201) pins 1,2,15,16.
there is a link there to 2 yahoo group messages about it, hope that helps
@schrab I hope to perform these tests in the next few days. Thank you for your input!
@dude163 thanks man!
@schrab I don’t get audio from the Q2/Q3 drain. Also, I do get a signal from U7 pins 7 and 8, but nothing from pins 1 and 14. I do get signal from pins 1,2,15, and 16 on U10 (AL1201). Does this mean that the TL084 is bad?
Or would I be better off replacing the TL084 with a TL074?
Nope. Theres a reason (whatever it is) they chose the TL084….
BTW swapping op amps just because they have a similar footprint is always a bad idea. If you don’t understand the circuit you can as well make things worse, it may begin to oscillate or pick uf strange noises or whatever bad things can occur….
@fcd72 this op amp TL084 is for the audio path. I read that Olivier uses the TL074 on some filter boards. Like in this post.
Note that the TL074 / TL084 thing is valid only when you use the highest grade versions.
@pichenettes highest grade as in brand name? The existing TL084 is Texas Instruments.
Look at the data sheets - there are different grade Chips with different store/soldering/working temperatures/inpuvoltage/outputVoltage/offsetVoltage etc……. and tolerances. And then theres always the MilSpec version
@fcd72 thanks guys! I think I will play it safe and go with the same TL084.
check U7 pins 3,12. If there’s no signal - replace U7
If signal exists - check volume pot and C44-C47 capacitors.
@xtrmnt – i’ve seen one Micron with fried headphones amp (U5 MC33079) shunting output signal path.
@schrab which pins on the volume pot cable should I expect audio from? I will check U7 again.
@kvitekp I will double check U5 again. Which pins should I look for a signal?
@xtrmnt: if you have signal at U7[7,8] the next thing to check is volume pot circuitry which is on the different pcb. Micron is known for volume pot issues. One way to check it out is to disconnect volume pot pcb connector and short pins 1,2 and 5,6 then check if output signal is restored. If it is, replace or clean the volume pot. I cleaned mine with DeoxIT fader lube and it holds. I’d eliminate volume pot before unsoldering any parts.
After that the primary suspects would be Q2 and Q3, but if you have them removed already they are out of question for now.
The next suspect is U7, however since you’ve proven that U7-B and -C work, I doub’t -A and -D died alone, so U5 si a suspect too. I don’t see a good way of checking it out though, do you’d probably have to remove it.