Echoplex Transistors

[TASBrendan]

Hi all-

I've got an Echoplex EP-3 high serial number on my bench and the input transistor Q5 has gone bad, putting out white noise. I'm wondering what others have used for replacements. The schematic lists Q5 as TIS58, but I swear the one in circuit reads TIS98. Maybe that's the problem...Either way, both are obsolete so I'm wondering what's currently in production. I have seen NOS lots on the bay for not too much, so I might risk it and take that approach.

Thanks

[52 Bill]

The TIS58 is an FET, while the TIS98 is a bipolar transistor. I think that both were used int the EP-3. Don't get them mixed up as they are not interchangeable.

[TASBrendan]

Thanks for the response, Bill! Strangely enough, this has a TIS98 NPN where the schematic has the TIS58...either way, I found a suitable replacement but it didn't make a difference- still have only white noise on the output. Further digging for me I'm afraid.

[g1]

What Fet did you use as a replacement? It's possible it needs a bias tweak to work.

[Helmholtz]

Useful items:

- Schematic(s),
- Datasheets of original and replacement Fets,
- Signal tracer or scope and signal source,
- Board pics,
- G,D,S voltages of Fet.

Did the original Fet measure bad?

Are you sure the circuit doesn't need a BJT?

[mozz]

The TIS58 is an FET, while the TIS98 is a bipolar transistor.
Shows the same as above in my cross reference books. Sure somebody didn't work on it and get it mixed up?

[TASBrendan]

Thanks all for the replies!
I ordered some J202 FETs as I found people have been using those to make an Echoplex booster pedal with an exact copy of the circuit- popped it in and bingo! The preamp works now... but the noise is still there as I move the Volume (really a mix pot) all the way to the echo side. So it appears the noise is coming from the tape head amp board. And, yes mozz, it appears that someone had just popped a TIS98 in the JFET position and I was assuming it was original. The schematic clearly shows a JFET at Q5 as the input.
I do have a scope and recently got a transistor tester. Solid state is pretty new to me, so there's a learning curve there for me and I appreciate all the help!
Thanks

[TASBrendan]

I'm sorry, I forgot who posted this in another thread- lots of work went into it, and I thank you. I do have a schematic, but this has been very useful, as the boards don't have any component labeling. Next step is to look at the left board- echo head amplifier Q1, Q2.

[TASBrendan]

Schematic attached
EP-3 schemo.pdf

[Helmholtz]

Schematic attached
[ATTACH]n978585[/ATTACH]

The schematic clearly shows that the input stage needs a Jfet. A BJT won't work.

Now do get the noise also with tape removed?

[TASBrendan]

Hi Helmholtz, yep tape removed and still white noise. I put a J202 in Q5 and that's fine now. There was no signal with the TIS98 there. I think next I'll test Q1 and Q2. Is there anything that would cause a transistor to self oscillate into white noise? I've seen opamps do it...

Thanks

[Helmholtz]

Did you verify circuit voltages around Q1/Q2?

What happens if you short point (8) to ground?

Freeze spray can help to identify noisy components.

[TASBrendan]

Ah freeze spray! I haven't had the pleasure of using that yet. Time to go pick some up. I finally have a few minutes today to go through that board...stay tuned!

Thanks

[TASBrendan]

So I shorted point 8 to ground and the noise did go away. There's still some hiss, probably too much, but a lot less than before. The voltages are all at 50-60% and I traced that back to the power supply. The last filter cap is in parallel with a Zener and the output there should be 22V, but it's 12.68V. There's two mystery diodes in series in there now. Schematic calls for a 1N4748. I don't think that has anything to do with the echo head noise. I did notice that the White and Black wires from the echo head are reversed. Also not sure if that would affect the output.
Thanks

[Helmholtz]

So I shorted point 8 to ground and the noise did go away.

What resistance do you measure from point 8 to ground in SOS or Echo mode?

Replace the 22V zener according to schematic. Do you see 35VDC at the rectifier output?

Is there any info about the circled circuit points?