Jim Cunningham Echoplate II needs TLC

[Radardoug]

Looking at that circuit I would throw it away. That method of drive overloads horribly. Just get a little amp kit off evilbay and build it. Nice little ten or twenty watter.
If you must persist with that amp, try a TL071 opamp.

[Richard Crowley]

The circuit may not be the latest design, but if you were happy with it before, it hardly seems like a reason to dump it.

Judging from the audio sample, it sounds like indeed the driver amplifier is the problem. What exactly did you replace the 2N3055 transistor with? 3055 doesn't come in a TO-99 package. Are you absolutely sure you replaced it with a 3055? The transistor and emitter resistor failure may not have been the only components damaged.

With no signal present, what is the voltage at the center node between the transistors (the output)? If everything is operating properly, it should be zero volts. If it is more than a fraction of a volt positive or negative, then the driver amplifier circuit is still not functioning properly.

OTOH, Radardoug is correct that it may be easier to just get a small power amplifier kit to replace the bad circuit rather than trying to repair it.

[Jonk]

Hey guys.

Thanks for the replies. I was wrong regarding TO-99 package for the MJE3055, it's actually a TO-220, which is smaller than the original part, but the replacement part number is MJE3055TG. Anyway, this is the part I replaced it with: MJE3055TG ON Semiconductor | Mouser

As of now, I've narrowed it down to definitely the driver amp or the line input stage.

When I use my hafler amp to drive the transducer the reverb works fine (the voice coil was blown on the driver, which I successfully replaced)

I don't have a scope, so. . .
(my rudimentary method of replacing things where I 'think' might be the problem hasn't gotten me anywhere.) But fwiw, I replaced A1 on the schematic, which is the uA759HC in the driver stage with the same part.

And, I've also replaced both LF351's chips in the line input stage (w/ the same part)

When I use the Echoplate's driver amp, the thing that's different now is that there is no thumping sound, instead I'm getting nothing, just the noise of the pickups on the output. No sound is going to the transducer.

Couple more questions. Radardoug, I have a bunch of TLO71's sitting in my parts box. Are we talking about replacing the LF351's?

Jim, which chips are you talking about specifically if you don't mind sharing?

And, also, anybody have a link to an appropriate driver amp/amp kit for that little transducer that's clean, cheap, and doesn't overload, if I end up needing to go that route? (not gonna spend a whole lot more time screwing with this thing) Unbalanced input is fine, as I have a balancing transformer in front of it already.

(Edit: Okay, I checked voltage at the center node between the transistors and I'm getting about 6.0VDC. So, yeah, that theer seems to be a problem)
Is the 5K trimmer at the output of the line amp stage the way to adjust the voltage to 0VDC at this spot?

[Richard Crowley]

No the 5K trimmer adjusts the audio level "drive" to the transducer driver amplifier. It should NOT affect the DC operating point. That is rather a strange amplifier design because it "shorts" the output of the 759 op-amp and depends on the power supply current drawn by the 759 through the 30 ohm resistors to drive the output transistors. The unusual topology of the amplifier circuit makes it more dependent on the characteristics of the components.

If you want to repair that amplifier, you should double-check ALL the components in the circuit, not just the components that were toasted. Direct-coupled amplifiers typically take out more components when they fail. Or there are other failed components that caused the fault that are not necessarily visible to the naked eye. At least replace the 2955 with a complementary version of your new 3055 device. And replacing the other emitter resistor would be in order as well.

Currently on eBay I found this...
HOOD JLH 1969 Class A Power amp kit 2 channel amplifier 10W+10W | eBay

Two 10W amplifier board kits ("stereo") for US$10. That makes them $5 each and you have a "spare"!

[Studerfreak]

Bad soldering joint ?
Sounds to me like a bad connection.