Jim Cunningham Echoplate II Needs TLC

[JW]

Hi all, Please see my last post and forget about all the other speculating I'm doing over the course of this thread.

I keep coming back to this plate reverb driver amp after long stints of not revisiting it, but I need help figuring out the best option for a driver amp for the transducer.

 

[JW]

Pic of the whole board

 

[JW]

Well,
I replaced the burned 1ohm 1/2 watt resistor and it's associated transistor, the MJE3055 (w/ a TO 990 type. The original was a bit bigger)
but I'm still getting the harsh distortion out of my plate. So far the new resistor and transistor haven't burned out yet. I wonder if I've killed one of the IC's?

I am getting +/-18V at the two .5watt 1ohm resistors, so the power supply seems okay.

Any ideas as to what this might be? Could I have killed the driver? The distortion is in both channels.

Sorry about directing to gearslutz. But the schematic and an mp3 of the sound it is making are here: http://www.gearslutz.com/board/geekslutz-forum/745213-jim-cunningham-echoplate-ii-needs-tlc.html#post8066424

 

[pucho812]

times like this a good 1K signal, a scope and some alligator clips will work wonders. Use the clips to hook in your one 1K signal to the input of the amp and scope until the signal gets dirty. then go from there. If you can scope the output and it's still clean then it's not the amp and your distortion lies elsewhere.

 

[emrr]

Yes, like pucho said, also I'd probably troubleshoot with a standard speaker so you can hear more clearly, eliminate amp problems.  Likewise, if curiosity killing you, rig a different set of amp/preamp to confirm driver is ok and it's truly in the amp or preamp. 

 

[JW]

It's been awhile, but I've returned to working on the plate.

I found out it had a blown voice coil which I replaced. However, I'm still getting some loud thumping when I turn up the aux feed to the driver amp.  It seems the problem IS the driver amp because when I swap in my hafler amp, the reverb works fine.

So, I imagine I must have a faulty A1 (on the schematic, the uA759CH opamp,) OR the opamp in the line input section on the schemtic? (which would be the LF351 chip)
Anyhow, I guess I'm wondering where to find a replacement for the uA759CH chip if that's the problem. Looks like the places that have them are large quantity type suppliers. Anyone know where/how to find a substitute?

Edit: Also, I wonder if A1 on the schematic is a typo. I'm definitely finding a lot more instances of uA759HC, which is a fairchild opamp part number. I must admit I don't know what the last two letters indicate, or for that matter, the other part numbers.

BTW, Pucho, my scope is broke, and I've yet to replace it.

 

[JW]

Bump,

I still need a bit of help on this one. Maybe just ignore some of my speculating above.
As of now, I've narrowed it down to definitely the driver amp or the line input stage.

If someone can take a look at the schematic I linked on my second post above and maybe suggest where to measure voltage or something else I'm not thinking of? (the +/-18V supply is definitely working)

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. I replaced the MJE3055 transistor in the driver stage as well (the one that had burned out, along with the corresponding 1ohm resistor)

And, I've also replaced both LF351's chips in the input stage.

When I use my hafler amp to drive the transducer the reverb works fine.

When I use the Echoplate 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.

????

 

[JW]

Bump.

So, I'm back at this one. I've decided just to replace the busted driver amp (can't figure out the problem) with something better. The pickups and pickup amps are working on this plate. It's just the driver amp that's the issue. Something like this would be great http://www.ebay.com/itm/SC-HOOD-JLH-1969-10W-10W-Class-A-amplifier-kit-/160886486402?pt=US_Amplifier_Parts_Components&hash=item257594f582

The power supply in the Echoplate, however is a +/-18V supply, so I'd need to locate a 24V single ended supply, (walwart? something easy?) or does anybody have a recommendation for something of similar ilk that could run on the existing +/18V rails?

ideas?

 

[Biasrocks]

Have you tried talking to Jim?

http://www.platereverb.com/home.htm

Mark

 

[dmp]

I think that website is out of date
You can try out that amp using the +18v and 0v (ground). At least you can get it working that way and increase the supply to 24v in the future if you want to for some reason.
I'm driving my echoplate with a cheap headphone amp and it has a Cunningham driver. Be careful not to over power it; mine doesn't need much.

 

[JW]

So, if I were to use that Hood amp, the difference in powering it with 18V would probably just be a loss of headroom? Which isn't a huge issue because not much is really needed to drive it? The Echoplate's pickup amps work fine so I'll still be using those. There's not going to be an issue with pulling more current from the positive rail?

BTW, if you follow that gearslutz link earlier in the thread (not that I'm trying to direct people there, but it was a place I could host the schematic) it IS still there, if anybody is interested.

 

[JW]

BTW,
I'm going to reveal how much I don't know, but. . .

Am I right to believe the Hood amp will draw around 416mA? That is, 10watts/24V=.416(Amps)

 

[radardoug]

You can buy a little amp board on Ebay for peanuts, put that in and all your problems go away.

 

[JW]

Thanks Doug,

That's what I'm talking about at this point. Just trying to figure out which one so I don't have to do this all over again again. I linked that Hood amp a few posts ago. Not sure if I need to find another power supply for it or if I can use the 18V and ground without affecting the rest of the Echoplate amp. The Hood amp schematic says it needs a single 24V rail.

 

[QUEEF BAG]

i had the same problem with an echo plate.

i was unable to find a replacement for the 759. they can put out a LOT of current,
and when they go they usually burn the voice coil on the driver.


my solution was to use a 10 watt amp card from an api console
that was used to drive the audition speaker in the meter bridge of a 1604.

it worked great, ran off the same power supply, and when i told the
guy he could boast of a plate with an api driver, he was stoked!

most any power amp will work.  i didn't do the math, but the power supply
in those plate electronics looked to be more than ample for what they were running.
pull all the chips in the driver section of the plate's electronics so they don't draw
power, and you should have room to have options. 

on a side note...there is another fella in town that likes the sound of his echo plate III
better with the side off...myself, i'm not convinced. (but that may be 'cause i think it's wrong)

good luck with that plate, and don't give up!  they are awsome! 
we've got 2  140s here and pretty much EVERYBODY uses them first.

 

[JW]

Thanks Your Queefness,

Yeah, mine definitely blew the voice coil. Luckily they're a standard 1 inch replacement coil. I found a local speaker guy here that sold me 3 for 2 bucks or something.

I've basically replaced all the chips and all the electrolytic caps (just because it was probably about time) but that still hasn't fixed the issue w/ the driver amp. Oh well. I'm moving on.

Did the guy say which side he likes to take off of the echoplate? (I'm assuming he means the side that the baffle is not on?) Hmmm. I think the III is smaller than the II anyway.

One more question. How do you go about tuning these guys? Feeding a pulsing 1-2K tone and just listening while adjusting evenly? I sure wish I'd bought one of those handy little tuning guages that they had on the Jim Cunningham site while it was still active.

Anyway, first I gotta get the amp going.