Need help with an Acoustic Guitar Pedal

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X100Ver10
Posts: 17
Joined: Mon Sep 18, 2017 2:49 pm

Need help with an Acoustic Guitar Pedal

Postby X100Ver10 » Sat Sep 23, 2017 5:17 pm

Hello. First time thread starter, long time lurker. :D

I need help from the Rockman electronic wizards here. I bought a Rockman Acoustic Guitar Pedal (blue button made by Dunlop) from eBay and it doesn't work. I tried two 9 volt batteries first. I then bought an 18 volt power supply (ECB004) from Dunlop via Amazon. Same problems. The issues are several:

1) The yellow (low battery) led and the red (effects on) led both come on when I plug in either the two 9 volts batteries or the power supply and then plug in a guitar cable in the input jack. Pressing the blue button does nothing. Both led's remain lit.
2) The diode at location D1 (1N4001) had a hole in it and had spewed silver gunk onto the green capacitor (2A103J) next to it at location C27.
3) The 3 IC's (TL074CN) and the 1 IC (TL071CP) get very hot to the touch. So much so that the solder starts melting at their pins and I can't touch it for more than a second without burning my finger.
4) If I plug in a guitar cable from the output of the pedal to the input of an amp, I get a loud buzzing. No guitar sounds (acoustic or otherwise).

I have tried the following without success:

1) Replaced the 1N4001 diode at location D1.
2) Replaced the 2A103J capacitor at location C27.
3) Replaced all 3 electrolytic capacitors.
4) Replaced the circuit board on/off switch.

If it is helpful, the part number for the Dunlop circuit board is CR 00013 Rev. C and dated 07-28-95.

Any help with diagnosing this would be appreciated.

GuitarBuilder
Posts: 169
Joined: Sat Aug 30, 2014 7:14 pm

Re: Need help with an Acoustic Guitar Pedal

Postby GuitarBuilder » Tue Sep 26, 2017 7:19 pm

Stop powering it up and send it to David Ferringer at Rockman-Repair.com

X100Ver10
Posts: 17
Joined: Mon Sep 18, 2017 2:49 pm

Re: Need help with an Acoustic Guitar Pedal

Postby X100Ver10 » Tue Sep 26, 2017 9:06 pm

GuitarBuilder wrote:Stop powering it up and send it to David Ferringer at Rockman-Repair.com


What fun is that? ;) I know David repairs these. I have a bunch of new parts for it (IC's, zener diodes, etc) and I just want to know if someone might have some suggestions, instead of continuing to blindly replace parts. Don't worry, I won't burn up the board. I'll send it to David if needed but, from what I've read on this forum, there are some pretty smart individuals here that can probably point me in the right direction. Anyone? Thanks.

RockmanCentralBob
Posts: 161
Joined: Fri Feb 06, 2015 2:10 am

Re: Need help with an Acoustic Guitar Pedal

Postby RockmanCentralBob » Wed Sep 27, 2017 2:22 am

I think Guitarbuilder is right, rather than guess, it's better to have a qualified tech diagnose the actual problem and fix it.

Btw, by continuing to power it up, you could cause more damage (cascading failure) from already damaged parts.

Just my opinion though....

GuitarBuilder
Posts: 169
Joined: Sat Aug 30, 2014 7:14 pm

Re: Need help with an Acoustic Guitar Pedal

Postby GuitarBuilder » Wed Sep 27, 2017 8:41 pm

You're shooting in the dark unless you have a schematic or serious reverse engineering experience.

X100Ver10
Posts: 17
Joined: Mon Sep 18, 2017 2:49 pm

Re: Need help with an Acoustic Guitar Pedal

Postby X100Ver10 » Tue Oct 03, 2017 10:29 pm

Does anyone have a copy of the schematic for the Dunlop AGP? The resistor at location R9 is out of spec. I tested it out of circuit and I get 74.5k. I need the schematic to determine what resistor goes in that location. The 1st color on the resistor is either grey or brown. Hard to tell. The 2nd, 3rd, 4th, & 5th colors are definitely green, black, red, & brown, respectively. That means the correct resistor is either 85k or 15k with a 1% tolerance. Either way, it is out of spec but I want to be sure to put a correct one back in. Thanks.

X100Ver10
Posts: 17
Joined: Mon Sep 18, 2017 2:49 pm

Re: Need help with an Acoustic Guitar Pedal

Postby X100Ver10 » Thu Nov 09, 2017 11:10 pm

Well, it's fixed. Don't asked me what fixed it because I'm not 100% sure. :?

I bought another Rockman Acoustic Guitar Pedal that was in working condition so I could compare things with the bad one. Measurements at the IC's and some of the resistors & capacitors were off.

The next step, after replacing the parts I already mentioned, was to test all the resistors, diodes, & tantalum, ceramic, & film capacitors. Holy unsoldering Batman! I unsoldered one end of each and every one of them to test them out of circuit. They all tested good (the resistor at R9 was actually good once I determined the correct colors under a bright light, magnification, and looking at the good pedal). Then I figured that, since I already had one side unsoldered, I might as well replace them all. Hello eBay and AliExpress. I bought resistor, ceramic, tantalum, & film capacitor kits for this project. On a side note: make sure your resistor kit contains every value used in the pedal. It blows when you get 3/4 of the way done and you realize you need two 11k resistors and the kit only has 10k & 12k. :roll: Hello again eBay.

Then I figured, I've done all this work, I might as well replace every component on the pedal. I already had the IC's & LED's. The Zener diodes were cheap. The power jack, input/output jacks, & trim pot weren't too bad. The damn 2N4340 transistors, however, must have been made out of gold though. :o Oh well. I'm in it now.

Fast forward to yesterday. I finally finished installing everything. Guess what? It works and sounds just like the other one. :D
I probably spent more on it than it is worth (definitely less than sending it out for repair though). I had fun with it.

Now, do I have an idea what was defective? Probably the red LED at location CR3. I could get it to light up while still in circuit. When I took it off the board, it wouldn't light up anymore. It is possible that the heat of the desoldering gun caused it to go bad. It probably was already going bad though. I also think it is possible that one of the IC's may have been bad. If you find yourself here from a search because you have a similar problem, and have made it through my novel, replace the red LED at CR3 first. Cheap and easy. If not, start with the TL071CP IC then try one of the 3 TL074CN IC's.

BTW - If anybody needs to know values, part numbers, component locations, etc., I have several pages of notes for the it. Now go forth and spread the word of the Rockman pedal gospel according to Mike. :mrgreen: