misterc57 6/4/2018 7:24 AM
EHX (Electro-Harmonix) Mike Matthews Dirt Road Special Guitar Amp Help
Hoping to find some help on this cool 70s EHX amp. It is a 1x12 combo with a solid state amplifier and a built in Small Stone Phase effect. A short story on the amp can be found at the below link

The Ol' Dirt Road (Special, that is)

My amp has at least two issues.

1. Phase effect makes all sorts of strange squealing sounds. I think I need to get issue #2 resolved before I work on the phase effect.

2. What I think is a 60 cycle hum. Similar or same sound you would hear if you touch both the tip and shield of an input cable plugged into an amp.

Phase effect is OFF. Nothing plugged in. The hum increases greatly as you dial up the volume. The hum increases if I touch any metal on the chassis (on/off switch, input jack, etc).

Probing around with a wood chop stick, if I move the white wire going to the indicator light I get a very loud snap sound.

Something I suspect may be a bad ground but I have visually seen nothing obvious. I have yet to pull the chassis and board out.

At the large power supply cap feeding the circuit, I have 55 VDC and 25 mVAC.

I have yet to locate a schematic but found something that may be very close (or actual) of the power amp drawn up, which I have attached along with some pics.

Thank you, MC

[ATTACH=CONFIG]49097[/ATTACH][ATTACH=CONFIG]49096[/ATTACH][ATTACH=CONFIG]49094[/ATTACH][ATTACH=CONFIG]49095[/ATTACH]
 
misterc57 6/4/2018 8:55 AM
schematic...
Schematic found and attached. Thank you to "oc disorder"!

[ATTACH=CONFIG]49102[/ATTACH]
 
J M Fahey 6/4/2018 9:14 AM
I have it on another computer, will search when I use it.
Although it died months ago, killing the Hard Drive, I lost a Ton of data, I am currently filling the new one with bits and pieces from older hard disks I have in storage, it "should" be on some of them.

Just from memory, yours looks close to the correct power amp.

If touching chassis *increases* hum, then you definitely have a ground problem.
Check ground continuity between that cheesy bottom mounted supply (it was an afterthought, original was designed to rely on 40 "D" batteries, which obviously was a flop), PCB ground, Chassis and input Jacks ground, at least one of them is broken.

Phase effect is killer , but if broken itīs sort of unrepairable, because of unobtanium parts.

*Absolute* worst case, gut it, mount an LM3886 amp on that chassis, adding a real power supply would not hurt it, you can make a *simple* guitar preamp for it and if a Phaser is a *must* , build one of the toner transfer pedal guys projects and call it a day.

You will still have that cool ultra portable and loud combo amp function.
What speaker do you have in yours?

Early ones had a real Celestion Greenback, which made 20/25W RMS sound like 60W with a regular speaker; but later he cheapened down to an Oxford or similar low cost driver, not bad but not the best either.


EDIT: I see you found it.
Killer little amp, if your came with a Celestion G12H it must be a firebreather.
beware of the "Bite" control, it will literally bite you.
The apparently simple "Tone" control also has teeth.
Excellent design and sound, marred by the cheesy supply and the klunky power amp.

test it and comment
 
misterc57 6/4/2018 9:29 AM
Thanks, will check out the grounds. I have the Rola Celestion G12H 30 watts speaker.
 
misterc57 6/6/2018 7:33 AM
switched input jack wiring?
I am getting closer to the noise problem. All 4 ground wires coming from the PS capacitor to the board tested good for continuity (see first pic 4 white wires on back side of main board). The chassis on this amp does not appear to be grounded (by design I think).

I found issues at the switched input jack. It has 2 black wires (tip and ground/sleeve) and 1 white wire (switch). One black wire (the tip) had a broken connection which I resoldered. I still had noise.

I noticed that the tip was connected to the ground side of the 330K resistor (this side of the resistor has continuity to ground) and the ground/sleeve was connected to the signal side of the same resistor. This did not seem right to me, so I reversed them. Now I had far less noise, but still some.

A little more probing, I turned on the phase, all of a sudden a huge amount of noise returned to the circuit. Shut phase off, still huge amount of noise.

If I pressure the input cable plug away from the tip and switch, and hold it there, I get no noise and the guitar signal is coming through loud and clear. If I release pressure on the cable plug, the noise comes back strong. The switch and tip are spaced properly with no contact with a plug inserted, and contact when no plug is inserted.

So I figure there is a fault on the 40 year old switched jack. I plan to replace it. However I am confused on the connections to this jack. Why was the tip going to the ground side of the 330K resistor? More confusing to me is the jack switch wiring.

Should not the switch be connected to ground? This switch is connected by a white wire to the middle of the phase board (last pic I drew a circle and line showing connection). And currently I suspect that the phase board is not getting the guitar signal (that is my next issue).

Thank you.

EDIT: Maybe that white wire is supposed to be the ground connection to the phase board. In which case if I connect a jumper from the sleeve to the switch I will have proper ground. Need to to some research on the phase circuit.

[ATTACH=CONFIG]49136[/ATTACH][ATTACH=CONFIG]49135[/ATTACH][ATTACH=CONFIG]49134[/ATTACH][ATTACH=CONFIG]49133[/ATTACH][ATTACH=CONFIG]49132[/ATTACH]
 
misterc57 6/6/2018 4:15 PM
>>>Maybe that white wire is supposed to be the ground connection to the phase board. In which case if I connect a jumper from the sleeve to the switch I will have proper ground.

Quick test and this worked! The phase is also working now. Sounds lush! Need to replace the input jack and hopefully done.

Thanks, MC
 
misterc57 6/11/2018 12:06 PM
All is sounding fantastic after replacing the input jack, and correcting the input jack wiring. I am surprised at how great this amp sounds and how powerful it is considering there is not much in there. The phaser is also amazing!

The amp makes a very unpleasant loud thump when first powering on. It does not do that when powering off and on quickly, I am guessing because the power cap holds a charge for a long time. I thought maybe the amp power switch was causing the thump but it also does it if using a power strip on/off button (with the amp switch left in the ON position).

Any easy way to block the thump?

Thank you
 
J M Fahey 6/11/2018 12:38 PM
Quote Originally Posted by misterc57 View Post
All is sounding fantastic after replacing and correcting the input jack wiring. I am surprised at how great this amp sounds and how powerful it is considering there is not much in there. The phaser is also amazing!
Glad you solved it.
A *true* 100/101dB speaker like G12H30 will make any amp sound louder, bigger, meanier.

The amp makes a very unpleasant loud thump when first powering on. It does not do that when powering off and on quickly, I am guessing because the power cap holds a charge for a long time. I thought maybe the amp power switch was causing the thump but it also does it if using a power strip on/off button (with the amp switch left in the ON position).
That thump basically does not come from main cap charging or some input signal (mhich might be muted fopr a couple seconds) but by the output capacitor charging to +30V *through* the speaker.
Any easy way to block the thump?
That big cap *musst* charge to+30V because itīs its operating point, so itīs unavoidable.

You may *hide* the thump, I do that often in amps which are used in VERY silent places like some Churches, live theaters, Opera/Symphonic concert halls, slide projection rooms, meditation rooms (Guru Maharaj Ji, Hare Krisna, Yoga) etc. by adding a "Standby" labelled switch (yes, on SS amps) which just opens the speaker hot wire.

Instructions mimic those for Tuibe amps: "turn Power on, let amp "warm up" for 10 seconds, turn Standby on."
"Reverse sequence on Turn Off" Guess what? ... absolutely thumpless ON/OFF switching.
Saves also from "squeal" shown by some amps on turn OFF, caused by discharging caps and low remining voltage making amp unstable for s couple seconds.

*** attention***: in that amp particular case, you MUST add a 100 ohm 2W discharge resistor from cap output to ground, before the switch, so it charges as needed while speaker is disconnected ... otherwise you will have a thump when you turn Standby ON because the cap will still be uncharged.

Or you might do like big PA amps do and add a relay and some delay circuit to do the same, automatically.

But doubt you will want to complicate such a simple circuit, I guess an extra switch and 100 ohm resistor will do fine.
Thank you
 
misterc57 6/12/2018 11:07 AM
Great idea!
I really like that switch idea. Finding a spot for that switch may be interesting.

I want to make sure I follow your idea correctly, I drew a before and after schematic (see attachment).

Also, is there a reason to use a 100 ohm resistor versus a 200 ohm?

Thank you

[ATTACH=CONFIG]49276[/ATTACH]