Lost the distortion on an Orange Crush 15R.

[bclemons]

I just traded a pedal for an Orange Crush 15R yesterday. I was playing it last night and was thinking how terrible the fuzzy distortion is on it. I found a partial schematic for it, and it uses several TL072 op amps in the preamp section and a pair of diodes for (soft) clipping before the EQ. I was going to open it up this evening, just to evaluate the board, but I decided to play it and have another listen to see what direction I wanted to go in modifying the clipping circuit. When I hit the button to engage the clipping, there was only fairly large boost in volume and the level control on the clipping had almost no effect on the sound. I'm guessing something went wrong with the diodes and that the output of that TL072 is reduced by something that is bypassed when the clipping circuit is engaged to make up for the signal being grounded out by the diodes. I don't think the reverb circuit likes the extra signal either, it started howling with the reverb pot about halfway up. I'm having a hard time following the signal on this schematic. Last night is sounded like some unclipped signal was mixed with the clipped signal like they were parallelled. What I'd ultimately like to do is maybe modify the clipping circuit to be like a Rat pedal(hard clipping) with a fixed filter value and output level limiting, and/or be able to drive the TL072's a little bit for distortion. I'd be willing to sacrifice the reverb, I'm not too fond of it really.

[Jazz P Bass]

Post the schematic?

[bclemons]

[bclemons]

Schematic posted.

[52 Bill]

Turning on the over drive switch SW1 does two things; one, it connects the two back to back diodes to the output of the second stage op amp and two, it connects the over drive control in parallel with R112 (100K).

The gain of the stage is partially controlled by the value of the two resistors (R112 and pot VR102). So with the switch on, turning up the control will both raise the gain of the stage as well as gradually add in the clipping diodes.

Check the pot or clean it, test the diodes and check for cold solder connections.

[km6xz]

A little correction, the clipping diodes are always in-line and are not switched in or out. They do not clip normally because the signal across them is lower than the forward drop. The switch increases the gain of that stage by reducing the negative feedback, which make the signal higher amplitude so it may clip. The pot is set up as a rheostat and its value determines the amount of feedback reduction. Since the gain does increase with the switch, the rheostat is conducting. But if the signal does not change with rotation of the control, the problem has to be an open wiper, which could be the pot itself or a cracked trace. Connecting an ohmmeter between the two outer pins of the pot should show that rotation of the pot does not change its resistance, but always 100k. The gain change without that pot working should be x2.

[bclemons]

Well, I figured out that I was mistaken about the od level pot not having an effect on gain. I must have bumped the switch inadvertently, before. I checked the pot at the lugs and at the connection to the switch. It tested fine. The diodes did not check out, I got the same reading both directions on each one. Is there anything that could be out of whack that killed the diodes, or do they just wear out?

[g1]

The diodes did not check out, I got the same reading both directions on each one.

If you are checking them in circuit, that is how they will read. The are in parallel with reverse polarity to each other. So with your meter leads one way, you forward bias one of them, then when you reverse your leads, you forward bias the other.
If you lift one of the leads of each diode, they should measure normally.

[bclemons]

OK, lifted the diodes. They check out. They read about 592 (mV?) one way only.

[bclemons]

So, since the gain boost is working and the diodes checked out, where should I look next?

[DrGonz78]

I am trying to follow along here on this thread, but not sure what we are fixing. So is the distortion absent or that problem was addressed on post #7 with the switch not engaged? Just gotta ask if you are wanting to mod the distortion clipping circuit and the original circuit is 100% working again? Don't know where to look at this point if we are not sure about the questions I posted, best regards.

[bclemons]

The distortion is not present. When the switch is engaged, there is only a volume boost that can be varied with the overdrive level pot ( I originally thought that the pot had no effect on the amount of volume boost, that's when I had accidentally disengaged the switch). The signal isn't being clipped. My original intent was to mod the distortion circuit due to dissatisfaction with the overdrive sound, but I'm thinking that something may have been messed up to begin with.

[Jazz P Bass]

Is the guitar volume all the way up?
Does the first Gain pot affect the signal at all?

[eschertron]

So is the amp behaving differently from the first night you played it, or was that fixed with the switch position?

With the gain pot all the way (or mostly all the way) up, does the boost control have much effect? Can you hear a clear difference in volume with the switch in/out? Do you get ANY distortion with the controls all the way up?

[bclemons]

I just powered it on to test the effect of the gain pot again and the clipping came on for just a minute, and then was gone again. The regular gain pot seems to be working fine and the overdrive level pot (when the switch is engaged) provides a significant volume boost when turned up, regardless of the regular gain pot position. Maybe the diodes aren't getting to ground properly? The soldering looks good on the diodes.