The wreck, a Marshall Origin 50 on the workbench.
Found that the EL34 power tubes had given up for some reason.
Had the tubes replaced, but the amp did not act as it should. Only produces about 2W in any of the power settings. Low, mid and high.
Setting the power scaling switch in low and mid position gives the bias circuit for the output tubes fixed voltages, in the high pos it’s adjusted with a bias trimmer.
Measuring the voltage over the 1 ohm bias resistor indicated it hardly pass any current. Voltage reading on the screens gave a hint of what’s wrong. At the high setting the screen voltage was about 140V lower than the plate voltage.
Now both the bias voltage and the screen voltage are modulated by a LTV826 Optocoupler, the screen voltage also uses some power transistors. Checking the 1M resistor feeding B+ to pin 6 of of the LTV826 solved the problem, it had an infinity reading.
Replacing the 1M resistor made all the difference. The new tubes are now rebiased and all three settings produce the wattage they’re meant to. The amp is fully functional again.
Chart for the voltage readings when everything is fine, bias is messured via the joint 1 ohm resitor for the cathodes. Hopefully it could help someone.
Got a MXR micro amp in for service and modification.
The fried original TL061 opamp is now replaced with an OPA134 and the old tantalum capacitors was replaced. Rewired the bypass with a new 3PDT switch for true bypass. Mounted a 2,1mm negative center DC jack.
View of the DC jack.
Zeta Sounds take on the proco Rat.
Pretty straight forward though. A three way switch for clipping options. No electrolytics in signal path. Now it’s time to tune the circuit.
And an another image of the pedal.
A custom built circuit this one, two clipping circuits in one.
Customer trying the repaired Colorsound Flanger pedal.
The circuit went through a massive repair and modification as seen in this image.
It’s been modded with a charge pump, lets it run from a standard 9VDC power supply. It also has trimmer that attenuates the output volume. Missing the bottom lid we got a new one manufactured.
An in depth look at the ramp down speed of the LFO in the dragonfly phase shifter. Going from fast speed to slow,
About the Colorsound Flanger in an earlier post here. Found some schematics that helped rebuild the circuit.
Gut shot of the pedal in a working state.
So pleased to here it producing flanging sound again. Here’s a short soundclip.
Here’s another Boss DD-3. The pedal gives signal in bypass, with effect engaged there’s just dry sound.
Someone has done some repair to the circuitboards power section seen in the upper left on this image.
The Mode selector rotary switch out of the chassie for replacement.
Old vs new rotary switch.
So what’s wrong with it. Found that C34 was broken, it did not pass any signal to the expander circuit. Had it replaced and got the delayed signal through the circuit, though at to high level and distorted. C31 was the cause of that problem. With C31 replaced the pedal works as it should. The tantalum capacitors C32 and C33 was also swapped with new ones as they do tend to fail in time. Checked and adjusted the high and low end of the Master clock frequency.
Here’s the service notes: Boss DD2-DD3.
Got hold of a powerstat, an old variable transformer from “The superior electric company Bristol Connecticut, U.S.A”
Also found an old chassie donator. Runs 3A 0 – 230VAC and 0 – 320VDC. Nice for tool for tubeamps. VAC outlet on the right side and VDC on the front panel.