Please help me mod my 1975 MXR Dyna Comp to true bypass with status LED

[sohosteve]

I wonder if someone could assist me as I’m not an electronics guy – just a strummer with a soldering iron and have got in a bit of a pickle?!

I went through all the steps here

http://gaussmarkov.net/wordpress/tho...ing-up-a-1590b

as the basis to mod my original 1975 MXR Dyna Comp to true bypass with status LED - but then hit a wall trying to hook up the circuit and pots as it would appear that:

1. The final stages of hook up seem to be omitted
2. I can't work out which wires I hook up from the MXR circuit to which connections in the new project box

To clarify, I left most of the MXR wiring intact and only de-soldered the original switch and jack sockets but then realised that I couldn't work out what would go where!

Can someone (anyone!!!) please help me get my fave old compressor working in the new box with the additional features?

Thanks

Steve

[52 Bill]

The blue wire from the switch needs to connect to the input of the circuit. The yellow wire connects to the circuit output. The red connects to the circuit battery positive pad. And the green connects to the circuit ground.

Did you add a dc adapter socket as shown in the diagram? If you didn't, then the red wire for the led should connect to the red battery connection pad.

[Mark Hammer]

Note that not all vintage pedals take to true-bypass equally well. Many DO, but often the output stage (which is what is switched in the pre-1980's type pedal) is predicated on how much passive loss there will be due to the input of the effect always being connected to the input jack. The result is that bypass (when converted to TB) can be louder than effect mode. That's not such a problem when the effect itself provides some gain and an output level control, as is true with the Dynacomp, but for many things like phasers (which rarely come with an output level control), there may be a need, following TB conversion, to mod the output stage in order to achieve bypass/effect volume balance.

I will also note that TB conversion can sometimes result in switch popping when the LED draws too much current. Keep in mind that the power is always feeding the effect, even when you can't hear it. But it doesn't feed the LED until you engage the effect. That sudden current draw can produce a spike on the power line and an audible pop.

Jack Orman has a suggested remedy for this over on the AMZ site, that essentially provides a slowed-down powering of the LED. Personally, I'd recommend getting the most efficient LED you can find locally (something at least 6000mcd is good), installing a black plastic, rather than shiny, bezel so that visual contrast is maximized, and using a 25k trimpot, with series 2k2 resistor to dial in the most series resistance you can, while achieving acceptable visibility. The greater the series resistance, the less current is drawn, and the lower the risk of audible spike/pop.

Starting out with the Dynacomp's dark-ish paint job is a leg up, in terms of LED visibility. You don't want it too dark, but you don't need to be blinded by it either. You should be able to get "in the right visibility zone" with a 6000mcd blue LED using somewhere between 10k and 18k total series resistance.

[sohosteve]

Hi Mark
Well I got it all working and it sounded great so I decided to seal the box up with the four screws.
Then disaster struck!
It seemed a bit of a tight fit and being an idiot I pressed on and screwed it shut.
BIG mistake as it now won't work - open or closed.
On closer inspection I discovered that my over eager screwing shut has snapped a resistor in half :-(
And on further inspection I've noticed that another resistor appears to be melting - probably caused by the formaldehyde based foam this was originally cased in melting down...
Could someone (anyone!!) please tell me which value resistors I need to replace
a. the one this idiot snapped in half - appears to be brown green yellow green?
b. the one that appears to be melting (I can't even see the colours on that one any more)

[52 Bill]

The cracked resistor is 150K (brown-blue-yellow) and the melting one is a 470K (yellow-violet-yellow).

I will add here that changing the 470K resistor will probably alter the bias setting of the input. So you may need to reset the trimpot.

[sohosteve]

The cracked resistor is 150K (brown-blue-yellow) and the melting one is a 470K (yellow-violet-yellow).

I will add here that changing the 470K resistor will probably alter the bias setting of the input. So you may need to reset the trimpot.

Thanks Bill,
Truly appreciate that.
I'll order both parts but only replace the cracked one for now and see how that works out.
Just one last question if I may:
If I do need to reset the bias what does that mean exactly? Does it change how hard the compressor is 'hit' - as in an input gain?