MXR Dyna Comp (1982) Pedal Repair Help

[Helmholtz]

I don't trust that schematic from post #1. Why would you need a diode in series there? The DC jack switch disconnects the battery line so the diode seems pointless.

Right, I meant in series with the ext. power supply, i.e. wired to positive DC jack terminal.

In series with the battery it can't protect the circuit against reverse polarity.

As a series diode, a 1N4148 is ok, as it will only see the low supply current of the circuit.

[misterc57]

The 10 uF is not shorted. Chip is still removed.

I am seeing the positive and negative rails are both shorted to ground. Regardless of switch position ON or OFF.

Should not be this way? If so, I could isolate the board by removing the red positive wire from the DC jack and again check for a short?

[misterc57]

Did I find the short?

There is a diode (I think it is a diode) that bridges the negative and positive rails. See pic below, diode in question is on the lower left corner of the board. It looks a bit toasted.

It is shorted in circuit and also with one leg lifted. With the one leg lifted the resistance across the rails changed from under 1 ohms to around 75K.

In looking at the original schematic I do not see this diode. According to some other schematics there are 3 diodes. I have looked at several different schematics, all slightly different, none seem exact to my pedal. Just adds to my confusion.

This third diode on my board looks different from the adjacent two diodes.

Is this the D3 1N 4148 that sits across the power rails in the below PS stage? And if so, if I put a new one in, I am not sure which direction to place it. The DC adapter is negative tip, and this black wire is the top rail in my original schematic, which gets me confused on which way the caps and diodes should reside.

Thank you!

[Helmholtz]

There is a diode (I think it is a diode) that bridges the negative and positive rails. See pic below, diode in question is on the lower left corner of the board. It looks a bit toasted.

Yes, that should be the parallel protection diode shown in the electrosmash schematic.
The diode being shorted confirms my concerns with this bad solution. Might have damaged a power supply as well.

I recommend to not replace it and use a series diode instead.

If you insist on replacing it, use a 1N4007 which has a higher current rating than a 1N4148.
Cathode (indicated by a ring or band) goes to positive.

[misterc57]

Yes, that should be the parallel protection diode shown in the electrosmash schematic.
The diode being shorted confirms my concerns with this bad solution. Might have damaged a power supply as well.

I recommend to not replace it and use a series diode instead.

If you insist on replacing it, use a 1N4007 which has a higher current rating than a 1N4148.
Cathode (indicated by a ring or band) goes to positive.

I am reading this after I have made some changes and got the pedal working.

So this pedal had two cooked diodes (one parallel on the board, and one in series at the 9V battery clip red wire).

Chip is back in the unit. I installed the 1N4148 parallel diode with the anode side on the negative rail (black wire). Cathode on the positive rail (red wire).

Connected to a DC power supply. Dialed up to 9V and saw no current draw. Footswitch now toggled the red led light. Clipped in a 9V battery, connected to amp, pedal is working!

This pedal also had a cooked diode in series on the red wire between the 9V battery clip and the connection at the DC plug. I ran my test without this diode. I think I should install this diode, any recommendations here?

Another point of confusion for me is the DC adapter socket. The tip is connected to the red wire. However I thought this unit used a negative tip adapter.

Thank you

[Helmholtz]

The pedal should work without any of these diodes. Older versions not having a power supply jack didn't have/need them.
They are only for protection of the circuit against wrong polarity power supplies, but can only protect if wired correctly (not as hown in the schematics).

To give furher advice I need to see a drawing of your actual wiring.

Some additional info here:
https://sound-au.com/appnotes/an013.htm

[g1]

Another point of confusion for me is the DC adapter socket. The tip is connected to the red wire. However I thought this unit used a negative tip adapter.

The pedal should work without any of these diodes. Older versions not having a power supply jack didn't have/need them.
They are only for protection of the circuit against wrong polarity power supplies, but can only protect if wired correctly (not as hown in the schematics).

To give furher advice I need to see a drawing of your actual wiring.

Is this a picture of your actual unit? If so, it uses a 1/8" type plug for the power supply, with tip positive. The adapter jack does not have the switch, which is why the series diode is added (to prevent charging of battery by supply).
The newer version pedals use a barrel style plug pwr. supply with switched jack, center negative.

[misterc57]

Is this a picture of your actual unit? If so, it uses a 1/8" type plug for the power supply, with tip positive. The adapter jack does not have the switch, which is why the series diode is added (to prevent charging of battery by supply).
The newer version pedals use a barrel style plug pwr. supply with switched jack, center negative.

Yes it is, below is a current pic without the battery clip installed. I will order the 1N5817 for the series diode. Thank you

[vintagekiki]

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/923139-mxr-dyna-comp-1982-pedal-repair-help?p=923199#post923199

[g1]

vintagekiki can your schematic magic locate this lost original schematic from 1975? It seems to have been completely removed from the internet. I am only interested in the original drawing:
https://music-electronics-forum.com/...648#post160648

[Helmholtz]

Do you mean this: (see below)?

It corresponds to my first generation DynaComp. It has a 1N 914 parallel protection diode (forgot about that). Obviously it is meant to protect the circuit against accidental contacting the battery with poles reversed.
The 1N914 is equivalent to the 1N4148. A 1N400X would be a better choice.

BTW, I hate (don't understand) the upload function of the new forum software.

[vintagekiki]

vintagekiki can your schematic magic locate this lost original schematic from 1975? It seems to have been completely removed from the internet. I am only interested in the original drawing:
https://music-electronics-forum.com/...648#post160648

Sorry, the link is not active.
On the subject of DynaComp this is all I have.
DynaComp.zip

[g1]

Sorry, the link is not active.
On the subject of DynaComp this is all I have.
DynaComp.zip

Thanks. The one in there called MXR DynaComp (760501).pdf appears to be an official MXR schematic from 1976, so that may be the one that was in the other thread. The 1/8" adapter jack and series diode must have been added a little later. So the versions with the 1/8" adapter would have had the series diode as well as the parallel protection diode (like the subject of this thread).

edit: missed your post Helmholtz , yes that is the one.

[J M Fahey]

I don't trust that schematic from post #1. Why would you need a diode in series there? The DC jack switch disconnects the battery line so the diode seems pointless.

Wrong drawing, probably by a user, definitely not Factory.
Even if seemingly barbaric, a reverse biased 1N4002 in parallel with supply input is common.

Better is a series Schottky one, but the parallel idea comes from the late 70īs, early 80īs, when Schottkys were unheard of.
Early cheap unregulated supplies could stand a shorted output for a minute, more than enough to notice something was wrong, and did not have enough juice to blow an 1N4002.

In the early days they even used standard 3.5mm jacks and plugs, which everybody knows get shorted on plugging in-out; only later unshorting hollow barrel plugs became the norm.
But many early Electro Harmonix and similar era pedals used the cheesy mini plugs, which were standard in "battery eliminators" for transistor radios.
Modern supplies of course can double as car starters he he.