ADA Depot Too - A Forum To Support Users of ADA Amplification Gear

Let's Get Technical => Troubleshooting Problems => Topic started by: red2203stack on January 08, 2019, 05:51:21 AM

Title: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 08, 2019, 05:51:21 AM
Good Morning,
I'm getting ready to recap an all original unmodified MP1 with a REV E board. Located at the positive lead of C131 (OEM; 3300uf 25v) is a 4ohm resistor that jumps to D38. Has anyone ever seen this in an original? What's it's purpose, to act as some kind of snubber to prevent poping at start up? Any replies greatly appreciated.

Joe
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: Dante on January 08, 2019, 06:10:02 AM
Hiya Joe, Glad you made it in here, welcome to the DEPOT!

I'm sure somebody will chime in with a response to your question very soon, I'm not the guy to ask. Best of luck with the project!

Dante
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 08, 2019, 08:28:20 AM
Thanks for the welcome Dante, been lurking for years. There's so much good info in this forum I never had to post before.

As far as this project, gonna keep it simple with all original values.

Replace front input jack
Partial electrolytic recap
Change 1N4007 rectifier diodes to UF4007
LED Upgrade
Replace pots
Battery receptacle upgrade
Check/clean/refresh solder joints & PCB & jacks

Looking forward to getting this bad Larry plugged back in.

Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 08, 2019, 02:15:36 PM
Found a chip in the circuit path above the diodes. Not sure if it was intentional. Also a green jumper wire. Everything in this unit appears to be original from the factory. Still not sure the purpose of the 4ohm resistor.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: rnolan on January 08, 2019, 06:40:29 PM
Hey Joe, welcome to the depot.  :thumb-up: :wave: you may want to pick up some bits and pieces from MJMP (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm) and he can probably enlighten you on the resistors purpose.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 09, 2019, 05:19:29 AM
Thanks for the welcome rnolan. I previously emailed MJMP about an effects return jack. The one part I can't find anywhere. Will ask him if he's ever seen this. Thanks again
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: MarshallJMP on January 09, 2019, 01:46:24 PM
Yep seen this before, ADA used a different brand of transformer (Taiwanese) and this one put out more voltage to the filaments and also to the 5V regulators. I think they put it in there so the 7805 won't heat up as much, you will get a voltage drop over the resistor so the 7805 get's a lower voltage which results in less heat in the 7805.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 09, 2019, 03:10:26 PM
Oh ok, that explains it. Thanks for the reply MJMP. I Replaced the 6 diodes with new 1n4007's and changed the 2200uf & 3300uf caps. Now i'm gonna have to wait on a new 4ohm resistor, closest i have is 5.6ohm. The original was 5w ceramic wirewound. Is that the best option for that?
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: Dante on January 09, 2019, 10:07:22 PM
You guys talk funny
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: MarshallJMP on January 10, 2019, 09:12:56 AM
You guys talk funny

 ;D :thumb-up:
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: MarshallJMP on January 10, 2019, 09:14:43 AM
Oh ok, that explains it. Thanks for the reply MJMP. I Replaced the 6 diodes with new 1n4007's and changed the 2200uf & 3300uf caps. Now i'm gonna have to wait on a new 4ohm resistor, closest i have is 5.6ohm. The original was 5w ceramic wirewound. Is that the best option for that?

Why would you replace it? Never seen these fail. You can also remove it but then you need put a wire bridge over those traces that are cut. Also be carefull with a bigger resistor, make sure the voltage drop isn't too big. The 7805 need a voltage difference of 3V between in and out.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 10, 2019, 04:14:14 PM
I cut it out to make for a clean removal with the 3300uf cap. Replacing that with a Sprague 105°. The old resistor still measures 4ohm on the meter without juice. I"ll order the correct replacement from mouser. Thought I had one. With this order might include replacement transistors, just to have. Are there any benefits in noise reduction by replacing those?
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 12, 2019, 05:58:40 AM
Slow is smooth, smooth is fast. Waiting on parts. The parts were reasonable so I decided to tighten up the power supply a little better before moving on..

New transistors (VR1, VR2, VR3, VR4) same values

New .047uf ceramic disc capacitors at (C119, C120, C122, C123, C125, C127, C129, C136). Replaced with .047uf 1kv 105° 10% ceramic

New 33uf/25v electrolytic caps rated at 105° at C121, C124, C126, C128

New 2200uf/40v 125° electrolytic at C129 & C130

New 3300/16v 105° electrolytic at C131

That should restore the power supply to its glory days. The original electrolytics were getting bad quick.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 13, 2019, 05:18:37 PM
One addition, don't try this type of work with desolder braid. It tends to lift the eyelets. For parts removal, A hot air rework station set to about 275° works well. For the iron around 350°.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: bunkyloo on January 13, 2019, 05:55:13 PM
What station are you using .
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 18, 2019, 04:00:05 PM
A cheap, high maintenance chinese 852D+. But it gets the job done.
Title: Re: 4ohm Resistor Across C131 & D38 ?
Post by: red2203stack on January 20, 2019, 04:36:13 PM
Using a cheap, high maintenance, Chinese 852D+ but it gets the job done.

So far I channged out the 2200uf and 3300uf caps. That made a huge improvement, but the original caps were on their last leg.

Then I changed all electrolytic's on the tube board with original values. Again a noticeable improvement in noise and the original tone is still there. It's not completely quiet, but back to within reason and my Boss NS-2 can handle the rest.

All thats left is the front input jack and pot..Still not sure if I should replace the .047uf ceramic disc and voltage regulators.