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Flickering mute illumination LED on 4 yr old McIntosh Preamp

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
There is no possibility tat board will work. https://www.electro-tech-online.com/attachments/c1100c-aee-service-p-04165600-1-pdf.128765/#page=13 , there is no power going to the LEDs.

Anyway, expect it's 5V. That would make the LED current about 1 mA, Usually, it would be around 10 mA, but there are some high efficiency LEDS that can use 1 mA.

Looking guarantees nothing. Freeze spray isn't a bad diagnostic too;
Resistor looks like it has a gap.

I actually have this https://www.circuitspecialists.com/bk5000.html as a re-work station. Surface mount basically forced me to get something.

It's not bad, but it has some glitches.
1. You can use solder/desolder at the same time.
a) I'd buy an integral desolder tool, maybe eventually
2. The desoldering tool doesn't work extremely well.
3. The soldering iron elements pop out easily.
4. No circuit diagram. helps when troubleshooting head problems.
4a. Had to figure out what it was.
5. The desoldering tool hung up in the holder
5a. I bought a reamer to fix that

Why am I saying this? Because many times you have to decide to purchase tools or have someone else do it.

I dug a drainage trench. I bought a used Bosch demolition hammer and re-built it. It was like$700-$900 new. I paid someone to remove the chuck. In the process I found that there are separate maintenance manuals available for Bosch stuff. I had to buy a clay spade as well which is great for digging. It gets used once and a while.

I just bought $225.00 of optician's tools. Why? I want to maintain my own glasses. The opticians keep destroying the frames. I have about 5 of the same frames. I like to have 3 pairs of glasses. Pair #1: trivex lenses, normal bifocal; paur 2. glass lenses, normal bi-focal and pair 3; glass lenses, high bi-focal (computer glasses). All have an AR coating. plastic doesn't require UV.

Back to your problem. LEDs have a forward voltage drop depending on color. They also have a range. Vf also determines how bright they are relative to another LED. Mcintosh may bin them to get even brightness or the LEDs have good factory tolerances. Chalk up a reason for a new board.
Typically you series connect LED's and have some wiggle room left over. e.g (5-2.2*n)/2.74k is about 1 mA. n=1 LED, 2.2 because it's green.

Most multimeters have a diode test mode which measures the voltage drop. I can also supply 5V externally after determining I can do so safely. My fluke 77, one of my meters (fluke 77) only goes to 2.0 volts, so that won't work.

You have the wiggle test available.

For the job, this https://www.amazon.com/Antex-G-3U-Watt-Pen-Sized-Soldering/dp/B00FS78QF6 iron would be suitable. I had one like it at work.

if your in the US and particularly if your nearby, I'll do it for you. If i get hit my a car in the time frame, you can buy a board.
 
Last edited:

gophert

Well-Known Member
Most Helpful Member
If DS9 (small, reverse surface mount green LED) is the issue, then you're in luck. All of the little green LEDs on that board are controlled by the same 5v circuit and all are switched by a single PNP transistor. The one you are after is DS9 and DS7, DS8 and DS10 are all controlled by the same transistor. So, if the others LEDs I listed are working, the. It can only be the LED or the corresponding resistor (R20) or the solder joint on either.

86E84824-32D4-4FEF-8255-B4205A5791DA.jpeg
 

richh

New Member
Can the resistor be replaced? Can the LED be replaced? They both seem to be indented into the board.
 

gophert

Well-Known Member
Most Helpful Member
Can the resistor be replaced? Can the LED be replaced? They both seem to be indented into the board.
Yes, they are just simple surface mount resistors and LED. The LED is pretty specific - called a down-firing LED. That means the light comes out on the side that is soldered to the board and illuminates through a hole in the board. You might want to order from McIntosch because there are so many options and matching the color of the LED, intensity and position (size) in the space provided so you don't get a shadow. The resistor is pretty common but, since your ordering from McIntosh anyhow, get a couple each (LEDs and Resistors) so you have a chance to practice your soldering skills. After all, there is a $10 minimum. Which city are you near? Maybe we can track down an experienced solderer for you. Otherwise we can get you some instructions. You should have a thermostat controlled iron.
 

richh

New Member
I have a thermostatic controlled iron but I am not well versed on soldering. I also have some 2.0% flux, rosin core 60/40 tin/lead solder wire.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
4 years old it will likely be Sn96 solder. High temperature stuff.

60/40 solder was nice to work with until SMT came along. 60/40 has a different solidus and liquidus point. 63/37 tin/lead has the same solidus and liquid us point.

R43 looks really suspect. Look at the gap carefully. You probably don't even need solder.

Sn96: https://princeizant.com/uploads/Sn96 (Flux-cored) TDS.pdf
Soldering temperature 480-520F which is HOT.

60/40: https://www.lucasmilhaupt.com/Lucas/Technical-Documents1/60_40TDS2018English.pdf
Flow point: Liquidus = 375F

There's a BIG difference,

Gauranteed, if you don;t hold that resistor down with a non-magnetic tool or cut off toothpick and wait a fair amount of time for the solder to cool. you'll pick the resistor up with your soldering iron. Make sure the tip of the soldering iron isn't huge.

So, tin the tip. Wipe it off. Hold don the component R43 with a non-magnetic tool. heat, whatever that means. Wait.

here http://www.proto-advantage.com/store/product_info.php?products_id=400014 is a practice PC. No components.

You would not be using a stencil.

these http://www.proto-advantage.com/store/product_info.php?products_id=400014 Are very god choices. I like the solder with the bismuth in it.

It's a paste in liquid form. For production, it's usually applied with a stencil and a squeegee and put in an oven under a profile.\

Hey, my very first SMT solder removal and install was a 20-30 pin processor in a car radio. I didn't have a temperature soldering station at the time. I've had plenty of other experiences that say I had the skills. My first weld with a stick, tig and mig welder in my hand I did a great job. It was set up for me and that was the harder part.

I was able to seal compounds in a quartz tube using a hydrogen/oxygen torch under vacuum. I picked that skill up quickly. i could work with glass. eg. neck down a square quartz tube and attach a round tube to it and seal off the other side. Then seall the whole ting under vacuum with red phosphoros and zinc in it. You don't screw up. then make it even more complicated and seal a tube with a controlled leak into a sealed tube under vacuum.

I could silver solder which is normally called brazing, I can do copper plumbing too.

I've probably been soldering for 50+ years learning the skill when I was 10.

Rich, i can still do it for you.
 

richh

New Member
Yes, I have ordered 4 resistors and 4 of the LED's, if I get the board to you with the resistors and LED's how long do you think it will take for you to complete and how much would your fee be for services? You mentioned pay pal, have you ever used Zelle? If your bank offers it my bank sends you an email you click on it and the money if deposited right into your account without any exchange of info other than your name & email address. Just a thought. I have never used Pay Pal to pay an individual but I am sure it is a breeze.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
My bank does offer Zelle. My smart phone is prehistoric, so if thats required I'm in trouble. I never looked into that. I tend to keep my "currency" and identifying email separate, so I use a non-identifying email for PayPal. I can use an identifying email for th emerchant and pay with a non-identifying one on the surface anyway.

Generally, it's just an email address. You can pay a merchant or "Friends and Family". You want the latter. email address is required. name - No, but you get it. There is a registered address and many PayPal merchants will generally only deliver to the registered address per policy.
if there is nothing going on, I'll turn it around as quick as I can. Most likely a day or so. You probably would want to ship priority mail whic is probably <$8.00. How is $20.00+Postage? so it will cost you at most $40.00. We can call it $30.00. Assuming 3-day ship and at most 3-day turn-around, your probably looking at 9 days max being conservative. You not shipping unobtainium to me.

Example glitches. Snow. A water leak. Just had the latter.

My plans would be to measure the resistance across the resistor and wiggle. You can do that if you have an ohmmeter. I'll look at the schematic and see if it's safe to apply 5V externally.

I just fixed a toothbrush that I thought was hopeless.

I'll send info by PM.

Give me any numbers/letters on U5.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Numbers/letters on U5? Need to get part number from the code and see if it's OK to apply 5V. You didn;t reply to a piece of our conversation. Tube amp storage.
 

gophert

Well-Known Member
Most Helpful Member
Can you take a nice clear close-up photo of d9 and R20 (Mute) circuit.
no flash, steady hand, far enough away to get perfect focus (digital zoom would be great - just not too close so the camera can't focus). Brighter lights make it easier to get a non-blurry photo if you are far enough away.

D0E261B4-3D1A-4BE6-B1EE-F3EB0DB0B24A.jpeg
 

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