Hello DIY People,
Can somebody help me figure out what this part is please. There are two of them, one for each channel, and they are mounted to the heat sinks right next to the power and output transistors of a McIntosh MC425 audio amplifier. It looks like a cap but there are no specs on it so even if it is a cap I don't know how to find the specs for it so I can either test or replace it.
I spent the last few days looking for a schematic for this amp so that I could figure out what this part is. But I can't find a schematic for this amp. I found some McIntosh amp schematics, just not for this one.
Here is the picture of the part.
Can somebody help me figure out what this part is please. There are two of them, one for each channel, and they are mounted to the heat sinks right next to the power and output transistors of a McIntosh MC425 audio amplifier. It looks like a cap but there are no specs on it so even if it is a cap I don't know how to find the specs for it so I can either test or replace it.
I spent the last few days looking for a schematic for this amp so that I could figure out what this part is. But I can't find a schematic for this amp. I found some McIntosh amp schematics, just not for this one.
Here is the picture of the part.
It is a NTC or a PTC, depending on the surrounding circuit to control the bias from temp-runoff.
On the Hifiengine there is an entry for a MC452 main amp... typo? But no ServMan available.
Can you provide a wider picture so an estimation of its function can be determined?
On the Hifiengine there is an entry for a MC452 main amp... typo? But no ServMan available.
Can you provide a wider picture so an estimation of its function can be determined?
It’s a motivator. It provides encouragement to the power transistors to keep up the good work during complex passages.
Don’t remove it. Your amplifier will be flat and listless without it, and will really only work on grunge and emo.
Don’t remove it. Your amplifier will be flat and listless without it, and will really only work on grunge and emo.
Your amp looks to be car power amp 2x50W. The MC420 is 4x 50 and you can find the schematic here McIntosh – Audio-Circuit [.dk]
Wow!
Thank You All. You guys rock!
@kokoriantz
Would I be able to con you into telling me the search parameters you used to find that website with the service manual? You have no idea how much time I burn on the internet looking for service manuals. Even though that service manual is for the 4-channel version of mine, it is still valuable and very helpful. Thank you thank you!
@DUG
I have seen lots of little caps that look like these. But thought it strange that it would be mounted to the heat sink next to the power & output transistors. So I suspected it could be some type of a thermal limiting sensor to shutdown the amp. From what I've read about these McIntosh amps, they are suppose to be high buck and provide near perfect distortion-less sound. So, it looks like you were most likely right on the money.
@susyj
Interesting. Not saying that I should. But I've never heard of a transistor needing some kind of stabilizing or starting device. I've seen certain electrical motors needing a big ol' starting cap.
ATM, I have not read about these motivators yet but I will. I did a quick search on the net and there is plenty to read about. So, thank you for opening my eyes. BTW... Fear not, for I have no intentions of removing it permanently from the board. Only to test the individual component.
I do have another question for you, if you have a moment. What is "emo"?
@MarsBravo
Thank You for your interest I took a couple more pictures today for you. If you would like other pictures, I will gladly upload as many as you want. This is the first McIntosh amp I have ever seen and I have no idea what this "temp-runoff" is, but I will definitely look into it..
Here are a few pictures.
You asked, or mentioned "typo?". I know what ya mean. I started thinking that myself when I could find no service manual or schematic for this thing.
I was able to find the owners manual without issue but those are almost useless.
I took a couple pictures of the amp with the model since we both thought typo, but we were both wrong. lol
If I didn't get enough, or etc of the board, my apology. Give me some direction as to bigger, closer, different area, etc. Thank you.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
image-of-half-the-mc425-board-showing-placement-of-item-in-question — ImgBB This link is for the full size version of the board. Can't upload pictures above a certain file size here.
Those little transistor looking items next to the item I asked about are I belive rectifiers. Both are double diode "facing in". On the other side of the board(not in picture), there is another one of those little orang-ish thermisters. But next to it are two power transisters.
(MOTOROLA MTP50N05E N-CHANNEL FET)
So, Diodes one side, FETS the other side.
As I said, if you want more pictures I will gladly upload them. Just need instructions on what you would like.
@OldDIY
Thanks for you response friend. I have not yet done anything to that part aside from de-soldering it from the board. I generally don't like to probe things until I know what they are and how to test them. Bad info in= bad info out. Plus there is always the possibility of damage when tinkering without. Now that I am armed with a bunch of info and a service manual I will be looking into it further and testing shortly after.
I had no idea I would even get a single response on this question. Let alone many good ones. Because you guys have been so great I will let you know what I have done so far and the reason for looking at this thing.
A little about me:
This amp came to me from a friend that has had me fix several old hifonics amps. I do not work on amps as a profession. My profession is ADT (Automotive Diagnostic Technician) for the last 30 years. I specialized in computer system and electrical diagnostics. I own two oscilloscopes, a fluke 87, and a snap-on scan tool.
1) This amp came to me with the power & the powergaurd light stuck on anytime power is applied to it.
2) Removed board from shell for visual checks. The board and all parts look pristine.
3) De-soldered rectifiers, ouput transistors, power transistors, and the item I asked about. The output and power transistors are testing in spec. And the rectifiers are also ok. So I gotta keep looking.
Thank you guys/gals/all 🙂
Oh... One more thing I did was to check the pinouts at the tl594 logic chip. Several pins are b+ and a couple b- and there is another pin that has a 3.2-ish volt sawtooth pattern on it. I don't remember off hand what they all were. So I'm sure there are some things I'm forgetting.
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@MarsBravo
I'm realizing that I misunderstood your message about the typo. Sorry.
I found that webpage from hifiengine before starting this thread. The MC452 is a different amp from the one I have here. I suppose that is why I thought you meant that maybe I might have typed it wrong. Because there is a MC452 and a MC425. Easy potential for typo. They may be good amps but they sure have some screwy model labels.
Thank you for taking time to respond to my thread. Fortunately @kokoriantz used his/her super powers and found a service manual for this thing. Well, the four channel version, but beggars can't be choosers.
When I get a little more time I'm hopeful that I will get all the info I need from that manual which has a full schematic of the amp.
Again.....Thank you for your time🙂
I'm realizing that I misunderstood your message about the typo. Sorry.
I found that webpage from hifiengine before starting this thread. The MC452 is a different amp from the one I have here. I suppose that is why I thought you meant that maybe I might have typed it wrong. Because there is a MC452 and a MC425. Easy potential for typo. They may be good amps but they sure have some screwy model labels.
Thank you for taking time to respond to my thread. Fortunately @kokoriantz used his/her super powers and found a service manual for this thing. Well, the four channel version, but beggars can't be choosers.
When I get a little more time I'm hopeful that I will get all the info I need from that manual which has a full schematic of the amp.
Again.....Thank you for your time🙂
Here is the spec for that thermistor. (hoping the 4-channel uses the same part).
The part number is 144279 and the parts list from the service manual states it is a 100 ohm thermistor. parts list and specs.png
@PPR
Thank you for analyzing this circuit for me. Unfortunately there are no fans in the MC425. Strange but I guess it is what it is. Like you stated, I'm sure that the logic senses hot and trips the shutdown to prevent a meltdown. At first glancing through this diagram myself I am seeing a lot of self protection circuits built into this thing. I'm kind of surprised I haven't heard of these amps before. Quality. I myself have been using Pheonix Gold for decades.
The part number is 144279 and the parts list from the service manual states it is a 100 ohm thermistor. parts list and specs.png
@PPR
Thank you for analyzing this circuit for me. Unfortunately there are no fans in the MC425. Strange but I guess it is what it is. Like you stated, I'm sure that the logic senses hot and trips the shutdown to prevent a meltdown. At first glancing through this diagram myself I am seeing a lot of self protection circuits built into this thing. I'm kind of surprised I haven't heard of these amps before. Quality. I myself have been using Pheonix Gold for decades.
Attachments
Well, thermally sensing what´s going on in a heatsink is always good, that can be used in different ways.
It is just a temperature / resistance transmitter that can be used as a temperature sensor in various stabilization or protection circuits.
I also have an MC425 amp with a Power Guard light that is always on at power-up. The main symptom is that one channel is playing much lower than the other. The PG terminal (output to head unit) is b- which I thought was odd.
Were you able to determine if it was, in fact, the thermister that's causing the problem? I have the MC425 service manual but haven't opened up the amp yet.
Thanks.
Were you able to determine if it was, in fact, the thermister that's causing the problem? I have the MC425 service manual but haven't opened up the amp yet.
Thanks.