Mackie 1400 Hot Light and going quiet

[NBD925]

Hello I have a Mackie 1400 SN: AH15857 and the hot light stays on. The amp also randomly goes quiet and then abruptly come back to normal volume while using.

I have seen a few repair suggestions mentioning Resistor R205 and Transistor Q91 but they seem to check good. But they mentioned both being on the smaller board mounted opposite the larger board on the heat sink and my amp does not have R205 and Q91 on the small board together. Q91 is there but R205 is on the larger board (rear right corner when viewing from the back of the amp) Non of the resistors I have checked have been open. A few have been slightly outside the 5% value range but nothing open.

Can anyone help???

[The Dude]

Schematic?

[x-pro]

You pointed them out because they were described in another thread. "That's fine".
Not even understanding what that transistor does in the circuit.
To fix the problem:
1. What is the load on each channel?
2. Is the output power reduction on both channels?
3. How do the fans behave? What state are they in? Does the ventilation flow vary with load and amplifier run time?
4. What position is the "AMP MODE" switch in?

Translated with DeepL.com (free version)

[x-pro]

https://music-electronics-forum.com/...-repair/47386-

[NBD925]

Non of the schematics I have seen match what I am seeing in the board. They are close but not quite the same. Here is a link that was given in a previous post that comes close but the R205 and Q91 and Q89 are in slightly different locations.

Example: My R-205 is in position R-198 shown in these diagrams.

https://music-electronics-forum.com/...etch?id=849038

[NBD925]

Here is the quote from a different Mackie 1400 repair thread that I saw.

Quote:

I thought I'd pass on a repair experience. I just fixed (I hope) a 1400i amp that had the hot indicator on at startup and full fan. The LM35 temp sensor had a strange reading on the output (-112mV vs ~200mV for room temp)and did not have a +16V supply on pin 1. The +16V supply was inop due to collector resistor (R205-10ohm) of Q91 being burned out and having a very high value (~4 Megs). This high resistance caused a 78V drop across it. I got the schematic from tech support (Thanks!). Just thought I'd pass on something to check if you ever see this problem. Initially I thought it was one of the Zeners (D88), but after replacing that and Q91(which had two leads busted also) with no joy, I measured the resistor and got virtually an open circuit. No signs of burn or anything on the resistor so perhaps the same shock that broke of the Q leads did that in as well. At least now the HOT light doesn't come on at startup. I have to do some more tests to make sure nothing else is wrong...

[NBD925]

Another Quote:

I realise the hot light is on immediately when it is not hot. My post above indicates which components may be triggering the fault. For instance check for dry joints around the LM35, check the base voltage on Q89 to see if the power transformer overtemp sensor is faulty etc etc. It is either coming from the power transformer sensor, the heatsink sensor (lm35 on main HS, or dry joint around there) or the opamp which triggers the overtemp. Test R205 to see if it is to spec and test Q91 also. Post some photos of the board (even the small ne) if you like.

[Pixel]

Just from the information you have provided it seems there are at least 200 resistors and 100 transistors and who knows how many other parts involved here. Do the math and think about the number of possible different problems there could be.

You can't troubleshoot this by looking at internet posts of how other people solved problems that most likely were not related to your problem.

[x-pro]

Example: My R-205 is in position R-198 shown in these diagrams.

can I get a picture of this little board

[NBD925]

X-Pro, Ill try to get a picture posted tonight. Most people online had honed in on that small board area for the Hot Light Problem.

My rub is my Mackie 1400 seems to be just a little different than the Schematics I have seen.

I have it all apart and have been scanning for bad solder joints. Also doing some testing on the resistors (tough because they are still on the board in the circuit) so tough to get actual hard readings that match the colored bands but I mainly looking for obvious opens.

Pixel: any ideas where to start? I just thought looking at people that have had this same problem might get me closer. This is apparently a common problem like the bad Ribbon Cables in these Mackies.

[Pixel]

Pixel: any ideas where to start? I just thought looking at people that have had this same problem might get me closer. This is apparently a common problem like the bad Ribbon Cables in these Mackies.

Ideally you start with the correct schematic and the ability to understand some of what is going on in the circuit. I would literally take a magnifying glass and look at every component and connection to see if you notice anything that looks suspect.

It would be helpful to know what your level of experience with repair is and what tools you have available to you.

[NBD925]

Here is a picture of the small board. I hope the upload works out.

[NBD925]

Another picture showing the whole board.

[NBD925]

I have the service manual and tried to attach it here but its slightly to large. What is the best way to get this document into the hands of someone that can help me??

[NBD925]

I hope this link works. Did a little reading and this was one of the options.


https://www.dropbox.com/scl/fi/hr9hz5gbiff3pe4dwbqbl/SM_FR1400.pdf?rlkey=0cqrc58rkv4nxhf9c13mr327n&dl=0