• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SAE Mark Nine Preamp

Shawnnnn

Shawnnnn

AK Subscriber
Subscriber
Popped the top off the preamp and noticed a couple very cooked resistors.

Looking for advice on replacing them. Were they getting too hot because they are just old carbon comp resistors? Or something else was perhaps causing it?

What kind of 100 ohm resistor will withstand the heat better? And maybe still fit on the board?

Thanks for the help!

I'm planning on changing those 4 big capacitors while the board is out as well. 20221214_212251.jpg
 
Register to hide this ad
Could be component failure causing resistors to drop more voltage than design or they could be undersized for the job or they could have drifted to cause higher voltage drop, all meaning more heat.

Measure voltage drop and calculate required wattage for replacement and mount them with longer legs to allow them to run cooler.
 
Does look like some component(s) failure/failing. It is the power supply board so you might want to check all the components to be safe. I would think you could fit a 3watt in without any trouble mounted above board a little like said above. Might be able to get a 5watt in if you can find one small enough.
 
Shawnnnn said:
Popped the top off the preamp and noticed a couple very cooked resistors.

Looking for advice on replacing them. Were they getting too hot because they are just old carbon comp resistors? Or something else was perhaps causing it?

What kind of 100 ohm resistor will withstand the heat better? And maybe still fit on the board?

Thanks for the help!

I'm planning on changing those 4 big capacitors while the board is out as well. View attachment 2740337
Click to expand...
Is there a resistor that you removed as the picture looks like one is missing next to the two 100 ohm ones?
 
I saw the empty holes as well but I haven't touched a thing.

I just bought it. Found it at a thrift store with the matching power amp in wood cases for a couple hundred and couldn't pass them up.

They both work but I only played them for a little bit. Was just happy both channels worked. I still haven't opened up the power amp but hopefully nothing is burned up in there like the pre-amp.
 
The resistor that looks to be missing was probably for the power lamp, power lamp works or missing? I have two 9Bs here and they are a bit different power supply wise. I can't find a schematic for either one. Both the MKIB/M and MK30 use the same basic power supply/regulator. The lamp is a 28V incandescent and the resistor is 1.5K/2W. Check the turrets on the end of the board, one should have +42VDC(MK30) or +45VDC(MKIB/M).

Craig
 
Thanks Craig. I'm not sure if there is a lamp in the preamp or not. I'll check it out when I get home from work tonight.


20221106_190429.jpg
 

Attachments

  • 20221106_190429.jpg
    20221106_190429.jpg
    81.4 KB · Views: 2
llwhtt said:
The resistor that looks to be missing was probably for the power lamp, power lamp works or missing? I have two 9Bs here and they are a bit different power supply wise. I can't find a schematic for either one. Both the MKIB/M and MK30 use the same basic power supply/regulator. The lamp is a 28V incandescent and the resistor is 1.5K/2W. Check the turrets on the end of the board, one should have +42VDC(MK30) or +45VDC(MKIB/M).

Craig
Click to expand...

The Mark 1 looks similar too.
 
It is indeed a Mark IVC amp.

No lamp that I could see in the preamp. There is just a long piece of plastic with nothing attached to it.
 

Attachments

  • 20221215_200448.jpg
    20221215_200448.jpg
    110.9 KB · Views: 18
You’re missing the bulb, wire & resistor. Probably would be a good time to switch to a LED instead if you want.
 
AD12FDBD-C391-4DC2-B07A-382E00D23DF1.png

A drawing I located on the interweb supposedly of the Mark 9 PS.
Doesn’t state voltages nor where exactly that light wire is coming from. Wouldn’t be hard to figure out looking at the trace side of that PS board.
 
Last edited:
Hopefully I have time to pull the board this weekend. I couldn't find any pictures of the Mark 9 , but the Mark XXX seems pretty similar. Looks like there is a wrap around the plastic piece that holds the light on.

Think I should just use tape to attach a 28v led to the end of it?

Screenshot_20221216_064740_Chrome.jpg
 
Yeah thats a better idea.

Interesting the burn marks are most likely from the missing light bulb resistor. Wonder why it got so hot.

Since I'll be taking the board out, any advice on anything else to address? I'm going to order the big capacitors and 100ohm resistors for sure.
I suppose I should get new electrolytic capacitors for the other board as well.


20221216_075319.jpg
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom