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SAE Mark 1B Preamp Repair

I have a silver faced with the EQ faders. It seems the board layout is a little different than the later MK1B. Not sure which one mine is.
 

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Looks like a normal IB to me. What makes you think it's different? Start a conversation with me and include your email and I'll send you the SM.

Craig
 
Bradw764 said:
I have a silver faced with the EQ faders. It seems the board layout is a little different than the later MK1B. Not sure which one mine is.
Click to expand...
Do you mean that cap that was added and is a service update on the amp board?
 
Hi: Sorry I missed your post earlier. I did see the cap that was updated.
I'm having a long fad out when powered off. Unless I pull the plug, the PS will sit there at 20vdc and keep playing while caps drain.
There is a bulletin that says to replace Q9 and Q10 with BC237 and BC 307. Mine has a 2N4036 for Q10 but the schematic shows 2N4250.
Have you come across this?
 
The fad out sounds normal on power down. Are you running the amp back through it with the special cable or just using the output to drive the amp. I believed this is always powered at reduced voltage even when off. Isn’t that bulletin for the X10 amp section if it starts to fad, I don’t think that’s what your talking about.
 
With most SAE preamps, the MK series anyway, the unit will have about half voltage when turned off and plugged in. This is normal and is said to limit turn-on thump. The preamp will still pass a signal when turned off.

Craig
 
Thanks for your help!
Have you done the light mod? I'm not sure what part of the switch they want me to cut the wires on. My unit has no power light indicator. I'm not sure what should be there for a lamp. I was going to put an led in but it never turns off.
Someone had the wrong PS transformer installed. was making 90 vac on secondary and they had an NE2 for a indicator lamp.
 
Bradw764 said:
Thanks for your help!
Have you done the light mod? I'm not sure what part of the switch they want me to cut the wires on. My unit has no power light indicator. I'm not sure what should be there for a lamp. I was going to put an led in but it never turns off.
Someone had the wrong PS transformer installed. was making 90 vac on secondary and they had an NE2 for a indicator lamp.
Click to expand...

That lamp was like a bulb butted up against a piece of plastic and heat shrink tubing to hold it together if I remember right. 28v bulb I think it was, but if you don’t have the piece of plastic not sure what to do. Probably could find an led that would work I’m sure.
 
Bradw764 said:
Hi: Sorry I missed your post earlier. I did see the cap that was updated.
I'm having a long fad out when powered off. Unless I pull the plug, the PS will sit there at 20vdc and keep playing while caps drain.
There is a bulletin that says to replace Q9 and Q10 with BC237 and BC 307. Mine has a 2N4036 for Q10 but the schematic shows 2N4250.
Have you come across this?
Click to expand...

That doesn’t seem right for that 2N4036 to be there but they made so many running changes on stuff it’ll drive you crazy at times. The one I worked on didn’t have a 4036 there. I’d probably pop in what the service up date says so they are a proper pair.
 
The LED doesn't draw much current so the power supply voltage doesn't drop like with an incandescent lamp. I just replaced the lamp in a MKXXX with a 28V/40ma lamp from Mouser, 560-7839SBP. If you don't have the clear plastic yellow rod to mount the lamp on use an LED of your choice and then parallel the LED with a resistor to get close to the normal current draw. Normally the 28V lamp is run from the raw +65VDC supply with a 1.5K/2W resistor in series. For the LED you'll have increase that resistor to about 6.2KOhms. That will get the LED functioning but won't let it turn off when the power is off. I based the the 6.2KOhms on about 10ma of LED current so now we need to add a resistor to get about 30ma more current draw from the supply by adding a 2.2KOhm resistor from the 65VDC point to ground.

Craig
 
Hello, when you turn on the bottom buttons, noise and track appear in the columns. in the right channel more than in the left. Most of all, when the TAPE / LINE button is turned on, it is treated with dioxite inside each button. everything shines without corrosion. It looks like new, but nothing has changed. I believe that the problem is more serious and for professional repair I need a service manual DSC_1196.JPG DSC_1157.JPG DSC_1160.JPG
 
I have the Service Manual, start a conversation with me and include your email.

Craig
 
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