V2A photos for a Mark IIB

[sdbthai41]

Hi Everyone,

I am in the process of restoring a "modified" Mark IIB back to original (serial #5947). I guess that this is a very early IIB.

I am having a problem with the V2A area especially the input (pin 1). Mine has a 0.1uF Ruby (Sprague) Orange Drop Cap with a 56K resistor parallel to the cap and a 47K resistor in series (i.e., = approx 100K). The IIB schematic indicates this as correct. The problem is that it looks like it was an afterthought - at least not factory done.

Can someone please post photos of what surrounds V2A on a Mark IIB should look like as per factory fresh?

Many thanks

 

[Dude2Dude]

Hi there,

Here's a gut shot of my 1980 Mk IIB. It's factory fresh, no mods.

https://imageshack.com/i/nfimg0037wlfj

 

[sdbthai41]

Thanks Dude2Dude, that's exactly what I wanted.

It looks like there is just a 100K resistor from the power rail to pin 1 on V2A which is different to the mark IIB schematic which has a 0.1 uF cap tied to pin 1 as well as two resistors that approximate 100K.

Can anyone advise what should I do

A) Tie a 100K resistor from the power rail to Pin 1 as per the mark II schematic
B) As per the mark IIB schematic, install a 0.1uF cap with a 56K resistor parallel to the cap and a 47K resistor in series from the power rail?

What is the function of the 0.1uF cap here? Blocking DC on the signal?

Thanks in advance.

 

[stokes]

That .1 cap is not tied to pin 1.It is a shunt/bypass cap around the 56k resistor.It is to prevent oscilations.Quite common and necessary in these high gain circuits.I would use it.

 

[Markedman]

http://www.flickr.com/photos/100168049@N04/12568243793/

Here is a 1981 Mark IIB gut shot.