Thanks Dave , Could I replace that resistor with a precision pot?

 

Thanks Dave , Could I replace that resistor with a precision pot?

an ordinary pot will do as long as it will fit.

 

I think the R2 resistor is hidden located under a large metal plate. I will upload an image of the area and let you take a look.

 

image of a3 vertical preamp channel A

I think R2 is located under this plate

 

image of a3 vertical preamp channel A

I think R2 is located under this plate

I shows the pcb A3 , its position on page 109, looks like you will have to remove the plate.

 

I think I can see it from under the side of the plate. It looks like a 1watt resistor
but I will have to remove the plate.
I ordered some 1watt resistors but it may take a few days to get here

 

Do you think R2 changed value over 30yrs. They look metal film type.
Or is it possible that old age effected other things as well ?

 

Do you think R2 changed value over 30yrs. They look metal film type.
Or is it possible that old age effected other things as well ?

everything ages over time, electronics especially where there is heat!!

i would remove it and replace it with a 1K preset.

 

I found something very interesting. There are 7 resistors and rectifier joint pins with no solder ! These joints were never soldered at the factory. the resistors can be
removed from the vertical amp board A3 with just a slight tug .

HP quality control my ass! Ain't that something. I think before I replace anything,
I must first solder the original components in place.

Can you believe this? those components were hidden under the metal plate we talked about.

 

I guess in 30 plus years the unsoldered joints developed a little oxide coating that
is causing much of the problem . More resistance

 

Identify which components are not soldered, please.
This can help us analyze the problem.

 

Identify which components are not soldered, please.
This can help us analyze the problem.

Yes compare them with the pcb layout on page 109.

 

Two resistors one on right at bottom is 123 ohm

the two components large circle one looks like Q2 6 pins other has Q1 3 pins transistor

 

Q2 Q1 R7 R127 page 109

none of this surprises me . It was manufactured in the 70s . this period was a low point in the auto industry as well.
Harley Davidson was at an all time low , Chrysler was bankrupt, fuel shortage lines at the gas pump ect...

 

Q2 Q1 R7 R127

All these components are in Chan A and will cause the fault your trying to fix, so get em soldered up!

 

Dave I got it , fantastic news ITS FIXED!
Finally , But the problem was not the unsoldered pins, It happen to be " R2 Like you
suggested". R2 was completely gone it was supposed to be 5.6K but it measured
open . It was a bad resistor. I replaced it with a 10k precision adjustable pot.
Set the pot for 5.6K and it worked!

YOU HAD IT NAILED ! R2 YOU ARE A GENIUS

 

Dave I got it , fantastic news ITS FIXED!
Finally , But the problem was not the unsoldered pins, It happen to be " R2 Like you
suggested". R2 was completely gone it was supposed to be 5.6K but it measured
open . It was a bad resistor. I replaced it with a 10k precision adjustable pot.
Set the pot for 5.6K and it worked!

YOU HAD IT NAILED ! R2 YOU ARE A GENIUS

WELL DONE!! pleased for you.

 

here are the screen images of channel A at all freq settings

 

Congratulations!

However, if R2 was open it does not explain how Chan A was able to respond to DC levels from the function generator.

 

MrChips When I removed R2 and measured the ohms with my fluke 23 .
There was no current flowing through it . Meter indicated 0.L "overload indication"
Dead resistor or the resistance was so high is was unreadable with the fluke. Color code was green,blue ,red,gold
1/4 w