My dadgum 1090 quit again

[petehall347]

ok so back to square one well done with that . did you check for dry solder joints and cracks in the circuit board ? only common thing i can see it is both channels that wont adjust and may well be something in common at fault or same components for both channels happen to be bad .i cant see what it may be without studying the schematic .it is getting my bedtime .
i am sure you will get this fixed soon ..

 

[adc002]

I will have to order some 5.6k resistors (1/2 Watt).. Do not have any on hand... probably be a week getting them here.

All I can say is Stay Tuned.

 

[ecluser]

It's not really critical. For example, if you have a 4.7k you will have an offset of -3V at the output. But as I said, the offset is not what I am looking for. I don't mind about the output offset at the moment. You see, a transistor behaves like a variable resistor. Its equivalent resistance varies with the base signal. By pulling the transistor out of the circuit and replacing the transistor by a fixed resistor we should have a constant current in the bias stage and the bias voltage (between the collector and emitter of Q712) should be steady, and variable with the bias trim pot.

PS Think about old electronic (television and computer monitors for example) gears for a source of components, rather than orderring a few components.

 

[adc002]

.696vDC with NO adjustment at all..............
danggit man!
I did find a decent pair of AR2ax's for 10 bucks tho....pretty solid.
Just looking for a silver lining ya know.

 

[petehall347]

looks like bias stage is bad as quoted in post 98 .

 

[adc002]

Where do I go from here or is it the end of the road?


I have the Traveling Wilbury's End of The Line song stuck in my head now!

 

[bktheking]

you do these yet?

"Changed all the transistors on the LH except 704 and 702." :scratch2:

Did you do what ecluser said to try? Far from the end.

 

[petehall347]

don't give up now ...ecluser is on to something .its a little beyond me to be honest but i would like to learn more . apart from checking individual components .cold solder joints etc . i don't know .unless if a transistor checks good with a meter and breaks down in use . voltage readings in a live circuit may well find the culprit .

 

[bktheking]

Pete- that's what my my amp did, tested fine but one was leaking. As soon as replaced the same referenced pair, 702 704 bam, everything came into adjustment. It may or may not be the problem but it's worth a try.

 

[adc002]

Correction:
Installed the 5.6 resistor in E and C and measured 53.6mVdc ccw and 55.5 mVDC cw. Dimbulb never even lit in the slightest during this test.

I had it in B and E to begin with.

 

[bktheking]

You can't test your reading's on a dim bulb, the bulb will mess with the readings. However it appears you have adjustment now so you need to focus in that circuit as told from post 98.

 

[adc002]

you do these yet?

"Changed all the transistors on the LH except 704 and 702." :scratch2:

Did you do what ecluser said to try? Far from the end.

Yes, everything on the even side has been replaced....I changed 702 and 704 over the weekend with no change in values

 

[bktheking]

So you`ve replaced every transistor on the even side is what you are saying.

 

[ecluser]

Correction:
Installed the 5.6 resistor in E and C and measured 53.6mVdc ccw and 55.5 mVDC cw. Dimbulb never even lit in the slightest during this test.

I had it in B and E to begin with.

Pardon me for my questions but I presume that you installed a 5.6 K resistor in E and C holes for Q710, as I suggested.

So where did you measured 53.6mv? Between which points exactly?

What voltage do you have between the collector and emitter of Q712?

 

[adc002]

Pardon me for my questions but I presume that you installed a 5.6 K resistor in E and C holes for Q710, as I suggested.

So where did you measured 53.6mv? Between which points exactly?

What voltage do you have between the collector and emitter of Q712?

I did what you said installing the resistor in E and C of 710 and measured the emitters of 718 and 720.

I did not check 712 at all but I will first thing in the morning.

 

[adc002]

Here's the resistor I installed.

http://www.mouser.com/Search/ProductDetail.aspx?R=271-5.6K-RCvirtualkey21980000virtualkey271-5.6K-RC

 

[adc002]

UH oh....1/4 watt not 1/2 watt.

 

[ecluser]

1/4 W is not enough. I computed a dissipation of 1/3 W with the ±44V supply

Here is a simplified schematic of the circuit decoupled from the front stage. RQ710 is the resistor in place of Q710. All voltages are referenced to ground.

 

[adc002]

I would guess paralleling two of the 1/4 watts will not work either?

 

[petehall347]

I would guess paralleling two of the 1/4 watts will not work either?

if you do that you will end up with 2.8k at 1/2 watt .