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Old 26th September 2021, 08:14 PM   #21
Mooly is offline Mooly  United Kingdom
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OK

Quick thought... has someone linked the 0.22 ohm resistors out? Are they really 0.22 and not something much lower?
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Old 27th September 2021, 06:56 AM   #22
poundy is offline poundy  United Kingdom
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so took both out and checked both packages and all 4 readings 0.3 0hm
nothing linked out
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Old 27th September 2021, 07:06 AM   #23
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They had to be checked

So I would just look at the voltages now across one of the 0.22 ohm and see if it alters as the bulb becomes brighter.
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Old 27th September 2021, 07:21 AM   #24
poundy is offline poundy  United Kingdom
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ok will do


i had the relay out and also checked across it


coil voltage 21v and operates ok
both pretty much equal voltages of 700mv from common to switched on both channels
and 0mv current across both
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Old 27th September 2021, 07:42 AM   #25
poundy is offline poundy  United Kingdom
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Quote:
Originally Posted by Mooly View Post
They had to be checked

So I would just look at the voltages now across one of the 0.22 ohm and see if it alters as the bulb becomes brighter.
well they are -71mv across and when you adjust them, the value changes and goes up as the lamp glows but then immediatly goes back down to -71mv


its the -71mv bit, its pretty much a constant


something isnt right here, when you test for the idle as per the instructions i can only get +5mv on the left test point by adjustment, but if i also alter the other channel i can get it to +11mv, but then the lamp is realy bright



could something be leaking to ground?


im going to get to the bottom of this because its something simple i recon and proberbly staring me in the face.

Last edited by poundy; 27th September 2021 at 07:50 AM.
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Old 27th September 2021, 07:59 AM   #26
Mooly is offline Mooly  United Kingdom
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71 millivolt across 0.22 ohms is over 0.3A. That is a lot. I can't really see any way for anything to flow to ground tbh.

If the transistors have been replaced for different types in the past then that might upset the bias adjustment range.

You are going to have to start at the beginning with this one. Concentrate on one channel and begin by forcing zero bias on both. You can lift this resistor to do that.

Bulb should be out and there should be no voltage across either 0.22 ohm.

If that is OK then refit the resistor and work on one channel. If the bias voltage across the resistor is to high then try a 390 ohm or 470 ohm instead of 330 ohm.
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Old 27th September 2021, 08:07 AM   #27
poundy is offline poundy  United Kingdom
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thats exactly what i have already done, checked all of the voltages right from the power supply up to the output transistors and all of the voltages check out


could the other channel that i think is ok be causing it somehow


i have only checked the right channel
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Old 27th September 2021, 08:23 AM   #28
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I'm getting really confused with what you are measuring here

There should be no interaction between channels. If you turn the bias preset on one channel up or down and the bulb goes brighter or dimmer then the control is at least doing something that it should.

Quote:
thats exactly what i have already done,
So have you forced a zero bias condition on both channels? and is the bulb out or at least very dim?

Lets get it to that state first.
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Old 27th September 2021, 08:37 AM   #29
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well ive been doing realy what you said was best, be methodical


i wasnt getting the correct voltages upstream so i went back to the start, found R851 open circuit, then checked all the voltages all the way through to the output transistors and they check out with the diagram
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Old 27th September 2021, 08:55 AM   #30
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Quote:
Originally Posted by Mooly View Post
I'm getting really confused with what you are measuring here

There should be no interaction between channels. If you turn the bias preset on one channel up or down and the bulb goes brighter or dimmer then the control is at least doing something that it should.



So have you forced a zero bias condition on both channels? and is the bulb out or at least very dim?

Lets get it to that state first.
it is zero bias on both, this is with the pots in the positions as instructed before you attempt to make adjustment vr603 fully clockwise and vr604 fully anti clockwise.
with these in those positions the lamp is very dim but as you adjust them the lamp gets brighter, but no change on the meter reading.



on the instructions it refers to TP1 and TP2 ok,

A-method-so ive been using the bottom pin common to both resistors and i get 0v and no movement.


B-method-if you use either of the top pins you can adjust it to say 8mv and what you end up with is -8mv on the other side which i guess you would expect


with both sides adjusted the lamp is still fairly well lit up.



so which pin would you use, it doesnt specify in the instructions
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