Greg:
No, it was copied, carefully, from the original schematic. I would have scanned the original schematic, but it is very fuzzy. I did check my schematic against the other channel's schematic, which was a bit clearer but did not have the expected voltages indicated, and was cluttered with the actual component values.
I have checked, re-checked, and RE-checked the accuracy of my hand- drawn schematic, comparing it to the original, and there are no mistakes.
I have just discovered that the voltage measured directly at the emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere from -300mV to -16mV.
The emitter of Q204 has a strange DC path. The amplifier appears non inverting. What is the voltage on Q204 collector ?
greg
Q204 is in a pretty simple DC feedback path with Q205 and the 207/8 combination. Its emitter currrent flows down through Q208 to ground.
The amplifier is non-inverting from the base of Q204, but inverting from its emitter, which is where both the DC and AC feedback end up.
d
Greg:
The voltage at the collector of Q204 measures +11.09VDC.
Thats no good. It seemed most was OK, and I imagined you have no scope. I was going to suggest injecting a signal at the bases of the imput stage. Holding a metal probe with your hand should give equal sounds from left and right on corresponding points of each amp.
You are going to have to check the Q208 more thoroughly.
greg
By subtraction from the 7.3 V you said was present at the junction of the two 3.3 ohm resistors there's nominally 7.3 to 7.6 V across the lower one.
So it looks like:
- that lower 3.3 ohm resistor is open circuit; or
- it's dissipating about 16 watts because of another fault and presumably getting rather hot which may lead to it going o/c soon (depending on its power rating which I would guess doesn't support a 16 watt dissipation); or
- it isn't actually connected to the emitter of Q208 so check the soldering.
You show the collector of Q208 as being ground. Is that same ground not the reference for all your reported voltage measurements. How could it possibly be negative? There are no sources of anything below ground shown in your schematic.
What is the voltage on the "to volume control pcb" node at the bottom left corner of the circuit? Is it ground/zero? Is that node *really* disconnected from the rest of the ground reference (collector of Q208, bottom of the 680 resistor, etc.)? It seems like it ought to be ground, but you show it as a separate node.
It seems suspicious that the base of Q204 is measured to be the proper voltage, but the collector seems to be pretty much up at the supply rail. That indicated to me that Q204 isn't conducting properly and preventing proper bias on Q205.
We have asked you several times about measuring the equivalent nodes in the other (working) side, but you seem to have ignored this valuable source of information about how a proper circuit behaves.
Last swap meet I went to I saw at least half a dozen scopes under 10 bucks. A couple of them were even decent semi-modern looking solid state things, probably 10-15 MHz, so this stuff does turn up.
Richard:
Sorry about not posting that information. I wasn't ignoring this request, I promise. Just hadn't gotten around to it yet.
I will post the same schematic, but this time with the measurements taken from the "good" channel.
And I will also take measurements at the node(s) you inquired about.
OK, here is the schematic of the "good" channel, with voltage measurements indicated.
I realize that some of the voltage indications are redundant, but I decided to "over-label" in the interest of clarity....
....And I should point out that ALL measurements taken in both channels were obtained using chassis ground.
I haven't read every follow up message but have you tried removing the electrolytic caps and retesting the d.c.? I've seen the d.c. stability been thrown off in many amps due to bad coupling caps.
Good luck.
-- David Farber David Farber's Service Center L.A., CA
I completely missed the star explanation. Why not put measured voltages there. What is the voltage at collector Q204 on other channel. Q205 doesn't seem to be conducting much.
greg
Are those the voltages you are NOW getting ?
Those look correct.
Graham
STOP THE PRESSES!
THE AMPLIFIER PROBLEM HAS BEEN *SOLVED* !!!!!!!!!!!!!!!!!!
SEE MY POST REGARDING THIS!!!!!!!!
MANY, MANY THANKS TO EVERYONE WHO OFFERED HELP, TIPS, AND SUGGESTIONS!!!!!!!!!!!!!!!!!!!!!!!
Maybe we can improve the design ?
greg
And the winner is...????
jak
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