All things G1176 - the new "repost" thread.

[gswan]

Q14/Q15 look fine, since I am assuming this is in GR bypass mode. Q15-C is tied to +30V anyway, as is Q13-C.

BTW, there's no such thing as 'bypass' on this compressor, only 'GR disable'. The signal still goes through the compressor, but the GR circuit is not engaged. If you are seeing a 6dB drop in signal between GR enable/disable then check your FET gate-source voltage difference between GR enable/disable (with no signal passing through). If it changes then you most likely have a wiring or component error in the ratio switches.

 

[dustbro]

I was comparing voltages to the schematic and other peoples findings and it seems that Q15 should read 17.25V - 16.72V - 30.3V ... and I'm over 10V off on this one.
Is R69 supposed to be a drop down resistor to get the 27.8V down to 17.25V? cause I'm only getting a drop of about 1V... seems more like it would have to be a 442K resistor (not 44.2) to get it all the way down there. Or does the voltage come from the other end and flow thru the transistor? Thru C23, R73, and R72?
Going to check the FET voltage when I get to the studio.
thanks again!

 

[dustbro]

[quote author="gswan"]If you are seeing a 6dB drop in signal between GR enable/disable then check your FET gate-source voltage difference between GR enable/disable (with no signal passing through). If it changes then you most likely have a wiring or component error in the ratio switches.[/quote]
Just tested... same voltage on the FET's when I switch between bypass and GR.

 

[gswan]

[quote author="dustbro"]
Is R69 supposed to be a drop down resistor to get the 27.8V down to 17.25V? cause I'm only getting a drop of about 1V... seems more like it would have to be a 442K resistor (not 44.2) to get it all the way down there. Or does the voltage come from the other end and flow thru the transistor? Thru C23, R73, and R72?[/quote]

R69 will only drop the voltage when there is current flowing through it. This needs Q14 turned on a bit more.

Take a look at Q12, your Base voltage seems a bit high. You might want to check the resistor values around here.

Also check R67, R71, R73 values.

 

[gswan]

[quote author="dustbro"][quote author="gswan"]If you are seeing a 6dB drop in signal between GR enable/disable then check your FET gate-source voltage difference between GR enable/disable (with no signal passing through). If it changes then you most likely have a wiring or component error in the ratio switches.[/quote]
Just tested... same voltage on the FET's when I switch between bypass and GR.[/quote]

Then you should not be seeing a 6dB drop in output signal level, since the FET is the only thing that is supposed to be reducing the level when GR is enabled/disabled. Where is the drop in gain happening? I assume you are measuring the input and output levels with an external meter.

 

[dustbro]

[quote author="gswan"]I assume you are measuring the input and output levels with an external meter.[/quote]
I am checking with an external meter from my console. If all of the resistors are the proper value, how would you suggest tracing the signal to find where the drop is?
Its funny that this drop didnt happen when I had a 47K in place of the 4.7K (r63)

 

[gswan]

Did you calibrate the Qbias it for a 1dB drop (GR disabled)?

It sounds like there are problems in the GR amp circuitry. Check your transistors as well as the resistors. Is R68 47K and R70 4.7K?

 

[dustbro]

[quote author="gswan"]Did you calibrate the Qbias it for a 1dB drop (GR disabled)? [/quote]
I did. Even if I crank up the Qbias so the FET is not conducting at all it has the drop. If I pull the FET, all is good. This is going to take some investigation....

 

[dustbro]

[quote author="gswan"]Is R68 47K and R70 4.7K?[/quote]
yup. both of these are correct
When I pop in my FET it drops volume. I think I remember reading something about that.

 

[gyraf]

When I pop in my FET it drops volume.

Verify that you have the BF245 "A" version - maybe try replacing.

Can maybe also be excessive leakage in the ultra-low-leakage FD333 diodes.

Jakob E.

 

[gswan]

[quote author="dustbro"][quote author="gswan"]Did you calibrate the Qbias it for a 1dB drop (GR disabled)? [/quote]
I did. Even if I crank up the Qbias so the FET is not conducting at all it has the drop. If I pull the FET, all is good. This is going to take some investigation....[/quote]
What do you get as Vgs on Q1 when you have Qbias fully CCW? It is starting to sound like the FET is misbehaving.

 

[gyraf]

If the fet conducts without q-bias (that is, reduces gain when inserted, not when removed), you either have a bad FET, or the q-bias voltage range is wrong due to some other error..

Jakob E.

 

[dustbro]

Wow! a doubleteam on my problem
[quote author="gyraf"]
Verify that you have the BF245 "A" version - maybe try replacing.[/quote]
Definitely BF245A
[quote author="gyraf"]Can maybe also be excessive leakage in the ultra-low-leakage FD333 diodes.[/quote]
Leakage... as in voltage? I have 6.66V at one end and 2.7V at the other end (on both diodes on all 4 units). does that look correct?
[quote author="gswan"]
What do you get as Vgs on Q1 when you have Qbias fully CCW? It is starting to sound like the FET is misbehaving.[/quote]
At the wiper of the Qbias pot I have 2.7 volts. byt the time it gets to the gate of Q1 it is 2.2V
[quote author="gyraf"]If the fet conducts without q-bias (that is, reduces gain when inserted, not when removed), you either have a bad FET, or the q-bias voltage range is wrong due to some other error..[/quote]
I still have my FET matching gig laying around and these guys seem to be reacting the same as they did before. For example, during the matching process it took 1.98V on the gate to make a 10% drop in mA at the drain.. and 1.84V to make a 50% drop. Do these voltages seem to be within reason of this transistor? or did I mess something up and fry them in the process?

 

[gswan]

You might need to re-check the FETs in the test jig. They might be conducting even when the bias pot is fully CCW. Check the pot value and R62/R63 as well, althout -2.7V is in the right range. Check to see whether the FET is conducting with -2.2V on the gate. I had a few FETs that would have been in conduction at the fully CCW setting had I not checked them first.

 

[dustbro]

[quote author="gswan"]Check to see whether the FET is conducting with -2.2V on the gate. [/quote]
Ah ha! Checking for conductance has give me a new piece to the puzzle. With no input signal there is no change in voltage at the gate. BUT!! with the tone generator feeding the input, the gate voltage changes from -2V to +.75V when I switch between bypass and GR.

 

[gswan]

How much signal are you feeding in from the generator?

Check the ratio switch wiring and resistor values first.
Particularly those between the output pot and the input of the GR amp.
Also make sure your BOM is correct.

 

[dustbro]

[quote author="gswan"]How much signal are you feeding in from the generator?
[/quote]
0dB... anything above the 48 marking on the input starts reducing the voltage. Midway between 48 and 36 the voltage becomes positive.
Am I reading the schematic right when I see that the GR bypass switch just pulls out R23-R25 from the circuit?
I pulled all the resistors from the ratio switch to double check, and everything looks fine there.

 

[dustbro]

Ok... finally solved!!
Very strange thing. a resistor has the color bands of a 7.68K resistor... it's in a 7.68k resistor bag... but it measures 224K. I never would have found this without dissecting one of my units.
that was not fun.
thanks for all the help along the way
Dan

 

[gyraf]

:shock: now THAT is one serious and luckily rare error!!!!

 

[gswan]

The resistor could be damaged (hence measuring a higher value) or, if it's consistent across both units you have really bad luck with your component supplier.

I always recommend measuring values before fitting them, just as a safeguard, since I've see reds, browns and oranges that all look similar from different manufacturers.

Glad to hear that the problem is now solved. Hopefully you can proceed right through the calibration procedure without a hitch.