1176 FDH333's?

scenaria · 02-09-2004 04:38 PM

quick question...

is the DC on these diodes suppose to go up and down proportional to the input level control?

scenaria · 02-09-2004 05:24 PM

soo heres the deal...

ssltech atcually took a look at this MC76 (I should mention this was one of the PCB's from andrew) today for me and demonstrated that the "pinch-off" voltage wasnt being met to control the fet....

My GR meter isnt sitting at 0-vu rather its all the way to the left when I switch to GR

the audio is passing through the compressor just fine (tons of gain)

what I notice is that the release knob will cause the gain to go up and down?

I have checked all of the wiring 4 times over .... even compared it to a Urei 1176.... and cant find anything out of order.

guess im looking for some thoughts and ideas on how to track this issue down...

I didnt wire in any of the stereo link circuitry....theres nothing I need to bypass in here is there?

any thoughts?

**Kev** · 02-09-2004 05:29 PM

Originally posted by scenaria:
... what I notice is that the release knob will cause the gain to go up and down?

I have a suspicion this is telling us something but I don't have a schem in front of me to help think it through.

scenaria · 02-09-2004 05:54 PM

http://www.purpleaudio.com/pdflib/PurpleMC76.pdf

page 17 and 18

drfrankencopter · 02-09-2004 06:15 PM

I was just reading this (the manual)....I take it you have set the Q bias according to the instructions on page 14.

You're supposed to use an external meter for this test, with the input of the 1176 set to 24, attack at off, and release on full with compression ratio set to 20:1, and the meter set to GR. With a 1khz signal at 0dB, adjust the output pot to read 11dB on an external meter, then turn the bias pot down (start fully clockwise) until the output drops by 1dB.

Next proceed to the meter calibration.

Or, have you done this already?

Cheers,

Kris

scenaria · 02-09-2004 06:23 PM

the problem is I cant even get to this point as when I switch the meter to GR it doesnt reflect a 0 VU level. it sits at the bottom of the scale... I have tried the "bias" trim and I cant get it up high enough (If I remember right keith got the fet to rspond at 1.6 volts..)

drfrankencopter · 02-09-2004 06:40 PM

I figure that's why they suggest using an external meter for the Q bias adjustment. They figure that the VU meter calibration is no good at this point in time. Note, that in the manual, all dB voltages are relative to 0.775V RMS. You should be able to just use a volt meter if you don't have an external meter handy.

Are you sure your meter switch is wired correctly?

Also, can you hear the unit compressing (i.e. is it doing any gain reduction)?

Cheers,

Kris

**deanp920** · 02-09-2004 10:46 PM

Scenaria,

is the DC on these diodes suppose to go up and down proportional to the input level control?

The FDH333's have a constant negative voltage on the common side, which is also the bias voltage for the Q1 BF245A and Q10 BF245A. At the diodes, this voltage is around -2VDC.

Some of this negative voltage is applied to the other side of the diodes through R74 and R75. This voltage sets the compression threshold, and changes as the ratio switch is adjusted. At the junction of R74/R75, this voltage is around -6VDC.

The GR control amp amplifies a sample of the audio input signal, such that the AC output of the GR control amp is rectified by the FDH333's, which creates a positive voltage that proportionally cancels some of the negative bias on Q1 and Q10, thus causing them to conduct more, fall in resistance, and attenuate the input signal(Q1), and control the GR meter driver amp(Q10).

The catch is that the differential AC output of the GR control amp must swing positive beyond the ~-6 volts of bias applied at the R74/R75 juncture before the FDH333's can rectify the signal and produce the aforementioned positive-going voltage that offsets the Q-bias, causing gain reduction(and meter deflection when in GR mode).

My GR meter isnt sitting at 0-vu rather its all the way to the left when I switch to GR

the problem is I cant even get to this point as when I switch the meter to GR it doesnt reflect a 0 VU level. it sits at the bottom of the scale... I have tried the "bias" trim and I cant get it up high enough

This problem sounds like something I've experienced with both of my clones. The symtoms are similar, at least.

In GR mode, I took a small screwdriver or my finger and put some gentle, yet firm pressure on the body of CR11(1N4148).

While doing this, I saw the meter jump up a few times.

I replaced the diode, leaving it up off the board a bit.

That fixed it.

Hope that helps,

Dean

Thread: 1176 FDH333's?

LinkBack

Thread Tools

Display

Similar Threads

1176 panel

1176 resistor value

1176 Hookup...

Overall costs on an 1176, LA- 2

LA-2 or 1176

Bookmarks

Bookmarks

Posting Permissions