Dave,
I really want to help, but it seems I'm not being specific enough about what measurements to take and where to take them. I understand the frustration of troubleshooting for several days and going nowhere with this thing. Leading you on a wild goose chase is the last thing I want to do here. However, I've got some data that I posted on the old forum that will most likely help you, providing we can take a breath and get on the same page in terms of how the measurements are taken.
To start with,
1. Take DC measurements from ground to specific test points, no signal applied.
2. AC signal measurements are taken relative to ground, except for the input and output jacks, which is across XLR pins 2&3. The differential output of the GR control amp is measured from the negative side of C19 to the negative side of C20.
3. When I refer to the high side of the input or output potentiometers, that is the pin where the signal enters the pot; the wiper is the adjustable point, and the last pin is connected o ground. With the pot at max, the high side and the wiper should be at the same point electrically.
So, for example, if you want to measure the signal level at the input or output pot, put one probe on the high side of the pot and the other probe to ground. If you repeat this procedure for both pots, you can divide the two results to obtain the gain factor between the input pot and the output pot(signal preamp). Once you have this number, hit log on your calculator then multiply by 20. this will give you gain in db's.
It sounds like you were literally measuring from the high side of the input pot to the high side of the output pot, which won't work.
What we're trying to do is measure signal level at specific points along the path, and then compare those numbers to each other as described above so we can see specifically where your gain loss is occurring. Once you find the spot, we can really focus in on the problem.
The data below was taken with no input transformer and without the output transformer reveresed. Installing input transformer only causes a small loss; abot 1 db or so. Reversing the output transformer gives a 12db boost at the output, so the measurement you get at the ouput XLR should be about 4 times greater than that listed below(1.25Vrms).
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Here are the results of the gain analysis of my G1176, as well as details of specific test parameters:
input XLR to output XLR gain = 29.9db
signal preamp gain(input XLR to high side of output pot) = 25.5db
signal line amp gain(high side of output pot to output XLR) = 4.4db
gr control amp gain[input of gr control amp (point 22) to differential output of C19 and C20] = 24.9db
The details of my testing procedure are as follows:
input pot = maximum
outpot pot = maximum
attack = full ccw
release = full cw
ratio switch = 20:1
meter switch = bypass(switched to gr for one phase of test)
input = 40mV RMS @ 1kHz at input XLR(pins 2&3, balanced connection)
output = 1.25V RMS across XLR pins 2&3
high side of input pot to ground = 40mV RMS
high side of output pot to ground = 750 mV RMS
juncture of R40 and R41 to ground = 1.2V RMS
With everything the same, except meter switch in gr position:
output = 1.05V RMS = 1.5db gain reduction(VU meter shows -1.75db)
high side of output pot to ground = 635mV RMS
point 22 to ground(input of gr control amp) = 320mV RMS
differential output of gr control amp(C19 and C20) = 5.6V RMS
DMM used for AC voltage measurments has 10M input impedance. It measures accurately at 1kHz(the manual states flat response up to 400Hz, but the difference at 1kHz is only about a millivolt).
Semiconductors substituted:
NTE 129p for BD518 (Q9)
NTE 128p for BD517 (Q
All static DC voltages check good per Jakob's measurements.
No input transformer.
No mods.
A quick synopsis of my exact Q-bias trimming procedure:
input = 50mv RMS @ 1kHz at input XLR(pins 2&3, balanced connection)
input pot = 12:00
Q-bias trimpot = full cw
output = 112mV RMS at output XLR(pins 2&3)
meter switch = bypass
ratio = 20:1
attack = full ccw
release = full cw
output pot = 2:00
I turned the Q-bias trimpot ccw until the output reading dropped to 100mV RMS(approximately 1db drop). This left the trimpot at about 2:00
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The checks I had you make before by removing Q1 and R26 are still in order, but the data needs to be collected and analyzed in this fashion.
Dean