G-1176 & unity gain

Has anyone run into this so far-

when inputing a .665Vac singal, with the unit in bypass and input and output half up, I get .129Vac output. With the input and output maxed, I only get .537Vac. Im using both lundahl input and output transformers with the output reversed. All my voltages appear to be close.

The other problem I am having which is driving me nuts, I cant get the 20:1 ratio to work, all the other ratios work, but no limiting in 20:1.

Any chance of these two things being related?

dave

Hi Dave,

The lack of unity gain is a result of an approximate 1.5db loss across the input transformer. You can measure to be sure. There should be no loss with the electronically balanced input.

I don't know what is up with the ratio problem. Trace the ratio switch, double check everything in the electronic viscinity...

Dean

hi dean-

thanks for the feedback- are you saying that with an input transformer, the unit cant even pass unity gain? I cant imagine that being right. In all my other 1176, there is tons of gain on hand, take a look at my voltages again, with both input and output MAXED, the unit isnt even making it to unity. I could see some loss from the transformers, but still would expect unity to be somewhere around 10 and 2 on the in out pots.

can anyone confirm that for me? Im assuming this is a problem.

dave

Woops! I misread your post. You have a serious lack of gain. With input and output at max, you should be getting a solid 40-42 db of gain at least(sorry, my calculator isn't handy to convert to mV, but the factor is around 100+.

I would only input about 50mV when cranking things to avoid possible clipping and false readings.

ok cool, thats what I thought.

I started off with the oscillator really low and then raised its output and the in/out tracked the same on the way up.

Assuming that my transistor voltages are good and the transformers are making a good connection, whats left? I think I have a short in the switches, I ordered new ones today so we'll see. Would it make sense that a bad connection in the ratio switch could lead to this? I get the same behaviour in GR and bypass mode. disconnected the VU, its the same.

whats odd is that the unit doesnt sound "broken", it actually sounds really fantastic, just low level out.

dave

Ok, with the input and output maxed, see what the amplification factor is between the input and the input level pot, then from the input level pot to the output. Those numbers are useful.

I need to find my calculator and my notes.

ok, with the input pot and output pot, with a .0602V applied to the input jack, I get:

input pot: .0402V
output pot: .0161V
input pot to output jack: .01V
outputt jack: .0498V

I need to check my transistor voltages again. How do I get "ok" looking voltages when there isnt amplification going on?

dave

Sh**t, I'm really getting ahead of myself. The number we're looking for is the gain from the INPUT to the INPUT POT, then from the INPUT POT to the OUTPUT POT, then from the OUTPUT POT to the OUTPUT. Three numbers.

Next, make the same measurements with R26 removed, or at least disconnected from one end. This will eliminate part of the ratio switch from the equation.

Depending on your findings, you will also want to repeat those measurements with Q1 out. This will eliminate problems the ratio switch entirely, along with any problems associated with the the Qbias supply.

What we need to see is where your attenuation is occurring. Is it in the signal preamp or the signal line amp? If it's the preamp, then look at Q1 and the associated Qbias circuit, which includes one half of the ratio switch. The other half of the ratio switch (through R26) forms a switchable voltage divider that determines signal level going to the GR conrol amp. A componet error or short here(including R26) might conceivably suck down your level.

Yep, all DC voltages need to be correct before starting. Just measure from ground to each test point as shown on Jakob's schematic. No signal.

did this before checking all the voltages but last time I checked they were good and I still had this problem, so-

I didnt know which side of the pot to measure, so gavve you both-

.06v applied to input

input to input pot pin1: .021V
pin 3: .06V

input pot to output pot, pins 1-1: .02V
1-3: .04V

output pot to output, pin 1: .034V
pin 3: .05V

removed R26 and got the same readings, removed Q1 (R26 out as well) and got the same readings.

stumper.

dave

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).

----------------------------------------

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
----------------------------

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

dean-

thanks for all the help, its appreciated. You have to remember Im a moron when getting into this stuff with me... Theres tons of good info in your post there, I need to go over everything but for now, here are some revised numbers-

input 50mv
input pot 33mv
output pot 13mv
output 82mv

q1 pulled
input 50mv
input pot 33mv
output pot 14.6 mv
output 91 mv

r26 pulled doesnt produce any change from the q1 pulled readings.

ok, so looks like the input to input pot gain is 3.6dB, input pot to output pot gain is 8.09 dB and output pot to output gain is 15.99dB.

dave

Dave
Have you checked out mnats' page on building the g1176? He's got some "what voltage should I see where" info up (there should be a link to the site somewhere in the Black Market--I'm too lazy to look). Also, I believe the schematic at Jakob's site has some further info of this sort marked up on it. All this stuff helped me a lot when troubleshooting my own G1176, though my problems were not the same as yours.

Tom

hey tom-

I had looked at mnats site recently and didnt find any voltage info, but the original schematic on the gyraf site includes voltages and is really helpful. All my transistors are reading close to what is documented, this is driving me nuts. I think I maybe have a bad solder on one of the transforfmers or a short in one of the switches.

here's my current voltages with +29.36V and -9.44V coming from the power supply:

E B C

Q2 .530 1.04 1.698

Q3 1.095 1.69 12.41

Q4 11.8 12.41 29.36

Q5 4.30 4.74 27.36

Q6 28.20 27.36 14.15

Q7 28.83 28.20 14.15

Q8 13.55 14.16 29.36

Q9 13.47 12.88 0

Q12 3.85 4.39 14.86

Q13 14.28 14.85 29.36

Q14 2.83 3.38 16.39

Q15 15.86 16.39 29.36


Im not overlooking something, those voltages look good, right?

dave

Dave,

Your DC voltages look ok.

I'll go over your gain numbers:

Input to input pot: -3.6db loss. That seems a little bit more than what I have, but is probably in the ball park. Use the 5532 input instead of the transformer input to eliminate this if desired.

Input Pot to Output Pot: -8db loss. Here is where the problem is. You should have an approximate +25db gain in this stage. I'm guessing a wiring or component error around Q1, Q2, Q3, or Q4 that would attenuate signal without greatly affecting your DC voltages(those look good). For a shotgun approach, you might try replacing Q2, Q3, and Q4 one at a time, after going over all your components and wiring/pcb traces.

Output Pot to Output: +15.99db gain. Looks fine

Dean

ok, restuffed the preamp, now it works... trial by fire. This was a good exercise though. Its fairly hissy with bc107's, anyone else feel that way? Im gonna try 2n3707's and see if there is any difference.

thanks so much for the help dean!

dave

what is the function of R11? Is that just a feedback gain set resistor for the preamp? Now that Ive got this thing working, Ive been playing with the value of that resistor, at its stock value, 562K I get about 20dB gain, but with a 90K resistor in there instead, I am seeing 19dB for the preamp. Thing is, the quality of the sound is totally different, way more than a simple 1 dB would suggest, the whole high end is smoother and the bottom is softer while with the 562K in there, its well more in your face kind of thing with a more full body. The quality of the sound gets creamier or less edgy as you lower the value of that resistor, this might be worth investigating the next time anyone of you guys have your box open, I just put a socket in there and started swapping ressitors.

whats that resistor doing exactly?

dave