[BUILD] 1176 Rev A - Back to the beginning...

[TillM]

My Old Fat Rack Revision A gets my hair off !
After I realize with my Oscilloscope that my Altran input transformer is broken, I ordered a new one from Mike.
Now this happen.
When I put a 1khz sine wave into my input, I can see the sine wave on the XLR pin, also the t-pad works, now I also have the signal at the input transformer pin 8. (With the broken Input transformer the signal don't)
Now this happen. On R5 and R4 I still get the input signal, but at R6 its gone.
I can't calibrate my Meter, cause also on R7 and R65 the signal don't come through.
I checked all resistor with the schematic and every single resistor is in the right position.
What could be the problem, that after R4 and R5 the signal don't come through ?
On which parts I should have an eye on?
I appreciate every help.

 

[scott2000]

Have you checked all of the DC reference voltages everywhere and compared them to the schematic?

 

[TillM]

yes, this is what I got.
And I think, it's okay for an uncalibrated unit.
I test in gr-Mode, the jumper was nearest the trimpot.
I hope, it's clearer now. I don't find the mistake since months.

 

[weiss]

yes, this is what I got.
And I think, it's okay for an uncalibrated unit.
I test in gr-Mode, the jumper was nearest the trimpot.
I hope, it's clearer now. I don't find the mistake since months.

bei dir scheint der Fehler auch am gleichen transistor zu liegen wie bei mir 

 

[TillM]

The same transistor ? That’s weird.
I buy super matched 2n5457, I hope that they are not broken.
I really don’t know why the comp don’t work

EDIT: When I test it now with my oscilloscope the sine wave stops after the transformer input again and don't react to the t-pad.

 

[Echo North]

The same transistor ? That’s weird.
I buy super matched 2n5457, I hope that they are not broken.
I really don’t know why the comp don’t work

EDIT: When I test it now with my oscilloscope the sine wave stops after the transformer input again and don't react to the t-pad.

Pull Q1, see what happens.

 

[echo zulu]

Trying to solve a Rev A build problem for a friend...

GR mode sort of works when the ratio switches are set to 20:1 or 12:1 but when set to 8:1, the meter won't zero; it only goes to about -7 on the meter. At 4:1, it won't even budge off the pin. The output is also very low on the 8 or 4 ratio settings, with weak limiting. Whenever those ratios are selected, it looks like FET Q1 is pulling the gain way down.

I have checked all the component values, transistors, etc. and they look like they are OK and installed correctly. The wiring also looks right. I've also checked the PCBs for shorts and bad solder joints - all look OK. I calibrated the comp using JBL's procedures and I think that's correct as well.

Could a failing Q1 be causing this?

 

[Echo North]

Trying to solve a Rev A build problem for a friend...

GR mode sort of works when the ratio switches are set to 20:1 or 12:1 but when set to 8:1, the meter won't zero; it only goes to about -7 on the meter. At 4:1, it won't even budge off the pin. The output is also very low on the 8 or 4 ratio settings, with weak limiting. Whenever those ratios are selected, it looks like FET Q1 is pulling the gain way down.

I have checked all the component values, transistors, etc. and they look like they are OK and installed correctly. The wiring also looks right. I've also checked the PCBs for shorts and bad solder joints - all look OK. I calibrated the comp using JBL's procedures and I think that's correct as well.

Could a failing Q1 be causing this?

Doubtful.  More likely an improper resistor value on the ratio PCB.  Probably on the DC side on that 8:1 position.

Confirm with the Pad 22/21 tests:
https://www.hairballaudio.com/blog/resources/post/fetrack-troubleshooting-guide

Mike

 

[echo zulu]

Thank you for responding. Except for the 20:1 range, the ac voltages are very low:

20:1 = .353
12:1 = .060
8:1    = .014
4:1    = .004

The resistors and connections appear to be correct but it looks like I will have to remove the ratio board and trace out the circuit to be sure.

Is there an assembly drawing available for this board? ...That would help immensely. This must be an early version, as there are no component designations or markings on it of any kind.

 

[echo zulu]

I traced everything out on the Ratio Board and the resistor values are all correct except for R19, which is 68k instead of 130k. I wouldn't think that would affect the issues at hand but I tried subbing a 120k for the 68K anyway (I didn't have a 130k handy) but it made no difference. In fact, completely removing R19 has no effect at all - the circuit still misbehaves the same way.

The connections to the board look correct so I'm not sure where to go from here...

 

[Echo North]

I traced everything out on the Ratio Board and the resistor values are all correct except for R19, which is 68k instead of 130k. I wouldn't think that would affect the issues at hand but I tried subbing a 120k for the 68K anyway (I didn't have a 130k handy) but it made no difference. In fact, completely removing R19 has no effect at all - the circuit still misbehaves the same way.

The connections to the board look correct so I'm not sure where to go from here...

http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

Something is bad at R19.  Removing it will kill all compression 12:1 and below.  You need to get a 68K in there and then confirm all the continuity around that switch.

Mike

 

[echo zulu]

I appreciate your help ...thanks for the link.

Continuity on the ratio board is all good. I put the 68k (5%) back in. It looks like there's a jumper on the right side in place of R78. I tried replacing that with a 47k (according to your schematic, I think that resistor belongs there) but it didn't make much difference.

Here's a photo of the board (the resistor in the shadow between the first two switches is a 56k):

 

[echo zulu]

Since the Ratio Bd. looks OK, any thoughts on what to check next?

>Thanks.

 

[Echo North]

Thank you for responding. Except for the 20:1 range, the ac voltages are very low:

20:1 = .353
12:1 = .060
8:1    = .014
4:1    = .004

The resistors and connections appear to be correct but it looks like I will have to remove the ratio board and trace out the circuit to be sure.

Is there an assembly drawing available for this board? ...That would help immensely. This must be an early version, as there are no component designations or markings on it of any kind.

These voltages still the same?

 

[echo zulu]

Yes.

 

[TillM]

So after I made a totally new rewiring of my unit the meter finally goes on zero when I start the unit in gr mode.
But, it still don’t react when I put the 1khz sine wave in.
I can see with my oscilloscope that the input react.
But i can’t bias the q-bias cause at the output, I don’t get the 2,775(?) vac.
But, when I turn everything on, the output transformer makes a high noise, which should be normal as i read.
Pulling Q1 change nothing, but now all voltages seams to be okay, after I checked them.
Where should I look next ?
Appreciate every help.

 

[Echo North]

So after I made a totally new rewiring of my unit the meter finally goes on zero when I start the unit in gr mode.
But, it still don’t react when I put the 1khz sine wave in.
I can see with my oscilloscope that the input react.
But i can’t bias the q-bias cause at the output, I don’t get the 2,775(?) vac.
But, when I turn everything on, the output transformer makes a high noise, which should be normal as i read.
Pulling Q1 change nothing, but now all voltages seams to be okay, after I checked them.
Where should I look next ?
Appreciate every help.

What voltages are you getting for test #2?

https://www.hairballaudio.com/blog/resources/post/fetrack-troubleshooting-guide

 

[TillM]

What voltages are you getting for test #2?

https://www.hairballaudio.com/blog/resources/post/fetrack-troubleshooting-guide

Hi Mike, thanks for your help.
Well, I already saw it, but could you give me an advice for the Testpoints ?
Testpoint at the Outputtransformer is clear for me.
Is TP1 on the Input transformer and TP15 and TP17 at the output potentiometer ?

Edit: If the Testpoints are the right one, I get this:
TP1 0,243vac
tp15 0,001vac
tp17 0,001vac
output 0,001vac

After check your recommends, I would say the signal section except some to high voltages is okay.
Wrong wiring of the output transformer I would rule out.
But it's definitely something wrong with my output section, thats for sure.

Q2
2,22V = 4,51v
6,73V = 13,98v
4,51V = 6,94V

Q3
13,74V = 13,32V
6,13V =  13,23V
R17 = 29,62V

 

[Echo North]

Hi Mike, thanks for your help.
Well, I already saw it, but could you give me an advice for the Testpoints ?
Testpoint at the Outputtransformer is clear for me.
Is TP1 on the Input transformer and TP15 and TP17 at the output potentiometer ?

Edit: If the Testpoints are the right one, I get this:
TP1 0,243vac
tp15 0,001vac
tp17 0,001vac
output 0,001vac

After check your recommends, I would say the signal section except some to high voltages is okay.
Wrong wiring of the output transformer I would rule out.
But it's definitely something wrong with my output section, thats for sure.

Q2
2,22V = 4,51v
6,73V = 13,98v
4,51V = 6,94V

Q3
13,74V = 13,32V
6,13V =  13,23V
R17 = 29,62V

15 and 17 are pads on the output pot.  You should have 2-4 VAC at 15.  If you have nothing there, your pre amp section is an issue.

TP1 is is at the input transformer output, so the side of R5 that connects to the input transformer secondary. 0.243 VAC is actually a little on the high side, but that can be because of you signal strength or tpad tolerance. Either way your input seems fine.

Seems like maybe your Q2 could be bad.

Mike

 

[TillM]

Hi Mike,
I changed Q2 but this change nothing. The voltage on Q2 is nearly the same.
VAC at my input is 0,089 VAC. So I guess this is good.
The funny thing is, that the compressor passes the sinus, but everything is really low noise with everything turned up to its maximum.
I already checked my Output potentiometer with my DMM, but the resistance is right.
I still get the same VAC at the test points.
I've really no idea. I even resoldered some areas, and I would say that I'm really expired with soldering.
Where can I look now ?