All things G1176 - the new "repost" thread.

[btproductionsaustralia]

No short on the E of q4...
r20 is 1.8k as required...

Should I replace q3 in case it is the issue as I have some spare BC107A?

 

[gswan]

And the voltage across R20?

 

[btproductionsaustralia]

Interestingly( i thought) the voltage on r20 was 1.19 on 1 side and 0 on the other...

Sorry for delayed reply
I have replaced Q3 to be safe...still read the same at Q3 and Q4 afterwards...

Ben

 

[gswan]

OK, it seems that Q4 is the most likley path for the extra current then. We should see about 11.8V on the base and 11.2 on the emitter.
Is R21 really 7.5K?
Does the R16 pot adjustment make any significant difference to the voltage read on Q4-E (it shouldn't)?

 

[btproductionsaustralia]

Geoff,
have checked R21, it is 7.5 k, and the voltages read 1.5 and 1.5.

The R16 trim does not effect the readings on Q4...

What is up stream from the Q4?

Ben

 

[Biasrocks]

I've inherited an aborted G1176 build.   :-[

When I received it, the secondary was shorted to ground by the previous builder. He soldered the ground and 25 volt wires together and connected them to ground. I was able to get the transformer sorted out. Unfortunately the entire board is populated so I can't just troubleshoot the PS by itself. I'm getting good voltages up until the 7824 reg. I'm getting my -10volt supply okay, and I have around 35volts going into the 7824 reg when I pulled the reg to check. When I reinstall the 7824 reg, where I was getting 35 volts it slowly ramps up to around 25 volts and on the other side of the reg, I'm only getting around 2.8 volts, middle leg of the reg is around .9 volts and the reg is running *very* hot. I suspect a short to ground somewhere, but there is nothing shorted to ground on the reg itself.

Any ideas where I can start to see what's pulling down the regulator? I've already lifted R49 at the 30volt tp. Can I lift other resistors to isolate the problem area?

This is the Mnats rev j version, is there a schematic of this beast anywhere?

Mark

 

[gswan]

Geoff,
have checked R21, it is 7.5 k, and the voltages read 1.5 and 1.5.

The R16 trim does not effect the readings on Q4...

What is up stream from the Q4?

Ben

Q4-E is decoupled from the next stage by C9, so it does not affect the DC bias conditions.
Check the value of R11 as well.

 

[gswan]

Mark,

I think you will find that the centre-tap of the transformer secondary is supposed to be connected to GND.

Firstly check that the electrolytic caps are inserted correctly. The regulator is clearly overloaded, so look for anything else getting hot. Check the PCB for solder splashes.

 

[Biasrocks]

Mark,

I think you will find that the centre-tap of the transformer secondary is supposed to be connected to GND.

Yes, I repaired that right away when I found it.

Firstly check that the electrolytic caps are inserted correctly. The regulator is clearly overloaded, so look for anything else getting hot. Check the PCB for solder splashes.

I made some progress. I  removed 4 BC107 transistors (Q12,13,14,15) and Q10 and managed to get the PS to spit out 35 volts, so I think I'm on to something. I'm going to put those back in and see what happens now. Keeping my fingers crossed.

Mark

 

[btproductionsaustralia]

Geoff,
have checked R21, it is 7.5 k, and the voltages read 1.5 and 1.5.

The R16 trim does not effect the readings on Q4...

What is up stream from the Q4?

Ben

Q4-E is decoupled from the next stage by C9, so it does not affect the DC bias conditions.
Check the value of R11 as well.

Geoff,

R11 is 560k as required. Voltage on R11 reads 1.6 and 1.7 

Any suggestions where to head next...
Is there anywhere that has a simple description of how to follow through the signal from power supply to end...Of course the schematic, but the only 1 I have is the original 1176 schematic, which of course is different to mnats J rev...

Ben

 

[Biasrocks]

Alright, some progress.

I've got the power supply issues sorted, my voltages seem to be in a good range.

My immediate problem is when switching the VU to monitoring gain reduction with no signal, the meter pins to past +3. I tried adjusting the ZERO ADJUST trimmer but it has no effect.

Any ideas guys?

Mark

 

[gswan]

Perform the test and calibration procedure.

 

[Biasrocks]

Perform the test and calibration procedure.

Are you talking about the Q Bias adjustment detailed here?

http://www.geocities.com/m_natsume/1176_revision_d.html#videos

Unfortunately, my meter is pinned so I'm not sure how I'm going to get the meter to drop 1db.

Is there another procedure I should be doing first?

Mark

 

[gswan]

Yes. First check the DC bias conditions of all your transistors.
Then wind Qbias out (inactive) and test the gain of the input and output amplifiers.
Now perform the Qbias adjustment. This is done using the +4dBu metering, since you are looking for a 1dB drop in output. Then you can calibrate the GR tracking.

 

[Biasrocks]

Yes. First check the DC bias conditions of all your transistors.
Then wind Qbias out (inactive) and test the gain of the input and output amplifiers.
Now perform the Qbias adjustment. This is done using the +4dBu metering, since you are looking for a 1dB drop in output. Then you can calibrate the GR tracking.

Okay, I'll give it a try.

I've seen various charts detailing the DC bias of each transistors, is there a definitive one that I can refer too?

Thank you.

Mark

 

[gswan]

I have one here:
http://www.axtsystems.com/index.php?view=article&catid=34%3A1176ln&id=57%3A1176lnproblems&option=com_content&Itemid=62

I wouldn't call it 'definitive' though, since it's based on actual measurements and you must account for component tolerances and temperature. However provided you are within about 5% of the values you are OK.

If something is way off (as is currently the case with Ben's unit) then it will show up straight away and we can set about looking at the cause. Most are caused by incorrect component values and insertion of transistors. Sometimes you can get a bad transistor from new, but that's not so common (unless you are using Chinese fakes).

 

[btproductionsaustralia]

Hey Geoff

R11 is 560k as required. Voltage on R11 reads 1.6 and 1.7 

Any suggestions where to head next?

Thanks for the help

Ben

 

[gswan]

Ben,

The input amplifier is DC decoupled by C1, C5 and C9. Somewhere in this 3-transistor amplifier is a wrong component or faulty transistor. It's about a dozen resistors and 3 transistors. Did you replace Q4 at all? Short of having the unit if front of me to examine and probe, I can only suggest the obvious areas to check. You will need to re-check all of these components carefully. Something may be causing the whole section to oscillate (which you will not see on your multimeter) so double-check the capacitors (C2, C6, C5, C8) just in case.

BTW, I have a circuit that I drew of my unit, which is much the same as Mnats Rev J except I added a couple more compression ratios and did not use the on-board PSU. It may help you during fault finding.
http://www.axtsystems.com/index.php?view=article&catid=34%3A1176ln&id=45%3A1176lncircuits&option=com_content&Itemid=62

Geoff

 

[btproductionsaustralia]

Geoff,

ok I'll replace Q4, I previously replaced Q3...
Thanks for the schematic...
It's become clear to me that I'm now out of my depth, as this is my first build...
I don't want to impose, but if I really can't get this going, would you consider being hired to check it out?
I live in Northern NSW so would ship the board down and at this point I'd give my first born to have it runnig...I know this is not really DIY etiquette, and I did really want to do it all myself, but it seems i've hit something that a newbie may never crack?...

Anyhow, let me know and thanks for the help

ben

 

[btproductionsaustralia]

Geoff,

I have just noticed on your site it says...
to check the voltages with the compression disabled and ratio set to 20:1...
I've been checking them with the ratio board etc all disconnected...this wouldn't have an impacts would it?

Cheers

ben