All things G1176 - the new "repost" thread.

[3nity]

sorry Geoff i just dont find it in the board it self (R50)

 

[gswan]

Maybe your board set does not cater for the +8dBm setting. I've seen a few clones that leave this out. I find it useful, so I left it in.

 

[3nity]

Anyone knows where can i find the 5M pot?

 

[gswan]

[quote author="3nity"]Anyone knows where can i find the 5M pot?[/quote]

I also had problems locating a suitable 5Meg pot.
Have a look here for some suggestions on how to overcome this.

http://www.axtsystems.com/index.php?view=article&catid=34%3A1176ln&id=47%3A1176lnreleasepot&option=com_content&Itemid=62

 

[kenrinc]

Well I've got some more time to troubleshoot so I'm going to need to start somewhere. I've removed Q1 I still get signal in and out what sections do I need to check? It's unclear to me where the amp section is (Q2 Q3 Q4?) I'll have a scope here this weekend and I intend to go through and check them against the pics on your site. So if the voltages I posted earlier look OK where do I move to?

Ken=

 

[gswan]

Ken,

Start by injecting a signal (1Vp-p 1kHz is a good start) into the transformer input. Probe at the top of the input pot (wrt GND) first to check that you have the correct signal level coming through the transformer.

Now probe the signal pre-amp section (Q2 - Q4) first to check that you are getting the correct amount of gain. You probably don't need the input pot wound all the way up here, just check the amount of signal on the wiper and the amount of signal at the top of the output pot. The gain should be about 26dB.

Next, if all looks well there, check the output driver amp section (Q5 - Q9). It has a gain of around 10dB. The waveforms should not be distorted at C14+ either.

Once you have the signal path amplifiers working correctly then you can start examining the gain reduction amp and rectifier. Use the 20:1 setting on the ratio switch for this check. You should see the signal being amplified by Q12-Q15 and then rectified and summed at the junction of the two diode cathodes.

Once you think it is working OK and calibrated you can also check your compression ratios using simple measurements as shown on my ratio setting page. Being analogue equipment with tolerance variations, don't expect them to be spot on or the same characteristics between different units.

 

[kenrinc]

I've got a scope, actually two and I have no idea if any of them work as they are "hand me downs". One is an HP 1740a and the other is an old heathkit. Seems simple enough to hook up but right now all I'm doing is hooking up to the built in calibration tone (sq wave 1v pp) and I for the life of me cannot get a square wave on the screen. Is it really this difficult? I've got the tim/div set for 2sec, I have the sq wave in input 1 set for 1v DC and I see dots moving across the screen at 2sec intervals. Turning up or down time/div just makes them faster or slower. Embarassing that I don't know how to work one of these things.

Ken

 

[gswan]

You'll need to set the timebase to around 1ms/div to see a 1kHz square wave. Set the trigger to 'auto' on channel 1 so it will trigger on the waveform and move the trigger level control until the waveform display stabilises. If the square wave output is 1Vp-p then you probably want the vertical set to 500mV/div (with a 1X probe)

The purpose of the square wave is to allow you to calibrate the probe. You will see a little screw adjustment on the probe. If the square wave has rounded corners or a lot of overshoot then this is adjusted to compensate the probe to give you a nice looking square wave.

 

[Echo North]

Hi All,

Just finishing up my first 1176 (and first major build). Things have gone well but I seem to have a problem.

I'm using the mnats boards and ratio assemblies from UA. My problem is that all the ratio buttons seem to have the same ratio. So lets say I input signal and measure its voltage while in GR mode. Gain reduction is activated...I can see it on the VU meter and I can hear it. With any of the 4 buttons I have the same output voltage. Voltage should decrease as the ratio increases correct?

Th odd thing in that if I have 4:1 selected then select anything higher (say 20:1) the signal decreases briefly then moves back to the original signal or voltage within 5 seconds. If I have 20:1 selected and move to a lower ratio the signal briefly increases then settles back to the original.

I assume this is not how it should work?

I'm pretty sure I have the UA boards wired correctly.

Thanks,

Mike

 

[gswan]

I'd suggest performing a ratio measurement:
http://www.axtsystems.com/index.php?view=article&catid=34%3A1176ln&id=54%3A1176lnratios&option=com_content&Itemid=27
to quantify what you hear and then we can see if there is a problem.

 

[Echo North]

Ah...thank you for the link gswan.

I performed this test and my ratios are fine...all is working perfectly.

Thanks again,

Mike

 

[kenrinc]

Jeesh, Ok, I figured it out. The ch 2 beam is on the screen even when nothing is connected. I know this is all probably common place but hey, it's my first time! :grin: I was never able to see the square wave from the calibrated output but I can now see a big beautiful 1k sine wave out of my cd player. So now I'm off. I'll be back.... :wink

On the square wave: I think part of the problem is for some reason the unit won't show the horizontal part of the wave. In other words it will only draw the horizontal part of a square wave (line, space, line, space ...) but won't show the vertical part connecting each horizontal part. I've tried everything. I think that part might be broke or out of calibration? Who knows. At least it measure sine waves ...

Ken-

 

[Echo North]

I'm having a baffling problem.

(Keep in mind I am still very much an electronics beginner)

I think I have current issues in my mnats 1176j.

Here is the back ground:

My unit seems to be working fine. Sounds great everything is good. The last thing I needed to do was hook up the VU lamp. I have sifam 12v and 39vdc running out of the VU lamp pads.

Here is the problem:
1. If I attach two 120ohm 2W resistors to the lead ends I get NO voltage drop. Early discussion seemed to indicated this would get me in the 12v ballpark.

2. My VU lamp pads seem to be reading 5Amps AC! How can that be? Can my Y236106 power transformer even draw that much current? I though the max in the secondary was .6A

3. My .331 Amp inlet fuse blows on occasion. I have to put in a 6amp fuse to maintain power.

4. Can I really be drawing 5amp of current?

5. If so how is my 1176 operating so well?

Mike

 

[gswan]

As a beginner, you are lacking a basic understanding of what you are trying to do and what you are measuring.

1. Your voltmeter has a very high impedance, usually >10^7 ohms. Effectively this looks like an open circuit when measuring a voltage. When you use series resistors to drop a voltage, the voltage dropped across them is proportional to the current flowing through them. This proportionality is linear and is known as resistance, measured in ohms. What you have done is measured the voltage with no current flowing through them, so it will be the same as the voltage going into them. Attach the load and current will flow through them and the voltage will drop, with I*R volts dropped across the resistor.

In your case you have a source voltage of 39V and a requirement for 12V for the lamps. So you need to drop 27V across the resistor. I don't know what the current or power rating of the lamps are, but let's say 100mA is required for the lamps to glow brightly. This means that the resistor value needs to be 27/0.1 = 270 ohms (approximated by a couple of 120 ohms resistors in series).

2. An Ammeter is effectively an almost short circuit to a power supply. It is inserted in series with the power lead and measures the current flowing into a circuit. It has a very low resistance shunt to pass this current through.

If you connect an ammeter across a power supply you are effectively shorting it out and risk destroying the power supply and/or your ammeter. Do not simply stick in another higher fuse. You will probably destroy the transformer or it will catch fire. The fuse is there to protect equipment from idiots that do things like this. Yes you are drawing 5A of current. It is probably overheating the transformer as the core losses are very high in overload.

Your 1176 is operating when you don't short out the power supply (ie when you leave it alone). Then the PSU can do its job properly.

I'd suggest reading up on ohm's law (very basic beginner stuff) and also find out how your measuring equipment works and what you are measuring.

Put the original low current value fuse back in.

 

[Echo North]

I see, thank you. I'm reading a bigger electronics book right now...clearly I need to review current.

What does the .6 amp on my secondary refer to then? Recommended maximum load rather then maximum possible draw?

By adding the "load" to you mean attaching the light?

I'm still blowing .331 amp fuses...and these are 250V fuses, so I assume the are .660 amps with my 118v mains?

Nothing should be sorted?

Off to by more fuses...

Mike

 

[gswan]

The 0.6A secondary current refers to the maximum secondary current permitted to be drawn and still remain within the manufacturer's specifications.

Yes, the load in this case is the VU meter lamp.

The 250V voltage rating of the fuse has little to do with the current at which it blows. The current rating will be the same regardless of the voltage rating. The current rating is calculated by the thermal capacity of the fuse wire. It heats up and after a period of time at elevated temperature it opens. The voltage rating has a lot to do with what happens when the fuse is opening.

A 0.6A secondary current at 40VAC (assumption here of your secondary voltage) is around 24VA and translates to about 0.2A primary current at 118VAC. Allowing for inrush a 0.5A fuse should do, provided it's not a fast acting fuse.

Have a look at the fuse after it has blown. Is it just a break in the wire or is the fuse blackened with metal sprayed across the inside of the glass? If it's the latter then you may have broken the transformer.

 

[Echo North]

HI gswan,

I think I have it.

I was getting the metal sprayed across the glass with the .331 amp fuse however it was a fast acting fuse (all I could find in the area).

It was intermittent though so I figured I was near the line. I moved up to a .5amp fuse and everything seems to work now. The lights is bright! 13v there now...I'll increase the resistance to drop it a little more.

Should I assume my power transformer is probably ok? Or should I get a new one in there?

Thank you again for your help....I promise I will put down the iron for the day and ready my intro to electronics book for a while.

I looked back at it and re read the part about measuring current without load...feeling pretty stupid now. Good news is that I won't make that mistake again!

Mike

 

[gswan]

Good to hear that it's progressing now. Transformer sounds OK if the light is working and the 0.5A fuse is OK. Toroidal transformers have a high inrush current which can blow a fast acting fuse. A 'slow blow' fuse is usually used instead.

It is better to run the meter lams a little dimmer. It prolongs the life of the lamp and is easier on the eyes. Pop another 100 ohm resistor in series and see how that goes.

You are lucky this time. If you had tried to measure the current on the primary side (ie 118VAC) by sticking your meter across the mains you would have blown up the meter, vapourised the probes and possibly hurt yourself.

 

[Echo North]

I understand, I never go near the primary side...I'm scare of it...which is a good thing.

The .5A fast acting blew after a few of start ups. I though maybe his was because of the surge you described. I have a 1A fast acting in there now and every thing is fine. I added a resistor and got the lamp voltage to 10V. The current draw there is 100ma.

I'm going to replace that 1A fast acting fuse with a .5A slow blow first thing tomorrow.

Thanks again,

Mike

 

[Echo North]

When applying gain reduction I am experiencing up to a 1.5db increase in high frequencies.  For example, lets say I send in a 1k signal at zero db with 10db's of gain reduction and monitor the output on an external meter set at 0db.  Then I start sweeping through frequencies..at around 9k I will start getting about 1/2 db increase.  By 16k it's up to 1.5db.  You can hear it on "s's" on vocals.

I compared it to an original Rev.D and it had a flatter response.  Maybe a 1/2 db increase at the most at 16K.

Could this by the 1:2 out put transformer configuration?  Is the increase in gain by flipping the transformer adding the increased highs?

Is this just the A/B fron end of the later 1176's?

Any ideas?

Mike