Voltages for T4B in LA3A?

[JW]

Hey guys,

I checked continuity on all the grounded T4B pins again, and they're all good. I am also getting about 850 ohms between pin 3 and ground. Don't know if that's significant.

I also checked the AC again at pin 3. I'm actually getting 196V's with everything cranked and a 1K 1.23V tone inputting.

I have the new T4B in. Never had the old one.

Also, the meter doesn't do anything in GR mode, except sit at 0VU like it should when nothing is being input. Even cranked it stays at zero. In output mode, the meter is operating normally.

Hmmm. 

 

[Greg]

Just for good measure...

Measure the resistance across pin 7 (or pin 6) and pin 8 as you increase reduction... this should decrease with increasing compression (IIRC). Also, I don't think it's necesarry to crank the reduction all the way. I don't know the max voltage for the electro panel in the T4B, but I think it might be best to not continually hit it with 200VAC.

Another way to test the T4B is get in a dark room and take the covering off. As you increase gain reduction, the panel should increasingly get brighter.

 

[Terby]

Greg is right, resistance between pin 7/8 and 6/5 decreases if compression increases, however I am not quite sure if we can measure the resistance directly in a powered up unit since our DMM introduces a small DC measurement voltage and will throw our results of.

Nevertheless, compare the DC voltage between pin 6/5 with Peak Reduction at zero with the reading at Peak Reduction at max, both with input signal applied.

Taking the cover off in a dark room is also a good idea, look if the brightness changes with the input signal ...

JW, just to make it sure, you have not only connectivity between pin 1,2,4,5 and 8 - they are all connected to ground as well ?

  Andreas

 

[JW]

Okay,
I took the top off the T4B and checked in the dark while applying gain reduction. There's no light at all at any point.

I also checked the resistances between pin 5 and 6 with signal applied.

Resistance with no gain reduction is 776 ohms
With full gain reduction, it's 791 ohms

Pretty minimal. How great should the difference be?

P.S. With the DMM leads applied across 5 and 6, this is actually the first time I've seen the meter move in gain reduction mode. It IS moving negatively like there is Gain Reduction when I put the DMM leads across pins 5 and 6.

 

[JW]

Just bumping this one again. ???

Does it look like I have a bad T4B replacement? Is there a test I can do on the T4B by itself to see if it's okay?

 

[Terby]

JW,

If the EL-foil in your cell does not glow with both input signal and peak reduction turned up (you had nearly 200V at pin 3 !) I am afraid your T4B is dead ...

  Andreas

 

[JW]

Hey Andreas. Thanks. 

Shoot. Well, that was pretty much the reason why I wanted to figure out if the voltages were okay. And I went ahead and put it in anyway. The problem was my first reading wasn't done with a sine wave. I was done using music as signal so I sort of tricked myself into thinking I was getting the range that you guys said I should be getting. What an idiot I am.

Anyway, later I was using a 0dB VU 1K sine wave (I said 1.23V earlier, but I wasn't thinking. It was .775) with everything cranked. That should not kill a T4B though, I wouldn't think.

I still need to figure out what is causing that much voltage at pin 3.

 

[Greg]

JW, I don't think there is anything wrong with the unit; other than the T4B. Cranking everything at the 50dB setting with a 0dB input signal might not be healthy for the unit since it puts a very high voltage (200Vish) at the T4B. If you want to crank everything, I'd suggest a much smaller input signal, perhaps <100mV.

But I'd recommend powering up the unit with both controls fully counter-clockwise and slowly bring them up. In general, this is always a better approach.

 

[Terby]

JW,

200V is not unreasonable in a schematic like this, the interstage driver stage is powerd by ~35V, it's output voltage is multiplied by 10 through T4, so apparently nothing wrong with your LA3 here. Most EL-foils I have seen are specified for an operating voltage of ~40V to ~110V AC so it might be advisible not to exceed this for longer periods...

Measuring the T4B out of the box might turn out difficult, at least I was not able to get any resistance readings between any of the pins of my new Urei cell with the meter set to 100Mohms range.

The T4B is a very simple device, you have:

- The EL-Foil between pin 3 and 2, supposed to glow with voltage apllied

- A light depedend resistor (LDR) between 4 and 6 to control the meter in GR mode

- Another LDR  between pin 7 and 8 for the gain reduction of the audio signal


What you could try to verify you LA3A is:

- Plug the T4B out, take the top of and measure resistance between pin 5 and 6, see if your readings change if you direct a light source at it (The LDR's are most sensible to green light, but I guess you should get some readings with a torch / lamp ...)

- Do the same between pin 7 and 8

- If you see resistance vary with light, put your T4B back in (top still of), switch unit on, meter to GR mode (all other controls are irrelevant) See if your meter moves with light directed at the cell

- Apply signal, see if your get volume reduction with light applied (Peak reduction setting irrelevant)


   Andreas

P.S.: While you are at it, you could do me a favour and measure the voltage at C10 (1000uF cap) 

 

[Greg]

Good advise, Terby. This can verify that the compressor is working... I did this test troubleshooting my LA-2A with a flashlight.

 

[JW]

Well the resistance is changing at both 5/6 and 7/8 when I apply light to the T4B .

And the meter is moving when when the same light source (flashlight) is applied with the T4B in it's socket.

AND I can hear it compressing properly with the light source applied.

So, if I'm getting proper AC V's with signal applied at pin 3, then the El-foil is the source of my problem right?

Voltage at C10 is 40V DC.

 

[Terby]

Thanks for the measurement JW!

Yes, it looks like the EL-foil is your problem, everything else looks ok. I cannot believe you have fried the element with 200V applied, I suppose you should at least have seen the meter moving and some drastic gain reduction ...

  Andreas

 

[JW]

Well, luckily I bought it from Drip, so I'll contact him. I started off with a very low signal before putting in a .775 signal and cranking it all the way, and it never worked in the beginning, so I don't think that that was the reason for it's demise, but who knows.

Anyway, thanks Andreas and Greg for helping me out with this. You guys are the greatest.

Jon

 

[mata_haze]

Hello,
i am experiencingsomething similar with an La3A of mine, no compression.
The T4B does not light up at all, but if I obscure the receiving element it starts compressing.

I went through the thread and did the measurements, and i am not getting any VAC on pin 3 of the T4B regardless of input signal of T4B being plugged in or not.
the maximum I get it's 1.45VAC (and it' not dependant on input gain).
SO I went on and measured few voltages around and this is what I have got:

660mVDC at the base of Q10
58mVDC at the base of Q9
28VDC at the emitter of Q9
0V after C9

now here is the tricky bit, if I lift C9 the voltage comes through and get about 24VDC after it (on the transformer side) but if I solder it back in it disappears, of course.

I have replaced C9 but nothing has changed, I have measured the static resistance of T4 and i have got:
800 ohm between ground and PIN3
69 ohm between ground and the "centre tap" of the transformer.

are we suspecting the trafo T4 is gone?

any help will be much appreciated,
Mattia.

 

[Greg]

hi mata,

28VDC at the emitter of Q9

now here is the tricky bit, if I lift C9 the voltage comes through and get about 24VDC after it (on the transformer side) but if I solder it back in it disappears, of course.

So even with the T4B removed no voltage on pin 3 of the T4B socket?

And no AC after C9 right?

Check to see if Q9 is shorted. Just check continuity from base to emitter, base to collector, and collector to emitter. If Q9 fails short it will put VDC (non-regulated DC) at C9. There will be some DC at Q9 but it should not be that high. And after C9 it should be 0VDC.

 

[mata_haze]

hi mata,

28VDC at the emitter of Q9

now here is the tricky bit, if I lift C9 the voltage comes through and get about 24VDC after it (on the transformer side) but if I solder it back in it disappears, of course.

So even with the T4B removed no voltage on pin 3 of the T4B socket?

And no AC after C9 right?

Check to see if Q9 is shorted. Just check continuity from base to emitter, base to collector, and collector to emitter. If Q9 fails short it will put VDC (non-regulated DC) at C9. There will be some DC at Q9 but it should not be that high. And after C9 it should be 0VDC.

Hello Greg, that was it!
Replaced Q 9 and Q11 problem solved!

thank you very much!

Best,
Mattia.

 

[Greg]

Glad I could help...

Cheers.