La-610 ld panel not working

[mrsoundb]

i have an LA-610 mk1 everything seems to work except the EL panel. I measured voltage on the EL terminals but I'm not seeing voltage at any gain reduction settings. What part of the circuit drives the panel? What would cause it to not receive power? I removed it from the circuit and turning a light on and off shows gain reduction from the photoresistors, so it seems that the only part of the circuit that drives the panel is bad. Any advice is appreciated.

 

[pucho812]

Check tubes.

 

[mrsoundb]

I replaced each tube so I don’t believe it’s that. Would love to know test points and voltages for el panel. I’m ordering a replacement to try.

 

[rock soderstrom]

Check the cathode resistor (and the rest) of the 6AQ5. See this thread for further info.

https://recording.org/forum/pro-aud...610-compressor-need-schematics-voltage-points

 

[rock soderstrom]

What part of the circuit drives the panel?

6AQ5

 

[mrsoundb]

Yes i believe so. Cathode was showing 105 dc. After r71 about 300v

 

[rock soderstrom]

Yes i believe so. Cathode was showing 105 dc. After r71 about 300v

Really, the cathode is sitting at 105VDC?
I expect this part similar to the LA2a, the 6AQ5 drives the cell with a plate out amp, not a CF, so 105VDC seems odd.

After r71 about 300v

Not really useful info, we can not see what you see. Tell us the voltages at G1,G2 and the plate of the driver tube. Some photos would help...

Follow the driver signal with a scope, check C11 (name will be different) for leakage.

Measure the resistors around the 6AQ5.

 

[Sidebstudio]

Repaired a pair during years with dead panels.

You can measured around the cell as already suggested.
Or if you want to see if panel work you have to open the black plastic case that cover the cell, send a sine wave and move gain reduction to see if lights on or not.
Of course out your lights off during the test or the light trigger the compression (photo resistors).
Maybe I have a video somewhere.

 

[mrsoundb]

Yeah I’m not seeing voltage around the el panel at the moment. My electronics knowledge is fairly limited but I’m still trying to troubleshoot this. I don’t own a scope, just a cheap digital multimeter. UA no longer services these and I can’t find anyone local that repairs this. I also can’t find a schematic but I see it should be similar to an la2a side chain . The original issue was the output volume was low and distorted but the compression still sorta worked when pushed hard. I pulled the t4 which is different in these units. With the t4 out of the circuit the preamp works great just no compressor. I clipped out one of the photo resistors (thru research I found that one is for the meter and one is for compression) thinking maybe one was bad and I replaced it with a non matched one that I ordered. Now it works in a similar way with it pulled. I turned off the lights and did what you suggested and the el panel is definitely not working. If I shine a flashlight on the light resistors it works (meter and compressor). I pulled the el panel and unfortunately ruined the one lead in the process. I ordered a new one to try but i feel it’s actually the driver circuit and not the panel because I’m not seeing any voltage ac or dc on the pins that feed the el panel, even with signal and turning up the compression. There’s a white four pin (h16) that feeds the panel. I’ll call the pins 1 and 4. 1, the top pin goes to a cap. (C1) then a lead on the el panel and 4 the bottom is the other lead. I also replaced r71 because that resistor seems to be know to go bad but the value I put in is slightly higher, I ordered the proper value. I also tried other tubes, I had one other 6a5q, I ordered another just in case…it’s definitely a trial and error style trouble shooting because my lack of knowledge but I am learning in the process.

 

[mrsoundb]

Changed the el panel but it’s still not working. UA will not share a schematic so that isn’t helping either.

 

[pucho812]

Seems to me that it’s voltage related.
Can trace from the panel to see where voltage and signal stop.

 

[mrsoundb]

I agree. Does the el panel received its voltage from the 6aq5 cathode? I I just don’t understand the circuit or circuits for that matter well enough to troubleshoot properly.

 

[rock soderstrom]

I agree. Does the el panel received its voltage from the 6aq5 cathode? I I just don’t understand the circuit or circuits for that matter well enough to troubleshoot properly.

Trace the tracks on the circuit board and you will see how the T4 cell is controlled.

I assume that this is done as in the LA-2a, i.e. from the anode.

Tell us the DC voltages at G1,G2 and the plate of the driver tube. Some photos would help...

Follow the driver signal with a scope your DMM or signal tracer, check C11 (name will be different) for leakage.

Measure the resistors around the 6AQ5.

That might help!

it’s definitely a trial and error style trouble shooting because my lack of knowledge but I am learning in the process.

At a certain point, however, a systematic search is much more helpful. Otherwise your troubleshooting will become a matter of luck and can frustrate you completely.

If you don't have an oscilloscope, you can help yourself with a signal tracer. Then feed in a music signal from your computer, smartphone or CD Player and trace the signal through the circuit with the tracer. This helps, but you should be careful because the high voltages are dangerous. (but it would be the same with a scope...)

Headsup: use a capacitor with 400V or more for the tracer!!!



https://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

 

[Brian Roth]

IF...IF...IF..... it's similar to a LA-2. the EL panel drive comes from the plate (anode) of the 6AQ5 after passing through a coupling capacitor. I haven't seen a listing of voltages around the 6AQ5 with no audio/idling. Too bad Bill Putnam, Sr. isn't still alive....from time to time, he would take a phone call to offer tech assistance.

Bri

 

[mrsoundb]

Thank you for the information.

 

[mrsoundb]

Trace the tracks on the circuit board and you will see how the T4 cell is controlled.

I assume that this is done as in the LA-2a, i.e. from the anode.

That might help!

At a certain point, however, a systematic search is much more helpful. Otherwise your troubleshooting will become a matter of luck and can frustrate you completely.

If you don't have an oscilloscope, you can help yourself with a signal tracer. Then feed in a music signal from your computer, smartphone or CD Player and trace the signal through the circuit with the tracer. This helps, but you should be careful because the high voltages are dangerous. (but it would be the same with a scope...)

Headsup: use a capacitor with 400V or more for the tracer!!!



https://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

So took some measurements of the 6aq5
g1 - 6.2v
k-143v
g2-9.8v

 

[mrsoundb]

So took some measurements of the 6aq5
g1 - 6.2v
k-143v
g2-9.8v

Plate -5v

 

[Brian Roth]

Those numbers make NO sense. How are you measuring them?

 

[rock soderstrom]

Those numbers make NO sense. How are you measuring them?

+1

Measured mirror inverted?

The best thing to do is to switch off the device, discharge the capacitors and then, armed with a continuity tester and your eyes, make a sketch of what is really happening around this tube!

 

[mrsoundb]

Got ya, I probably had it opposite, mirrored. It would be g1- 6.7, g2 9.8, plate 118, cathode 143, does that make more sense.