UA LA-610 Died......Trying to revive.

[goldstache]

Got a buddies Universal Audio LA-610 Grey Face Rev C board (Rev J written on PCB), that had the Compression portion tank.
It's the 6AQ5 variant, and that stage had an open Plate Resistor (R71, I believe stock value is 22K 2W). Swapped that component and now have about 100VDC on the plate. Didn't restore comp however. Looked under the hood of the T4 Opto, and direct light does influence Resistive change in the LDR portion. But, I'm not seeing much illumination at any Peak Reduction Knob setting.
Anybody got any pointers here? Don't really want to tell this person that they gotta pay $175 bucks for two Lamps on a card.
I don't think the Opto part is documented as service docs don't seem to exist in the Public Domain.
Any advice or pointers are greatly appreciated.
Also, found the 5VDC heaters to be odd too, but I guess that's how they chose to get up to temp, and is normal for these units.
Looking forward to anybody's input!
Thank you,
Joe

 

[beatnik]

5V heaters ?? I don't think tubes would work properly at that voltage

It's not uncommon to find dull el panels on T4Bs that have been used a lot.

A forum member sells el panel replacements that are drop fit for original T4Bs.

https://sites.google.com/view/curiousaudio/catalog/new-t4b-opto-el-panels?authuser=0

 

[abbey road d enfer]

Also, found the 5VDC heaters to be odd too, but I guess that's how they chose to get up to temp, and is normal for these units.

No, this not normal. Underpowering heaters is not unknown, particularly for VLN applications, but here it's not justified.
It may well be the predominant cause for compression not working. I guess the 6AQ5 with 5V heater is not capable of driving the EL panel.

 

[Lampie519]

Underheating tubes is not unusual (especially microphones).
If the voltage on the cathode of the 6AQ5 is about 7v it does it's job fine.

 

[beatnik]

I am not sure the two 12AX7 would work properly with 5.0V heaters.

Standard NOS 12A_7 typically have +/- 10% tolerance for the heater voltage.

In some occasions I have experienced modern Chinese made 12A_7 to work well with underheating , while the Russian or Slovak tubes did not tolerate the same voltage

 

[Lampie519]

Just measure the current flow of the tube, if it is within the tolerances it is ok no matter what heating voltage is applied. Naturally overheating will shorten tube life.

 

[abbey road d enfer]

Underheating tubes is not unusual (especially microphones).

Indeed. As I mentioned, it's often done when noise performance must be optimizezd, which is not the case in the 610.

If the voltage on the cathode of the 6AQ5 is about 7v it does it's job fine.

It's a big if...

 

[goldstache]

Thanks for all the replies. I'm aware of the 10% heater voltage tolerance, which is why I mentioned. I went gleaning forums and found that many owners sighted a 5VDC filament voltage, which is why i decided, although strange and definitely effecting emission at that potential, perhaps intended????
I will check actual filament draw, but in my experience, if they look heated they are often pulling the data sheet current spec regardless of potential served up.
Lampie, I had around 10VDC at idle, on Cathode.
Cap that feeds lamps checks out. I will measure AC out of this cap to make sure it's moving.
Again, thanks.

 

[Lampie519]

If you know the voltage over the cathode resistor plus the value of the resistor then you know the current. Looks to me that there is enough current flow. Now the question is if this current is too high or low.

 

[goldstache]

Yep, but math aside, i don't see any illumination from the Bulbs. I see AC swing post coupler to Optical units. So that's why I suspect them. Where is the resistive LDR portion tied to in this puppy, output section probably but in parallel with what resistors? Cathode??? I don't see fixed bias voltages on any grids, but perhaps its in parallel with a leak resistor or something. Should just trace it out, but I aint really got the time right now.
I'll get back in there later tonight and get more data together.
Thanks again, don't feel so alone with this one cuz of yall!

 

[Lampie519]

If your cathode is on plus 10v and grid is 0v then this is your bias. The grid is 10v minus against the cathode.

 

[mrsoundb]

I’m having a similar issue. Did you ever find out what was causing the issue?