LA610 optocircuit diagnosis and repair

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andyfromdenver

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*edit: problem solved. read on for fun*

Hi everyone, hope you are well.

Symptom: 1st ed LA-610 The compression stopped working.

observation/ tests: the anode resistor to the 6aq5 (6005) appeared a little worn, and the trace approacing the lead is darkened.  The 6aq5, like in the LA2A provides gain to the illuminating EL in the optocircuit.
With a known good 6aq5, I tested little/ no DC voltage on the anode.  With the pcb pulled and the resistor out of circuit, it was an open circuit.
Fortunately, I had mapped much of my old La610 before selling, and I knew that the value of the resistor was 210ohm 1W (edit: this value is incorrect, I was fooled by cooked color bands see below).

This was a first for me to see a resistor fail open.  and it looked like brwn-brwn-brwn, 110ohm, so clearly it took some heat damage to brown that first color band from red.

So, I swapped in a 2W 220ohm as you can see in pic.
Now the tube is getting proper voltage, furthermore the blocking cap is working, and on the other end of that, I get an increase in AC voltage for increase in input signal and peak reduction knob.
But the EL does not light up. 
Does this conclude that I will need a new t4ua (the opto circuit drop in)?  They are $170.

Could a tube failure or the anode resistor failure cause damage to this? how so if the blocking cap is still good?

thank you so much for your time ;-)

Andy

edit: link for optocircuit part: https://www.studioelectronics.biz/sunshop/index.php?m=&c=158&p=4569&l=product_detail&my_rate=TWD
 

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JohnRoberts

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andyfromdenver said:
Hi everyone, hope you are well.

Symptom: 1st ed LA-610 The compression stopped working.

observation/ tests: the anode resistor to the 6aq5 (6005) appeared a little worn, and the trace approacing the lead is darkened.  The 6aq5, like in the LA2A provides gain to the illuminating LEDs in the optocircuit.
With a known good 6aq5, I tested little/ no DC voltage on the anode.  With the pcb pulled and the resistor out of circuit, it was an open circuit.
Fortunately, I had mapped much of my old La610 before selling, and I knew that the value of the resistor was 210ohm 1W.

This was a first for me to see a resistor fail open.  and it looked like brwn-brwn-brwn, 110ohm, so clearly it took some heat damage to brown that first color band from red.
I have seen resistors fail open.  Some on purpose like flame proof or fusible resistors, and some because they were too cheap to work, (like in a chinese battery charger I fixed once.)
So, I swapped in a 2W 220ohm as you can see in pic.
Now the tube is getting proper voltage, furthermore the blocking cap is working, and on the other end of that, I get an increase in AC voltage for increase in input signal and peak reduction knob.
But the LEDS do not light up. 
Does this conclude that I will need a new t4ua (the opto circuit drop in)?  They are $170.

Could a tube failure or the anode resistor failure cause damage to those light up diodes? how so if the blocking cap is still good?

thank you so much for your time ;-)

Andy

edit: link for optocircuit part: https://www.studioelectronics.biz/sunshop/index.php?m=&c=158&p=4569&l=product_detail&my_rate=TWD
I am not familiar with that product  but to confirm if gain element is working or not, defeat it temporarily and see if signal level through the circuit changes. vs when it is connected.

LEDs should be pretty robust and hard to kill, but I am not familiar with that product so others may have more informative advice.

JR
 

andyfromdenver

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JohnRoberts said:
I am not familiar with that product  but to confirm if gain element is working or not, defeat it temporarily and see if signal level through the circuit changes. vs when it is connected.
Thank you for the response JR, I truly value your time and input :)

To that, as I am away, some additional data. There is no audible gain reduction, or change in output lvl when the Peak reduction knob is cranked. Furthermore, shining a bright light upon the exposed device does indeed move the needle and reduce the signal.

edit: as I read back, I will still confirm the results of your test :)
 
Sincerely :)
Andy

I am feeling a small pang of concern that pehaps
my long ago read on the anode resistor for that tube is incorrect.
The LA2A uses a 22k anode resistor and shows a quiescent DCV of 150V (from memory)
This 6aq5 has ~300V on the anode @ quiescent.
Perhaps another important step is to collect the cathode voltage drop and doulbe check that. However, I felt the amplification of AC signal (varied roughly from <1V to 4-5V with shouting into mic) perhaps alleviated a concern of an unbiased gain stage.
tbc! and please keep em coming!

If some kind soul could double check R71 on your MK1 and confirm if it is 210ohm.
I don’t want to believe it is the t4uA optocircuit either cause it will be a costly swap and I am trying to barter services for mixing, hate to hit him with a wicked bill. He just wants to sell it anyway!! rats.

Andy
 

pucho812

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I worked on an LA610 the other day. This was a MKII and had the el84 instead of the 6aq5. This one had the same issue of  the comp not working, but in this case it was simply a faulty  el84.  I did notice the metering was pretty sh*tty and not very accurate though, maybe it was just how I was testing it with the AP, but it was really hard to get it to 0 properly.
 

andyfromdenver

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pucho812 said:
I worked on an LA610 the other day. This was a MKII and had the el84 instead of the 6aq5. This one had the same issue of  the comp not working, but in this cause it was simply a faulty  el84.  I did notice the metering was pretty sh*tty and not very accurate though, maybe it was just how I was testing it with the AP, but it was really hard to get it to 0 properly.
I wish I was that lucky! when I first determined that the tube plate was not getting any B+ I was so confident that replacing the resistor and using a good tube was going to nail it :-(. . . .

I really have a small love for this box, because I spent hours tracing the pcb to get perhaps 25% of it done.  I always wanted to start an ultimate mod thread.  I feel like many find the unit underwhelming in some way.  A lack of "bigness" to the sound that one might attribute to such legendary gear and design topologies.

 

shabtek

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What is b+

If 300vdc on anode; likely not conducting.

If you have variac or  signal generator and can put~80 vac  to the opto el panel, you can see if it lights up and measure if/what the photoresistors  ohms change to.
 

andyfromdenver

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Athens, GA
shabtek said:
What is b+

If 300vdc on anode; likely not conducting.

If you have variac or  signal generator and can put~80 vac  to the opto el panel, you can see if it lights up and measure if/what the photoresistors  ohms change to.

B+ is ~350vdc

It just hit me as I was driving to the grocery store that I was thinking of the optocircuit incorrectly.  an EL panel and 2 LDRs. sorry lol

hm I’ll test the EL panel, I do have a variac.

I haven’t done any electronics in a few months cause I have been trying to write songs. my thinking cap is settling back on my head  ;D

This box intimidates me d/t the lack of schematic, so I have to pace myself and break down what I know about the circuitry. 

tbc!

thanks so much all :)
I’ll report back if I get it going for him. With much more difficulty, I’m simply going to return it to him. He’s a super great friend, he’s used to me letting him down.......*these are tha jokes people*

andy
 

JohnRoberts

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andyfromdenver said:
I did, and it does move the needle and compress.

I’m going to try and borrow my old one and swap in the t4ua from a working unit.
Nothing else on the board looks suspect in the slightest, so I feel it’s a decent risk vs. reward.

*flashes peace symbol*
thanks :)
If the LDR is working with a flashlight, coming up with a replacement light source may be easier...(cheaper).

JR
 

andyfromdenver

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Athens, GA
hi solder-slingers.
happy to report it’s fixed.
I borrowed a working unit (not my old one) and tested the voltage drop.

ON ALL 3 that plate resistor (R71 version mkI) had color changed bands from heat.
I had a wrong read from my trace out way back, and I simply did not bias the 6005 stage correctly.

doing a final test cause I don’t want to lift the resistor on the working unit to read w/ a meter. the value is more like 22k-39k.

thanks for the input

I urge users to make sure the ventilation is good above this box, and perhaps put a higher wattage resistor.

I can testify that under use, that plate (anode) resistor will cook open.


yay!
20 cent part ;-)

thanks for the input fellas, peace to you in the world.

Andy
 

andyfromdenver

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so I used 33k for R71 (anode/plate resistor of 6aq5/6005 tube) and it responds exactly like the working test subject.

Shout out to Shabtek for helping me realise I wasn’t getting near enough AC voltage to light up the night (as it were), when i measured after initially installing a 220ohm resistor for R71.

Thanks so much all!

have a lovely day
Andy
 

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