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pcrecord 20 June 2016

Hi guys,

Like the title says, I got one of my two LA-610 who doesn't compress anymore.. the action faded over a few months.
I tried to change the tubes and even exchanged the opto cell with the other LA-610 and it did no good.

So I guess a cap or a resistor failed.. I'd need some guidance and some precise schematics

Thanks in advance.

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Comments

Boswell Tue, 06/21/2016 - 07:29

Depends on your definition of "precise"! There are several schematics on the web if you Google for "LA-610 schematics" and select Images, but you have to beware that one or two of the ones it brings up contain glaring errors (such as a cathode connected to ground via a capacitor only).

It was many years ago, but I seem to remember that in the last one of these that I had in for repair with the symptoms you describe I found the cathode resistor of V4 had gone high resistance, biassing V4 almost off, and hence not able to supply any change in the drive to the photocell. It's worth unplugging V4 and checking that the resistance to chassis from pin 2 of the valveholder is 1K Ohm or less. The fitted value of this resistor varied from issue to issue of the LA610 - on some it was 1K Ohm and on others it was 470 Ohms.

If this resistor checks out OK, then I think you are going to need a signal generator and a scope to start looking for the problem elsewhere in the circuit. I'm happy to guide you through a checklist. Fortunately, there are not that many components in the gain reduction control path.

I'm assuming that the quality of the signal passing though the unit is OK with no distortion.

rmburrow Mon, 07/18/2016 - 08:38

pcrecord, post: 439980, member: 46460 wrote: You are very kind Bos ! I think I'm gonna use it without compression for a while.
A scope would be a big investment to repair just one unit.. but I keep an eye open for a used one. ;)

I don't own a LA610, but I own LA 2 units. My guess is you should be able to repair the LA610 with an audio generator and a good digital multimeter ("DMM"). My guess is the no compression problem is associated with the T4C (opto cell) or associated circuitry. I don't have any schematic for the LA610, but my schematic for the LA-2 (from 21 Dec 1965) shows a 470K resistor (R28) between 275 volt B plus to a NE2 neon bulb (used to regulate voltage) and the wiper of a 1 meg ohm potentiometer (zero adj). The schematic shows +65 volts at this junction. When the unit is fired up, and the neon bulb is NOT lit and you don't find +65 volts at the "hot" end of the neon bulb, you have a problem. (The ground side of the neon bulb is grounded.) Also test the 1 megohm zero adjust potentiometer (R4) to make certain it is not open to either side or the wiper. The value of R25 was chosen to work with the particular photo cell in use; don't change this unless it measures open.

The T4C is driven by a 6AQ5 in the LA-2, so DC signal varying at audio rate at the "hot" input of the T4C should be found there...

PLEASE NOTE that I took my information from a LA-2 manual; the part numbers and values may not correspond to the LA610, but the operation principle should be similar.

If you don't have an audio generator, do you have an old test record or CD with clean audio tones (like 1 kHz) recorded on it? That should serve as a audio source for testing. BTW a good DMM, audio generator, and a scope are good to have around. Look around at local "hamfests" or electronic surplus dealers.

ADDENDUM: Noted your comment about a 470 ohm resistor. Wonder if that is a misprint? 470 ohm resistor is yellow purple brown (and either gold or silver) color code (in that order), or the value is printed on the component. 470 K ohms is yellow purple yellow (and either gold and silver). Check carefully, if similar circuitry is used in the LA610 as used in the LA2, the 470 K ohm resistor may be open, the NE2 lamp blown, and hopefully the T4C (light cell) wasn't damaged by excessive voltage....

rmburrow Mon, 07/18/2016 - 09:32

DonnyThompson, post: 439988, member: 46114 wrote: I hate you. LOL ;)

LOL...Just looked at the schematic of the LA610 compressor online; the 470k resistor is there and still designated as R28.

BTW, if you have to change R28 to the correct value of 470K, the NE2 neon bulb, or other components, MAKE CERTAIN your LA610 is UNPLUGGED and there is NO DC on the 275 volt B plus rail. You can get a healthy shock off the stored energy in the capacitors, even with the unit turned off!

Boswell Mon, 07/18/2016 - 15:27

rmburrow, post: 439986, member: 46233 wrote: Noted your comment about a 470 ohm resistor. Wonder if that is a misprint? 470 ohm resistor is yellow purple brown (and either gold or silver) color code (in that order), or the value is printed on the component. 470 K ohms is yellow purple yellow (and either gold and silver). Check carefully, if similar circuitry is used in the LA610 as used in the LA2, the 470 K ohm resistor may be open, the NE2 lamp blown, and hopefully the T4C (light cell) wasn't damaged by excessive voltage....

The 470R is the cathode bias resistor in the cell drive stage; I'm told some versions of the LA610 have this as 1K. In a 610 I serviced some while ago this part was marked as 470R but had gone high resistance and was starving the 6AQ5A valve, giving symptoms very similar to the fault Marco was describing.

I suspect the 470K resistor you describe is the one feeding current from the 275V HT into the slave photo panel and on to the meter circuit. If that part went open circuit, it would sabotage the meter readings when the meter was switched to gain reduction, but the audio compression action would be unaffected.

rmburrow Mon, 07/18/2016 - 18:19

Boswell, post: 439998, member: 29034 wrote: The 470R is the cathode bias resistor in the cell drive stage; I'm told some versions of the LA610 have this as 1K. In a 610 I serviced some while ago this part was marked as 470R but had gone high resistance and was starving the 6AQ5A valve, giving symptoms very similar to the fault Marco was describing.

I suspect the 470K resistor you describe is the one feeding current from the 275V HT into the slave photo panel and on to the meter circuit. If that part went open circuit, it would sabotage the meter readings when the meter was switched to gain reduction, but the audio compression action would be unaffected.

Agreed, if R36 (the 470 ohm 6AQ5 cathode resistor) increases value, the operating point of the 6AQ5 changes, and that will "starve" the 6AQ5 as you mention. R34 (10 k, 2 w) shows a 120 v drop across it (275 supply-155 v anode); therefore 12 mA of anode current and 1.44 w dissipated in R34. There is a greater chance R34 could open or change value before R36 due to the power dissipated in those resistors. Look for discoloration of any resistor and test resistors using a reliable DMM.

Sean G Mon, 07/18/2016 - 18:38

dvdhawk, post: 439992, member: 36047 wrote: How lucky are we to have guys contributing here on a regular basis, with this level of knowledge and experience, who are so generous with their time and talents??

We are very lucky indeed

dvdhawk, post: 439992, member: 36047 wrote: Gentlemen, you are greatly appreciated.

I second the motion put forward by the member from Western Pennsylvania :D

pcrecord Tue, 07/19/2016 - 05:11

Thats very good info guys..
One thing I should say is that I idendtified the problem: There isn't enough current to light up the opto cell. If I remove the cover and use a flashlight on it, a fair amount of compression is applied. No, the light isn't broken, I tried the Opto cell in an other LA-610 and it works fine.

There isn't any discoloration or visual indication any part was damaged or blowned. (would have been too easy...)
I have many projects to attend in the next weeks, I'll get back to it when I get time.

Thanks again

Boswell Tue, 07/19/2016 - 07:14

pcrecord, post: 440026, member: 46460 wrote: Thats very good info guys..
One thing I should say is that I idendtified the problem: There isn't enough current to light up the opto cell. If I remove the cover and use a flashlight on it, a fair amount of compression is applied. No, the light isn't broken, I tried the Opto cell in an other LA-610 and it works fine.

There isn't any discoloration or visual indication any part was damaged or blowned. (would have been too easy...)
I have many projects to attend in the next weeks, I'll get back to it when I get time.

Thanks again

Great work! That's completely consistent with one or more of the cell driver components having developed a fault. You said in your first post of this thread that you had swapped the valves (tubes) with the ones in your other (working) 610 and the fault did not move, so you have a limited number of passive components to investigate when you next have the time.

Sean G, post: 440027, member: 49362 wrote: Just leave the cover off and the lights on in your room.

How did I not think of that? A complete set of disco lights should do the trick, and with careful choice you could get multi-band compression for free.

rmburrow Tue, 07/19/2016 - 08:27

pcrecord, post: 440026, member: 46460 wrote: Thats very good info guys..
One thing I should say is that I idendtified the problem: There isn't enough current to light up the opto cell. If I remove the cover and use a flashlight on it, a fair amount of compression is applied. No, the light isn't broken, I tried the Opto cell in an other LA-610 and it works fine.

There isn't any discoloration or visual indication any part was damaged or blowned. (would have been too easy...)
I have many projects to attend in the next weeks, I'll get back to it when I get time.

Thanks again

OK..the next step is to get out the DMM and check for 155 v at the anode and 100 volts at the screen of the 6AQ5. If R34 (10 k 2w), R35 (220 k 0.5 w), or R36 (470 ohm 0.5 w) change value, these voltages will be incorrect. Next, check C9 (0.02 uF) for leakage or short; leakage through C9 will put DC at the top of R3 (stereo adj), upset the 6AQ5 grid bias, and perhaps "red plate" the tube unless R3 was set to ground. If C9 tests good, then check that R3 (stereo adj) isn't set all the way to the ground end, since this effectively shorts the 6AQ5 control grid to ground and removes drive signal.

Check C11 (0.01 uF) for correct value and no leakage/short. C11 couples drive from the 6AQ5 anode to the T4C cell. If the 6AQ5 screen voltage is incorrect (low), check C7D and C13 for short, and R38 (22k 0.5 w) for open or changed value.

C7 is a multisection electrolytic capacitor, in a metal can. These capacitors are expensive and hard to find now, unless UREI stocks them.

Be careful when measuring the DC voltages...

pcrecord Thu, 11/24/2016 - 05:21

I just thought I'd give you some news about this.
I finally got it to work last weekend. Opened the beast and inspected all parts.
I found a small discoloration near a resistor. Tried a few replacement I had that weren't the exact same value and I was able to get compression but at an extreem volumes and with the knob to maximum.

Intriguing enough, I first thought a smaller value with send more current to the light in the compressor but no it was the other way around. I guess it uses some kind of signal cancellation and the current needs to be lower at one end to let it hit the Opto cell. So after a few tests, I ended up with a 4.99k resistor instead of the original 1.2k. Man these old preamps circuits are alive and changing overtime and that's why we love them !!! As for now, the action of the compressor knob gets better precision that it ever did since I got it. (used) For exemple ; to get the same degree of compression I needed to put it at 7 when I got it and now at 4 (very similar to my other LA-610)
I've used it everyday all week without a glitch, so I think I'm good ! :love:

Thing is, this unit sounds better than the other one I have. Clearer, better transients, more in your face. It's a big relief that I got it to work ok.
When coupled to my T47 DIY mic, it's a blast !!

Lately I was talking about changing one of them for a ISA220 or 430 but I'm not so sure anymore. . . They sound so different but yet alike.
I'm thinking that I should wait for something totally different that what I have in my preamp list. Like a Millennia STT-1 Origin. At around 2.1k used.. it will take a while to get money for it...

Anyway, my LA-610s work so I'm a happy man !!

rmburrow Fri, 11/25/2016 - 18:50

pcrecord, post: 443690, member: 46460 wrote: I just thought I'd give you some news about this.
I finally got it to work last weekend. Opened the beast and inspected all parts.
I found a small discoloration near a resistor. Tried a few replacement I had that weren't the exact same value and I was able to get compression but at an extreem volumes and with the knob to maximum.

Intriguing enough, I first thought a smaller value with send more current to the light in the compressor but no it was the other way around. I guess it uses some kind of signal cancellation and the current needs to be lower at one end to let it hit the Opto cell. So after a few tests, I ended up with a 4.99k resistor instead of the original 1.2k. Man these old preamps circuits are alive and changing overtime and that's why we love them !!! As for now, the action of the compressor knob gets better precision that it ever did since I got it. (used) For exemple ; to get the same degree of compression I needed to put it at 7 when I got it and now at 4 (very similar to my other LA-610)
I've used it everyday all week without a glitch, so I think I'm good ! :love:

Thing is, this unit sounds better than the other one I have. Clearer, better transients, more in your face. It's a big relief that I got it to work ok.
When coupled to my T47 DIY mic, it's a blast !!

Lately I was talking about changing one of them for a ISA220 or 430 but I'm not so sure anymore. . . They sound so different but yet alike.
I'm thinking that I should wait for something totally different that what I have in my preamp list. Like a Millennia STT-1 Origin. At around 2.1k used.. it will take a while to get money for it...

Anyway, my LA-610s work so I'm a happy man !!

Glad to hear you got the LA610 units working again. Also glad to hear the problem was an inexpensive resistor. DIY repair work saves $$...the investment is time and perhaps some money spent on a digital multimeter, audio generator, and a scope.

pcrecord Sat, 11/26/2016 - 06:07

rmburrow, post: 444246, member: 46233 wrote: DIY repair work saves $$

I couldn't have done it myself with new gear that are based electronic chips. A circuit like the LA-610 may seem old fashion to many but it has the advantage of having nearly all its part exposed and easy to reach. :)

I had far less chances with my focusrite Saffire 56 which now serves as dual headphone amp...

pcrecord Tue, 03/06/2018 - 07:06

djmalo, post: 456096, member: 51179 wrote: This has been a very helpful thread. In fact, having the same exact problem with my LA610 currently. Wondering if you remember the number of the resistor you replaced that fixed it? Saw some discoloration on R20 on this board, but didn’t wanted to reach out first.

My resistor blowed because the circuit became out of specs with time. If your unit has the same problem, I can't give you an exact value at the risk of causing more damage because in my situation I had to test 5 value starting with the original before I got acceptable behaviours...

This was the answer I sent in private :

Hi DJMalo,
Not being an expert in electronics, there is one thing I know ; it's rare that the lightning hits twice at the same place :) Unless it's a design flaw.

I had the chance (or not) to have a blown out resistor in my unit and I could see the damaged part being carbonised.
Since it was visually evident, I proceeded in trying the same value, then lower but it wasn't helping.. so I went higher value until the compression had a normal behaviour. One thing I did was to remove the black cover of the T4 to test if it was defective. By pointing a light at it, it made the compression react. So I knew that part of the circuit wasn't blown out.

My LA610s are old and this unit I bought from ebay might have been runing non stop 24/7 in a studio for what I know.
So the specs changes.

Now be carefull if you don't have experience working in such complex circuits. To make the tubes work it uses high voltage that can kill someone...
Good luck

pcrecord Tue, 03/06/2018 - 12:46

djmalo, post: 456096, member: 51179 wrote: This has been a very helpful thread. In fact, having the same exact problem with my LA610 currently. Wondering if you remember the number of the resistor you replaced that fixed it? Saw some discoloration on R20 on this board, but didn’t wanted to reach out first.

Again a copy of the conversation with DJMalo, for the sake of having this thread with the complete informations :
----
In any case here is what I said in the thread :

Intriguing enough, I first thought a smaller value would send more current to the light in the compressor but no it was the other way around. I guess it uses some kind of signal cancellation and the current needs to be lower at one end to let it hit the Opto cell. So after a few tests, I ended up with a 4.99k resistor instead of the original 1.2k. Man these old preamps circuits are alive and changing overtime and that's why we love them !!! As for now, the action of the compressor knob gets better precision that it ever did since I got it. (used) For exemple ; to get the same degree of compression I needed to put it at 7 when I got it and now at 4 (very similar to my other LA-610)
I've used it everyday all week without a glitch, so I think I'm good ! :love:

Now the danger is that with adjusting this value it may impact the the longevity of other components.. So if you do so, you do at your own risk.
Start at 1.2K or around and go up with different resistor value. . .
Don't you ever short the chassis and any of the surrounding circuit near the tube and capacitors... Those capacitors hold a lot of voltage even when not plugged to AC current

Boswell Fri, 10/16/2020 - 03:01

GeryGuitar, post: 465783, member: 52091 wrote: Dear fellows, I do have a LA-610 with an open/burnt R71 on my bench. The colour code seems to show 110 kohm. Do one have the correct value for me? Thanks!

It's not easy to tell from your photo. If the multiplier stripe was originally yellow, then it's 110K 5% 1W. If the stripe was orange, then it was 11K.

The un-burnt resistor at the top of your photo is 220K from the E48 range, and the orange on that one looks similar to the E24 multiplier stripe on the burnt one. So I would go for the burnt one being 11K. That's not a usual value for equipment from that era.

pcrecord Fri, 10/16/2020 - 04:49

GeryGuitar, post: 465783, member: 52091 wrote: Dear fellows, I do have a LA-610 with an open/burnt R71 on my bench. The colour code seems to show 110 kohm. Do one have the correct value for me? Thanks!

I can't remember what I put in mine. I had the exact same problem. I tried many values until I found one that made the compressor work again.
I'm guessing my unit was out of specs because of the long hours of use.
If you don't find the answer, I could open mine. I got 2 LA-610 MK1 and only one had the problem.
Let me know (I'm at work now)

pcrecord Sat, 10/17/2020 - 08:50

GeryGuitar, post: 465806, member: 52091 wrote: Thanks! Could you measure the ohm value as a reference for me?
violet, violet gold would mean 7.7 Ohm!?

Sorry, I replaced everything in the rack now..
Yes it seems like a 7.7... but if you unit is out of specs like mine, I ended up with a 4.99k value.
so I guess starting with a low value and go up might be what you need to do..
I wonder if it could be replaced with a variable resistor.. ;)

pcrecord Sat, 10/24/2020 - 06:09

GeryGuitar, post: 465856, member: 52091 wrote: Thank you all! I found a 33k 3W metal film resistor working fine. But it gets pretty hot, I hope it will survive some time.

I'm glad you made it work !!

One thing to remember is that the resistor have blown up for a reason. Often many other parts have gone out of specs and it sends additionnal stress to the whole circuit.
If it fails a few times, I'd send it for a complete overhall...

Boswell Sat, 10/24/2020 - 09:48

I would be a little wary of replacing the 22K with a 33K. Valves (tubes) that run in conventional grounded-cathode configuration are approximate transconductance devices, i.e. they turn an input voltage into an output current. That means two things in this case: (1) with the same output current, the power dissipation (I^2 R) in the resistor will be 50% higher (as you may have noticed!), and (2) the valve will have only 2/3 of the designed voltage headroom to operate in. These effects could both be corrected by adding a 68K 1/2W resistor in parallel with your 33K.

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