Fixing UA La-610 Compressor (need schematics with voltage points)

[Boswell]

Great work, Marco!

Did you measure the resistor you took out, or was it obviously open-circuit?

 

[pcrecord]

Yes, the circuit was open.

 

[rmburrow]

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 !

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.

 

[DonnyThompson]

Great thread!

 

[pcrecord]

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...

 

[djmalo]

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.

 

[pcrecord]

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]

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 !

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

 

[GeryGuitar]

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!

 

[Boswell]

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]

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)

 

[GeryGuitar]

Dear pcrecord, that would be very kind of you!

 

[pcrecord]

Alright I opened my LA-610 the one that never had the issue.. To this is the original resistor :
Seems to be violet, violet gold and then grey ?? .. not sure..

 

[GeryGuitar]

Thanks! Could you measure the ohm value as a reference for me?
violet, violet gold would mean 7.7 Ohm!?

 

[pcrecord]

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..

 

[ronmac]

It is not unusual to be fooled by the colour code on a burned resistor.

A little sleuthing on the internet shows typical value of R71 to be 22k~33k

See posts #9 and #10 in this thread... https://groupdiy.com/index.php?topic=67189.0

 

[Boswell]

It may be a trick of photography, but the first two bands do appear to be brown and not red. If it is 22K, that would be red-red-orange.

The resistor R71 burning out seems to be a common problem in that model. How about seeing if you can fit a 22K 2W resistor as a replacement?

 

[GeryGuitar]

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.

 

[pcrecord]

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]

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.