LA3a problem

[John Noll]

I have an LA3a that just went down.
The meter hangs at around -5db when the unit powers up
instead of zero.
The unit was recapped a couple of years ago, so I'd
assume that the caps are OK. The plug-in opto unit is
original and pretty old. What are the symptoms of a bad
opto? Thanks.
--
--
John Noll

[Animix]

What are the symptoms of a bad
> opto? Thanks.

When my LA3A opto went down the gain reduction ceased to function, but the
unit would still output/amplify a signal.


"John Noll" > wrote in message
...
> I have an LA3a that just went down.
> The meter hangs at around -5db when the unit powers up
> instead of zero.
> The unit was recapped a couple of years ago, so I'd
> assume that the caps are OK. The plug-in opto unit is
> original and pretty old. What are the symptoms of a bad
> opto? Thanks.
> --
> --
> John Noll
>

[Animix]

What are the symptoms of a bad
> opto? Thanks.

When my LA3A opto went down the gain reduction ceased to function, but the
unit would still output/amplify a signal.


"John Noll" > wrote in message
...
> I have an LA3a that just went down.
> The meter hangs at around -5db when the unit powers up
> instead of zero.
> The unit was recapped a couple of years ago, so I'd
> assume that the caps are OK. The plug-in opto unit is
> original and pretty old. What are the symptoms of a bad
> opto? Thanks.
> --
> --
> John Noll
>

[Gareth Magennis]

If the Opto isn't working, i.e. it is not lighting up, you should still get
audio through, but obviously the unit won't compress. If there is no audio
at all you probably need to look elsewhere.

If you open the Opto up you should be able to see it light up with a signal
and the unit set to compress a lot. (you may have to do this is the dark).
Be careful though as I seem to recall there are lots of ways to get nasty
electric shocks from around the mains transformer.

http://www.vintagedesign.se/schematics/la3a.pdf will get you the circuit
diagram.

Check the power supplies, and especially check you have 27 volts accross the
Zener diode at the bottom of the page.


Good luck,

Gareth.




"John Noll" > wrote in message
...
> I have an LA3a that just went down.
> The meter hangs at around -5db when the unit powers up
> instead of zero.
> The unit was recapped a couple of years ago, so I'd
> assume that the caps are OK. The plug-in opto unit is
> original and pretty old. What are the symptoms of a bad
> opto? Thanks.
> --
> --
> John Noll
>

[Gareth Magennis]

If the Opto isn't working, i.e. it is not lighting up, you should still get
audio through, but obviously the unit won't compress. If there is no audio
at all you probably need to look elsewhere.

If you open the Opto up you should be able to see it light up with a signal
and the unit set to compress a lot. (you may have to do this is the dark).
Be careful though as I seem to recall there are lots of ways to get nasty
electric shocks from around the mains transformer.

http://www.vintagedesign.se/schematics/la3a.pdf will get you the circuit
diagram.

Check the power supplies, and especially check you have 27 volts accross the
Zener diode at the bottom of the page.


Good luck,

Gareth.




"John Noll" > wrote in message
...
> I have an LA3a that just went down.
> The meter hangs at around -5db when the unit powers up
> instead of zero.
> The unit was recapped a couple of years ago, so I'd
> assume that the caps are OK. The plug-in opto unit is
> original and pretty old. What are the symptoms of a bad
> opto? Thanks.
> --
> --
> John Noll
>

[Don Cooper]

Gareth Magennis wrote:

> If the Opto isn't working, i.e. it is not lighting up, you should still get
> audio through, but obviously the unit won't compress. If there is no audio
> at all you probably need to look elsewhere.


That's the problem I've been having, and I've been meaning to get it
fixed forever. I have Alesis (!), Aphex and dbx compressors, which I
hardly ever use, and I miss my LA-3A!

I bought an additional opto, but plugging it in doesn't solve the
problem either. I wonder if I can swap out the parts inside each can.
There are two in each.

But I will try looking in the dark first.


Thanks,

Don

[david]

In article >, Don Cooper
> wrote:

> Gareth Magennis wrote:
>
> > If the Opto isn't working, i.e. it is not lighting up, you should still get
> > audio through, but obviously the unit won't compress. If there is no audio
> > at all you probably need to look elsewhere.
>
>
> That's the problem I've been having, and I've been meaning to get it
> fixed forever. I have Alesis (!), Aphex and dbx compressors, which I
> hardly ever use, and I miss my LA-3A!
>
> I bought an additional opto, but plugging it in doesn't solve the
> problem either. I wonder if I can swap out the parts inside each can.
> There are two in each.
>
> But I will try looking in the dark first.
>
>
> Thanks,
>
> Don




If you want it fixed right, including having the whole signal path set
up properly, send it to Bob Alach.


http://www.alactronics.com/contact/contact.htm




I wouldn't want to record vocals and bass without my la3's.






David Correia
Celebration Sound
Warren, Rhode Island


www.CelebrationSound.com

[Gareth Magennis]

The Optos use an electroluminescant strip which lights when fed with a
(large) ac signal. There are then 2 Light Dependent Resistors attached to
the glowing strip. The more it glows the more the resistance changes, and
thus the more the circuit will compress. One LDR is used to compress, and
the other drives the VU meter circuit to give a reading of gain reduction.
Swapping parts from one Opto to another is unlikely to help, in my opinion,
as it's more likely its the electoluminescand strip which won't glow. I
would guess that the LDR's are quite robust, but that's purely a guess. You
will have to find out why there is no light. Either the Opto is faulty or
the circuitry that drives it is.

Does the unit pass audio?



Gareth.



> That's the problem I've been having, and I've been meaning to get it
> fixed forever. I have Alesis (!), Aphex and dbx compressors, which I
> hardly ever use, and I miss my LA-3A!
>
> I bought an additional opto, but plugging it in doesn't solve the
> problem either. I wonder if I can swap out the parts inside each can.
> There are two in each.
>
> But I will try looking in the dark first.
>
>
> Thanks,
>
> Don

[Don Cooper]

Gareth Magennis wrote:

> The Optos use an electroluminescant strip which lights when fed with a
> (large) ac signal. There are then 2 Light Dependent Resistors attached to
> the glowing strip. The more it glows the more the resistance changes, and
> thus the more the circuit will compress. One LDR is used to compress, and
> the other drives the VU meter circuit to give a reading of gain reduction.
> Swapping parts from one Opto to another is unlikely to help, in my opinion,
> as it's more likely its the electoluminescand strip which won't glow. I
> would guess that the LDR's are quite robust, but that's purely a guess. You
> will have to find out why there is no light. Either the Opto is faulty or
> the circuitry that drives it is.
>
> Does the unit pass audio?


Yes. Although I haven't tried in a year or two, so maybe I should say "I
hope so".

[Gareth Magennis]

> > Does the unit pass audio?
>
>
> Yes. Although I haven't tried in a year or two, so maybe I should say "I
> hope so".

Well if it does then there's a very good chance it's just the Opto not
lighting.

If it's down to the luminous strip, it is actually possible to make your
own. Except that the kits you can buy from electronic suppliers come in
different colours and luminosities and I have no idea which type would be
closest to the original, and you would have to recalibrate everything. I
believe most of the attack/decay characteristics are down to the LDR's, but
you could have an interesting time experimenting if you are that way
inclined.

[John Noll]

Gareth Magennis wrote:
>>>Does the unit pass audio?
>>
>>
>>Yes. Although I haven't tried in a year or two, so maybe I should say "I
>>hope so".
>
>
> Well if it does then there's a very good chance it's just the Opto not
> lighting.
>
> If it's down to the luminous strip, it is actually possible to make your
> own. Except that the kits you can buy from electronic suppliers come in
> different colours and luminosities and I have no idea which type would be
> closest to the original, and you would have to recalibrate everything. I
> believe most of the attack/decay characteristics are down to the LDR's, but
> you could have an interesting time experimenting if you are that way
> inclined.
>
>

Thanks for all of the suggestions in regards to my
original post. Thanks also for the manual .pdf file.

I adjusted the bias trip pot and zeroed out the meter
and all seems OK. When I get some time I'll try a full
calibration.

--
--
John Noll