LA500A stopped working

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bobbriessinck
Posts: 4
Joined: Tue Oct 15, 2019 2:51 am

LA500A stopped working

Post by bobbriessinck »

Hi!

Been using my LA500A quite frequently and lately had to do a lot of moving around between studios and live recording sessions where I always brought a 500 rack.

Recently the VU of the LA500A stopped going to 0dBVU when powered on and in both GR and VU positions. At first I thought the VU ribbon cable got loose (happened before once) but it was still plugged in. Then when trying to get audio through some really weird stuff happened.

So:
- The VU LED lights up when in both GR and VU and cuts out when in Off mode. VU meter stays all the way down
- audio goes through but comes out very quiet, and the volume depends on the selected ratio
- when in the Off position, audio comes out at the same level it is going in, so bypassed.
- makeup gain doesn't work

It has been in the last slot of the 500 rack and I had to take it out and back in recently (for the mobile 500 rack). It is possible that because the last slot is the trickiest to slide the unit in, it accidentally saw the wrong voltages on the wrong PCB lines. Also possible is that the power rails didn't discharge fully and when taking it out some pins touched the wrong rails. Anyway, pretty sure it is on me... :roll:

- What could be the issue here?
- Hopefully this is something that can be repaired?

Thanks for helping out!

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Joe Malone
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Re: LA500A stopped working

Post by Joe Malone »

Please attach some photos of the LA500A inside do I can check it over for you.
Joe :-)
JLM Audio
Capturing Audio without Injury

bobbriessinck
Posts: 4
Joined: Tue Oct 15, 2019 2:51 am

Re: LA500A stopped working

Post by bobbriessinck »

Thanks for getting back so fast.

As requested!
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Joe Malone
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Re: LA500A stopped working

Post by Joe Malone »

HI Photos don't show anything unusual.

With the LA500A removed from rack on bench let me know the ohms you get between P- & P+ ? and S- and S+ ? pads behind the transformer plug on the main pcb. Do not unplug and ribbon cables.
Joe :-)
JLM Audio
Capturing Audio without Injury

bobbriessinck
Posts: 4
Joined: Tue Oct 15, 2019 2:51 am

Re: LA500A stopped working

Post by bobbriessinck »

P+ and P- measure 11.1 Ohm
S+ and S- measure 22.4 Ohm

Thanks for helping out!

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Joe Malone
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Re: LA500A stopped working

Post by Joe Malone »

bobbriessinck wrote:
Tue Oct 12, 2021 10:01 pm
P+ and P- measure 11.1 Ohm
S+ and S- measure 22.4 Ohm

Thanks for helping out!
Ok output transformer is ok.
Listening to the output at high level can you hear any thin low level audio? If you turn threshold fully CCW and turn up the makeup does the bad audio level go up at all? If yes does the thread hold turn the bad audio level down?
Joe :-)
JLM Audio
Capturing Audio without Injury

bobbriessinck
Posts: 4
Joined: Tue Oct 15, 2019 2:51 am

Re: LA500A stopped working

Post by bobbriessinck »

Sorry for the late reply-

- audio does not sound thin, but has a 20dB drop in volume
- turning the threshold and/or makeup gain does not change anything
- when on ratio 3:1 there's a 20dB drop in volume
- when on 5:1 there's a 40dB drop
- when on 10:1 there's a 50dB drop
- when in OFF mode audio passes through at unity level

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Joe Malone
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Re: LA500A stopped working

Post by Joe Malone »

bobbriessinck wrote:
Sat Oct 30, 2021 8:01 pm
Sorry for the late reply-

- audio does not sound thin, but has a 20dB drop in volume
- turning the threshold and/or makeup gain does not change anything
- when on ratio 3:1 there's a 20dB drop in volume
- when on 5:1 there's a 40dB drop
- when on 10:1 there's a 50dB drop
- when in OFF mode audio passes through at unity level
From your description, there is dc offset getting to the ratio switch so as the ratio resistor gets smaller the comp increases.

Are the 2 x 10R resistors on the LA500SC pcb ok and still measuring 10R?

Check DC voltages to 0v with no input and threshold and makeup fully CCW at pin 1 to 8 on the NE5532A and let me know what voltages you find on each pin.
Joe :-)
JLM Audio
Capturing Audio without Injury

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