A500B-1 Switch Issue in LB75

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mellowmike
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A500B-1 Switch Issue in LB75

Postby mellowmike » Sun Jul 19, 2020 5:01 pm

I have an LB75 that I'm the original owner of. It's been a closet dweller for most of its life. I take it out every few years to check everything, tune it up, and do a little noodling. I put in a new 9v battery about 2 years ago. (It is the old style with the harness only - not a compartment). I keep a small flat piece of foam between it and the circuit boards, etc.

Well, the other day I plug it in, and as I'm testing the electronics, I notice the Deep mini-toggle switch is not functioning properly. With the Neck PU selected - there is only sound in the ON position. Switching it to OFF kills the signal entirely. This never used to happen. No sound from the PU... but there IS a lot of static "noise". Everything else on the bass works like it should. I even replaced the 9v battery with a fresh one, just in case. No difference.

What do you think it might be? A mini-toggle that has gone bad...? A short circuit...?

The neck PU sounds great in Deep Mode.... but I'd like to have the option to turn it off restored.

The Bright switch works (although brittle and slightly noisy), and the Phase switch works fine.

Pickups are H50B's. This has the A500B active module.
Last edited by mellowmike on Thu Jul 23, 2020 12:44 pm, edited 1 time in total.

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Toptube
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Re: A500B Issue in LB75

Postby Toptube » Mon Jul 20, 2020 11:29 pm

If a fresh battery didn't fix it, then Its likely a little bit of corrosion in a switch or a solder joint gone bad. De-oxit should fix up swiches and pots nicely. and chasing a bad solder joint shouldn't be too bad. You know the problem happens after a certain switch/setting. So try solder joints which make sense from that circuit.

mellowmike
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Re: A500B-1 Switch Issue in LB75

Postby mellowmike » Thu Jul 23, 2020 12:46 pm

Would anybody have the PDF wiring schematic for this A500B-1 module (circa 1989-1990)...?

I contacted Kiesel/Carvin and they only have the A500B-3 (without the Deep-Bright-Phase).

mellowmike
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Re: A500B-1 Switch Issue in LB75

Postby mellowmike » Tue Jul 28, 2020 11:31 am

Well... I poked around inside my the LB75 and upon diligent scrutiny - I noticed one of the PU leads unconnected. The one that is the coax braid with a tail wire added. That probably was causing my issue, with the Deep/Normal switch. Right?

Putting in the 9v battery dislodged it. Gotta be more careful.

After soldering it back on, I still have the issue. Deep switch only does Deep. Normal is no signal.

I hate to do it, but it looks like I need to replace the mini-toggle itself on the PCB board.

Update: I did a re-flow of the "slightly crusty" solder on the switch lugs. Then shot some contact cleaner into the switch itself. (I didn't want to flood it - jut a spritz or two). Now I have an improvement of sorts. The switch is no longer a "kill switch". There's a little sound coming through the amp from the Normal position. Thanks Toptube. The Deep position is still working perfectly.

That's maybe telling me that the switch is not fried or broken. At some point I might put in the new switch.... but for now I'm a little relieved. Just for the record, this bass puts out a monstrous sound. I put a compression pedal into my amp's effects loop, and all of the goodness increased two-fold. Your aggressiveness may vary. But for old H50B pickups... these most likely will be keepers...! :D

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gumbynotpokey
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Re: A500B-1 Switch Issue in LB75

Postby gumbynotpokey » Fri Aug 14, 2020 7:51 am

Mini-toggle on an LB? Is that not a splitter for the HB at the bridge position?
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mellowmike
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Re: A500B-1 Switch Issue in LB75

Postby mellowmike » Mon Aug 17, 2020 5:56 pm

You would assume it was - but nope.

On LB75 basses with the first A500B modules, the 3 minis are

(1) Deep ON/OFF neck humbucker J-PU

(2) Bright ON/OFF bridge humbucker J-PU

(3) Phase IN/OUT, when both PU's selected.

And of course, the standard sized PU select toggle.

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Re: A500B-1 Switch Issue in LB75

Postby mellowmike » Fri Sep 04, 2020 4:46 pm

Further update...

I replaced the mini toggle switch. Nothing changed at all. It's still either Deep - or kill switch. No normal PU tone.

AND... now the mini toggle switch next to it is doing the same thing. (Bridge PU). Bright - or kill switch. No normal.

I swear I never touched it, or bumped it. What the heck is going on...?

Therefore, the problem must be somewhere else in the circuitry, not the switches. And this is about the limit of my electronic skill and repair. I put in another new battery, just in case. Checked for loose or wiring not connected. Totally baffled.

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Re: A500B-1 Switch Issue in LB75

Postby mellowmike » Sat Sep 05, 2020 9:53 am

Here's something I should add...

The A500B-1 active module was designed to go with the older Carvin bass pickups that had 4 wires.

Braided Shield
Black
Red
White

My LB75 has H50B pickups which only have 3 wires. They lack the white wire. On the guitar version, white was the tap wire that was used when configuring a coil tap. I read in the museum that if you weren't coil tapping, you left it disconnected.

When I got the H50B pickups, the counter person at Carvin told me to leave the solder point for the white wire empty, and all will be fine. I wired it up, and it was fine. Then this year, it wasn't. On the PCB board, there are letters next to the solder points for the pickup wires. B for black... R for red... W for white... and G for ground. So there is no way to hook them up wrong. I double-checked that.

The only thing I haven't considered, is if the H50B pickups have failed, in terms of now only having one sound.


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