Carvin Pro Bass 500 Bridge Mode

[stoneattic]

I'm not sure if this a repair or I'm just missing something.

I have a Carvin Pro Bass 500 that works in every way except in bridge mode. Each individual channel works fine, but when I put it in bridge mode I get no sound at all. It does not trip the Protect light and I hear the relay click, but no sound. I have the bridge switch set to "mono/bridge", the biamp switch to "off" and I have a banana plug speaker cable in the bridge binding posts. The speaker cab is 8ohm, which is the minimum for bridged mode. I know the speaker cab is good as it works fine with the individual channels and I know the cable is good as it works with other amps (and I check for continuity and proper polarity).

I got the amp fairly recently and when I first got it I THINK I tested it in bridge mode and it did work, but I might be mis-remembering.

The only attempt at "repair/troubleshooting" was pulling the chassis and measuring the poles in the bridge switch. It is a DPDT and when in "normal(bi-amp)" mode it tests as it should (center pins to end pins on one side, open on the other). When is "mono/bridge" mode the all the pins on each side have continuity. I don't know if that's related to the rest of the circuit (my guess) or an indication of a bad switch.

Any ideas on what I might be missing or what might be wrong?

[mhuss]

If you have a scope, check to see that the two outputs are out of phase with respect to each other when in bridge mode.

What's up with that black wire jumper in the schematic? If that is in place as shown, it would pretty much guarantee that bridge mode won't work!
I hope 'N/U' means 'Normally Unplugged'...

[g1]

The only attempt at "repair/troubleshooting" was pulling the chassis and measuring the poles in the bridge switch. It is a DPDT and when in "normal(bi-amp)" mode it tests as it should (center pins to end pins on one side, open on the other). When is "mono/bridge" mode the all the pins on each side have continuity.

This does not sound right. Remove the switch and hard wire the board contacts the way a normal DPDT would work and see if that helps.

[Dave H]

...pulling the chassis and measuring the poles in the bridge switch. It is a DPDT and when in "normal(bi-amp)" mode it tests as it should (center pins to end pins on one side, open on the other). When is "mono/bridge" mode the all the pins on each side have continuity. I don't know if that's related to the rest of the circuit (my guess) or an indication of a bad switch.

Any ideas on what I might be missing or what might be wrong?

Is trace [9] cut as it says in note 9 on the power amp schematic?
It's drawn as a link just below the bridge switch.

[stoneattic]

If you have a scope, check to see that the two outputs are out of phase with respect to each other when in bridge mode.

What's up with that black wire jumper in the schematic? If that is in place as shown, it would pretty much guarantee that bridge mode won't work!
I hope 'N/U' means 'Normally Unplugged'...

Thanks for responding!

I have a scope but really don't know how to use it (it's on my long list of "things to do") or if it works properly.

I don't see a single wire jumper like that shown in the schematic, but two wires in the area of the L1 and L51 that go to the output board (black and brown in the attached pic). However I think they are the channel 1 and 2 outputs to the output PCB.

I've included a pic of the entire power amp PCB. The other side has the power transistors under a huge heat sink. I can't get a good pic but can see there is no jumper there.

While looking more closely I found a bad/broken solder joint where the channel B binding post was connected to the output PCB. I was all excited thinking I found the problem, but I resoldered it and tested with no joy. If I turn the volume about 3/4-full up I can hear a low output. I can hear the notes I'm playing, but they are WAY too quiet and distorted.

[stoneattic]

This does not sound right. Remove the switch and hard wire the board contacts the way a normal DPDT would work and see if that helps.

Thanks for responding!

It doesn't seem right to me either but I don't know enough to be sure if there isn't something in the rest of the circuit that would make it test that way. I disconnected what I THINK are the channel outputs from the power amp PCB to the output PCB and retested and got the same result. I will try hard wiring.

[stoneattic]

Is trace [9] cut as it says in note 9 on the power amp schematic?
It's drawn as a link just below the bridge switch.

Thanks for responding.

I'm not sure. I'm having a tough time figuring out where that trace is. I don't see any obviously cut traces anywhere on the board though. I believe, based on the file name, that Carvin used that PCB on more than just the Pro Bass 500 and certain notes and/or features might not pertain to the PB500. Case in point, it shows Ch1 & Ch2 inputs, but there is only one input. I'm guessing Carvin had 500W a PA power amp that used this board. Based on the file name they had a powered stereo mixer that used this board (CX1252) and possibly the MX842 although that appears to be a 400w mixer.

Also, even though I mentioned previously that the bridge switch is a DPDT it looks like the poles are wired together. It seem like they just used the DP for a more stable PCB mounting.

[Dave H]

Thanks for responding.

I'm not sure. I'm having a tough time figuring out where that trace is. I don't see any obviously cut traces anywhere on the board though. I believe, based on the file name, that Carvin used that PCB on more than just the Pro Bass 500 and certain notes and/or features might not pertain to the PB500.

I'm sure you are right. There are also unused transistors in the output stage. If that trace is not cut I think it will put the outputs in phase in bridge mode so you won't have any output with the speaker 'bridged' between the outputs. I'd check it by connecting the speaker for normal operation (one side grounded) with the amp set to bridge mode. If both outputs work OK like that but not with the speaker bridged then the outputs must be in phase and you'll need to cut the trace.

The clock has just ticked over to Christmas day here in the UK so Merry Christmas to you all

[Mick Bailey]

Take care with scoping a bridged output as both terminals are 'hot' with respect to ground. you either have to use an isolation box or ground the scope lead to the chassis and scope each output individually. You can't just clip across the output.

[Dave H]

I'm having a tough time figuring out where that trace is.

Have a look at the first picture in post #5. Zoom in halfway down on the right and you'll see two connectors H1 and H2. Just to the left of H2 is an arrow pointing to a trace linking H2.3 and H2.4. I think that is trace [9]. Check for continuity between H2.3 and H2.4 with the switch in bridge mode. If it measures short circuit cut the trace.

[g1]

Check for continuity between H2.3 and H2.4 with the switch in bridge mode. If it measures short circuit cut the trace.

His resistance checks in post #1 seem to agree with this (trace not cut). I had mistakenly thought he measured the switch out of circuit, but it was measured in circuit when all contacts read short to each other.

[stoneattic]

You guys are awesome. Apparently Carvin never bothered to cut the trace.

I disconnected the ribbon cable that connected to the trace Dave H pointed out and checked for continuity at the bridge switch. The switch acted the way it should. So I cut the trace and now have the correct output in bridge mode!

[gcavy1]

Still around to help with this?

You guys are awesome. Apparently Carvin never bothered to cut the trace.

I disconnected the ribbon cable that connected to the trace Dave H pointed out and checked for continuity at the bridge switch. The switch acted the way it should. So I cut the trace and now have the correct output in bridge mode!

I know this post is a couple years old now, but wondering if you're still able to help. I have the same problem with my trusty old Carvin ProBass 500 as well that it doesn't work in bridged mode. I've opened it up and located the arrow mentioned in the picture as well as the H2 section on the poweramp circuit board. I'm just unfamiliar with the term "trace". Can you tell me exactly what you cut to get the amp to work in bridged mode? Are you cutting part of the ribbon cable coming off the H2 connector section? Or is it on the connector itself? And what did you cut it with?

I've got a gig tomorrow that I had to pull out this old amp of mine and would love to get it to full power if possible!

Thanks in advance!

[Enzo]

A printed circuit board is a slab of fiberglass or other insulating material with thin copper plated lines running around on it instead of wires. We all those copper lines "traces".

If I have to cut a trace - meaning to cut away a portion of one to leave a gap - to break a connection, I use my Xacto knife. A sturdy single edge razor blade would work, like in a box cutter would work. I also sometimes use a cutoff wheel in my Dremel tool. I grind a gap in the copper trace.

[gcavy1]

o.k. I just did some more digging online and now understand the term "trace". I looked at my amp again and it appears mine is already cut? (there's a white line going through it, more like a scrape, I'm assuming that's the cut) I tried to add a pic but it's not letting me for some reason.

I was hoping this was an easy solution, but I must have a different problem. Thanks for the help!