Mark III no distortion channel 3

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woodbutcher65
Mark III
Posts: 194
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III no distortion channel 3

Post by woodbutcher65 » Fri Feb 01, 2019 7:38 pm

Just because the LDRs may not be working doesn't mean they're bad. Are they getting their driving voltage when the lead channel is engaged? That's the question. (One of them.)

On my Black Stripe, both pull brights and the treble shift only work when lead mode is engaged. (I did not check R2, I don't have any use for it.)

So there's that test to perform. If you go to the lead mode, and those functions don't work, then the lead circuit isn't even being cut in.

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Mon Feb 04, 2019 10:11 pm

Hi Guys, I took a video to make sure about the problem.
The video will shows that Lead Channel Engage the Lead Drive is not working and Master Lead works but bright is not. While the Lead Channel is engage Treble Shift, Volume Bright, Bass Shift are working. Even the R2 and Deep are also working.

Link: https://drive.google.com/file/d/17_349W ... p=drivesdk

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Tue Feb 12, 2019 5:19 am

It's difficult to establish from the video as I think the video/audio are out of sync? Anyway ignore everything woodbutcher65 said (sorry assume you just quick read this) as we've basically already established what is switching in lead mode and exactly what to test for (see below)... but I'll make it even clearer:

Please do a video, or confirm 100% with the amp, where you ONLY do the following:

Make sure no footswitch is plugged in
Pull Lead ON
Show that the Lead Master controls the volume
Show that Lead Bright changes the sound when pulled (double check it does actually change the sound)
Show that Treble shift changes the sound when pulled (double check it does actually change the sound)

Turn lead channel off by pushing the pot back in for Rhythm 1 mode.
Show that Lead Master has NO control at all over the volume.
Show that Lead Bright does NOT change the sound at all when pulled
Show that Treble shift does NOT change the sound at all when pulled

That's it. Do not pull any other pots in or out as all the other push/pulls activate on both channels and is confusing.

My money is on LDR2 or a bad resistor around V3 as mentioned by lovetoboogie
On my Black Stripe, both pull brights and the treble shift only work when lead mode is engaged
This is how every single III is set up. And the IIC+. Read the schematics... as just mentioned all other PUSH/PULL pots are physical component switches and have nothing to do with the LDRs. Apart from R2 which is engaged by a relay and can be engaged at any time in any channel.

woodbutcher65
Mark III
Posts: 194
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III no distortion channel 3

Post by woodbutcher65 » Fri Feb 22, 2019 9:44 pm

So has this problem been solved yet?

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Feb 27, 2019 8:30 am

Not yet... I will make video again to confirm as per jrb32. I will post it here tomorrow...

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Fri Mar 01, 2019 7:58 am

You should be able to work this out without needing a video from the info I've given you.

I'd recommend taking it to someone who repairs valve amps at this point. Replacing LDR2 and physically inspecting around V3 is the first place of call if that doesn't fix it. There was also a case where an LDR had been put on backwards in another thread so don't trust anything! But it's probably something simple. LDRs (or optocouplers technically) basically act like ON/OFF switches and are used to reduce audible popping when switching channels, so you should be able to work it out from the schematic and go from there.

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Mon Mar 04, 2019 6:47 am

Hi Everyone!!!

Good day... Here is the 2nd Video just to be sure. And I have already ordered VTL5C1 for LDR2 3weeks delivery...

I hope this video will help.

Thank You So much for the help. I'll keep you all posted.

https://drive.google.com/file/d/1Ywdbee ... p=drivesdk

woodbutcher65
Mark III
Posts: 194
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III no distortion channel 3

Post by woodbutcher65 » Wed Mar 13, 2019 9:54 pm

That video doesn't tell us anything other than the lead isn't working. For all that video tells me, it might even be that the switch on the lead drive pot itself could be the faulty part.

You turned the lead master up, with lead engaged. But you didn't turn the lead master up with the lead NOT engaged.

Anything you do you have to do BOTH ways, with lead on and with lead off. Otherwise we don't know what effect is really occurring.

On MY Mark III, a black dot, here's what works when and where:

In lead mode, volume 1 pull bright works. This does not have any effect when the amp is NOT in lead.
In lead mode, treble pull shift works. This does not have any effect when the amp is NOT in lead.
In lead mode, lead master pull bright works. This does not have any effect when the amp is NOT in lead.

All other pulls work when the amp is in the clean channel.

Do exactly those tests.

In clean, pull volume 1. Pull treble. Pull lead master. They should have no effect.

In lead, pull them. If they have some effect, then SOME of the lead circuit switching is working.

When in the clean channel, turning up the lead master should have NO effect on volume. It may add a little bit of noise but it won't increase volume.


But you've got to test everything between the clean and the lead channels in the same way and in the same order.

I can tell you now, you need to take the amp to an electronic technician. This isn't going to be something you are going to fix yourself unless you plan to learn electronics, and quickly.,

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Thu Mar 14, 2019 4:28 am

In lead mode, volume 1 pull bright works. This does not have any effect when the amp is NOT in lead.
I think you mean Lead Bright - Pull Bright on Volume 1 does actually work in both Lead and Rhythm modes, although it doesn't have much of an effect after around 8 on Volume 1.

Apart from that I second this entirely:
you need to take the amp to an electronic technician. This isn't going to be something you are going to fix yourself unless you plan to learn electronics, and quickly.,

woodbutcher65
Mark III
Posts: 194
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III no distortion channel 3

Post by woodbutcher65 » Thu Mar 14, 2019 6:07 am

Yeah, on my amp at least, volume 1 pull bright has so little effect that I actually thought it doesn't work at all unless in the lead channel.

It's more apparent in the lead channel but it's subtle at best.

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Mon Mar 18, 2019 2:50 pm

Just waiting for the LDR to arrive. Resistors are ok... Already opened it and test the resistor. LDR VTL5C1 would be the last shot. Thanks!

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Fri Apr 05, 2019 12:35 am

Hi guys,

I have replaced every resistor and LDR2... The same no Lead Channel. Do you have any other solution?

Thank You!!!

diancarlo81
Bottle Rocket
Posts: 15
Joined: Fri Jan 25, 2019 1:33 pm

Re: Mark III no distortion channel 3

Post by diancarlo81 » Fri Apr 05, 2019 9:04 am

I have replaced LDR2 and LDR5, R119, checked other resistor the reading was Good.... Any other idea for the problem?
Thanks guys... Will it fix the problem If i replace R26, R153, and C32?

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