Mark III no distortion channel 3

[jrb32]

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]

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/1YwdbeeOfsiePPpT4XbTSBLMDCOIPAe3I/view?usp=drivesdk

 

[woodbutcher65]

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]

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]

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]

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]

Hi guys,

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

Thank You!!!

 

[diancarlo81]

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?

 

[wlk66]

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?

has the problem been solved yet? I have the same thing

 

[Henz]

has the problem been solved yet? I have the same thing

Although I have no experience with III, when my C+ lost its lead channel, it was due to the plate load resistors failing on preamp board. But it appears that this approach has been done.

 

[Henz]

I would just like to chime in again...

In my experience the LDR's in the Mark IIC, C+ and Mark III's are very hardy. It's rare that I have had to swap any out EVEN with a total meltdown on the low voltage rail. These are good suggestions you cats are making. The OP has been unclear at best about what is actually happening when he pulls the lead drive. Without having the amp in front of me I can only speculate... But no lead gain leads me elsewhere, particularly if I can hear the lead drive pot engage the signal path change, and/or the rotating of the lead drive pot alters the volume level and/or changes the tone somewhat(as in adding any noise,etc...). I would be snooping around V3 first taking voltage measurements. A cooked plate load resistor on V3 will yield the same symptoms if it's determined that the LDR's are switching via voltage checks on the relay rail.

All this could be nought as well because I have found several early Mark IV's with bunk Vactrols, and the OP said this is a late Mark III...

Just trying to help with the troubleshooting since the OP is in another country and his tech or service person might need some suggestions.

Old post but I'm thinking its just a preamp plate resistor.