Mark III no distortion channel 3

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

Mark III no distortion channel 3

Post by diancarlo81 » Fri Jan 25, 2019 1:53 pm

Hi Guys,
I have this purple stripe Mark III amp and the problem is the channel 3 it has no distortion, it only started when I change it to R2 mode. The distortion was not good anymore and suddenly went out. I have replaced all the tubes and reCap. Please help me... Im from Philippines Mesa Support is not available. If some knows the contact details of Mike B. Kindly pm me. And i hope someone here can point out the specific problem.

Thank You so much!

Carlo

jrb32
Mark III
Posts: 193
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Sat Jan 26, 2019 10:53 am

Which channel isn't working? Lead channel or R2 channel?

Do you get any sound at all? If so please be as specific as possible with what happens and whether the volume levels of each channel have been affected. Does it definitely switch channels etc? R2 isn't really a 'channel' is just adds a boost to the Rhythm channel and the Lead Channel.

woodbutcher65
Mark III
Posts: 180
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III no distortion channel 3

Post by woodbutcher65 » Sat Jan 26, 2019 8:20 pm

It seems most likely that LDR2 has taken a dump. It's what switches the signal into the lead drive circuit. LDR4 then directs the lead drive signal back into the main circuit.

This means "Take it to a qualified amp tech who understands LDR switching.".

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III no distortion channel 3

Post by lovetoboogie » Sun Jan 27, 2019 6:41 pm

Another thing to look at which is notorious on Mark III's is the under spec'd 680 ohm resistor on the low voltage rail... If that starts to cook out the first symptom will be switching problems...shortly after that it will burn right through the board.

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 10:45 am

jrb32 wrote:Which channel isn't working? Lead channel or R2 channel?

Do you get any sound at all? If so please be as specific as possible with what happens and whether the volume levels of each channel have been affected. Does it definitely switch channels etc? R2 isn't really a 'channel' is just adds a boost to the Rhythm channel and the Lead Channel.
Only Channel has no distortion. Its like clean channel... No dirt at all... I

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 10:48 am

woodbutcher65 wrote:It seems most likely that LDR2 has taken a dump. It's what switches the signal into the lead drive circuit. LDR4 then directs the lead drive signal back into the main circuit.

This means "Take it to a qualified amp tech who understands LDR switching.".
LDR2 and LDR4 that needs to be replaced? I can replace the LDR myself. Hope this will work out fine.. THANK YOU SO MUCH!!!

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 10:50 am

lovetoboogie wrote:Another thing to look at which is notorious on Mark III's is the under spec'd 680 ohm resistor on the low voltage rail... If that starts to cook out the first symptom will be switching problems...shortly after that it will burn right through the board.
Hi,
Do you know which resistor? Resistor Number? R20 example...
Thank you so much!!!

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 10:57 am

By the way guys... It's not Purple Stripe (mistake) it's Green Stripe mesa boogie Mark iii. The last production of mark iii series...

Thank you so much!!!

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 1:57 pm

jrb32 wrote:Which channel isn't working? Lead channel or R2 channel?

Do you get any sound at all? If so please be as specific as possible with what happens and whether the volume levels of each channel have been affected. Does it definitely switch channels etc? R2 isn't really a 'channel' is just adds a boost to the Rhythm channel and the Lead Channel.
Yes, there is sound... Lead channel has no distortion. The volume is good, it's like a clean channel. I have tested the lead channel and it turns out the same as Channel 1 (clean channel). R2 is working also (no problem). Only the lead channel has the problem. LEAD CHANNEL - Clean sound intead of Dirt.

jrb32
Mark III
Posts: 193
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Wed Jan 30, 2019 2:00 pm

LDR2 and LDR4 that needs to be replaced? I can replace the LDR myself. Hope this will work out fine.. THANK YOU SO MUCH!!!
DO NOT REPLACE ANYTHING UNLESS YOU'RE SURE! But there is no harm in replacing the optocouplers (LDRs) like for like for the Lead Circuit if you can get hold of the same part numbers as before. Just make sure you put them round the same way as you took them off... the LDRs are LDR2, LDR5, LDR6 though technically. LDR3 and LDR4 are for the R2 channel. I would HIGHLY recommend NOT doing anything and getting an amp tech who understands LDRs to test these first and diagnose your problem before doing ANYTHING!!!!!!
Do you know which resistor? Resistor Number?
R119. See below for where it is. It's pretty obvious if it'll be burnt... also test the diodes and do a visual check of the board to check for any burnt or black components.

GREEN STRIPE CP1G LAYOUT AND SCHEMATIC: 1989-1997

SCHEMATIC: https://drive.google.com/open?id=146K06 ... _5CU9_hXlV
BOARD VIEW: https://drive.google.com/open?id=1MmBK- ... idtRplSIIl

Without diagnosing it on here by saying exactly what works and what happens for every single thing and using logic to work it out, or going to a decent amp tech to test things like the LDRs, you're probably going to do more harm than good randomly replacing things, but if you do decide to go ahead best of luck!
Yes, there is sound... Lead channel has no distortion. The volume is good, it's like a clean channel. I have tested the lead channel and it turns out the same as Channel 1 (clean channel). R2 is working also (no problem). Only the lead channel has the problem. LEAD CHANNEL - Clean sound intead of Dirt.
Ah cool thanks for the info. OK let's start diagnosing this through logic does Lead Master, Lead Gain or Treble Shift have any effect when Lead channel is on?

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Wed Jan 30, 2019 2:31 pm

jrb32 wrote:
LDR2 and LDR4 that needs to be replaced? I can replace the LDR myself. Hope this will work out fine.. THANK YOU SO MUCH!!!
DO NOT REPLACE ANYTHING UNLESS YOU'RE SURE! But there is no harm in replacing the optocouplers (LDRs) like for like for the Lead Circuit if you can get hold of the same part numbers as before. Just make sure you put them round the same way as you took them off... the LDRs are LDR2, LDR5, LDR6 though technically. LDR3 and LDR4 are for the R2 channel. I would HIGHLY recommend NOT doing anything and getting an amp tech who understands LDRs to test these first and diagnose your problem before doing ANYTHING!!!!!!
Do you know which resistor? Resistor Number?
R119. See below for where it is. It's pretty obvious if it'll be burnt... also test the diodes and do a visual check of the board to check for any burnt or black components.

GREEN STRIPE CP1G LAYOUT AND SCHEMATIC: 1989-1997

SCHEMATIC: https://drive.google.com/open?id=146K06 ... _5CU9_hXlV
BOARD VIEW: https://drive.google.com/open?id=1MmBK- ... idtRplSIIl

Without diagnosing it on here by saying exactly what works and what happens for every single thing and using logic to work it out, or going to a decent amp tech to test things like the LDRs, you're probably going to do more harm than good randomly replacing things, but if you do decide to go ahead best of luck!
Yes, there is sound... Lead channel has no distortion. The volume is good, it's like a clean channel. I have tested the lead channel and it turns out the same as Channel 1 (clean channel). R2 is working also (no problem). Only the lead channel has the problem. LEAD CHANNEL - Clean sound intead of Dirt.
Ah cool thanks for the info. OK let's start diagnosing this through logic does Lead Master, Lead Gain or Treble Shift have any effect when Lead channel is on?
Lead Master = Yes
Lead Gain = No (it plays like Volume no Distortion)
Treble Shift = Yes

jrb32
Mark III
Posts: 193
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Thu Jan 31, 2019 10:28 am

Lead Master = Yes
Lead Gain = No (it plays like Volume no Distortion)
Treble Shift = Yes
Cool and Lead Master, Lead Gain and Treble Shift have no effect when in Rhythm 1 mode?

LDR2 looks like the first thing to swap then. Should be a VTL5C1 :)

May as well replace R119 if it's never been done too as precaution. 680 Ohm Resistor. Wirewound 4W or higher should do fine, just make sure it fits as crazy high W versions are giant ;)

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

Re: Mark III no distortion channel 3

Post by diancarlo81 » Thu Jan 31, 2019 10:55 am

jrb32 wrote:
Lead Master = Yes
Lead Gain = No (it plays like Volume no Distortion)
Treble Shift = Yes
Cool and Lead Master, Lead Gain and Treble Shift have no effect when in Rhythm 1 mode?

LDR2 looks like the first thing to swap then. Should be a VTL5C1 :)

May as well replace R119 if it's never been done too as precaution. 680 Ohm Resistor. Wirewound 4W or higher should do fine, just make sure it fits as crazy high W versions are giant ;)
Yes... Thank you so much!!! Keep you posted for the update.

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III no distortion channel 3

Post by lovetoboogie » Thu Jan 31, 2019 11:34 am

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.

jrb32
Mark III
Posts: 193
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III no distortion channel 3

Post by jrb32 » Fri Feb 01, 2019 5:41 am

Please chime in as much as possible! Yeah snooping around V3 also sounds like a good plan.

The plate load resistors for V3 are R281 and R272 (check schematic and location of these above). As mentioned, a visual check of the entire board especially around V3 should be done. Then multimeter measurements. There is no point blindly replacing things on the board without diagnosing first with a visual and voltage check. If in ANY doubt get an amp tech to look at it. These carry crazy high DC voltage that can kill you, but hopefully this will get you in the right direction and it'll get sorted :)

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