1996 Marshall 6100LM Presence Pot Not Working / Mode Volume Levels

30th Anniversary 6100 range LE/LM amps

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 12 Nov 2016, 12:43

Will do... Thanks!

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 12 Nov 2016, 17:31

Is it just me, or does channel 3 sound a bit too hot? I'm sure many people like it, but it almost sounds like there's some blocking distortion happening. I do know that tube changes make huge differences in this amp.

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Re: 1996 Marshall 6100LM mode volumes...

Postby Anitoli » 13 Nov 2016, 07:37

Glad you got it up and running. The presence control is only active when the damping switch is in auto ( works only on channel 2-3) and low, works on all channels.

For myself i find that the Chinese 12AX7's work the best in this head. This is what they shipped with from the factory and they seem to take the higher voltages quite well.

Channel 3 has a shitload of gain. If it seems too hot try it without the gain boost pushed in this will lower the gain somewhat.

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 17 Nov 2016, 13:35

I tried all the damping settings, but the presence doesn't seem to be doing anything. I borrowed my wife's ears... She said that she couldn't hear any difference. Any ideas as to what is causing this?

Update... The damping switch is functional, but the presence has very little effect, if any at all.

In case anyone runs into an issues such as this:
One of the fasteners at the base of the OT was loose. I tried to remove the dedicated pcb that holds the pots (to get access the other end of the fastener), but there's no way to remove it without removing the entire power section pcb! I started to remove it, but decided there just had to be a better way... I ended up wedging a flathead screwdriver underneath the OT base, putting upward pressure on the head of the fastener, and successfully torqued the nut. Wow! What an amp! :)

Hard to believe these amplifiers sell for as little $ as they do. I've been playing Marshall's since '78, and this is by far one of the best MV Marshall's I've ever played, if not the best.

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Re: 1996 Marshall 6100LM mode volumes...

Postby Anitoli » 18 Nov 2016, 12:52

It is possible that the relay could be bad or have corroded contacts causing the signal to not pass very well. On all of the 6100's i have worked on the presence controls effect is very noticeable. Are you hearing the relay click as it shuts on and off? This would be easy to hear in auto mode by switching between channel 1 and channel 2. Have you tried removing the ribbon connectors, cleaning the contacts with a good contact cleaner like Deoxit D5 and reinstalling? This might get you somewhere. If not we will need to poke around a little more.

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 18 Nov 2016, 17:15

Tests results...

Low and High Modes:
- No relay activity is audible when switching the amplifier on.
- No relay activity is audible when switching between all three channels.

Auto Mode:
- Relay clicking is audible when amplifier is switched on.
- Relay clicking is audible when switching from channel 1 to channel 2.
- Relay clicking is audible when switching from channel 2 to channel 1.
- No relay clicking is audible when switching from channel 2 to channel 3.
- No relay clicking is audible when switching from channel 3 to channel 2.
- Relay clicking is audible when switching from channel 3 to channel 1.
- Relay clicking is audible when switching from channel 1 to channel 3.

All large block (ribbon) sockets and presence pot have been sprayed with Deoxit D5 + G5. I tested the presence control, but there is no change. The presence effect very slight, to nearly non existent.

MIDI board is functioning normally.

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 19 Nov 2016, 18:55

Is it possible that only 1/2 of the relay is working? Also... I've heard that some Marshall amps require that the MV be set to specific level, before the presence control becomes functional. I haven't pushed the amp yet, as I want the tubes to break-in a bit.

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Re: 1996 Marshall 6100LM mode volumes...

Postby Anitoli » 20 Nov 2016, 02:15

The relay appears to be functioning properly. The presence control is non-dependent on the MV, it doesn't matter what the MV is set to it still should work. Is there any way you can verify the function of the presence pot? Those pots can and will go south and this problem could be related to that.

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 20 Nov 2016, 07:55

Thank you, Anitoli.

I'm looking at the schematic now. I'm a novice, but should the presence control not be indicated at the PI tail circuit? I would expect to see a 25k linear (presence) pot between the 4 Ohm tap and V7b PI. I suppose, this is where the relay and component boards become relevant... :doh

The Presence pot appears to be tagged as VR107 (10k maybe?) with contact points at pins 5 and 7 @ CON104. I cannot get my probes underneath the pot it self. Assuming CON104 is a male-end socket w/pins, I will try to take a reading there.

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 20 Nov 2016, 12:35

Okay... I was successful at checking the 10k presence pot @ CON104. At first, the reading was 4k3 with the presence turned to 100% CCW. After rotating the dial a few times, the impedance increased to 9k5/100% CCW. Close enough for rock. I had treated the pot with Deoxit, so maybe it took some time to clear the debris?

I also tested the Effects pot, which testes good @ 10k/100% CW. The only thing I don't dig, is the way the dry signal volume increases (in parallel mode) along with the effects. Maybe I need to use kill-dry on the effects? It seems I recall the Effects volume acting as a MV in parallel... Of all things, I suppose I should take a look at the owners manual. :winking

I found that most of the 14AWG slip-fit connectors were loose... I cleaned w/D5 and tightened the connectors. Some of the insulation wasn't applied properly, so I had to do some minor trimming for 100% coverage onto the terminals. I'm going to let it sit for a few hours. Until then, I'm going to enjoy a bowl of some homemade Skyline Chili w/angel hair pasta + shredded cheese.

Story...
When I purchased my '87 SJ 2550 stack, I found that Marshall had neglected to solder one of the wires onto the standby switch. WTH? The former owner had complained about "popping at power-up, and volume loss..." I used the old power-it-up in the dark trick, which pointed me right to the arch point. Just a bare wire placed through, and then wrapped around the standby switch terminal. Zing!

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Re: 1996 Marshall 6100LM mode volumes...

Postby Anitoli » 20 Nov 2016, 12:42

Are you getting any response now from the presence pot?

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 20 Nov 2016, 13:05

Anitoli wrote:Are you getting any response now from the presence pot?
It's not working. Doesn't sound like it's doing anything. The Damping seems functional, but that's it. :(

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Re: 1996 Marshall 6100LM mode volumes...

Postby MacFangus » 20 Nov 2016, 13:46

Found it!

If I apply downward pressure on pcb (outer edge, opposite the output jacks), the presence control works. Apparently, there is a cracked solder point on the pcb. Looking at the area near the PI tail nest and relay, I can see where a cracked solder is probable. Taking the PI tube in and out visibly causes the board to flex! Two additional anchor points would have prevented this. There is a 1/2" easement where standoffs could have been placed. If I have to remove the board, I'm going to install one standoff at the center, and one at the corner of the board.

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Re: 1996 Marshall 6100LM Presence Pot Not Working + Mode Volume Levels.

Postby MacFangus » 20 Nov 2016, 19:51

Update:
Repairs went great! Found a cracked solder joint at the relay (source of issue), and one at the fuse LED. Repairs really aren't bad at all on this amp. Once you get the cables disconnected, it's a breeze! disassembly of the power-tube base assembly would be a more complicated, but still acceptable as a home repair. You don't need to remove the power board junction wires... Just flip the board to the right, leaving the wires connected. Place a pad between the populated board ends. I didn't add any standoffs. I decided it best to leave the amp stock.

Thank you everyone who helped me with this! :beerme

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Re: 1996 Marshall 6100LM Presence Pot Not Working / Mode Volume Levels

Postby Anitoli » 21 Nov 2016, 13:05

Cool! You now have the most kick ass Marshall ever made up and running. :Thumbs

That lead free solder that is used leaves allot to be desired. I have fixed many issues resulting from it cracking.

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