MkIII lead channel master volume problem

[Ed Mullins]

My green stripe MkIII head freaked out this weekend. When switching to the lead channel the lead master knob no longer had control over the level. It was as if the knob was on 10 regardless of it's position. The drive control functioned as expected but not the lead master. At practice the next day the amp seemed fine for an hour or so. All the channels were balanced and functioning as expected. Then the problem reared it's ugly head; the lead channel was screamin' loud as if the lead master knob was on 10. Is it a bad pot, a bad relay, a bad tube? Where should I look?

 

[Restless Rocks]

LDR #5 grounds the lead master volume when in lead mode. The leads on the LED side are somewhat fragile you may check to see if one has come loose or broken from vibration. Intermittents are sometimes a real pain in the ***. Probably a replacement will solve your issue.
Good luck!

 

[Ed Mullins]

Mike at Boogie agrees that it's probably the LDR associated with the lead master. A replacement is in the mail. Hopefully that will fix the problem. Thanks for your input.

 

[Boogiebabies]

If LDR 5 stays grounded it will bypass the Lead Master and basically go at full volume to the master. If it's an intermittent problem, it may be trying to bleed past the 10M dark/off resistance or the photo coupling section of the LDR is dying. Be sure to use heat sinks when you replace it.
They don't like heat.

 

[Ed Mullins]

Boogiebabies,
If the LDR doesn't like heat would that explain why it seems to fail only after the amp has really warmed up? It works fine when the amp is first turned on. I'm sure the component will eventually fail leaving me with a permanent problem so I intend to replace it ASAP.
Thanks for the advice.

 

[disassembled]

Boogiebabies,
If the LDR doesn't like heat would that explain why it seems to fail only after the amp has really warmed up? It works fine when the amp is first turned on. I'm sure the component will eventually fail leaving me with a permanent problem so I intend to replace it ASAP.
Thanks for the advice.

No he means when you are soldering it in (500+ degree blasts don't feel too good). A tube amp under normal operating conditions won't see this type of heat.


Greg

 

[Restless Rocks]

Boogiebabies,
If the LDR doesn't like heat would that explain why it seems to fail only after the amp has really warmed up? It works fine when the amp is first turned on. I'm sure the component will eventually fail leaving me with a permanent problem so I intend to replace it ASAP.
Thanks for the advice.

You'll want to use a 15watt (not a soldering gun or pencil hotter than 20watts) soldering pencil with a very sharp tip. The leads on the LED side (the ones that look kinda squared) are a bit brittle. when you bend them down sideways to mount to the board be very careful that you don't snap them right off. If you're not versed at soldering or working on very small components on a circuit board you should probably have a tech perform the surgery for you. The board is very crowded with wires and other small components that are easily melted and shorted when poking around with a soldering tool. You'd also need a good "solder sucker" to remove the leads from the old LDR. Good Luck!!!

 

[Boogiebabies]

Boogiebabies,
If the LDR doesn't like heat would that explain why it seems to fail only after the amp has really warmed up? It works fine when the amp is first turned on. I'm sure the component will eventually fail leaving me with a permanent problem so I intend to replace it ASAP.
Thanks for the advice.

LED's theoretically do not go bad. They either dim with age or die from too much voltage. In an LDR they are rated for 10,000 to 200,000 hours. The photo cell has a lifecycle which decreases with use and ultimately increases in resistance to the point where it becomes a closed circuit even if the LED is lit.

 

[Ed Mullins]

I installed the new LDR provided by Mike B. Learned a couple things. 1) I don't have the steady hands of a surgeon and 2) Leave electronics repairs to the professionals. Anyhow, the problem I was experiencing with the lead master seems to have disappeared but a new one is in it's place. The lead master knob works as expected but when channel switching from the lead channel back to the clean or R2 there is a slight volume swell then quick shrink back to the expected level. I don't remember the amp doing this before. Any suggestions?

 

[Restless Rocks]

LDR #5 should be a "5C4" which has the correct timing factor for the lead master switch. I beleive MkIII Green Stripe owners experience this same phenomenon to some degree.

 

[Boogiebabies]

Sounds normal to me. Break it in for a while.

 

[Ed Mullins]

Thanks for the suggestions guys. I'll play the amp at tomorrow night's gig and see if it does better or worse throughout the night. Just spent a few minutes comparing the channel switching on my blue stripe head and green stripe combo to that of the green stripe head (the one that's giving me trouble). The other two amps seem normal (IMHO) and switch to the clean channel from the lead instantaneously with no volume swell. Maybe I'll try to record the problem and post an mp3 so that you can hear it. Doesn't seem normal to me. Thanks for all the input though. Looks like I might send it back to Mike B next week.

 

[Restless Rocks]

Hey Ed, I've not experienced that volume swell on any mark amps myself but have heard of some who claim to have it. I can see how it would annoy me as well. I'm starting to think about a 15 volt zenor diode that regulates the voltage seen by the LDR circuit, that if open could raise the voltage being seen by the LDR switches. If it were shorted you would not have any switching at all and the LED on your foot switch would not light up. Your LDR 5 must be "going dark" sooner than the others in the switching circuit. Just for fun you might try and first use the footswitch and cable from one of your other amps that are working properly. If not the problem; use a DMM to test for about 12.2 volts DC across the tip to ground of the plug going into your footswitch while in R1. Since your technical abilities are limited as you have professed in previous posts I'd find a good tech or just go ahead and send it over Mike B. and let the master himself go over the amp. Good luck and I'd be curious to hear what you find out.
John