Mark IV trouble shooting help

[tim6string]

I have not spent much time trouble shooting a recent issue I had, here are the symptom.

The other night after playing a full set (amp working fine) I went to start the second set and got nothing out of the amp on full power. The loop LED (Orange) was dim. I switched to tweed power, the loop LED was no longer DIM and the amp worked perfect. For the last set I went back to full power and the amp worked fine the entire set. At the end of the night I noticed the Loop LED was very DIM. The amp seemed to work.

All of the tubes are new, maybe 9 hours and the amp sounded fine even at full power the third set. Any thoughts. I do not use the effects loop and don't care if that LED is dying, but no sound on full power is a little bit of a worry.

 

[Boogiebabies]

Whats the serial number?

Does it have a satellite send?

 

[tim6string]

No, I believe the serial number is under 1000. I got it in around 1990, they were very new then.

 

[Boogiebabies]

OK, It's a MK IV A with the 561136 power transformer. I am wondering if the full power switch is going bad. The amp would have low voltage to the heaters, LED's and footswitch regardless as they are secondary taps.
I would clean it first and see if the problem persists. It should be a three way Carling. As always, a visual inspection of the power board and main board would not hurt to see if anything is burnt and the filter caps are not leaking crud. The schematic is very simple on this one. Full power is up, off is in the middle and tweed is down. Both modes run off the same caps and circuit, but the Tweed is a tap in the PT that lowers the voltage. This is what would lead me to the switch.

 

[tim6string]

Thanks for the input. I will start with the switch. I recently had the amp checked out and they said the voltages/caps and all seemed perfectly in spec. Savage Audio did the inspection and retubed it.

That is what made me a little nervous. I could see the switch going bad, but based on your description, that doesn't necessarily explain why the LED would work fine in tweed and not power if they are secondary taps. The TUBES looked to glow ok in both On and Standby, so I am ASSUMING the heaters were working fine.

But if it is just a case of the LED dying that is cool. I do not use the Mark IV foot switch, so I am not sure about the LEDs on it. I use the Axxess Electronics CFX4 with the Mark IV cable.

THANKS for the input.

 

[Boogiebabies]

I mean it could be trying to make the connection in full power and pulsing the voltage whick is causing the LED's to dim... There is a ton of circuitry in that amp and the best we can do is start with the basics. If it gets into
the switching matrix, it is you and mesa that will be doing the repair.

 

[tim6string]

One interesting side note. I at this point I cannot confirm how all this went down (a little panic'd that my amp wasn't working as we had to start)

I switched the amp off and back on once and it seemed to have some sound in the full power position (LED seemed to get brighter than dimmed), then I just went to tweed and it worked. So it could be a switch issue.

I am just not 100% about how all this went down. I will be setting up the rig again this weekend and playing around with it.

 

[Boogiebabies]

It is a 16 year old amp. Even on a Boogie the Gremlins can pop up.

 

[tim6string]

I agree, it all just freaked me out a little, I love playing through that amp and my back up is a JCM800 which while cool pales in comparison.

It Sounded great for the last hour in full power, so it does seem like a switch problem.

I chalked it up to the LED dying, seeing as everything else seemed to work flawlessly in full power for the last set.

One other question, the AMP seemed warm. My first step was to make sure the fan was still plugged in, could this also be a heat issue?

Again, the sound was fantastic.

 

[Boogiebabies]

It could be a heat issue with the switching system, but the PT switch is hooked directly to the PT. How hot was the PT? A really hot tube may cause them to get very hot.

 

[tim6string]

It is rack mounted, so it just seemed that there was more heat at the back of the rack than usual. I noticed all this at the end of the night when full power worked perfect and the LED was dim. Nothing out of the ordinary on tube glow, but unfortunately I had put it on standby before I had really thought to look in back again.

 

[Boogiebabies]

It might not be a bad idea to see what those tubes are drawing. If it is running that hot it may be due to the tubes. I mean, your tubes may be drawing -78 ma and putting a hell of a strain on the PT in terms of heat and bias voltage. In the MK IV A, the bias has a seperate tap and the heaters and the footswitch and othe LV circuits are run off the two 6.3V taps. Also, LED's never go bad. They either dim or die. They should last for ever.

Just a thought.

 

[tim6string]

I will look into it. When they retubed it they verified the BIAS for the tubes, they are 5881.

I do use a hotplate and run the master around 6.5, so the power tubes get used. They did not bias it at that power level I am sure. It is a brand new problem though and I have run this set up for a while. The 5881s are new (9 hours) before that it was always MESA tubes.

Again, thanks for the input. I have some work to do on my end no doubt, but it sounds like there is possible hope that nothing serious is going down here. I guess I will find out.

 

[Boogiebabies]

OH NO !!!!!! A HOTPLATE !!!!!


Now I see.

I would never ever use a hotplate. Most say they are safe, but I could nor would ever trust them.

 

[tim6string]

I have had no issues and it works fine on tweed, so I doubt that is the issue.

It is not like I have the master on 10 or anything. The amp should run fine at 6.5 master, just too loud.

Plus I use the line out feature to drive the effects and reamp.

 

[tim6string]

The one issue I see is that I have been using the 4ohm out to a 4ohm load or cabinet, but I was told that was fine.

I was just never sure if you had to use both 4ohm outs to 4ohm loads to match what the amp wants.

 

[Aart]

Mark IV speaker outs:


4 Ohm cabinet on 4 Ohm output

8 Ohm cabinet to 8 Ohm output


2 8 Ohm cabinets both in a 4 Ohm output.


Then it should be fine.
The manual says so.

 

[tim6string]

Thanks for the input, that is what I thought, but it always helps to get more input.

Actually I thought the manual said 1 8ohm cabinet - 8ohm jack, two 4ohm cabinets - 2 4ohm jacks and overlooked the 1 4ohm cab situation I was in. Although it seems that would not be a problem.

 

[Boogiebabies]

Super. I still hate hotplates.

 

[tim6string]

Just because you hate them doesn't make them a problem.
So many great tones have used a device similar to this, and it makes too much sense in certain applications.
And it is not for everyone I admit.