Mark IVA loop feedback issue, eq led. Are my LDRs going?

[elegost]

Hey folks, I have a 1993 Mark IVA that I bought new and have played the crap out of.
Recently two issues have cropped up:

1) It seems like I'm getting feedback from the fx loop return back into the send (wtf!). When I use a delay in the loop, I get a weird, choked sound like there's a phase issue, and the pedal goes into oscillations, even with feedback set very low. Taking the delay pedal with the same settings and putting it in front of the amp = no oscillations. Running a straight cable from fx loop send to return, things sound okish, maybe still phasey? I am not getting this problem on the non-switchable loop jacks. Does this sound like my LDRs are going? I'm looking at the schematic and if LDR19 is not turning off properly when LDR17 and LDR18 turn on, it seems like it could cause this problem...

2) The EQ LED on the front of the amp has stopped working. The eq still seems to function ok, and the eq LED on the footpedal works. From the schematic this seems like maybe the LED has just burned out? Does that happen? I thought LEDs were essentially indestructible!

 

[Eevil]

…

2) The EQ LED on the front of the amp has stopped working. The eq still seems to function ok, and the eq LED on the footpedal works. From the schematic this seems like maybe the LED has just burned out? Does that happen? I thought LEDs were essentially indestructible!

Every electronic component has expected lifetime. LEDs have over 100000h but still those go south at some point..

 

[Rob S]

2) The EQ LED on the front of the amp has stopped working. The eq still seems to function ok, and the eq LED on the footpedal works. From the schematic this seems like maybe the LED has just burned out? Does that happen? I thought LEDs were essentially indestructible!

Got two Mark IV A and at some point had the same thing noticed. A slight tap with the finger and / or a bit of light wobbling at the LED did the repair for me. Works fine for quite a while now without any flashbacks so far.

The problem with the loop is weird tho, I’d check the jacks first with the chassis pulled out. Then the switch (normal/switchable).

 

[Appleridge]

I have a IV B (1996) and also had LED issues, specifically ch 3. I sent it to Mesa for a cap check, and they replaced this (along with a few leaky caps). May be worth sending in for a tune? I am a big fan of the mark series amps. I sent it in and asked them to restore any questionable components to new. It was relatively in-expensive, and if I choose to sell this one day, I have documentation from mesa (and mike B) highlighting the replaced components. I just sent my mark III in for the same service along with the III+ mod. Looking forward to operating these classics for years to come! Happy these services are still being offered by experienced techs.

 

[Rob S]

I wished I could get my Mesa collection serviced at HQ, but I don’t live in the US so it’s a pricey thing with all the extra shipping and custom clearance stuff along the way. I try to maintain a good state of my stuff on my own and had received great support from HQ when one of my Mark IV was at an authorized dealer for a repair job I couldn’t perform on my own.
Awesome how they are still standing behind their older products after so many years

 

[J.H.]

For the loop, did you try cleaning the jacks? That would be a good thing to try.

With the front LED(s) if I remember correctly there's a crimp on the legs, and sometimes those loosen up.

 

[Appleridge]

I wished I could get my Mesa collection serviced at HQ, but I don’t live in the US so it’s a pricey thing with all the extra shipping and custom clearance stuff along the way. I try to maintain a good state of my stuff on my own and had received great support from HQ when one of my Mark IV was at an authorized dealer for a repair job I couldn’t perform on my own.
Awesome how they are still standing behind their older products after so many years

Ah, copy that. Didn't see your location, but agree it is nice they still offer these services. I'm even more blown away that Mike B is still in the mix! I purchased both my Marks used, so hard to say what they have been through. They need a "once over" and I'm happy that Mesa is doing it. Pretty cool you have owned yours since new...

 

[Rob S]

Yeah that’s a bit of a problem but it’s just fine that you get support when needed. Got mine used, too btw.
I’m mostly a Mark IV guy because this was my first Mesa amp. It was well used and had some issues after a while with some leaky caps that were hard to identify. As it took some time and I was loving it already, a second Mark IV was found soon. This was a lucky find because it was looking and performing perfectly and had very low mileage for its age. Both Rev A (Serials between 3000-4000) and Combos which were converted in heads later.
Out of interest I got a Mark III and a Dual Rectifier later on, but have used the first Mark IV most of the time since it was back in working shape.
I really like the balance between the classiness and the advanced features over the previous Versions because you have a proper Footswitch and the ability to dial in all channels individually at balanced volume levels. The only thing I miss is an assignable Reverb to each channel e.g. Rhy 1 ON and Lead OFF with no extra tapping…
Mark V seemed to be an option for a while but I didn’t get my hands on one yet to make my decision clear to really go for one. Technically the Mark IV is kind of the last one with the original Mark roots and architecture.

 

[elegost]

Ok thanks for the feedback guys. I'll let you know when I get it figured out what it was.