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PostPosted: Fri Jul 13, 2018 11:12 pm 
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Joined: Sun Aug 29, 2010 10:02 pm
Posts: 87
Hi to everyone on the board! I haven't posted in around 5 years but I'm back after picking up another quad pre amp to add to my rack rig.

I was expecting to run into some issues due to the age of these units but the new unit I bought is in worse condition than the other one I have. The LDR's in my other unit still need replacing due to the GEQ fading in and out randomly but the problem doesn't happen all that often and it hasn't really gotten worse since I have owned the unit. In the new quad I just bought after a few minutes of running it the volume drops to a fraction of the normal level and it's not playable at that point. This happens across all channels and I've tried swapping in new tubes without any improvement. The settings are exactly the same as my other unit so I have to assume that some of the capacitors or other parts are starting to wear out.

I am thinking that most other quad and studio pre owners are going to be dealing with these issue soon if the lifespan of the capacitors is 20-40 years. I am hoping enthusiasts on the board here can help put together a parts list for rebuilds of these units.

With limited time to work on this I am wondering what my options are? I can send it to Mesa which is going to be expensive. If I had a good idea of the first to fail parts I could try and just replace them. Any ideas? I can do minor repairs but my electrical knowledge is limited.


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PostPosted: Wed Jul 18, 2018 12:46 am 
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Joined: Sun Aug 29, 2010 10:02 pm
Posts: 87
I'm going to start replacing parts since most of the capacitors are probably due. I'll start creating a parts list here to help make it easy for other quad owners that want to rebuild their units. I'm starting with the large blue caps.

2 of 220MFD - 300VDC (these are the largest blue ones on the circuit board, the closest I could find were 450VDC so I bought those, larger cap values should be ok)
4 of 30UF 500VDC (second largest blue ones)


On the LDR's I've found the descriptions below, what do these numbers mean? How do I know the values the codes represent and find an alternate to replace them with if I can't find the originals?

21L883 - 9107 - ASSM.IN RP
21L884 - 9040 - ASSM.IN RP


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PostPosted: Fri Nov 09, 2018 6:44 pm 
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Joined: Sun Aug 29, 2010 10:02 pm
Posts: 87
I wanted to share some information here after solving the volume drop issue in my latest quad pre amp. I took the quad pre to a Mesa authorized repair shop in my area and they cleaned the entire unit out with contact cleaner, tested the unit and weren't able to find any problems. The volume drop issues have not returned after months of having the unit back so a thorough cleaning seemed to have fixed the problem.

A few other notes of interest I found out after asking various questions is that the big capacitors I listed in the last post are not part of the audio circuit and would not cause a volume drop. The tech told me that the orange caps are usually the ones to fail.

One other detail I found out about the LDR's causing the problems in my other quad are that they are no longer in production. A replacement of similar value could be substituted but could change the tone of the pre amp.


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PostPosted: Fri Nov 09, 2018 9:59 pm 
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Mark II
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Joined: Fri Nov 16, 2012 4:55 pm
Posts: 126
Location: East Coast, New Jersey
Wow, I had the exact same issue about 5 years ago. Same thing. Brought it to an authorized repair shop. Explained the problem.

The tech called me up the next day and said he cleaned it and couldn't recreate the volume drop after playing though it for over an hour.
Haven't had the volume drop happen again in over 5 years.

You have me wondering how many techs ripped off owners when all it needed was a simple cleaning.

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