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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: 86
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: 86
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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