Should i have my Harmon Kardon repaired or replace

I have a Harmon Kardon 730 amp that was my brothers. I'm using it as my phono preamp. It has a short somewhere and one channel fades down to an unlistenable level. Is it worth trying to repair? or should I start looking for a dedicated preamp for my turntable? If I need to replace it, what would a comparable preamp cost? The HK 730 is probably from the early eighties.
No, dump it. Repairing vintage electronics is not cost effective, especially since parts are generally not available.
I would replace it, if your brother gave it to you, at this point it owes you nothing. As old as it is, chances are repairing it, may only obligate you to invest more if it breaks again. Especially if you're only use is the phono stage, depending on your budget, you can obtain a phono stage that will either equal or exceed the performance for little more than the repair bill would be.
As Theo said, your repair bill may be just about as much as buying something new. However, it also may not - you might have someone look at it and give you an estimate? Especially if you like the sound of it.
This amp is ancient history DUMP ASAP!!!
Actually, it is from 1977. The nice thing about it is that it has dual power supplies and it is a receiver, which can be a great all in one unit for an extra room.

You didn't say what is wrong with it and it just might be an easy fix. If you like the receiver and it does what you want then you should explore a repair. Take it to a competent repair person and have them diagnose it and give you an estimate. If it is above $100 or so for the repair it might be time to move on. If it is below that, then, you might be able to get a few more years out of it.
Only keep it if it has personal meaning. You can get a better sounding phono pre for the cost of a professional inspection.
I'd use this as an opportunity to audition a couple current phono preamps in my system if you haven't already before even considering repairing the HK. If the sound quality is superior to what you're getting from the HK you'll have your answer. If you don't have a dealer with something in your price range to audition you can try Audio Advisors or others that offer 30-day money-back guarantees if you aren't won over. There are several sub $300 pres from the likes of Cambridge, Vincent, Parasound, NAD, etc. that get good reviews and would be worth trying just for reference if nothing else. I'd read the reviews and pick one that sounds like it has the characteristics that are important to you and give it a try (but be aware most pieces will require 100 - 300+ hours to break in before you can accurately assess them). Also, last time I got something repaired they wanted something like $100 just to look at it that could be applied to the repair if I went forward, but if not I was out 100 bucks. Just something you might want to check on if you haven't already. Best of luck.
i own the same rcvr and sorta agree with all of the above--if you're just using it as a phono preamp you're probably better off with a modern nad/parasound, etc. however, to me it's real value is as an amp/tuner--like raymonda says it's a very good all-in-one unit and would be worth $100-150 to fix for that purpose. it's not very rare, however, and i do see 'em all the time on ebay for very modest bucks.
Thanks for all the advice. I think I will start a search for another preamp.