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how do i figure out 500-c channel volume discrepancies!!

S

shrimpton

New Member
well.......

i'll cut to the chase:

left channel is about half the volume of the right channel in every setting (i think) - there may be a tone difference too but i can't tell for sure. i just purchased this receiver and was told it was "in perfect shape" - i don't think the seller was intentionally trying to deceive as any un-discerning ear could easily have not noticed this issue unless they listened carefully or paid attention and the dude who sold me this unit was definitely NOT a audio nerd so i'll give him the benefit of the doubt...

even though i'm no technical wizard (i can use a screwdriver, replace a tube and do a little soldering) i do have a great ear and love hi-fi so i thought i'd try and tackle this myself.

are there any suggestions on how to figure this out ie. how to tell where the drop in volume is coming from? weak tube? dirty volume (or balance or treble etc) pot? dirty switches?

i know this is a vague question with a LOT of possibilities but i'd thought i'd give it a shot...

also, any recommended fisher techs in the los angeles area if/when i can't fix it myself???

many thanks in advance!!!!
-shrimpton
 
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hi shrimpton,

i just had a similar issue; though for a different amp, the idea is the same.

i used a multimeter to double check my pot's resistance and they were off. cleaning the pots with caig detoxit fixed it for me.

looks like you've got the right idea already. do the easiest/cheapest thing first.

1. clean those pots/switches.

2. even without a tube tester, you could swap around the 12ax7 tubes to see if it makes any difference. Same goes for the 7591 tubes. Without the schematic on hand, i can't inform you of the correct ones to switch but i'm sure the fisher aces here are going to jump in to assist soon, if not already.

*edit: V12 (left) and V13 (right) are your phase inverters

good luck
 
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Shrimpton -- Using the Fisher tube placement guide, first determine which 12AX7s are the Tone Control Amplifiers, and which are the Phase Inverters for each channel, and which output tubes serve which channel. The first order of business is to swap those tubes between the channels, and see if the problem moves with the tubes to the other channel. If so, then tubes are the problem. If not, then using nothing more than some alligator clip leads (available at Radio Shack), we can trouble shoot further to localize the problem internally.

Congrats on getting a 500C!

Dave
 
wow, thanks everybody!

i love this forum, i signed up about a week ago and i'm hooked.

ok! i'm gonna give this fisher the all-american try and attempt to do it myself seeing as there seems to be a wealth of information and support from all you fellow members out there.

it's been 20 plus years of listening to music on a fisher!
whenever they break i go out and get another one as it always seemed cheaper to buy a new (used) one than to fix what i had but the price ratio seems to have changed since my last 500-c purchase, looks like it may be cheaper to fix mine than get another one.
the prices have really gone up in the last 10 years!!!
luckily for me i kept all my old units so i have a back-stock of 500 and 800 c's that i can use for parts!!!!

thanks again for all the help/advice/support!!!
this is gonna be fun.

regards,
-shrimpton
 
shrimpton said:
luckily for me i kept all my old units so i have a back-stock of 500 and 800 c's that i can use for parts!!!!
Click to expand...



:yikes: Errr, uhhhh.... And how many might that be? I mean, you could give them away as gifts :D

Seriously, once you catch the DIY bug on the first one, it won't be long before you start attacking the rest of the lot. It's really kinda addicting :yes:
 
i now have two 500-c's and one 800-c and an hh scott (i can't remember the model number) and a marantz 2385. the problem is that they ALL have issues. i'm going to try and figure out which one is the easiest to fix and do it.
i never set out to be a "hi-fi nerd" but once you get used to hearing rudy van gelder sessions played back through a 500-c and a pair of altec 604-e's OR black sabbath played back through the marantz and a pair of jbl l-100's it's pretty hard to turn back if you know what i mean.
once you get used to things sounding so rad you can't "un-hear" how spectacular this stuff is and when you play back wes montgomery through a crappy amp it can be pretty depressing...
 
You hit the right place here Mr. Shrimpton. Folk here are very generous with their time and knowledge. So long as you're willing to put in the legwork, you'll be more than rewarded for your efforts. It's all about the karma :yes:

As far as where to start, start with the one right in front of you; get to the others later. They all have their issues. Once you work on one, the second one becomes easier.

Dave and cgoroo already started you off. Clean the controls and swap the tubes. You'd be surprised how many times those simple steps addressed apparent problems.

Amend your profile to include your location. It'll help if fellow AKers are close by.

And consider subscribing to audiokarma. The twenty five bucks will be returned many times over.

BTW: The marantz is a piece of junk. I'll be happy to recycle it for you :)
 
thanks again notdigital! i'm going to start in on it this week. i picked up some deoxit D100L, can i use this on the pots as well as the switches? i've read conflicting threads where some people seem to think the D100L can mess up pots and should only be used on switches? any thoughts? thanks again!
 
Deoxit D100L is the 100% solution in a bottle or tube, right? It is fine for exposed metal surfaces where you can contact the surface directly with something like a swab. The selector switch comes to mind. However, for pots, the surfaces are buried in the housing and, as such, are inaccessible to mechanical cleaning. That's where the D5 spray comes in. The violent flushing action of the spray is what helps clean buried surfaces. You can do it with the pure liquid products if you can get it in there somehow but in some instances might be difficult (volume pot.)

The only thing I'll say about the debate between switches and pots is this: if it is a carbon trace, DO NOT use any of the D products. Caig has something else for that. Don't even worry about it with a Fisher because I do not believe there are any carbon tracks anywhere.

Here's a sticky on the subject. Check it out for helpful guidance:

http://www.audiokarma.org/forums/showthread.php?t=207005

One last thing: on any switch or pot you clean, surround it with tissue to absorb the overspray/overflow. IMHO, it just makes for neater work.

Good luck :thmbsp:
 
after spending several hours reading any post i could find on deoxit here on audiokarma i was really surprised at the lack of consensus!
thanks again for all the info, it's time to start cleaning...
 
My 400 receiver also has a similar channel imbalance. I've just learned to be less OCD about it and I just use the balance control to offset the difference.

It probably needs a volume control update and some tube swapping, but it sounds so good that I am not wanting to mess with it right now when there are more urgent matters to procrastinate.
 
50-60 year old pots and switches can't be expected to work as they did back when new. We can get them close, which in 99% of the cases is good enough. Also the QC on the tracks back then wasn't as tight or as good as now, so they will be off somewhat. Occasionally you'll find one that is super close. But for the most part all are off a little bit at the extreme's of travel.

The way DeOxit works is to soften the sulfates(environmental pollution) that congregate on the traces, contacts on the pots and switches. And God knows, you've got them in LA!!! spritz the DeOxit on the contacts and let them sit for a 1/2 hour or so. Then give each another spritz, then EXERCISE the POT or switch 40-50 times. The friction plus the fresh application will scrub the sulfates off the contact in what looks like a very small line (this is the contact area). In some instances a second or 3rd application and scrubbing is warranted, so don't get discouraged if it only partially works. Let it sit a day or so and do it again. I've seen them so bad that they need to be dis-assembled and physically scrubbed so they will work. (Some guys do this as a matter of course.) There is a STICKY on how to take apart a 500-C pot on the Main FISHER page. This sticky can be used for all pots of similar construction.

The main idea is to get them clean. That means ALL of them. If it goes round and round, up and down, or in and out, it needs to be cleaned. This will rule out the switches and pots during later troubleshooting.

You should be able to get all 4 up and running with minimal to no parts swapping. It sounds like they all need their 100,000 mile overhauls. Which really isn't that bad.
Some caps, a few resistors, maybe a couple tubes per unit, and you are set. But testing everything is the hardest part if you've never done it before.

Concentrate on 1 unit for now and get it back to battery. Bring it up to current standards, (new tubes if necessary), caps, resistors as needed, rectifier diodes, and the few mods that are necessary for it to live comfortably and safely. They aren't really hard to integrate into the 500-C/800-C. Once you've done 1, you should be able to do the rest without too much problem.

Larry
 
thanks for all the info!!!
i really appreciate it, without the encouragement i doubt i would ever get started.

here's the update:

i opened it up and for starters i began just switching around tubes. after swapping the tubes back and fourth that i thought would affect the balance i had no luck at all. then i swapped the v4 and v3 12ax7's and blammo! the volume imbalance was back to normal, everything sounds amazing now!

i think i got REALLY REALLY lucky that the fix was so easy...

one more question since i feel like i'm on a roll:
i've heard that if certain components blow or go bad it can take out other parts of your setup like my speakers or maybe a transformer in the 500-c.
are there any recommendations as to the most important parts to replace just as a cautionary move? the receiver is up and running beautifully right now and sounds great.
thanks again everyone!!!!!
 
ooops, i was looking at the wrong schematic. it was v14 (left and right low filter amp) and v15 (left and right high filter amp) - initially i didn't touch these tubes because i did not think they would effect volume or channel balance but i was obviously wrong. and i have my doubts that switching them was what worked., i think it was just the fact that i took them both out and put them back in which i'm guessing made previously dirty tube pin connectors make a better connection in the socket, loosened up some grunge inside the socket or something. although it could have been something else entirely, this stuff is all new to me...

here's the correct tube layout from my 500-c handbook.
 

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Shrimp -- Clean the Tape Monitor switch real good. When these become dirty, they cause the classic condition you are dealing with: One channel plays louder than the other. If not already done, then all the switches and tube sockets should be given a good cleaning!

Dave
 
The "round and round, in and out, up and down" also applies to tube sockets with regard to where to put DeOxit. Basically if it moves or can be moved, it gets DeOxit and exercised. EXCEPT FOR THE TUNER VARIABLE CAP! NEVER HERE!
This will get you started with cleaning everything on the 500-C, 800-C, Scott?
You can add/subtract items as needed. But this will get you started.
Small set for cleaning:
91% Ispropyl Alcohol.
Large box of Q-TIP brand cotton swabs.
DeOxit in the following Flavors......D-5, F-5, G-5
A bucket of Voltage Drops.
Dental Braces Brushes (tube socket pin cleaner)
couple of old toothbrushes.
1 qt. can of Relative Bearing Grease.
MicroCloths
Scott SHOP TOWELS (Blue one's). These work better than regular paper towels and in the long run are cheaper.
Blue Magic Metal Cleaner.
5 gallons of Live Steam

Larry
 
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