The No.1 Website for Pro Audio
 Search This Thread  Search This Forum  Search Reviews  Search Gear Database  Search Gear for sale  Search Gearslutz Go Advanced
Input oscillation problem when gain is turned up Modular Synthesizers
Old 15th December 2010
  #1
Gear Maniac
 
Rylan's Avatar
 

Input oscillation problem when gain is turned up

Hi again slutz, I am attaching an audio clip of an input problem I am having with an old tube preamp. There is some noticeable 60 cycle hum that gets worse when you turn down the input gain on the mic circuit, THEN when you turn it to about half way it lessens, but if you turn it just above half way you get this oscillation that sounds almost like something is shorting out at a variable frequency, which seems to go faster and slower depending on the level of the input gain above the 5 mark... Anyone know what this might be caused by before I go nuts testing components?

Bad capacitor somewhere maybe?
http://allhousemusic.com/randomcrap/Voice057.wav
Old 15th December 2010
  #2
Lives for gear
 
brianroth's Avatar
 

The "clues" are somewhat excessive hum and "motorboating". That almost always points to bad capacitors somewhere along the power supply rail(s).

What make/model is the unit? Perhaps we can find a schematic somewhere.

Best,

Bri
Old 15th December 2010
  #3
Lives for gear
 
Wavebourn's Avatar
 

If it is Ampeg SVT, it's a birth defect, bad ground layout.
Old 16th December 2010
  #4
Gear Maniac
 
Rylan's Avatar
 

it's actually an Ampex 600 and I believe you posted the schematics already somewhere around here Brian! (thanks for that!) hehe yes motorboating sounds about right, it sounds like a 2 cycle engine starting up. upon further reading I am thinking I need to replace the coupling caps? I just need to find the values, I understand the thing to recap with are Auricaps as far as quality.. thanks for the replies!
Old 16th December 2010
  #5
Lives for gear
 

Hi
You don't need to use 'fancy' caps for repairs, just ones that are not 40 years old and 'cream crackered'.
Matt S
Old 19th December 2010
  #6
Lives for gear
 
brianroth's Avatar
 

Dear Rylan,

Coupling caps aren;t nearly as likely the problem as are the electrolytic caps that "bypass" the B+ power supply.

Referring to this schemo:

http://recordist.com/ampex/schematic...n/600schem.pdf

...those caps would be the ones right after the 5Y3 rectifier (multi-section cans, and called out as C114B, C114C, C1-3C, C103D) as well as C103A up there in the line input section. Also, cathode bypass caps can be suspects as well.

I suggest replacing ALL of the electrolytics.

Best,

Bri
Old 19th December 2010
  #7
Lives for gear
 

yes... just like Brian suggests, replace power caps, coupling caps, and bypass caps. One of the major components that can cause this circuit issues is c101b (according to schematic in this post) mainly because the buffer stage is getting 100% negative feedback naturally to cancel common mode noise and the capacitor stabilizes gain. However, when this capacitor goes bad, this circuit tend to transform itself into an oscillator when the signal is applied.
Old 19th December 2010
  #8
Lives for gear
 
12ax7's Avatar
 

Quote:
Originally Posted by brianroth View Post

I suggest replacing ALL of the electrolytics.
Yep.

I became a convert to this strategy when I was working for a company that did maintenance for numerous radio stations (back when dinosaurs roamed the earth).

I actually got to the point where if there was a good chance that the problem the call was about involved a bad cap, I just re-capped the whole thing, and went on with life.

It took less time than specifically diagnosing the problem, and reduced the chances of another call on the same unit.

...Besides, caps are pretty cheap compared to hours.

Nine times out of 10, the problem was solved, and I was out the door.

(Of course, this strategy breaks down with something like a large-format console.)
Old 19th December 2010
  #9
Lives for gear
 
brianroth's Avatar
 

Hmmmmm..C101B is "way downstream" for a unit working merely as a "Phat mic preamp". But, I dunno.....

I still say "recap the electrolytics" for something that was built almost before Mom and Dad Roth had their first child! <g>

Best,

Bri (born with hopefully Good Caps in 1954............)
Old 19th December 2010
  #10
Lives for gear
 
12ax7's Avatar
 

Quote:
Originally Posted by brianroth View Post

I still say "recap the electrolytics" for something that was built almost before Mom and Dad Roth had their first child!
Another advantage to doing this is that modern electrolytic caps are (physically) a whole lot smaller, given the same voltage ratings.

This allows you to put higher-voltage caps in their place. I always try to put the highest voltage-rated cap that will easily fit back in.

.
Old 21st December 2010
  #11
Lives for gear
 
mjrippe's Avatar
 

I actually had a 600/601/602 (I forget which) where there was hum on the mic pre but the line input was opposite phase, so when you turned up the line in the hum canceled! Of course replacing all of the electrolytics fixed it for real...
Old 7th January 2011
  #12
Gear Maniac
 
Rylan's Avatar
 

ah you guys are the best, thanks for all the solid advice!
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Similar Threads
Thread
Thread Starter / Forum
Replies
gimboid / So much gear, so little time
3
dickiefunk / Low End Theory
14
skiroy / Rap + Hip Hop engineering and production
0
barryjohns / So much gear, so little time
4

Forum Jump
Forum Jump