From: info@turneraudio.com.au   
      
   On Sunday, 8 June 2014 07:22:16 UTC+10, ultrap...@gmail.com  wrote:   
   > Hi all,   
   >    
   >    
   >    
   > I posted before while I was fixing up a couple of Quad II amps. After   
   working fine for a number of weeks, one of them just started oscillating like   
   mad before I even could put on a record. The "starting up a harley   
   davidson"-type of oscillation, ending    
   with a squeal when I shut it off. Now, the circuit is almost original, but a   
   lot of things and wires have been resoldered and I have replaced the OPT with   
   another original one, which I strapped for 9 ohms.   
   >    
   > Swapping out all the tubes for known good ones did not help.   
   >    
   >    
   >    
   > Last time I checked, all the original resistors were ok and less than 10%   
   from spec. I have raised the shared cathode R to 220 ohms to be able to run   
   6L6GC. The C's are all renewed: C1 is a Wima pp, C2/C3 are NOS Good-All glass   
   sealed PIO with metal    
   case, C4/C6 are replaced with modern 105C 33 and 47uf. C5 is also modern, but   
   raised to 100uF/100v.    
   >    
   >    
   >    
   > I know I will soon have to fire up the scope and do my own work, but have   
   any of you had this happen before? Is the most likely cause a bad solder   
   joint, or something like a shorted resistor in the global negative feedback   
   loop?   
   > > Thanks!   
   > > Martin   
      
   Phil could be right, you just need to eyeballs and a multimeter.    
      
   BUT, so many ppl do not have huge experience with repairing electronics of   
   many varieties, so they will struggle to see anything as they gaze at an   
   amplifier upside down on the bench without any single question appearing in   
   their mind.   
   You may have a crook output tube with loose wire inside a pin perhaps a socket   
   has a loose gripper, perhaps a resistor somewhere is intermittent, or a   
   capacitor.   
      
   Unless you replace ALL the old R as well as the C which you have replaced,   
   expect troubles with intermittent connections which can be a royal pain in   
   arse until you assume shit will happen with old junk unless serious time is   
   spent doing all you can to    
   keep it road worthy. I'm no fan of paper in oil PIO caps.   
   I've seen to many of them fail in the past, and Wima polyproylene MKP rated   
   for 630V are the best.    
      
   I've never heard a Quad-II imitate a Harley starting during turn on and then   
   squeal at turn off. The grumbly start sound indicates something reluctant to   
   connect until some voltage is across the connection long enough to make tiny   
   little arc to cause a    
   little local heat and the connection is made, which then conceals the   
   whereabouts of the fault. So you need to be able to quickly check rising Vdc   
   everywhere just after turn on. And try putting 2uF from each output tube grid   
   to 0V then turn it on when    
   cold. If it grumbles&squeals, the problem is in the output stages, not in   
   EF86s. Try change of output tubes, it that makes not difference, then you have   
   a dodgy old resistor.    
   The old resistors may not be more than +/-10% away from correct value, but   
   that doesn't mean they have connection integrity. The old R used in Quad-II   
   fail randomly after 55 years. Modern 1 Watt rated metal film should be used   
   and in another 55 years    
   they will probably have exactly the right value and not be faulty, unless they   
   have suffered many heat cycles.    
      
   The only thing which makes Quad-II oscillate continuously is a 0.22u cap   
   across the output when strapped for 9r, and the oscillation is at F well above   
   20kHz so you won't hear it.    
      
   Does the amp grumble&squeal with a pure R load, say a 10W 8r2 resistance? Does   
   it happen only with a speaker?    
      
   Once turned on, how do the Vdc everywhere around the circuit compare with Vdc   
   in the other channel? I assume you have 2 Quad-II amps. Is there a damn Quad   
   22 control unit involved? Is the problem in a preamp?    
      
   Fact is, your plea for help here raises more questions than I have time to   
   poke a stick at.    
      
   I always found life was to be figured out, and unless I befriended Unkel   
   Doubt, I could figure nothing out.   
      
   Patrick Turner    
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)