What would make a Quad II oscillate?
On Sunday, 8 June 2014 07:22:16 UTC+10, 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 **** 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
|