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hrad 15th March 2004 12:52 AM

General Question/Old Threshold Equipment
Nelson, it is nice to see you giving your time to this forum. We may have communicated years ago when I had problems with my 4000 (I have one of your Threshold business cards).
I've had a Threshold 4000 and Cas2 since the late 70's early 80's. I used to push them hard like everything else in my youth, and the 4000 failed several times back then. They are used with a stacked pair of DQ10's and Dahlquist active crossover. I just replaced my Bryston 1B with an old NS10. I have always left my pre-amplifier and crossover powered on.
I have two questions: Should pre-amplifiers be left on all the time? How much better would it be to send this equipment out to Jon at Vintage Amp for preventative maintenance than to wait for it to fail? Thanks for all opinions. :bigeyes:

Nelson Pass 15th March 2004 06:13 AM

I leave preamps on all the time. 15-20 years is the range where
we see power supply caps beginning to fail - "dry out" as it were,
and I recommend that if you have the amp in for service in this
range, you definitely want to replace the power supply and any
tantalum caps (very early Threshold) in the circuit.

niconoise 7th April 2004 12:00 AM

What causes the electrolytic caps to fail? Heat? Ripple current? Materials failure? Do they age just sitting on a shelf unused? I also have several older Threshold products and am considering refreshing them. I have suitable NOS caps, but they haven't been used in 10 years. I would like to know if I should avoid using them.

Are there other components that should routinely be replaced? Any that should be routinely checked? (transistors, diodes, etc)

wrenchone 7th April 2004 05:16 AM

I don't know the degradation mechanism for tantalum caps, but electrolytic caps can be considered as bags of water. The rubber seal around the leads is only so good at retaning the water, so the caps will dry out over time. Elevated temperatures of course will speed up the process. Long-term interaction of the electrolyte with impurites in the aluminum foil (especially iron) can also cause failure.

Solid tantalum capacitors have a distressing habit of "scintillating" - the tantalum oxide breaks down in a localized area, and if the enough current is conducted, thae capacitor shorts. I have an 80's scientific instrument (multichannel analyzer) with a veritable forest of teardrop-style tantalum bypass caps in the analog section These fail and yank down the supply rails. One of these days I need to strip them all out and replace them with something decent.

niconoise 13th April 2004 11:28 PM

Are there tests that can be done on the electrolytic capacitors to determine their age and condition, and to indicate the necessity of replacement?

Nelson Pass 16th April 2004 12:18 AM

Meaure their capacitance. As they dry out, the capacitance
declines dramatically toward the end. You can also observe
the ripple voltage on the + and - sides of a bipolar supply, and
if one of them is dramatically different and the amp is old, then
you probably have a cap drying out, or in the case where there
are parallel caps, one may already be dried.

When your old amplifier starts to buzz at 120 Hz for no new
reason, think about the ps caps.

I emphasize that this is an exercise for people with old amplifiers.
The caps do in fact last quite a long time - most of them well
beyond 20 years.

Nelson Pass 16th April 2004 12:22 AM


Originally posted by wrenchone
I have an 80's scientific instrument (multichannel analyzer) with a veritable forest of teardrop-style tantalum bypass caps in the analog section These fail and yank down the supply rails. One of these days I need to strip them all out and replace them with something decent.
Last time I encountered a forest of tantalums with some unknown
shorts, I hooked the board up to a high current supply and
burned them out of the circuit. :devilr:

wrenchone 16th April 2004 03:59 AM

Boy oh boy, if I did that I'd have to wear protection against shrapnel. Really and truly, I'd still consider the caps that survived the test to be time bombs. I'll just clear the whole lot out and replace them with something like FC switcher electrolytics. The ESR for a small FC cap is on the same order as a reasonable teardrop tantalum of the same capacitance and voltage rating. One of my colleagues nearly set his cubicle on fire with a largish teardrop tantalum. He accidentaly put the cap in backwards and blew it up. He shrugged and kept on working. A flickering in the corner of his eye got his attention - the red-hot tantalum slug from the capacitor had landed on a stack of schematics and set them ablaze...

Brian Donaldson 16th April 2004 04:20 PM

Nelson, You're the man. I too am a fan of full power acid tests.

A old mechanic I knew as a youngster always said " Use a big cheater (pipe over a ratchet wrench for leverage) and make it come or bleed"

shriekback 26th June 2009 03:18 PM

Hi all,

I am hoping that someone might give me some guidance in replacing the PS caps in a Threshold CAS 2 poweramp.

The original caps are 6900uf 60v Mallory that are no longer available. Would 15000uf 50v Spargaue caps work okaY? They are the same size (2" D by 2.625" Tall--with raised screw terminals).

If those would not work, any suggestions?


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