The sound of dry capacitors?

[Elbert]

Hi!

Anyone familiar with the symptomes of dried out electrolytes in power amps?

I have an old Tandberg TR2075 receiver.
When cranking it up, I first noticed qhat sounded like a blown midrange/ tweeter, but switching speakers I discovered that it came from the receiver.

When playing at low levels, everything sounds ok, its only when playing a bit louder that this becomes noticeable.

Since the receiver is years old, I figured it might be the big power supply caps that have dried out, but thought I might check here in before I spend $$ on a new set.

There's two 4700 uF caps for each channel.

Any ideas or suggestions would be welcome!

[h_a]

Dry caps? Of course they give dry sound

Sorry for my weak joke. But hey, 2x 4700uF caps cost next to nothing, so why don't you just try it out?

Have fun, Hannes

[wg_ski]

The amp will have a hard time putting out full power if the PS caps are bad. If they're *really* bad it will hum. If the power supply voltage is dropping like a stone under very light load, I suspect the caps are bad. Just "distortion" could be other things - bias out of adjust or bad coupling caps.

[Elbert]

Thanks for the reply wg,

Well, since the power supply capacitors can't be elliminated from what you say I'll take a shot at replacing them. Being manufactured in 1976 I guess it's a fair assumption that they are not quite what they used to be anyway..

[latala]

Going back a no of years Tandberg used to use some lamps as emmitter resistors on the output transistors it was no uncommen for the lamps to fail and the output would be distorted !
I can remember having to resolder the lamp tips and this cured the problem I do not know if this is your problem
Regards Trev

[Elbert]

Haha!

Dug out some of the old schoolday knowledge and tested one of the caps.

With the help of a power supply a voltmeter and a resistor I tested the capacitance value based on the RC time constant..

Well, the cap should have had a value of 4700 uF... torned out it was about 500 uF!!

.... as far as I could estimate, the short time made it difficult to measure, and the cap once charged instantly discharged it self!!

So now I'll be surfing e-bay for a suitable set of replacements!

Only problem is that the original Siemens capacitors are of that kind with one ground pin and 3 plus pins, so finding some caps that will just snap in might be difficult....

But at the end of the day, very pleasing to find the fault is so "easy" to fix, much better than messing around with bias adjustments and decoupling caps and god knows what other stuff I'm totally lost at!

[latala]

to be surei would tack any suitably rated capacitor across the old 1 first !
Do you have a signal generator and a scope?
If so i would monitor the output while driving an 8 ohm load you would get a lot of 100/120 hz ripple if the capacitor is bad i would not expect mid distortion !!
regards trev

[Elbert]

Well, as sure as the bear s**** in the woods, that cap was dead! Tried with a 1000 uF cap in the same test set up and that was way better than the original 4700 uF cap!

If all fault-finding had only been that easy!

[ampliton]

These 4700 uF caps could be the output coupling caps to your speakers, otherwise you will have some hum on the power rails. Do you have scope? The voltage on the coupling cap is at about 1/2 of your Vcc. I hope this helps!

[Elbert]

Well, these 4700 caps are actually the main power supply caps, two for each channel. Might sound a bit flimsy by todays standards, but 30 years ago it was probably considered rather hefty!

Found some caps on e-bay that should fit in nicely and they are allso a tad beefier at 6800 uF.

Anyway, looking at the state of these caps I'll change all the other electrolytes in the output stage as well while I'm at it!

but thanks for the advice anyway!