ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all... I have a Beolab 150 amplifier, long overdue for replacement trimmer potentiometers. They have now been replaced, but when I had a look on the scope, I can see RF oscillation at around 2MHz. This instability is marginal, as simply switching an adjacent lamp On or Off can stop or start this oscillation.
Unsurprisingly, trying to check the no load current is misleading because it varies by 50mV or so depending upon whether the oscillation is occurring.
I have not done any recapping so far, and strongly suspect that this issue is not related to dried up electrolytics, but I could be wrong.
Does anyone have any suggestions as to the cause ?
Tony
Seems like a classic example of the need for re-capping?
Member Dillon will be able to supply a kit for this I imagine.
The time to do the work will be well spent I'm sure.
Nick
Hi Nick.... thanks for your prompt response...
You could very well be right about recapping...
I shall make my way down that road...
Hi Nick,
as you suggested, I did a complete recap & trimpot replacement on the Beolab 150s, and discovered that the easiest way to work on them was to desolder all the power devices and remove the PCB...
Once I had done that, I could see that the Zobel resistor had burnt out (on both amps), and that 'it' was the real cause of the RF oscillation...
Of course, the RF oscillation will itself create excessive power dissipation in the power devices and the Zobel failure may well explain why other members have dead power devices...
As a matter of interest, I measured all the removed capacitors for capacitance and leakage, and they were all o.k...
I hope this will be of help to other members...
The burned resistor is not the cause of the oscillation.It is the result.The reason will have to be found, and it's typically bad decoupling of voltage regulators.Self-oscillation in DC-coupled output stages like this can also occur if transistors with too high HFe or capacitors with too low ESR are fitted.
Martin
Hi Martin,
thanks for the reply... It did puzzle me a bit, as I had always thought the Zobel network was there to help deal with capacitive loading such as electrostatic speakers...
In any event, as the amp was fully dismantled, I have done a full recapping with good quality caps, and I can can see no further sign of instability on my scope, and the quiescent current sets up very stable...
I have run the amps for days now and they run luke warm, so hopefully the issue has been fixed...
Good new's that the Beolab Penta's are back in service!
We've all been fooled by capacitors that check out "ok" when tested,but when you think about all the problems that can occur with them such as,intermittantancy,noise,and even becoming microphonic,I rarely bother to test them.Just replace with good quality modern equivalents.
With the Penta's in particular,you don't want to have to go in there again for the sake of saving a little.
After all,the originals have probably been in place for 30 odd years!
Hope you enjoy your excellent speaker's.