The Workbench

Re: Penta Bias Adjustment

egoh — Fri, 20 May 2011 11:11:21 GMT

Sorry to talk out loud, but I figured it out and perhaps it may help someone else. 5 minutes is just about perfect (as mentioned before) to set the final bias.

Obviously, set it as soon as you power up as to not fry the amp. Then let it stabilize which is just about 4-5 minutes and do your final adjustment.

Re: Penta Bias Adjustment

egoh — Thu, 19 May 2011 16:08:50 GMT

I actually asked this in another thread but thought you guys might have some more insight.

The manual says to set the bias to 30mV without load and while the amp is "cold."

How cold do they mean? How many minutes after startup? At start, I'm at 22mV....rises to 27mV after 1 min., then 30mv after 2 min., 31mV at 3min.

Thanks!

Re: Penta Bias Adjustment

PL212 — Wed, 27 Aug 2008 18:43:08 GMT

Update! It appears the vintage 1986 trimmers are having no effect on setting the bias correctly. I have heard that these are a common point of failure ("a complete nightmare," according to one authorized technician,) and so will replace both the 220Ω and 100Ω units with new, high-quality components. I am looking for ones which can be adjusted "through the board" as the originals were, because otherwise you would have to remove the whole transformer assembly to adjust them.

Those of you that have done this before -- any particular manufacturers and part numbers that you've used? Is there a big difference between the (cheaper?) round trimpots and the more deluxe, enclosed rectangular ones?

(I also wonder if the failing pots could have caused any damage to the transistors or other components either before or after this set of speakers came into my possession?)

Thanks again!

Re: Penta Bias Adjustment

Dillen — Tue, 26 Aug 2008 07:06:49 GMT

Good thought Peter !

Place the trimmer in center position and have the meter leads connected, ready to adjust when powering up. This way you won't go wrong. The design of the amp calls for a resistor about 50 Ohms so the trimmer is 100 Ohms and the correct adjustment shouldn't be much off center. In stereo machines you will do one channel at a time to avoid cooking one channel while adjusting the other.

Martin

Re: Penta Bias Adjustment

PL212 — Tue, 26 Aug 2008 06:37:00 GMT

You're right, there are apparently two versions of the service manual -- and one of them lacks that clarifying part! Thanks...

If I'm replacing or adjusting potentiometers, is there a way of ensuring I don't send the whole thing up in smoke -- some default adjustment (all the way to the right, or to the left) so that I don't blow anything while starting to measure?

Re: Penta Bias Adjustment

Dillen — Tue, 26 Aug 2008 05:37:56 GMT

Exactly. Or measure across just one resistor and set to 15mV. Remember to do the adjustment while the amp is cold, with no signal and preferably with no speakers attached though it really doesn't matter much. Leave it on for 5 minutes and check again.

Martin

Re: Penta Bias Adjustment

ProGram — Tue, 26 Aug 2008 04:03:23 GMT

You say, that you have the service manual. So read the page 5-1. There you can read: "...that is from the emitter of 1Tr32 to the emitter of 1Tr24...."

In other words from one end of R103 to the opposite end of R104.

Penta Bias Adjustment

PL212 — Tue, 26 Aug 2008 02:50:35 GMT

Can anyone detail how to properly measure and adjust the offset and bias on the Penta amplifies (BeoLab 150/200)? The service manual says to measure over R103/R104 and adjust for 30mv, but I'm not quite sure if they mean that both resistors should be tested separately or somehow across one another...?

Thanks!