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 Everyone!
I’m having difficulties finding the correct idle current measuring points for the BM4400. I’m reading different ways of doing this and also that raises questions from a design standpoint! By reading the multiple threads about this subject, I’m confident that 50% of us got it wrong resulting in too high or too low idle current.
Let’s get the facts first!
From the service manual:
Adjust no signal-current with receiver cold and volume control turned down. SPEAKERS MUST NOT BE CONNECTED.
With 5R177 (5R277) adjust within the area 25-40mA in collector of 0IC100 (0IC200), or adjust to 10-15mV measured across both emitter resistors 5R178 and 5R179 (5R278 and 2R279).
My understanding from the above is that measuring points are represented by the red dots on the schematic.
Now if we transpose that to the real life, this is where it gets weird for the following reasons:
- If we look carefully at this picture, we see both emitter resistors are mounted vertically were the long leads (top of the resistors) are pointing opposite directions… apparently purposely.
- From various readings I’ve made, the multimeter probes should be clipped to the resistor’s long leads for the idle current adjustment.
- If we look carefully how the emitter resistors are “linked” to the rest of the circuit, we see that R178 (R278) long lead connects to R172 (R272) and, to me, follows the logic of the service manual and diagram.
- However, if we look carefully at R179 (R279) we find that the long lead connects instead to the emitter of TR116 (TR216) but the short side of the emitter resistor connects to R171 (R271), which is the opposite of the diagram measurement logic.
So, if I understand correctly, measurement taken at the top of the emitter resistors equals to only half the recommended value of the service manual, which seems in line with Beolover recommendation and also some other members BUT I’ve also seen adjustment set according to the service manual resulting into twice of the recommended value. Additionally, I’m far from certain that this is the most accurate way of adjusting the idle current because we are ignoring one half part of the output circuit
But why B&O oriented R179 (R279) long lead the opposite way if they purposely « exposed » the long leads for adjustment purpose? Why isn’t the long lead of R179 (R279) connected to R171 instead? This is what I find weird…
Looking forward answers from the gurus… ;-)
Real life!
I don't know why they oriented that way but I measure my no-load current adjustment emitter to emitter.
Depending on the orientation of the emitter resistors I set up to 5.5mv across the tops of the resistors, if they are connected in opposite directions one way (not the other) your only measuring across one resistor. That how it appears to me....
That is right. If the emitter resistors are not oriented for a full measurement across both then one has to be used and the millivolt adjustment range halved.However, the way the output transistors are mounted on the heat sink I think it is easier to just clamp a probe on the two emitters involved. In doing it that way though I strongly recommend having power turned off the Beomaster whenever attaching/removing the DMM probes. It is easy to slip and have the probe short something. Don't ask how I know this.
Thank you guys for answering my question.
I actually measured from the leads going to the output transistors as Sonavor suggested and I was spot on.
So measuring from the top of the emitter resistors and halved or directly at the output transistors emitters produces the same result.
Cheers!
Charles
Did both on my BM 4400 and same result as Charles, but I wasnt wondering about it, just applying Ohm law
Collecting Vintage B&O is not a hobby, its a lifestyle.
I could not match the new output transistors within 10% like I'm usually doing and I just wanted to confirm that my adjustments were good.
Thanks to all!