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
Doing a full overhaul of my old 6621 Pentas, the usual refoaming and new display windows. Also full recap of the amplifiers etc. But struck a problem with the crossovers.All caps measured, capacitance and DF looks OK on all of them so I'll keep them. Some overheated resistors will be replaced as a precaution.Problem is the inductors L3 and L6. For some reason these two use very thin wire.
One L3 is black and shorted, the other one is dark brown but measures OK. Both L6 measures OK but has taken on a slight brownish color and has clearly been overheated. All 4 of these inductors will be replaced.Now to the core of the problem:
New inductors have much thicker wire and therefore lower DCR.
Eg these from Audio.nl:L3 100uH & 0.44R and L6 1.8mH & 0.74R. The DCR of the respective OEM inductors are 2R8 and 14R. This difference is far to large to be ignored in a lo-Z circuitry like this crossover filter.What to do... Is the solution to wire in a suitable resistor in series with each of the new inductors??Other observations regarding this crossover filter:A) Useless excess components?B) Design flaw / error?C) Messy schematic drawingA)
Look at all the components across the input (PCB 14): R6,C9,C10,L6,C7,L7,C8,R5. I expect the Zout of the amplifier to be very low, probably on the range 0.001R - 0.05R. So placing any(reasonable) load across its output will only draw current, but will not alter the output voltage seen by the rest of the filter. I'm no expert in this field, but can only see 2 possible reasons for these components:
-When the amp is clipping hard its servoloop is out of lock and Zout will go through the roof, maybe to several ohms. And in this case the components may serve to dampen the woofers since the primary damper (the amp) is 'offline'.-Power Factor Correction. The components may serve to present a less reactive load to the amp. But the amp already has a RCL Zobel network and should therefore handle any reasonable resistive or reactive load.B)
The 4 midrange 4ohm drivers are all connected in series, giving a total Z of 16ohm. Across pair AB is R3/C6 and across pair CD is R4/C7. But look at L5/C8! They are connected only across pair CD. Given that all 4 drivers are in series and placed in a common box/compartment I find this unsymmetry very strange. Maybe they meant to connect the top of L5 to P18 instead of to P20?I had to check the board, and found that the board is actually laid out as the schematic.This almost looks like they made a drawing error in the (CAD)schematic and unknowingly transferred this error to the physical PCBs. Go figure!C)PCB 15: If they had moved driver D to below C, where it belongs, the schematic would be easier to read.Here we see the crossover boards. Eh... no seems I can only attach one picture, schematics are more important so it takes precedence.L3 is shorted and only 16uH / 0R5, it should be 100uH / 2R8.L6 is electrically OK at 1.8mH / 14R, but has leaned over due to overheating.
The drivers A, B, C and D are drawn how they actually are placed in the Penta.
They filter more midrange on the outer 2 drivers to give a more smooth transition soundwise from the tweeter to the woofers.
If you start with your ear next to the tweeter and then move along the drivers towards the woofers, then you will notice the sound wil be more gradual.
A burnt L3 is caused by playing the Penta's on high volume for a longer period and/or the combination with the shot midrange drivers where the induced power of their malfunctioning is brought back to the amplifier and probably "collides" in L3.
As long as the new L3 has the same value of 100uH, you will be good. It's " resistance" is frequency dependent with WL (2*pi*f*L). A thicker diameter of the coil is capable of handling more power. Same with normal resistors with a power rating from 100mW to much larger several Watts.
I had the same coils fail in a pair of Beolab Penta 1's,so it seems to be a common problem?
Beobuddy's insite into the reasons for the failure is useful too.Both amplifiers of course needed refurbishing,and the wrong cable(unscreened) had been used for years,to connect to the Beocenter,so I assumed that instability was to blame.
I had some new coils made for me by Falcon Acoustic's,here in the U.K.
Falcon were also puzzled by the thin wire used in the originals,but the replacements seem to work ok and are much more substantial than the originals too
I aim to check the response with instruments in due course,but an initial listening test suggests that all is well.
Nick
The potential problem with lowering this speakers impedance is that it was designed to be a part of a link system, wherespeakers are working in parallel.There has to be a certain resistance, but I don't think replacing the coils and/or the internal wiring will damage anything..
Martin
I dont dare to take any chances to the electrical functionality of the filter so I must work around the limitations of the coils that are available as replacement... That means external series resistors must be used to create an total equivalent resistance in line with the OEM design.
Adding an external series resistance to a coil is by all practical means indistinguishable from a copper resistance distributed among the windings.
Look at PCB14 - L6/C7, when these two are in (series)resonance at about 1500Hz they essentially short-circuit the amplifier output with an impedance I guess is around 0.5 - 2ohms. With the correct coil DCR this will never drop below 14ohms.
Similar with L3, ignoring the DCR of nearly 3ohms would increase the power to the mids by almost 50%...Not good!
- Or find a set of original boards with unburned coils.