Some pictures from my (unexpected) Beovox Penta restoration project:
What they looked like when I got them.
Well known problem, I think. For increased difficulty level, two of them midranges were completely shot (open voice coil).
Purists and haters will hate, but I think those drivers work absolutely great. You are invited to visit Switzerland and listen carefully, if you are not a believer yet. The connector is a high current automotive type (interlocking on connection).
New 2.5mm2 wiring for the bass drivers, connected up in a star topology, instead of the original daisy chaining through what appeared to be 0.75mm2 wires, across dozens of spade connectors.
Crossovers needed total recapping and some tuning for the new midranges. Simulation & lots of trial listening used to find ideal configuration.
All done and set up. If you must know: The display is a 56" Philips Cinema 21:9 screen.
The once presumably copper colored deco stripes had become...ugly.Redid them in glossy, dark metallic red to match my living room interior. Other plastic parts were painted maté black.
30 year old preciousssssss, in all new and shiny glory!
There's more info on my blog. Not sure if may link here, just google for "niston cloud".
Some points I wonder:
You're decreasing R3 on board 14 from 1.8 to zero ohm. At the same time you're increasing C4 from 2,2uF to 4,7uF?
That means that you're shifting more energy to the tweeter with a bigger frequency range at the bottom with more lower frequencies.
Is it neccessary to lower the crossover frequency of the tweeter? Limited range of the mids
Be carefull with turning up the volume
On the mid board 15 you're also decreasing R1 from 2,2 (older boards have 3.3. depends on tweeter used) to 0.68 ohm. If have already had several boards from customers with a burned coil L3. So, the same counts here, you're allowing more power to the mids, but it has to pass L3.
Sorry the schematic in the photo above is somewhat unclear.
First, I scribbled the original value if R1 on it, as actually found on my PCBs (I checked against different schematic versions). Second, I wrote some recommendations by DieBogener (I believe, IIRC - not sure). The new resistor values are finally written with the computer (as opposed to the other hand written stuff). I should probably have photographed a nicer schematic!
I didn't actually follow the recommendation and change C4 in the tweeter circuit, because as Beobuddy suggests, that'd lower the crossover point for the tweeter, which I don't think is useful. If anything, I would actually have liked to try and shift the crossover point for the midranges upwards, so that they'd reach a bit higher. The new drivers could theoretically do quite well up to 9kHz, although because of the line array arrangement,very much above (specified) 5kHz would probably not be a good idea. In any case, I don't think it really can be done without changing L3 on the midrange board; At least I couldn't do it in XSim.
The modifications to the midrange PCB are 0.68Ohm instead of 2.2 Ohm for R1, because replacement drivers have supposedly (listening, driver data sheet) a tad bit lower efficiency than the originals. According to XSim, 1.4Ohm would have been the optimum value for R1, because lower values seem to create a peak around 2400Hz in the simulation. Listening to both, I however found that I preferred the 0.68Ohm configuration.
One thing to note about my simulations is, I have no data whatsoever (except DC resistance) on the original drivers, so ran simulations with flat 4Ohm drivers. Thus, the simulations are really nothing more but extremely rough approximations! I still found them useful to get an idea of how the crossovers work and what might actually help to attain desired changes, before soldering and listening.
I've also changed the 6.8 Ohm resistors in the boucherot circuits (R3, R4 on midrange PCB) to 7.5Ohms, roughly twice a new driver's DC resistance (measured 3.7Ohm). Something I have not done so far is adapt C6/C7 to the new voice coil inductance. Not sure if I ever will, Im happy with the midranges. If I'm gonna change something at all, it'll be reviewing the 0Ohm in the tweeter circuit.
The new drivers are Visaton FRS8 (4 Ohms). They need a bit of mechanical trimming to fit; I used metal shears which worked a treat. The surrounds have sort of a ring around them on the front. Pay close attention as not to damage it while trimming (I ruined one driver like this). Conveniently, the original driver mountings can be reused for these drivers. It's advisable to have some kind of o-ring between driver and assembly though, as there might result an open sub-millimeter gap between driver chassis and assembly panel, probably when the trimming is not done "round" enough or something. For me, this was a non-issue, my pentas still had (the supposedly original) o-rings. My coworker found it to be a problem though, the o-rings in his pentas had mostly disintegrated.
Also checked his amplifiers for DC offset and found nothing above few millivolts. I have no idea what happened there. The coils themselfes weren't burnt, also their lower ends still stuck firmy to the PCBs. I somehow doubt that heating of the large coil (located below L6 when mounted) caused this. Maybe it was heat from the amplifiers during heavy duty use? They didn't produce much heat when I had them here for testing, but who knows what the previous owner used them for.
Here are some XSim graphs. As noted above, the Y axis values have no absolute meaning and can only be used in relation to each other and only among the same type of PCB drivers (midranges or woofers or tweeter). Concerning the tweeters, does somebody here happen to know more about them, and/or what influence the mounting baffle has on efficiency in particular?
On another side note about the sometimes found "SEL" drivers: SEL stands for "Standard Elektrik Lorenz",a German manufacturer that was acquired by ITT in 1930. The brand name remained until the late 80s/early 90s, if memory serves me right.
Hello just think your Penta's are looking beautiful, I have a set as well working with my Beosystem 5500 in my drawing room, the sound is amazing. My vinyl collection has been enhanced by using these speakers. The crispness of sound linked using the Beogram 5500 turntable is brilliant. My visitors are all pleasantly surprised at the sound.
Wish you all the best with these speakers.
Professor David A Flynn JP LM
Allthough from a time long gone,- they really are forever....