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!
I'm Keri from Chicago. Last year, I was adopted by a rather battered Beocenter 7700....
The initial assessment was grim; the turntable deck had separated from the turntable chassis, the connections cover hinges were shattered, the controls cover spring and damper gone, the tape deck noisy and slow, the turntable belt gone, FM stereo mode dead, the left channel much louder than the right, and to add insult to the many grave injuries, the MMC4 cartridge was missing its cantilever.
The good news: the metals, plastics and wood veneers were mostly intact and the Master Control Panel 7700 as well as the Terminal remote control were intact and working. Also, all of the LED segments work!
Quickly, Craigslist supplied a donor 7700 that looked like it had been in a flood externally, but had intact hinges, serviceable circuit boards (except for a corroded main board 3) a totally trashed tape deck and a tolerably intact stuck shut turntable which ended up hiding a perfectly useable MMC4! All for just $20US!
After dismembering the donor unit outside and cleaning the parts carefully before bringing indoors, my 7700 began to shape up.
First, re-glue the turntable deck, replace the belt and install the good MMC4 restored the turntable to some semblance of function. Then, replacing the connector cover hinges, though the donor control cover damper was not in useable shape. Next, out with circuit boards, one by one for evaluation of problems and likely age-related failures. Having an almost complete set of intact donor boards is great, as it allows one to restore them at leisure and try two sets of solutions. Eventually, eBay supplied the coveted control cover damper, and BeoParts supplied the necessary rubber belts and tires for the tape deck.
Some highlights:
Turntable:
Re-glued deck, disassembled, cleaned motor, re-impregnated the oliite bearings with oil under vacuum, re-capped the motor control / pre-amp board, replaced the jittery trim potentiometers. Eventually, I got a new Soundsmith SMMC4 with sounds better than the tired MMC4.
Tape deck
Yuck! this thing is a mechanical nightmare with broken plastics everywhere and even a broken metal capstan mount! Repaired lots of cracked plastic, sometimes with metal backing, replaced all of the rubber tires, the belts, disassembled motor, replaced its tiny internal electrolytic caps, re-impregnated bearings of motor, capstan and countershaft with oil, replaced light bulb with LED. It now runs swiftly and silently in rewind or FF, plays at exactly the right speed and sounds fantastic. Replaced suspect power-related electrolytics on the tape control board, as well as ones in signal path.
FM tuner
Radio electronics has always fascinated, me, but has always eluded my understanding. But I did manage to replace the FM board's electrolytics and the stereo decoder IC. AND I got the decoder adjusted right! I hope.
Amplifier / power supply
Arrgh! Board 3! Eventually, I ended up: Replacing the failing main power replay, replacing all the electrolytic capacitors, (two outright failures) re-mounted all the power transistors and regulators with modern silpads instead of messy white silicone grease , replacing the trimmer potentiometers, setting the idle current and the 6.5 standby voltage.. Those big "amplifier reservoir capacitors" seem happiest with Elna Tonerex 6800uf.
CPU / Display board Power backup board and IR transceiver board
Recapped these boards, as the CPU is made of Unobtainium and the main filter cap (an EK) had gone out of tolerance. Eventually, going to redo the spare display board LED segments.
Sound Control board aka Capacitor Central
Arrrgh! Why can't ANYONE make a compatibility chart for replacing 80's era capacitors with modern ones?
Ten bipolar electrolytic caps along with four polar low-leakage in signal path, two plain polars at the "Loudness" switch and some power filtering as well. along with a zillion tiny film caps... what could go wrong?
Both of my boards had at least two shot EKU bipolar cps with high ESR or just plain completely open-circuit. ALL of the EKU caps were out of tolerance.Tried MUSE Biploars which somehow sounded worse, then went with SU bipolars / KL low leakage polars and finally Elna RBD bipolar / Silmic 2s which sound wonderful. Up until this stupid board came clomping into my life, I was convinced that "Audio" capacitors were someone's idea of a joke. So far as that compatibility chart: ROE EK - Use PW. ROE EKU = Use RBD. Orange Rubycons - use KL or Silmic. For those great big reservoir caps, use Elna Tonerex. For sale: one box full of barely used capacitors of various types. I hate capacitors!
My Beocenter 7700 now functions perfectly, or nearly so. The FM radio sounds very good, the tape deck works and sounds great and the turntable sounds best of all. RPM phone app indicates the platter speed is within .01% and the wow & flutter at 0.2%... a little room for improvement. The wonderful MCP7700 remote, clock and timers work perfectly. Plus, it is my first stereo that I don't have to hide in a closet!
For speakers, S45.2s seem to be happier than Redline RL60.2s.
So, maybe this can give someone some insights as to just some of the work involved in restoring the magnificent Beocenter 7700. A well made system but even the newest is roughly three times older than their expected life of ten years. Though complicated, the 7700 is repairable, and worthy of restoration.
¡No entiendo Español!
NEIN! Nicht Versteh!
Я немного говорю по-русскии но не очень хорошо... и
I'm not very good at English either! Maybe someday I'll find a language I'm good at?
Wow, well done, it is indeed a beautiful machine and I was always very happy with mine - sounds great too
An update on the trusty Beocenter 7700....
I've wondered why the main heatsink gets quite warm when the system is on, even with no music playing. For example, I play a record, after it finishes, the record player shuts off and the Beocenter just sits there at idle. with no audio audio input and consequently, no output to speakers. If I come by an hour later, the heatsink is still quite warm in the middle and right hand sides, though the left side by the output transistors has cooled down.
Now, the right side is populated by the voltage regulators for 6.5v Standby, +15v and +14.3v rails... some heat comes from them, and the right side gets fairly warm. But the middle, or center contains only the main bridge rectifier, which powers the +30v and -30v amplifier supply and the 15v and 14.3v rails. With the system at rest and drawing very little power, this section should be nearly as cool as the output transistors section on the left. Good heat transfer is no surprise, as I had used modern heat transfer Silpads during the overhaul.
Inspired by my recent rebuilding of a Beomaster 5000, I took a look at the bridge rectifiers, both 80v parts, very similar to the ones in the Beomaster. Unlike the Beomaster, the standby power bridge rectifier, 3D1, shows no sign of overheating and darkening the PCB, and its standalone heatsink remains only slightly warm. However, the main power bridge rectifier, 3D6, was quite warm, even with the system at idle. Ah-Hah! could this be partial breakdown of the diodes causing excess power consumption and heating? The 7700 is only at a combined 60v on this 80v rated part, but it is 35 years old and has had a rough life. Although 3D6 is the same rating, 80v / 5A as the Beomaster 5000's counterpart, it was made by Hermann instead of Siemens and is a lot thicker, So, in with my spare modern and much thinner 600v / 6A replacement, along with some ingenuity to firmly clamp it to the heatsink and away we go....
Replacing the main bridge rectified resulted in a large decrease in heat radiating off of the heatsink! To make sure of the thermal contact, I checked and the rectifier itself is quite cool at system idle, or for that matter, when under considerable load. The regulator section still gets quite warm, but with the system energized and at rest, it is the only warm section.
The sound quality is unchanged, as it the idle current. It seems like the only difference is several less watts are being radiated as heat.
This could be a trend in older equipment.... "leaky" bridge rectifiers that still function tolerably well, but generate considerable amounts of heat, even when under little load. If only I had taken before and after wattage readings or maybe looked at the ripple, I could give some objective measurements.
Thanks!
And another update....
Hooked up a signal generator to the 7700 expecting the worst after my many repairs and updates... I felt it was almost certain that my substitution of modern components, particularly capacitors in the signal path, would snowball into horrible distortion.
And got a nice surprise! The Left channel has almost no distortion, even with a square wave, and only slight distortion of the Right channel only on a square wave, the Sine and Sawtooth waves appearing perfect.
So this is "High Fidelity" ... what comes out is what goes in. Pressing the "Loudness" switch distorts the square wave, but it's supposed to!
Test parameters:
1khz squadre wave fed into the Tape 2 input, and Channel 1 of the scope (top trace) with a 50ohm terminator on the coax.
Channel 2 of scope (bottom trace) hooked to speaker output in parallel with a Beovox 45.2
Varying the frequency from 300hz to 3khz mode no significant change in shape of waveform
Another update, but first a question...
Should this thread be in the "workbench" section?
Anyhow, I was meaning to get around to some more upgrades to the Beocenter 7700...
So, out with Board 3, or the amplifier / power supply board and get around to looking at a few things...
First, I replaced Bridge Rectifier D1 for the Standby power with a modern one, just because it is on 24/7 holding the main relay open. The original was rated at 80v / 3700ma, and only had 12v across it, but hey, its dedicated heatsink was always a bit warm and it is 35 years old.
Second, I've been curious about the output silicone. This Beocenter has had a rough life, and I suspected the original 1983 date-coded parts might be degraded so I got some new ones on hand, even though it's been sounding OK and the measured distortion on the Right channel(above post) seemed small. Of course, it could be compensated for with the Bass control but then the Left side goes out a bit.
So, I got ahold of some modern replacements for the TIP141 and TIP146 output transistors, as well as their drivers, four MPSA06 transistors.
Checking the new parts resulted in consistent Hfe Gain readings of around 200 on the MPSA06's and about 44- 50 on the TIP transistors.
Pull out the old ones and... Surprise! 3 of the eight are reasonably close to new parts. On the right channel, both the TIP 141 and 146 tested at 20 Hfe and the MPSA06s test at 100. On the Left channel, both MPSA06s seem OK at 220 hfe but the TIP 141 is at 50 and the 146 - at 20. Ah-Hah! I knew that right channel was a bit weak! All of the output resistors seemed OK at around 0.39ohm.
Put in well-matched pairs of new parts, put it all back together and... as expected, the Idle current was way higher, especially on the Right channel. Surprisingly, the 6.5v standby voltage went a bit high with the new bridge rectifier. As expected, its heatsink remains cooler.
Test listen, and it seems OK, and as a bonus, no popping noises or smoke. Out with the signal generator, and no significant change to the distortion of a 1khz square wave, though both channels match a bit better. Somehow, I managed to not make it worse!
I think that it's time to stop messing with Board 3 before I break something! It seems OK now.
My point is posting this is to illustrate just how much of a project a complex machine like a Beocenter can become. It's been fun!
Long-term update on the trusty BeoCenter 7700 restoration.
Still sounding AWESOME after some time... about a year and a half since the most recent post...
The Tape Deck still functions perfectly.
The Amplifier still sounds great
The Radio continues to work well.
The Phonograph had taken to being slow for the first 30-60 seconds after a few days of sitting, so I went through the motor once more, this time infusing the bearings with a lighter oil under a much higher vacuum of 150 microns or so, and replacing the spark snubbers on the armature. Re-assembled and re-adjusted, and it now works very well indeed, ranking fairly well on RPM cellphone app at 33.33 -0.02% speed and 0.28% w/f. Of course, it starts at the correct speed now, even after a week!
an original Pink Panther soundtrack LP playing very well right now!
Brilliant work, I have just purchased a 7700 and in the process of repairing a few things, do you have any links to tutorial videos or guides for replacing the cassette belts and also replacing the damper on the right hand side lid. any help would be appreciated.
Hi and Thanks
I can't think of any video tutorials, but I got the parts from Beoparts.com and a salvage 7700.
For some step by step instructions on the tape deck, try searching Beolover for "Beocord 9000" ... he has documented repairs on the tape mechanism, which is similar.
My tape deck was in sad shape. Hopefully, yours is better.
For the Tape deck, you will need the belts and tires at a minimum. The tape deck has a lot of very brittle plastic parts, one of which, a critical bearing holder, has a great tendency to crack around where the bearings are pressed in. I repaired mine with industrial epoxy reinforced with short pieces of metal tubing around the outside of the cracked bearing holder.
The Damper for the tuner / tape deck lid is considerably easier. You need the reproduction "Beocenter Friction Plate" available at Beoparts. The old one is likely to broken off or severely weakened by congealed viscous grease causing the lid to stick. Forcing it open or closed cracks the friction plate that attaches to the lid, but not the corresponding plate that is fixed to the chassis.
After removing the lid, which is a bit tricky, the old friction plate can be coerced out of its slot. Be careful not to break the hinges, which do double duty as hinges for the connections cover.
The chassis mounted plate should also be removed, and the old and congealed grease turned to glue removed.
Viscous dampening grease can be re-applied to improve the dampening, but I run mine dry.
If you don't see a part available at Beoparts.com, contact them! they can usually get whatever it is you need.
Good Luck!
Thanks very much for the help and advice. All parts ordered, let’s hope it all works when put back together.