After the success of the Beolit 505 and 707 range of transistor radios that ended in 1981, Bang and Olufsen introduced the Beosystem 10 in 1984. As the name suggested this was more than just a radio, it combined a stereo cassette recorder and stereo FM preset radio in one albeit wide slimline cabinet.

Beosystem 10 showing FM presets, centre controls and cassette

Beosystem 10 FM presets and cassette mechanism

David Lewis’s design evolution from Jacob Jensen’s Beolit 400 range was evident with top slide rule style tuning and black anodised aluminium push buttons. Its sleek lines echoed the thin Beolit, extending to twin speakers for stereo broadcasts and the cassette tape source. Internally the set was completely different though. It was Japanese. Integrated circuits were now employed instead of the simple discrete transistors of the previous range.

Don’t be fooled by the sleekness into thinking the Beosystem 10 is light-weight. With a complement of six D cells, over the Beolit’s 5, it weighs over 5kg. The optional wall bracket hinted at the lack of portability but arguably lead the way to integrated mini-hifi systems and the wall mounted BeoSound 9000 CD changer.

Sound quality is not really much of an evolution from the 1 watt germanium transistor powered Beolit. The Beosystem used Sanyo’s chip-set manufactured for the booming radio cassette market. B&O’s ghetto blaster if you like. Sanyo’s audio chip, the LA4505, claims 5.5 watts (peak power) driving a 3Ω speaker. With the Beosystem’s 4Ω speaker and a 9v DC supply expect somewhat less, still loud enough to fill a living room. THD figures of 0.3 to 1.5% barely improved from the Beolit and to my ears suffer from the faint raspy cross-over distortion inherent in these mass market audio chip-sets. It’s not hifi and not lofo, somewhere in-between.

Beosystem 10 showing FM presets, centre controls and cassette buttons

Beosystem 10 tape cassette controls and FM preset tuning

The cassette tape transport looks pulled directly from a Japanese boom box and although there are controls for chrome and metal tapes the lack of Dolby and overall quality is disappointing. I’m probably being too critical, spoilt by CD fidelity, the warmth of analogue audio technology is certainly nostalgic. My example’s drive belts probably need replacing to improve the wow and flutter. The lower acrylic flap is missing in this photo – it unclips and I know I put it somewhere…

Given all its faults and charms the Beosystem still looks good in 2020 unlike many radio cassette players that have ended up in the recycling centre or 90s landfill.

FM radio fault

Do you happen to have a Beosystem 10 that’s no longer working on FM radio — goes it go silent with just a faint hiss?

TL;DR skip to end if you want to try the fix that solved this Beosystem’s FM radio tuning fault.

My Beosystem 10 was bought for a few pounds nearly ten years ago as another restoration project. After an initial clean up and replacement telescopic aerial sourced from the now defunct Maplin, it’s laid at the back of a desk all that time.

Lockdown lunacy had me looking to focus on clearing out space when I stumbled on the Beosystem and plugged it in. Silence. On all FM frequencies only light static and the sound of the same distant station all across the tuning dial. Strange, as I’m fairly sure the radio was working perfectly when put away years ago. After hunting down a cassette, the audio playback sounded good if a little wobbly, speed wise. So just the FM radio was faulty, it failed to tune in to any stations, although AM tuning worked well, if you wanted to listen to AM.

Downloading the Beosystem 10 service manual from the excellent beomanuals.com site gave me some ideas what to look at inside and identify what could be wrong. B&O helpfully have coded the position of each component with a two character identifier. For example, a suspect 100μF electrolytic capacitor C39 is D3 so I can look for column D, row 3 as printed on the circuit board and locate C39. Shame B&O no longer include the service diagram in a small envelop inside the sets like they did in the old days but heyhoe the internet makes it redundant I guess. Perhaps B&O foresaw it.

Opening the back, by removing the 3 screws inside the battery compartment, I found a large PCB packed with components squeezed from one side to another. The FM tuning is varicap controlled which means the top tuning slider is really a big variable resistor rather than the old style tuning cord driven variable capacitor. Variacaps change their capacitance with voltage so there’s no need for a mechanical tuning capacitor. These were a revolution in the seventies when they became popular in UHF TV tuners as a way to preset channels from an array of simple preset potentiometers. I built an FM tuner using varicaps as a teenager, even had a new fangled red L.E.D as a tuning indicator… Nostalgic trip over, back to post.

So if the FM tuner would not tune maybe the voltage was not changing on the varicaps? No, a multimeter across D2 at E3 said it was swinging all the way from zero to 22v as I moved the slider. 22v you may ask, how’s that as the radio only has a 9v DC supply? Well there’s a small inverter, IC1 to generate the higher voltage for the varicaps. I mention that as it could have a faulty reservoir capacitor so there’s not enough voltage generated to tune through all the FM band. Or with lack of smoothing voltage ripple modulates the local oscillator and you hear a squeal instead of LBC.

An FM radio that does not tune can be for several reasons. As the LW and MW bands were working the fault must lie in the FM stages. On the service sheet someone had marked C39  as a possible failure for radio faults so I checked the voltage and it measured 2.2v as expected from the schematic that includes test voltages. I dusted off the Tektronix ‘scope and checked the FM IF signal. It should be 10.7MHz and there it was, but at low amplitude. And the signal did not change when I moved the tuning slider or switched to any of the preset stations.

A puzzling fault. My ‘scope does not have the bandwidth to look at the RF input signal which is above 88MHz or the local oscillator which is above 88MHz by +/- 10.7MHz. (In a superhet design the oscillator frequency can be either higher or lower to mix down to the intermediate frequency of 10.7MHz.) So I could not tell if the local oscillator, transistor TR2,  was changing frequency as the variacap voltage changed. I measured all the voltages around TR2 and they were okay.

Maybe the RF amplifier FET TR1 was to blame? A low input signal could explain the low IF signal… I measured the voltages and noticed the drain was sitting at 4.7v not the 0v stated in the service sheet. Hmmm, something’s wrong.

You have to check yourself sometimes and not get mislead by the apparent ‘facts’. When I looked at the schematic, B&O had helpfully told me to expect that TR1’s gate and drain were both at 0v. But its source was connected to ground. How could this be? My FET theory was a bit rusty but I could not understand how it could work as an amplifier if all three connections were sitting at 0v – ground potential. The FET would not function.

It must be a typo and indeed it was so don’t be fooled if your Beosystem’s TR1 drain measures 4-5v it’s working fine. There’s a similar circuit in the AM stage with TR11 that shows the drain at 3.4v not 0v.

After the FET red herring I looked again at the circuit to think what could be wrong.Then I saw a preset capacitor C6, a trimmer across the local oscillator coil. Supposing that had tarnished contacts and open circuit wiper – it would be too high capacitance and the oscillator would be at the wrong frequency. I found a small slotted screwdriver and gave it a tweak through the small hole in the PCB. The Beosystem roared into life. Then went quiet. I moved it back and forth to try and clear the contacts and it started to tune across the stations. Success.The fault was a simple preset capacitor that was intermittently open circuit.

After a few more turns and adjusting to align the tuning with the dial frequency, all seemed well. I tuned each of the three FM presets to our favourites, replaced the back and set up in the living room to use for a while. Next day it was still singing and I though I’d found the fault. Then after a couple of hours FM went quiet again and each preset was silent. Opening it up and moving C6 restored the tuning so it could be a dry joint or needs squirt of contact cleaner to clear the wiper. It’s back in the list of things to do but either way it will work again and the trimmer capacitor C6 was the culprit – see Updates below for a fix.

Updates – November 2022

Adding Bluetooth

As the Beosystem 10 has an AUX input I decided to add a small Bluetooth 4.0 receiver to bring the Beosystem in line with the latest Beolit 20. The results are impressive allowing stereo playback of digital sources through the analogue B&O. I’ll feature the upgrade in a future post or DM if interested.

FM Sound fault solved

This fault with FM radio cutting out was due to a poor solder joint (dry joint) on the variable capacitor C6. Gentle pressure of the screw driver on the wiper was enough to make contact again temporarily. So after removing the old solder joints, folding the leads close to the PCB and then re-soldering both sides the FM radio is working again with no interruption.

Circuit diagram showing location of preset capacitor C6

Location of preset capacitor C6 – grid ref F3 on PCB

Volume dial cord

Dial cord for FM tuning

Volume cord for FM tuning

Frequency Cord for AM tuning

Frequency Cord for AM tuning

Output stage showing headphone and external speakers

Shows Headphone and External Speaker Switching

Headphone and External Speaker Switching