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MBLE kit BBO840 FM tuner (1963-65)

The BBO 840 is a home built FM tuner, built from a kit by MBLE. I bought one at the NVHR swapmeet in september 2008.
MBLE was a well known brand of radio's and radio kits in Belgium and less in The Netherlands. In the september 2008 issue of the magazine of the Flemish historical radio association (VLHR) there is an article on MBLE kits. The BBO840 FM tuner is listed in the 1965 MBLE catalogue. It has a tuning range of 88-104 MHz. According to the magazine article, the sound quality is reasonable, but not up to today's standards.

This tuner contains a number of valves:

Two OA85 Germanium diodes are used for the ratio detector.

The case is built simply and effectively. It consists of a “tunnel” from perforated steel sheet. The front plate is attached to the frame. The frame assembly is slid ito the case and then the back plate is screwed to the frame, effectively keeping the case squeezed between front and back plate. MBLE used this type of case on all of their kits. Philips used a similar style of case in their contemporary kits.

    A look inside.

A look inside.

I bought this FM tuner on the september 2008 swapmeet of the NVHR. I recognised it as an early 1960-ies kit. The seller then told me that the VLHR were writing feature articles on MBLE kits and this tuner in their magazine.

At home, I opened the case and found that there was an ample layer of dust inside. The case was a bit rusty and the innards also showed the influence of moist. I googled for some information and found reference to a feature article on this tuner in the RB magazine of december 1963. I happened to have a copy of it, which contained a review and a schematic diagram of the IF and AF part. As the front-end is a ready-made Philips unit around the ECC85, no schematic or alignment procedure of the front-end was included.

Two weeks later. I started to fix it. After removing the dust, I reformed the power supply electrolytics. Then I connected a high impedance headphone to the output and switched on the BBO840.

Not a sound. I checked the voltages but all looked well. I fumbled a bit with the output valve, but no sound at all. I used an AF generator to inject some signal in the line amplifier, but it did not work. Using a scope, I saw that the output capacitor wasn't connected to the output. Peeking under the PCB, I saw the soldering work did not look very well. Getting the PCB out was the only way to get at the soldering side, so I disconnected the wires going to the board and took it out. (I should have made a photograph of the PCB in the works). There were a number of bad joints. There also was some green corrosion on some of the traces, which I removed using a glass fiber brush. After that, I covered the board with a clear coat of colophonium resin. I put the board back in place and resoldered the wires.

Then I turned the tuner on. It did not work. Fumbling a bit with the ECC85 valve in the front end caused some cracking and then I received a station. But the sound was distorted and the sensitivity was poor. But suddenly the sound became stronger and the tuning indicator closed, after some stronger fumbling with the ECC85. Cleaning its pins made all cracking noises disappear. Bu every now and then the signal dropped again. I discovered that the signal came back when I touched a small tubular ceramic capacitor across the antenna input on the front-end. I resoldered the capacitor. After that, the tuner was more stable.

It isn't perfect yet, though. The alignment of the front-end seems to be off by 2 MHz and I don't know why. I will try and align it when I'll find more information on the front-end. I am looking forward to the december 2008 volume of the Flemish radio society magazine. In the mean time, I am using it regularly to see if it will keep on working.

Copyright © 2008 by Onno's E-page         published 2008-09-13, last updated 2008-10-18