Music centre modifications

Submitted By Member: Nuvistor

We sold a lot of music centres in the 1970’s, some inexpensive others quite expensive. One problem with many of them was poor reception of MW, LW was not good but acceptable. There wasn’t one cause of the problem, each had there own issues. For most it wasn’t that much of a problem, VHF/FM was used most of the time for the radio and the record player was the most important part of the unit.

However for one customer it was a major problem and he wasn’t happy with the unit, in fact he had tried a couple of makes and none perform as he wanted on MW. He picked the unit he liked best and said if we can make it work to his satisfaction he would buy it, I forget the price but it would have been at least £200. A call to the makers service dept said they couldn’t help, it is what it is. The boss wanted a sale.

After listening to the receiver, the biggest problem was heterodyne whistles across the band, I found the performance very poor. So what to do, IF and RF alignment were checked but were in specification, then a thought, is the Local Osc injection too large. I obtained a portable transistor radio. And found I could receive the LO at the other side of the workshop, 30-40 feet away, there were probably harmonics as well. It was a strong signal.

How to tame the LO, now I am not a radio designer, so I couldn’t use lots of theory to help. I took the empirical approach, first was a resistor of a couple of hundred ohms in the base of the mixer/osc transistor, a base stopper. Struck gold first time, heterodynes gone, LO now only receivable next to the music centre. The set had enough gain and worked quite well, it was never going to be a world class in performance but it was much better than before.

So delivered to the customers and let him test it for himself, couple of days later he came into the shop with the cash and very happy. He knew it was not a communications receiver and wouldn’t expect it to be but no excuse for designing sets like that.

I contacted the service dept with the information but again they were not interested.

There may have been a much better technical fix, but it did the job, customer happy, boss happy, I was happy with a something different to fix, but no bonus in the wage packet.