Decca SRG899
 

I've recently been restoring one these grams for a chap who's into 70's retro furniture and equipment. The performance is actually very good in terms of volume, tone and overall features, but it was suffering from a raucous hum on both channels even at zero volume, more pronounced on the left channel. That should have told me something but I missed the obvious. I went through the usual checks for earth loops, faulty capacitors, badly positioned cables etc. to no avail. Then I noticed what should have been obvious from the start (not thought of as Decca equipment was usually very well designed, but not in this case!) The mains on/off switch is adjacent and very close to the scratch filter switch which is in circuit regardless of being set for gram, radio or tape. and the filter capacitor for the left channel was actually laid on top of the mains switch. Just moving that helped but a much better result came from adding a metal screen between the 2 switches - see pictures. I insulated the mains switch side for safety.

Re: Decca SRG899
 

I used to have a an SRG899 and it too used to hum quite badly. Even when I switched off it continued to hum on the left channel for a good 20 to 30 seconds. I never did find out what the cause was. but like you say it did sound quite good otherwise.


Cheers
Neil.

Re: Decca SRG899
 

Probably the same issue. When turned off the amplifiers stay live whilst the capacitors discharge and the mains switch remains adjacent to the scratch filter. Presumably you no longer have the gram so can't try a screen.

Re: Decca SRG899
 

I would have expected Decca to have spotted that clanger possily in the initial design stages, maybe even before a prototype was made.
How did they miss it ? It must have been the same on all sets of that type, but it appears no mods were issued for that problem.
Surely customers must have complained? It was one of their "better" models.

Neatly solved, Colin. Tony.