Roberts R700.

[Sideband]

Well a few weeks ago I asked for information on this set with regard to an annoying hiss (see thread https://www.vintage-radio.net/forum/...ead.php?t=8069 ). The suggestion from several of you (thanks guys) was to check resistors. I had not suspected resistors and, at first, had substituted transistors in the audio amp with very little success.

Most of the noise seemed to be around TR10 so with the subbed transistors in place, I set about changing resistors. After a few checks R54 (82K) seemed to have a marked effect on the noise so a new one was fitted. Now the crackle part of the hiss was gone but the hiss remained. In fact it was worse ! After more checking I discovered that TR10 was generating a lot of noise....a new AC126 was in this position at the moment. I've discovered that the transistors used in the TR9 and TR10 positions are quite critical...the circuit shows OC71 and OC75 respectively. In fact my set used two OC45's in these positions. These transistors are more commonly found in I.F amplifiers rather than audio stages.

Well Roberts must have known what they were doing because when I fitted the original OC45 back in TR10 position, the bad hiss disappeared . I then fitted the original OC45 back in TR9 position (which was also a subbed AC126). This had far less effect but took the hiss down slightly lower. So now the set has both it's original transistors plus a new 82K in position R54. The hiss is now very much in the background so it must have been R54 causing most of the trouble. Unknowingly I had masked the effect of the replacement resistor by fitting substitute transistors which acted more like noise generators!

Under normal circumstances, I would have thought that AC126's would have been OK in these positions. Maybe mine are just noisey. Anyway it's fixed, it sounds good and I'm off for a cool from the 'fridge! .

Thanks for the tip regarding the resistors. Just be careful when you sub transistors!

Rich.