|
Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
|
Thread Tools |
10th Feb 2022, 4:12 pm | #1 |
Pentode
Join Date: Jan 2013
Location: Nottingham, Notts. UK.
Posts: 228
|
Defiant A55BS
Recently acquired a Defiant A55BS so decided to see what I could do with it.
Hooked up to a 9V PSU it was completely dead apart from a slight crackle when the volume pot was rotated. Touching a wet finger on the input to the audio stages did cause a hum from the speaker so immediate suspicion fell on the RF and IF stages which use the dreaded AF117 and its well known tin whisker issues. Cutting the screen lead of all three AF117s brought the radio to life and a number of stations could be heard on both Long and Medium waves albeit very crackly and distorted. Having removed all three AF117s and tested them on a simple Chinese component tester all were showing an hFE of well above the typical 150 you might expect from an AF117. I suspected this was due to leakage or tin whiskers and this was confirmed by testing with a multimeter. Not having any suitable germanium replacements I replaced all three AF117s with silicon PNP BC560s and things were much improved. I find that in transistor stages with an emitter resistor that this trick pretty much always works due to the self biasing effect of the emitter resistor leading to the silicon transistor being pretty well biased. Following on from this I tested all the electrolytics and found them all to be leaky to a greater or lesser extent. The more critical ones (Speaker coupling, amplifier stage coupling and RF circuitry decoupling) were replaced. Finally, a squirt of Servisol into the volume pot and wave change switch finished off the electronic part of things for now. At some point I may check out the calibration of the dial and set up the IF stages but this doesn't seem necessary at the moment. That left the case. I decided to do just a light restoration of this. The brown, possibly smoke stains, were removed using a paste of washing powder followed by a damp cloth and then spray furniture polish. Around the volume pot the grill gold paint was worn so I gave the grill a spray over with some gold paint. I'm quite pleased with the result. It really doesn't sound bad at all and brings in a good number of stations. The looks have grown on me too! All I have to do now is find a knob for the wave change switch which was missing when I got the radio. |
7th Mar 2022, 8:26 pm | #2 |
Octode
Join Date: Feb 2006
Location: Ilkeston, Derbyshire, UK.
Posts: 1,397
|
Re: Defiant A55BS
Very nice! I used to have one of these years ago so it's good to see one again. Rather a stylish set and so very 1960s. Well done on the repairs and thanks for an interesting write-up too, particularly with regard to the AF117 substitutes.
Steve |
8th Mar 2022, 8:59 pm | #3 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
|
Re: Defiant A55BS
very pretty
__________________
Kevin |