Bush TR130 manufacturing fault. - UK Vintage Radio Repair and Restoration Discussion Forum

Leon Crampin · 5th Aug 2010, 5:37 pm

I don't collect transistor radios, but this was handed to me by my sister for repair - so it had to be done.

Cosmetically good, the set was dead and unmolested inside. TR1 had the AF117 syndrome; replacing this device restored operation. No surprises here - a 2 minute repair.

The RF performance was normal but the audio was distinctly poor and distorted and nowhere near as good as I remember these sets to be. The quiescent current was about right at 7 mA.

An audio generator and a 'scope showed severe distortion - not clipping but asymmetry of the type caused by low gain in one half of the output stage. Moving the 'scope probe to the bases of the output transistors showed normal drive to one and partially rectified drive to the other.

Investigation showed the output transistor with the distorted drive to have been soldered in with the collector and emitter connections reversed - hence the rectification of the drive. Reversing the connections to this device restored undistorted sound.

The quiescent current adjuster then required considerable movement to restore the specified bias current but could be set correctly within the range of adjustment provided. As the blue locking paint was intact on the preset pot, I can only assume this was a mis-build - there was no sign of any work being done on this set.

It's worth noting that many junction transistors will operate in reversed mode - although with severely degraded gain as this case shows.

Leon.

Steve_P · 5th Aug 2010, 5:41 pm

And it slipped through the net! It's amazimg what does isn't it. Well found.

Cheers,

Steve P.

Hybrid tellies · 5th Aug 2010, 6:54 pm

Thats an interesting fault, congratulations on this one. Its just amazing that this one got through the net and has not been repaired earliar.I bought my TR130 in great condition about 17 years ago, like you it suffered the AF117 syndrome but with mine it was TR3. With its appearance and performance it has been one of my favourite radios and has been in frequent use eversince.

julie_m · 6th Aug 2010, 9:47 am

Any transistor will work -- after a fashion -- with the collector and emitter reversed, but it will have a hFE in the single-digit range!

A company I worked for once shipped several batches of boiler control PCBs with a BC547 (this has the base in the middle, rather than the collector as you might expect) incorrectly inserted, before anybody noticed. They all passed end-of-line test and functioned fine in the field .....

Tazman1966 · 6th Aug 2010, 10:33 am

Amazing that the poor performance was just accepted like that. Well done in finding it out.

ppppenguin · 6th Aug 2010, 11:36 am

A very few transistors, notably the OC140, were symmetric so you could interchange C and E.

If you swap C and E on most transistors you also get a lower saturated voltage drop. I'm sure I've seen some designs that take advantage of this.

5th Aug 2010, 5:37 pm	#1
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Bush TR130 manufacturing fault. I don't collect transistor radios, but this was handed to me by my sister for repair - so it had to be done. Cosmetically good, the set was dead and unmolested inside. TR1 had the AF117 syndrome; replacing this device restored operation. No surprises here - a 2 minute repair. The RF performance was normal but the audio was distinctly poor and distorted and nowhere near as good as I remember these sets to be. The quiescent current was about right at 7 mA. An audio generator and a 'scope showed severe distortion - not clipping but asymmetry of the type caused by low gain in one half of the output stage. Moving the 'scope probe to the bases of the output transistors showed normal drive to one and partially rectified drive to the other. Investigation showed the output transistor with the distorted drive to have been soldered in with the collector and emitter connections reversed - hence the rectification of the drive. Reversing the connections to this device restored undistorted sound. The quiescent current adjuster then required considerable movement to restore the specified bias current but could be set correctly within the range of adjustment provided. As the blue locking paint was intact on the preset pot, I can only assume this was a mis-build - there was no sign of any work being done on this set. It's worth noting that many junction transistors will operate in reversed mode - although with severely degraded gain as this case shows. Leon.

5th Aug 2010, 5:41 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,645	Re: Bush TR130 manufacturing fault. And it slipped through the net! It's amazimg what does isn't it. Well found. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

5th Aug 2010, 6:54 pm	#3
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,967	Re: Bush TR130 manufacturing fault. Thats an interesting fault, congratulations on this one. Its just amazing that this one got through the net and has not been repaired earliar.I bought my TR130 in great condition about 17 years ago, like you it suffered the AF117 syndrome but with mine it was TR3. With its appearance and performance it has been one of my favourite radios and has been in frequent use eversince. __________________ Simon BVWS member

6th Aug 2010, 9:47 am	#4
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Bush TR130 manufacturing fault. Any transistor will work -- after a fashion -- with the collector and emitter reversed, but it will have a hFE in the single-digit range! A company I worked for once shipped several batches of boiler control PCBs with a BC547 (this has the base in the middle, rather than the collector as you might expect) incorrectly inserted, before anybody noticed. They all passed end-of-line test and functioned fine in the field ..... __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

6th Aug 2010, 10:33 am	#5
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,194	Re: Bush TR130 manufacturing fault. Amazing that the poor performance was just accepted like that. Well done in finding it out. __________________ All the very best, Tas

6th Aug 2010, 11:36 am	#6
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Bush TR130 manufacturing fault. A very few transistors, notably the OC140, were symmetric so you could interchange C and E. If you swap C and E on most transistors you also get a lower saturated voltage drop. I'm sure I've seen some designs that take advantage of this.