|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
3rd Apr 2008, 7:42 pm | #1 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
NKT transistor problems.
In case this Thread title puzzles some, I'll explain. A Dynatron HFC 13 was brought to me for repair, and, for a short while I couldn't understand why, when I screwed the amp. PCB down to chassis, one channel cut out, accompanied by a crackling sound. Checking the centre rail, I found it was only about 0.5v(should be 15!) Then the penny dropped. The SRX 26 series chassis uses germanium transistors(NKT212, 717, & 279)which are fitted with cooling clips secured to the chassis by threaded studs. Two of these measured about 30 ohms between the case of the transistor and the collector-hence virtually short-circuiting the centre rail to chassis(Positive in these amps.) The NKT 212, etc., for those not familiar with them, are similar to AC128s and the like, but in 36 years of working with electronics I don't recall any AC1xx developing this fault.(Someone will say I'm wrong, of course!!)
|
3rd Apr 2008, 7:50 pm | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: It's not only AF117s which fail......!
It would be interesting to know if this was caused by the same AF11x tin whisker problem or some other fault.
Paul |
3rd Apr 2008, 8:06 pm | #3 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: It's not only AF117s which fail......!
I'm not sure, Paul. Might inspect one under a magnifying glass! Of course, as you know AC12x and NKT2xx, etc. transistors are 3lead devices, i.e. they don't have a shield(or screen)lead like the AF11x types. As an aside I don't remember this problem with the OC170(which was essentially the same as an AF117) either
|
3rd Apr 2008, 8:10 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: It's not only AF117s which fail......!
|
4th Apr 2008, 1:15 am | #5 | |
Dekatron
Join Date: Jan 2003
Location: Bradford on Avon, Wiltshire, UK.
Posts: 3,310
|
Re: It's not only AF117s which fail......!
Quote:
We have some equipment at work, the OC170's go faulty in the same way as AF117's etc. and have to be replaced with CV1089 (AF125)
__________________
"Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly." |
|
4th Apr 2008, 7:03 am | #6 |
Octode
Join Date: Mar 2006
Location: Bracknell, Berkshire,UK.
Posts: 1,172
|
Re: It's not only AF117s which fail......!
I have also had failing NKT germanium transistors, if you do a web search you will find in fact it is a pretty common problem.
Dave |
4th Apr 2008, 4:44 pm | #7 |
Retired Dormant Member
Join Date: Jul 2003
Location: N.E. Surrey, UK.
Posts: 361
|
Re: It's not only AF117s which fail......!
I seem to be forever changing AC127 transistors these days... The PNP Germanium output transistors always seem to be the most likely to fail..and these are the worst of their kind! (or may be just the most common!).
Roberts collectors do seem to have it tough with transistors! |
4th Apr 2008, 5:31 pm | #8 | |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Quote:
I have an old Fidelity 12 radio chassis with AC128/127 and it has given years of trouble free operation, being used as a workshop radio, on all day. Geof |
|
4th Apr 2008, 7:52 pm | #9 |
Retired Dormant Member
Join Date: Jul 2003
Location: N.E. Surrey, UK.
Posts: 361
|
Re: It's not only AF117s which fail......!
You are quite right of course - it was a long day!
Substitute NPN where I said PNP though, and I'll stand by all else I said about them being troublesome little devils! |
4th Apr 2008, 8:37 pm | #10 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: It's not only AF117s which fail......!
It was one of the first Ge NPN output transistors so I guess there could have been manufacturing problems with the early ones.
Paul |
4th Apr 2008, 8:44 pm | #11 |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Don't I know it But I don't think they are a particularly troublesome transistor, considering they where one of the original output devices they have proved reliable as far as a small germanium transistor can be. I have a number of transistor sets with this combination that are still working after forty years or more. They can be damaged if overdriven because they are not intended to be a high power device.
Geof |
4th Apr 2008, 9:07 pm | #12 |
Octode
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
|
Re: It's not only AF117s which fail......!
In my (some say miss-spent) youth, I repaired many early British transistor radios due to semi-conductor failures. Everything with 0A, 0C or AF were the worst offenders leaving me to believe that while I accepted Mullard valves to be superior to others, their semiconductors were very opposite. It was usually just the switches on volume controls that caused failure in Japanese radio's , mostly pocket models at that time.
Whenever a British radio came into the shop with "No Go" written on the job card, one instinctively knew it would be a Mullard transistor at fault. The best way to quickly find the culprit was to give it a flick with the fingers or a gentle tap with the Steadfast (remember them?) screw driver. I don't think it was tin whiskers in the 0C45's, probably the nature of the beast, early PNP junction technology i.e. Indium Pellets etc. but the Jap ones seemed so much more superior and reliable. The Mullard Lockfit range were not so bad but I had a fair share of those failing too! To avoid the barbs on their leads ripping the PCB pads to bits, I used to rock them back and forth until they broke away, then solder sucking the leads away on the print side. My favourites were the Ediswan Top Hat devices, never had a failure with those but I know that others have. Maybe it's just down to luck but whatever, I shall go to the grave with 100% contempt for those early Mullard Germanium Transistors. Different with tubes though, what engineer would buy a TV with a Mazda tube in it if a Mullard one was available? I felt very guilty sometimes selling a TV to a customer with a Mazda Tube in it, they seemed to have awful crushing of whites just out the box!
__________________
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way! |
5th Apr 2008, 12:35 pm | #13 |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Although the AC127-AC128 are used as a complimentary pair the AC127 has a Ptot of 340mW compared with the AC128 Ptot of 1W. This is probably why the AC127 is the first to fail as they are not well matched for power especially if the output stage has been set up to deliver 1 Watt nominal, the AC127 will be working beyond its design limit if the volume is used at high levels. It could be that NPN germanium transistors were more difficult to manufacture than PNP types.
An alternative as replacements might be the AC187-AC188 matched pair. The collector voltage is lower at 25V but more than enough for transistor radios (the Grundig Yacht Boy is one receiver with this output stage). Geof Last edited by geofy; 5th Apr 2008 at 12:40 pm. |
5th Apr 2008, 2:38 pm | #14 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: It's not only AF117s which fail......!
I recall that a (fairly) common output pairing was AC128/AC176. A quick look at a data book confirms that they match better than AC127/128, both having Pt of 1Watt. Having said that I still have a homebuilt SW superhet which uses ac 127/128 output(and the dreaded OC170(AF117) in it's RF/IF stages. It's still working more than 30 years after I built it(or it was last time I switched it on!)
|
5th Apr 2008, 4:02 pm | #15 | |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Quote:
Probably just as damaging to any transistor is if the quiescent base current is set too high. I hope the home built SW set still works ok Geof |
|
5th Apr 2008, 8:16 pm | #16 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: It's not only AF117s which fail......!
The data I quoted is from a 'dov' transistor 1(A-Z) book (on the back cover, in German, it states(quote) dov...das intelligentere buch(the most intelligent book) Info for the AC176 includes Vc32/18 Ic 1A Ptot(Ta,$Tc, &Tk)1W($45deg.c). These figures are similar to those given for the AC128. (I have looked in the abbreviations list and nowhere does it explain what the $ symbol means!) Don't have a Towers or Mullard book handy to compare notes. As to the SW Radio, I'll power it up sometime soon and find out!
|
6th Apr 2008, 10:52 am | #18 | |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Quote:
AC176 Germanium npn for use in mains operated audio amplifiers, (not sure why this should differ from battery powered ones apart from longer and more constant operation times) with class B output stages. VCB 32 V ICM 1.0 A Ptot 155mW Tamb 45C hFE (ie 500mA) 52 -180 ICBO (VCB = 10V ) 12.5mA |
|
6th Apr 2008, 11:27 am | #19 |
Retired Dormant Member
Join Date: Jun 2006
Posts: 1,798
|
Re: It's not only AF117s which fail......!
Correction; the ICBO should be 12.5uA (VCB = 10V : IE = 0 )
Not sure what $ could mean, possibly absolute ! Geof Last edited by Dave Moll; 6th Apr 2008 at 10:11 pm. Reason: remove unnecessary quote |
6th Apr 2008, 12:38 pm | #20 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
Re: It's not only AF117s which fail......!
Geof, I've no idea which is right, either, or why Mullard should specify the AC176 for use in Mains Powered Amplifiers. All that the dov book states is 'Ge npn AF-driend(driver-endstuck, i.e. driver or output) Dov by the way is 'Deutsch-Osterreichische Buchhandels-und Verlags gmbh, Munchen'=German-Austrian Book Distributors, Munich. I obtained 'Transistor 1(A-Z), and 'Transistor 2(0-mju) some years ago from Willow Vale. The two books give data on abt 40000 transistors, with equivalents.
|