Dodgy NPN? - UK Vintage Radio Repair and Restoration Discussion Forum

Bufo Bill · 28th Jan 2022, 3:36 pm

If an NPN transistor has the same voltage on all 3 legs does that mean it's a dud?
Cheers from Bill.

markfff · 28th Jan 2022, 3:45 pm

Sounds like it's shorted. What resistance do you measure between leads? Do you have it out of the circuit?

RogerEvans · 28th Jan 2022, 3:56 pm

Almost certainly yes. You could imagine multiple failures in the surrounding circuitry that caused all three connections to be at the same voltage even with the transistor removed, but apart from damage such as a large solder splat or severe flashover I can't think of anything remotely likely. Remove the transistor and check for shorts between the wires and also between the pads on the PCB.

Roger

Ambientnoise · 28th Jan 2022, 4:05 pm

As previous replies say, probably faulty but you do need to measure with reasonable resolution. Assuming a silicon transistor, C to E could be about 0.2v whilst B to E could be 0.6v if the transistor is saturated on (and OK).

Bufo Bill · 28th Jan 2022, 5:16 pm

Hi guys, thanks for the help. The voltages were measured in circuit. I will do as you advise and test out of circuit for shorting of transistor or PCB.
Cheers from Bill.

28th Jan 2022, 5:33 pm

If it is "dead" short, say less than an ohm, you can leave it in circuit.

G6Tanuki · 28th Jan 2022, 6:17 pm

How are you measuring it?

Is it Silicon or Germanium?

A lot of the generic component-testers tend to be inept when faced with the low forward-voltages associated with Germanium junctions.

Bufo Bill · 28th Jan 2022, 7:00 pm

Hi, it is a silicon NPN 2N3053 transistor, I believe. I am using a DMM. I get 30V on each leg whilst grounded to Chassis. I am going to remove it and test properly this evening. Should I only test out of circuit? I am more used to valve technology, but I am very willing to learn here.
Cheers from Bill.

paulsherwin · 28th Jan 2022, 7:16 pm

As said in #6, if it's failed short circuit you should be able to establish this in circuit. Just measure the b-c and b-e resistance both ways round - the resistance should be markedly different depending on the polarity. If it's the same, and especially if it's very low, then the transistor is probably bad. Take it out to confirm.

Bufo Bill · 28th Jan 2022, 8:11 pm

Yup, in no way is this a keeper. Thanks so much guys.
Bill

28th Jan 2022, 3:36 pm	#1
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Dodgy NPN? If an NPN transistor has the same voltage on all 3 legs does that mean it's a dud? Cheers from Bill.

28th Jan 2022, 3:45 pm	#2
markfff Tetrode Join Date: Aug 2021 Location: Virginia Beach, Virginia, USA Posts: 79	Re: Dodgy NPN? Sounds like it's shorted. What resistance do you measure between leads? Do you have it out of the circuit? Last edited by markfff; 28th Jan 2022 at 4:15 pm.

28th Jan 2022, 3:56 pm	#3
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 382	Re: Dodgy NPN? Almost certainly yes. You could imagine multiple failures in the surrounding circuitry that caused all three connections to be at the same voltage even with the transistor removed, but apart from damage such as a large solder splat or severe flashover I can't think of anything remotely likely. Remove the transistor and check for shorts between the wires and also between the pads on the PCB. Roger

28th Jan 2022, 4:05 pm	#4
Ambientnoise Heptode Join Date: Feb 2006 Location: Marlborough, Wiltshire, UK. Posts: 917	Re: Dodgy NPN? As previous replies say, probably faulty but you do need to measure with reasonable resolution. Assuming a silicon transistor, C to E could be about 0.2v whilst B to E could be 0.6v if the transistor is saturated on (and OK).

28th Jan 2022, 5:16 pm	#5
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Dodgy NPN? Hi guys, thanks for the help. The voltages were measured in circuit. I will do as you advise and test out of circuit for shorting of transistor or PCB. Cheers from Bill.

28th Jan 2022, 5:33 pm	#6
Guest Guest Posts: n/a	Re: Dodgy NPN? If it is "dead" short, say less than an ohm, you can leave it in circuit.

28th Jan 2022, 6:17 pm	#7
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Dodgy NPN? How are you measuring it? Is it Silicon or Germanium? A lot of the generic component-testers tend to be inept when faced with the low forward-voltages associated with Germanium junctions. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

28th Jan 2022, 7:00 pm	#8
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Dodgy NPN? Hi, it is a silicon NPN 2N3053 transistor, I believe. I am using a DMM. I get 30V on each leg whilst grounded to Chassis. I am going to remove it and test properly this evening. Should I only test out of circuit? I am more used to valve technology, but I am very willing to learn here. Cheers from Bill.

28th Jan 2022, 7:16 pm	#9
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,969	Re: Dodgy NPN? As said in #6, if it's failed short circuit you should be able to establish this in circuit. Just measure the b-c and b-e resistance both ways round - the resistance should be markedly different depending on the polarity. If it's the same, and especially if it's very low, then the transistor is probably bad. Take it out to confirm.

28th Jan 2022, 8:11 pm	#10
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: Dodgy NPN? Yup, in no way is this a keeper. Thanks so much guys. Bill