Transistor Query ref. Trio/Kenwood KR5200 - UK Vintage Radio Repair and Restoration Discussion Forum

'LIVEWIRE?' · 6th Oct 2018, 11:19 am

One channel of this 40 year old receiver is dead, due to a faulty transistor (Qe5), which is a 2SC1451. This silicon device has a Vce rating of 120v. The various listed equivalents also are rated at between 80 & 150v. Since the amplifier runs from a +32/0-32v supply, and the transistor has, from the cct. diag. only 30v across it, would it be O.K. to replace it with a device rated at (say) 40 or 50v max? I don't presume to know more that the designer(s) who, many years ago, specified higher voltage transistors, but, having found some lower-voltage devices in my stock (2SC18xx, from memory) I'm inclined to fit one. Qe5, b.t.w., is a driver transistor in a typical complementary transformerless audio amplifier circuit.

paulsherwin · 6th Oct 2018, 11:31 am

I would fit your lower rated part and measure the operating voltages. It should be OK if there is some headroom, but remember that it needs to survive any voltages applied under fault conditions as well as those present under normal operation.

The 2SC1815 can take 50V.

'LIVEWIRE?' · 6th Oct 2018, 1:00 pm

Basically those were my thoughts, Paul. The transistors I have are in my workshop, not here at home, but I think they are 2SC1815's.

paulsherwin · 6th Oct 2018, 1:05 pm

That's very likely. They are a standard general purpose transistor in Japan, like the BC108 family in Europe or the 2N2222 in the US.

Radio Wrangler · 6th Oct 2018, 1:14 pm

In a normal complementary stage or quasi complementary stage the power transistors and their drivers will idle with a single rail voltage across them, but as the output swings to what it does on full power, the transistors go up to the sum of the rail voltages 32+32=64 in this case. You certainly don't want a lower rating than this.

32v will be nominal, so on a day when the mains voltage is feeling frisky it will be higher.

Roll in exports to countries with dodgy mains systems, private generators and transients from inductive loads and designers learn to play it safe.

80v transistors are risky, 100v have a bit of margin. Beyond that it's a question of how safe do you want to play it?

David

paulsherwin · 6th Oct 2018, 1:19 pm

You would certainly be pushing your luck with a 50V transistor in that position, but you may well get away with it as the ratings are usually conservative.

'LIVEWIRE?' · 6th Oct 2018, 1:46 pm

Perhaps I'd be better off, and the repair would be more reliable/longer lasting, if I sourced a higher voltage transistor. A quick look on eBay shows several suitable types to be available.

paulsherwin · 6th Oct 2018, 2:37 pm

Towers gives the BC394 as a suitable replacement. They are available from UK eBay sellers at the moment for under £4, not cheap but not expensive either and definitely the safest option.

Many other types are likely to be OK of course.

Herald1360 · 6th Oct 2018, 4:44 pm

It wouldn't hurt to try a boggo BC546. Good for 80V (>64V max available in amp).

They cost pence, too!

6th Oct 2018, 11:19 am	#1
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Transistor Query ref. Trio/Kenwood KR5200 One channel of this 40 year old receiver is dead, due to a faulty transistor (Qe5), which is a 2SC1451. This silicon device has a Vce rating of 120v. The various listed equivalents also are rated at between 80 & 150v. Since the amplifier runs from a +32/0-32v supply, and the transistor has, from the cct. diag. only 30v across it, would it be O.K. to replace it with a device rated at (say) 40 or 50v max? I don't presume to know more that the designer(s) who, many years ago, specified higher voltage transistors, but, having found some lower-voltage devices in my stock (2SC18xx, from memory) I'm inclined to fit one. Qe5, b.t.w., is a driver transistor in a typical complementary transformerless audio amplifier circuit.

6th Oct 2018, 11:31 am	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Transistor Query ref. Trio/Kenwood KR5200 I would fit your lower rated part and measure the operating voltages. It should be OK if there is some headroom, but remember that it needs to survive any voltages applied under fault conditions as well as those present under normal operation. The 2SC1815 can take 50V.

6th Oct 2018, 1:00 pm	#3
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Transistor Query ref. Trio/Kenwood KR5200 Basically those were my thoughts, Paul. The transistors I have are in my workshop, not here at home, but I think they are 2SC1815's.

6th Oct 2018, 1:05 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Transistor Query ref. Trio/Kenwood KR5200 That's very likely. They are a standard general purpose transistor in Japan, like the BC108 family in Europe or the 2N2222 in the US.

6th Oct 2018, 1:14 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Transistor Query ref. Trio/Kenwood KR5200 In a normal complementary stage or quasi complementary stage the power transistors and their drivers will idle with a single rail voltage across them, but as the output swings to what it does on full power, the transistors go up to the sum of the rail voltages 32+32=64 in this case. You certainly don't want a lower rating than this. 32v will be nominal, so on a day when the mains voltage is feeling frisky it will be higher. Roll in exports to countries with dodgy mains systems, private generators and transients from inductive loads and designers learn to play it safe. 80v transistors are risky, 100v have a bit of margin. Beyond that it's a question of how safe do you want to play it? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Oct 2018, 1:19 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Transistor Query ref. Trio/Kenwood KR5200 You would certainly be pushing your luck with a 50V transistor in that position, but you may well get away with it as the ratings are usually conservative.

6th Oct 2018, 1:46 pm	#7
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Transistor Query ref. Trio/Kenwood KR5200 Perhaps I'd be better off, and the repair would be more reliable/longer lasting, if I sourced a higher voltage transistor. A quick look on eBay shows several suitable types to be available.

6th Oct 2018, 2:37 pm	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Transistor Query ref. Trio/Kenwood KR5200 Towers gives the BC394 as a suitable replacement. They are available from UK eBay sellers at the moment for under £4, not cheap but not expensive either and definitely the safest option. Many other types are likely to be OK of course.

6th Oct 2018, 4:44 pm	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Transistor Query ref. Trio/Kenwood KR5200 It wouldn't hurt to try a boggo BC546. Good for 80V (>64V max available in amp). They cost pence, too! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O