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Old 5th August 2004, 08:01 PM   #1
bfisk is offline bfisk  United Kingdom
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Default Phonic 740

Hi all,

I'm in the process of repairing this model after a burn up of one channel. After tracing out the circuit (no schematics available) I've since discovered this is a class G (I think) design which is something I've never seen before.
Doing a search in this forum I've come across some posts which have referred to a link at Phonic.com, which is no longer available.
Would anyone have the information from this link saved at all?

As I say I've drawn out the circuit but it's a bit of a spiders web and a proper one would be nice.
The o/p consists of a 2SC5200/1493 with the second voltage rail feeding a pair of RFP50n06 mosfets to the collectors of the o/p semis.

As a sideline it also uses an MJE802 which I am having no end of trouble sourcing in the UK due to it now being obsolete. The recommended replacement is a BD679 but I've noticed the internal resistors are different values. I suspect this is going to cause some grief, any thoughts?

Many thanks
Bob
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Old 6th August 2004, 06:07 PM   #2
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mje802 cross references to an ecg 253 or NTE253 which is an NPN darlington TO-126 case style. I have used some of these in one of my circuits along with it's complement. I also had trouble getting any more than 4 of them although I only checked with 1 distributer. Oddly enough there was no limit to how many of the NTE254 PNP devices I could get. I would bet that you could substitute a different device for this one, as long as the Vce limits and current Ic limits are not exceeded. If you match beta Hfe values somewhat closely, a device could even be a TO-220 style componant if you can rig it. This of course is dependent on your circuit but if this is not the output driver, I bet it would be OK to sub in a different part with comparable charactoristics if you can't get the exact transistor. Who says that analog electronics is an exact sport!


Get an ECG cross reference book and look through the different types of transistors that also cross to an ECG 253 device and see if you can track one of those down. There may be several crosses.
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Old 7th August 2004, 01:44 AM   #3
anatech is offline anatech  Canada
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Hi bfisk,
Never, never, never "cross" in and out of ECG or NTE or whatever! If possible, use original numbered devices. At least you know what you have.
The problem with crossing in & out is that the listing in the replacement manual is not 100% accurate, and the device in your hand may have lower ratings, but cross to the same part. See the problem?
Anyway Bob, the internal resistance values may not make any difference at all, give it a shot.

No offence cunningham, but I've never seen quality with replacement brands. Maybe they are okay for TV's, but not audio. Also, just 'cause something works, doesn't mean it works right.
-Chris
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Old 7th August 2004, 09:39 AM   #4
bfisk is offline bfisk  United Kingdom
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Thanks for the replies guys.

I would be happier using the originals because at least I know it's not going to add to my fault symptoms.

Out of interest I finished the repair on this last nght and whilst testing, everything appeared OK. Various tones were checked on the scope and everything looked nice and linear. I started uping the power level and 'POP!', one of the o/p semis went short (2SA1943) along with one of its bais semis (BC560) actually this one exploded!!
I thought I'd open up the 1943 to have a look inside and it looks like a fake to me. The actual die is tiny and a lot smaller than the semi that was originally fitted.
I'm just in the process of taking some photo's to send of to Toshiba for verification.
I'll post here the outcome.

Bob
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Old 7th August 2004, 01:37 PM   #5
anatech is offline anatech  Canada
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Hi Bob,
Sounds like a fake to me. Die size is a pretty basic indicator of current capacity.
-Chris
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Old 7th August 2004, 11:12 PM   #6
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I am not a fan of replacement parts, but if the circuit is designed well, and not dependent on the devices to be exact, then the slight difference between replacement parts and originals should not affect the circuit operation if the replacement parts are of any quality. That of course depends on the manufaturer.
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Old 8th August 2004, 04:47 PM   #7
anatech is offline anatech  Canada
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To cunningham
Actually, circuit performance is enhanced when the correct devices are used. Matched devices will have a positive effect on performance. Rest assured that a replacement brand device is the least expensive thing that could be sourced. Kinda like using MJ15015/15016 to replace MJ15022/15023. Close specs, totally different animals and behaviour. But the circuit would work, just not anywhere near the level it could.
Sometimes, the devices do matter. So does how you install them and whether or not they need to be matched. Therein lies some of the skills of the technician or designer.
-Chris
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Old 13th August 2004, 08:41 PM   #8
bfisk is offline bfisk  United Kingdom
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Hi all,

A big thanks to all who responded on and off line.

An update on the fake semis......

The distributor has sent me two new 2SA1943's free of charge after I sent them a mail with a picture of the semi opened up. No appologies or reply to the mail though!! Also they are not Toshiba semis but Fairchild (I think).
Toshiba replied too but only to say they would only deal if the distributor contacted them.
Hmm, now if it was Pound notes or Dollar bills what do you reckon the response would be then?? I reckon people would be knocking on my door.

I shall fire the amp up again tomorrow and see what happens. Lets hope the 2SC5200's aren't fake too!!!!!

Bob
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