Need an Equivalent Transistor

[bloxstompboxes]

Hey guys, long time no see I know. I have a non pedal related question. I need an equivalent transistor for what I believe is silicon based on the voltage drop of 670mV. The number on it is FS24053, but I believe the F to be the Fairchild logo, so maybe drop it. I only know that it is a PNP with EBC pinout and my good one has a hfe of 66. It's used in a vintage Lafayette receiver I beleive as a driver for output trannies. Those are originally 2Sd92 but I have replaced those with NTE175. I did that years ago and it was shelved to get to some other more important projects. I know one of the original output trannies was shorted along with the one in question and maybe another. I remember it was coming on for a second and then blowing the main fuse.

EDIT: I have no schematic! But, it is the https://www.radiomuseum.org/r/lafayette_lr500alr_500.html

[Rob Strand]

My initial thoughts are:  assuming the amplifier output stage has just 2x2SD92 transistors.  Those are TO-66 packages so the amplifier is probably around say 10W, perhaps a tad more or less (25W max).  Those are rated at 100V/55V but I have doubts a small amp like that is operating anywhere near those limits.   The replacement NTE175's are much higher voltage parts and I'm assuming you got those because the sources of TO-66 transistors have been quite limited for many years.

I'm also assuming those Fairchild S24053's are TO-92 packages?  and not TO-5 or TO-126.

If I had to pluck a common transistor out of the air that would fit those parameters I'd be thinking BC640 (PNP); TO-92.   I think those are still available.  Be very careful of the pinout.     A BD140 would also work and perhaps easier to get or cheaper- that's a TO-126;  the BC640 are kind of reduced power versions of the BD140.   The gain of those is about 60 to 80 depending on the current.

With more info about the output power and supply voltages (single or dual) it might be possible to make an even looser choice of replacement part.

My gut feeling is there's probably posts on the internet for similar repair jobs and they might offer other options.

I found this stating it's 60W into 4 ohm and 50W into 8 ohm.
https://hamoperator.com/Files/1968%20Lafayette-Catalog.pdf

However, I have doubts those poor TO-66 transistors can really do that.

I found a post saying that the Lafayette power specs are exaggerated.  There were also sort of tricks: total power which is twice the *per channel* power and various peak power rating schemes.   I don't know what applies in this case - I lost the link while surfing randomly.
See,
https://electromarket.co.uk/blog/amplifier-power-ratings/

Here is the post,
https://audiokarma.org/forums/index.php?threads/lafayette-lr-500b.794009/

That "120 watts IHF into 4 ohms" power claim for the LR-1000B translates into ca. 22 wpc into 8 ohms (probably at 1 kHz).   So true power is 0.183 times the claimed.

The equivalent rating for the LR500 is 60W so we end up at 11W which is more on the money for those transistors.

More here,
https://audiokarma.org/forums/index.php?threads/ihf-compared-too-rms.329286/

"Power rating's were a real trip back then with some IHF, some peak, some inverse peak, and most being totally meaningless. I had an old Lafayette amp rated 100 watts IHF +/- 1db total for both channels combined. Macintosh used to have test clinic's that would sweep across the the frequency range into a dummy load. Bass distortion fell off the map at 15 watts rms, one channel driven. Still have the test chart. When the FTC stepped in, this same amp was marketed as 10 watts rms/channel. "

Here, per channel is 0.15 IHF power (one channel driven) and 0.1 IHF power (both channels driven).  That's would push the 60W down to 9W and 6W respectively.

[bloxstompboxes]

Thanks for the reply Rob. I have some BD140 already and was going to use them to replace some 2N4356 in this receiver as well. The FS24053 are a round dome shaped tranny with gold legs that the package designation escapes me at the moment.

Holy crap am I a moron! I was looking at this sheet on hifiengine linked below to get the part numbers for all of the trannies and just realized the 2n4356 are the FS24053 in my unit. Basing my decision on the datasheet of the 2n4356, I had already made a good choice to replace the FS24053, not realizing I was looking at the same thing, lol.

Have any suggestions on the CS1503 on the same receiver? They are marked Q110L and Q110R. I haven't pulled them yet. I was going to replace all the trannies in this part of the receiver just due to age. I might leave those in though if they check out okay.

https://www.hifiengine.com/manual_library/lafayette/lr-500t.shtml

[Rob Strand]

Thanks for the reply Rob. I have some BD140 already and was going to use them to replace some 2N4356 in this receiver as well. The FS24053 are a round dome shaped tranny with gold legs that the package designation escapes me at the moment.

Probably TO-5 or TO-39 packages.  There was a heap of common parts like that: 2N4033, 2N4037, 2N2904, 2N2905.  I haven't checked the voltages and some were somewhat faster than others.

Holy crap am I a moron! I was looking at this sheet on hifiengine linked below to get the part numbers for all of the trannies and just realized the 2n4356 are the FS24053 in my unit. Basing my decision on the datasheet of the 2n4356, I had already made a good choice to replace the FS24053, not realizing I was looking at the same thing, lol.

It's easy to do.

From this datasheet it has Cob_max at 30pF, so it's not one of the fast versions.  The Cob in the order of BD140's but some of the replacement BD140's are up around 100pF.   Bottom line is BD140 should work OK.   Worst-case is the distortion could rise up at 10kHz - I doubt you will hear it but you might be able to measure it.
https://www.datasheetarchive.com/pdf/download.php?id=50e121704878821f3045746c50e0905b75b3f2&type=O&term=2N4356

Have any suggestions on the CS1503 on the same receiver? They are marked Q110L and Q110R. I haven't pulled them yet. I was going to replace all the trannies in this part of the receiver just due to age. I might leave those in though if they check out okay.

https://www.hifiengine.com/manual_library/lafayette/lr-500t.shtml

I don't have an account so I can see it.

If it's not part of the RF circuit I wouldn't be too worried but if it's amongst the RF stuff some care is required.   Most generic TO-92 package transistors are very similar.  Also you don't want start changing too much in the RF area as that can trigger re-aligning and calibrating the RF stuff - fine if you have a schematic and the equipment but not great if you don't.

[Rob Strand]

Most generic TO-92 package transistors are very similar.

I should have said you need to to get the right voltage rating.

If you have a schematic and the schematic has symmetrical parts on the amplifier like a PNP in one place and an NPN in a similar place.  Then if you can work out a substitute for say the NPN then you can look-up the complementary PNP to that NPN and hence have some idea of a substitute PNP.  (visa versa for other polarity.)

[bloxstompboxes]

Have any suggestions on the CS1503 on the same receiver? They are marked Q110L and Q110R. I haven't pulled them yet. I was going to replace all the trannies in this part of the receiver just due to age. I might leave those in though if they check out okay.

https://www.hifiengine.com/manual_library/lafayette/lr-500t.shtml

I don't have an account so I can see it.

If it's not part of the RF circuit I wouldn't be too worried but if it's amongst the RF stuff some care is required.   Most generic TO-92 package transistors are very similar.  Also you don't want start changing too much in the RF area as that can trigger re-aligning and calibrating the RF stuff - fine if you have a schematic and the equipment but not great if you don't.

[/quote]

The layout of the PCBs in this thing are kind of annoying. The RF section and the main amp are all on one board and there is no real separation either in placement or in silk-screening to show where one begins and the other ends. However these two are marked either L or R at the end so I am assuming that is left and right and probably not part of the RF section. Either way I'll check if they are bad and hold on to them just in case.

As for the specs for each replacement, I have been trying to pay special attention to Vcb, Vce, Veb, Ic, and Hfe depending on where they are in the circuit. I just want to get the damn thing working first and then I can tune it, no pun intended, later if needed. The NPN and PNP symmetry you mentioned is correct in some amplifiers. I have been reading up on, and need to get back to doing so, amps and push/pull configurations and have seen this is the case. I have been designing a push/pull replacement circuit for an STK pack recently and have to get back to it too.

Thanks for the help! I probably won't get to actually physically working and testing this thing until the weekend but I'll post an update when I know something.

[Rob Strand]

The layout of the PCBs in this thing are kind of annoying.

Yes, a schematic would be a big help.  I found part of a schematic for the LR-1500T but nothing for any of the 500 models.   So there are schematics somewhere.

I guess schematics do exist but they aren't so easy to find on the web.  Maybe they were available at some point but I've noticed a lot of info helpful for repair of consumer equipment has disappeared.    I think  lot of repair guys had sites and created their own community but since nothing is fixable these days these businesses have thinned out.

As for the specs for each replacement, I have been trying to pay special attention to Vcb, Vce, Veb, Ic, and Hfe depending on where they are in the circuit.

OK, cool.

Thanks for the help! I probably won't get to actually physically working and testing this thing until the weekend but I'll post an update when I know something.

No worries.   Good luck with it.

[Rob Strand]

There's a few schematics here,  not your model, but very likely to be similar and have similar parts,

https://www.vintageshifi.com/repertoire-pdf/Lafayette.php

Maybe the LR-200?
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Lafayette-LR-200-Schematic.pdf

I found this snippet of the LR-775 you can see a similar circuit but with some different part numbers,




Some common generic parts are the 2SC945's (NPN), the 2SC828 (NPN) and complement 2SA564 (PNP).   BTW, they have hFE category codes.

[bloxstompboxes]

So a little update. A week or two ago I swapped most of the trannies previously discussed and unsoldered two of the output transistors for the right channel. I knew they were shorted on that channel. Now I have a unit that doesn't blow the fuse but has very low output on the left channel. Pretty sure I am gonna have to do some rebiasing, guessing on the output trannies since they are hotter than the originals. It's certainly a step in the right direction. Just need to find the time to sit down with it for more than a few minutes without having to get up to go do a hundred other things.

[Rob Strand]

Now I have a unit that doesn't blow the fuse but has very low output on the left channel. Pretty sure I am gonna have to do some rebiasing, guessing on the output trannies since they are hotter than the originals. It's certainly a step in the right direction. Just need to find the time to sit down with it for more than a few minutes without having to get up to go do a hundred other things.

One of the advantages of a stereo amp is you can compare DC voltages on one channel to that on the other channel.   It can quickly narrow down where the problems are.   

The DC voltage on the transistor side of the output cap should sit mid rail.   Amps with fried transistors (including smaller transistors) won't bias to mid supply.

Some of those older amp designs need a speaker connected for the amp's DC voltages to be correct.   That LR200 schematic looks like it's one of those.

Rebiasing sounds like a good move for the hot transistors.   There's an emitter resistor on the output transistors.  The DC voltage across that can be used to measure the bias current (Ibias = Vmeasured / RE).

There's something wrong with the set-up instructions for the LR200 it's saying to use BIAS trimpots to set the DC output voltage - no way!   

The trimpots marked BIAS should be used to set the bias *current* in the output transistors as mentioned.   I can't see a figure but 10mA to 20mA  should do it.  If you set it too high you might find the current wanders upwards when left on for a long period.  You need to monitor it.  If it rises too much you might need to back off the bias current.   The BALANCE trimpots are used to set the DC voltage (transistor side of the output cap) to mid supply.

The LR-775 has similar trimpots but they are not labelled on the schematic.