transistor replacement old to new SCHEMATICS ADDED - Page 1
Forum section: electronics / beginners

author: Nikan
author_rank: Regular Contributor
timestamp: May 06, 2022, 01:23:01 am
content: Hi there,I have been given an old audio amplifier board no clue what brand it is etc...There are some dead transistors.I am looking for replacements on mouser but I am kind of overwhelmed.Can anyone point me to the best replacements forBF417andBF418I would prefer a TO126 case, but a TO220 case would also be fine.I'm not into new old stock and am looking for a part which is avalible on mouser.Is it true that measurements for transistors were done more precisely in the past than they are today? I was told so, but tbh that sounds really odd.Thanks in advance and best regards from Germany

author: bdunham7
author_rank: Super Contributor
timestamp: May 06, 2022, 03:55:06 am
content: Quote from: Nikan on May 06, 2022, 01:23:01 amIf you are going to be servicing old audio equipment, you may need to set aside your distaste for NOS. Sometimes it is the only way. In this case you can probably come up with a substitute, but if not there is an eBay seller in Spain with both transistors available, although it looks like they need a good cleaning.For substitution, it may not be possible to find replacements that have all of the same attributes as your originals, so you need to understand your circuit so that you know which ones need to be the same and which do not. And often minor modifications are needed to make new parts work, since amplifiers tend to oscillate when you use transistor with different gains or transition frequencies.In this case, the transistors in question were designed for use in an analog CRT television or monitor and they have a C-E voltage of 300V and a transition frequency of 70MHz--both of which I suspect are unnecessary in your audio amplifier. Can you clarify that and make sure that is true? If you do need the voltage for some reason, look at the MJE340 and MJE350 from Central, but watch the reduced E-B reverse voltage, it is 3.0 vs 5.0 for the original. Otherwise maybe the MJE180 and MJE170. Onsemi and Central are your friends.Otherwise, let us know what type of audio equipment, what the transistor does (output transistor driver?) and what it's power supply voltages and such are. A schematic would be very helpful.

author: strawberry
author_rank: Super Contributor
timestamp: May 06, 2022, 04:56:21 am
content: old days semiconductor manufacturing was not as precision as now , so reason why measuring/sorting done more in old daysHfe 20...80 could heat driver stagebut most modern transistor have less power dissipation (smaller die) for same parametergood for up to 50V rails TTC004 , TTC008...board picture could tell what brand and decade of manufacture

author: Zoli
author_rank: Frequent Contributor
timestamp: May 06, 2022, 06:04:07 am
content: Personally, I would replace them with the KSA1381/KSC3503 pair; typically Mouser has them in stock.Good luck with your repair!

author: strawberry
author_rank: Super Contributor
timestamp: May 06, 2022, 06:51:15 am
content: KSC3503( D )available = expected Hfe 60...120 , capacitance 1.8pFTTC004B = expected Hfe 140...280 , capacitance 17pF

author: Zoli
author_rank: Frequent Contributor
timestamp: May 06, 2022, 07:31:51 am
content: Quote from: strawberry on May 06, 2022, 06:51:15 amBF417/8 capacitance 5.5pF; TTC series most likely will need extra CB compensation capacitor to cut oscillations, KS series don't - IF the transistors are used in the VAS(yeah, I've saw them used as input differentials).

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2022, 01:12:30 am
content: Quote from: bdunham7 on May 06, 2022, 03:55:06 amCRT drivers are used in audio VAS stages because of their low capacitance. They are useful anyplace where high voltage combined with high frequency capability is required.Quote from: Zoli on May 06, 2022, 06:04:07 amThe KSA1381/KSC3503 are good up to 300 volts and the KSA1142/KSC2682 are good up to 160 volts. Their capacitance is like 1/10th that of other similar transistors.

author: Nikan
author_rank: Regular Contributor
timestamp: May 08, 2022, 07:56:18 pm
content: Quote from: bdunham7 on May 06, 2022, 03:55:06 am

author: Nikan
author_rank: Regular Contributor
timestamp: May 08, 2022, 07:58:26 pm
content: Quote from: Zoli on May 06, 2022, 06:04:07 amGreat thynk you. I'll have a look into it!

author: Nikan
author_rank: Regular Contributor
timestamp: May 08, 2022, 08:04:22 pm
content: If there is anything I should change to make it easier to read just let me know!This is just a submodule of the amplifier where the dead BF417's and BF418's are sitting

author: srb1954
author_rank: Super Contributor
timestamp: May 08, 2022, 10:43:00 pm
content: Quote from: Nikan on May 08, 2022, 08:04:22 pmYes, the way this schematic is drawn it is barely recognisable as an amplifier. A schematic should be drawn to clearly and simply indicate the circuit function. The convention is to draw schematics, as far as practicable, with the signal flow from left to right with the most positive supplies to the top and most negative supplies or ground to the bottom. Have a look at how other people draw similar circuits to see how it should be done.For example, in this schematic the input transistors should be rotated clockwise and the immediately associated connected transistors appropriately arranged above and below those input transistors. With this rearrangement it can be easily seen that the input stage is a simple differential amplfier and would be easily recognised by most EEs.Adding reference designators to all components makes it easier to refer to parts of the circuit.Lastly, avoid 4-way junctions such as at the audio IN connection. With the very small connection dots used it can sometimes be confusing whether it is a connection or a crossover.

author: Nikan
author_rank: Regular Contributor
timestamp: May 08, 2022, 11:28:40 pm
content: Quote from: srb1954 on May 08, 2022, 10:43:00 pmThank you very much. I will work on it probably tomorrow.

author: bdunham7
author_rank: Super Contributor
timestamp: May 08, 2022, 11:38:18 pm
content: Quote from: Nikan on May 08, 2022, 11:28:40 pmI realize you are reverse-engineering the drawing from the physical device, so it can be tricky. A few additional points:1) Unless your "0V" point is actually the ground reference or something weird like that, this point should probably be "+47V" and placed near the top of your page. The -47V would become your ground. Of course you might check to see how it is actually set up. I've never seen an amplifier with a single supply at negative voltage from an actual ground, but there's always a first time.2) your A and B points should end up on the right side of the page, the audio in should be on the left.Perhaps take a look at a drawing from another amplifier schematic to get a general ideas of the normal way to illustrate them.As far as transistor replacements, they're cheap enough that you might want to order two different sets in case one doesn't work as expected. Perhaps the KSA1381/KSC3503 and the MJE170/MJE180.

author: Nikan
author_rank: Regular Contributor
timestamp: May 09, 2022, 12:34:48 am
content: Quote from: bdunham7 on May 08, 2022, 11:38:18 pmYes I just drew it from the PCB... The board is also pretty weird, all the parts connected to 0 have their own traces even though they are next to each other. I understand that the current flow can superimpose signals, but would it have been such a big problem if they thickened the traces and connected the parts together or just use a ground plane? (The current flows in the same direction and the devices are next to each other).I will take a picture of it tomorrow.It seems like 0 is actually the reference. The transformer has several windings, two of which are high power windings that end up on two diode bridges. Then the power supplies are buffered and connected in series.I will have a look into it again tomorrow as of now I have to say good night it is already 2:30am

author: strawberry
author_rank: Super Contributor
timestamp: May 09, 2022, 05:38:07 am
content: Quote from: Nikan on May 09, 2022, 12:34:48 amdistortion low limit could be limited to ~0.5% even with the best amplifier. amplifier peak current for 40V and 4Ω = 10A and input sensitivity expected about 1000mV so 1mV on that copper trace = 0.1% change to input signal. high frequency is even worse than that(phase shift , positive/negative feedback)could be power amp supply and preamp power supply

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2022, 01:33:08 pm
content: Douglas Self had some things to say about layout and star grounding in his Audio Power Amplifier Design Handbook.