Help needed to find modern equivalent part - Page 1
Forum section: electronics / repair

author: woox2k
author_rank: Regular Contributor
timestamp: September 06, 2024, 10:34:48 pm
content: Hello!I'll give the full story here for those who like a backstory but TLDR will be in the end!First off, i'm no engineer. I have messed with electronic stuff as a side hobby for many years but usually just with the stuff that needed to be done around the house (and some fun projects). Usually my electronic repair skills end when there is no obviously bad capacitors in sight or some faulty mosfets/diode bridges in power supplies.Today, a vintage receiver landed on my bench (from a friend). I usually keep away from power amps because they are way too much for me and usually just contain bad output transistors that are unobtainable. This time i thought i'll at least have a go with it. If i diagnose it carefully and don't harm it further then no harm done and i may learn something new?!The fault symptom was described as a random very loud pop in the speakers occasionally. Then some other, more professional...ish, guy had a go with it, changed some capacitors and then the speaker protection did not disengage anymore at all. I thought "great, finding faults is hard enough, finding faults that are introduced by changing random parts is even worse" I still thought hell with it, i'll look at it at least.I hooked up a throwaway speaker and tried to turn the thing on (owner did the same so no harm there). Surprisingly enough it did start up for few seconds and then a loud pop came through the speaker and the protection engaged. Then few seconds later the protection disengaged again and after another few seconds it engaged again with a loud pop, then it did not disengage anymore until i turned the receiver off and let it cool down a bit. Since the receiver has preamp output and power amp input on the back (connected with a jumper) i thought it's a good idea to try to see if the problem comes from the preamp. It did not, disconnecting the links and attaching another amp to the preamp output produced clean signal while the speaker protection still didn't disengage. Note that i did connect another working but silent preamp output to the power amp input just in case, i remembered that at least some soviet power amps liked to self generate and die when there were no input in them.Then i went a bit deeper by taking the covers off, figured out where the power amp outputs (before protection) were and attached my oscilloscope to both channels. Sure enough, one channel went crazy occasionally and the longer the amp was on the more i saw huge spikes on one output, while the second was okay. I thought "great, some capacitor is going haywire there" I had heard that capacitors (even ceramic ones) can randomly short like that but usually this behavior is temperature dependent. Since temperature is easy to manipulate, i blasted the power amp board with canned air (upside down)... The fault did disappear for a while! YAY! Then, using trial and error, i managed to find the area on the circuit board where the cooling produced the best effect. Sadly there were transistors, ceramic capacitors and resistors in that particular area. Then i had an idea to use a soldering iron after cooling to heat up those parts one by one and see what made the fault appear the soonest. It appeared that one transistor looking thing was generating problems ("dam!") Just to be sure, i de-soldered it, attached longer leads to it and tried the heating/cooling method again without affecting the surrounding parts. Sure enough this was it!Now i had a problem, i know nothing about transistors other than the basic working principle. I did have a service manual and i could get the full part number off that (did not have to rely on the few number description on that small 3 legged thing) but quick google did not produce any useful purchase links (nor the datasheet to the exact part)... So i don't even know where to start finding good replacement for it, or if i need to change out both Q3 and Q4 together? (see images) Anyway, help me!TLDR: I found a faulty 2SC1885-R transistor from an amp board and need help finding modern replacement for it i can buy!PS. I'm not here just asking a "one link" answer. If possible, can anyone explain me, using this particular example, how to find transistor replacements. What parameters are the most important, what can be substituted with more powerful ones and so on...

author: amyk
author_rank: Super Contributor
timestamp: September 07, 2024, 06:53:45 am
content: Just searching online for "2SC1885 substitute" finds plenty of discussions about the same.The specs are here:https://alltransistors.com/transistor.php?transistor=12509The same site has a suitable list of replacements:https://alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=0.75&ucb=150&uce=150&ueb=5&ic=0.1&tj=150&ft=100&cc=3&hfe=150&caps=TO92&cf=onQ3 and Q4 may need to be a matched pair, based on how they're used in the schematic.There are sellers claiming to sell 2SC1885 on Ali and eBay but those may be fake.I think 2N5550, 2N5551, 2N5832 might work, but no guarantees.

author: woox2k
author_rank: Regular Contributor
timestamp: September 07, 2024, 07:50:33 am
content: Quote from: amyk on September 07, 2024, 06:53:45 amWhat confused me was the "-R" in the parts list. I was not sure what that meant and if the generic 2SC1885 is the same part at all. It seems to be critical enough that i would not like to just try some random replacements and possibly damage the output transistors that really are matched pair of magic containing bricks made out of unobtaniumI looked away from those Ali links quite fast because even if they are real they may be sourced from scrap sellers who "desoldered" them from old equipment using fire and they may be more damaged than the part i have.Anyway thanks for answering but i feel like i should wait and see someone answer who can definitely say if those replacements will work and do they really have to be a matched pair. I do feel like i should give it to someone else to finish up since my luck may run out. As in i was incredibly lucky the glitchy fault was so stable when i got my hands on it. Other people who worked on it were not that lucky and i understand why they couldn't find it if the fault occurred like once in 4 hours or so. Then again, i am interested of learning a bit from all this.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 07, 2024, 08:45:17 am
content: will need to check with the datasheet, but for JIS transistors, the suffix indicates the hfe grade.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 07, 2024, 08:59:23 am
content: copied the databook I have since I could not find any information on the internet.I hope it will be helpful.can see the characteristics and equivalent parts.But the data is from 1986.characteristics table is helpful.

author: woox2k
author_rank: Regular Contributor
timestamp: September 07, 2024, 07:32:13 pm
content: Quote from: squadchannel on September 07, 2024, 08:59:23 amHuge thanks for taking the time to do so, this is very useful!I'm still not sure that i want to proceed with this project (would not want to mess this one up) but at least i get some useful info for the future.

author: fzabkar
author_rank: Super Contributor
timestamp: September 07, 2024, 10:05:14 pm
content: Quote from: woox2k on September 07, 2024, 07:32:13 pmThat's too bad, because you've already done all the hard diagnostic work.

author: woox2k
author_rank: Regular Contributor
timestamp: September 08, 2024, 05:10:33 pm
content: Quote from: fzabkar on September 07, 2024, 10:05:14 pmYeah, but that was pure luck. I would love to continue on this and learn something new but i feel that this quite rare vintage equipment is not the right thing to practice my skills on. Since there was no "doh, it's so easy, that's how you do it" in this thread i assume it's not that easy thing to do and i should find something simpler to mess with.Heck i too have an old soviet amplifier with pretty much the exact same issue and i have a working spare currently in use (lovely sounding amp) i could try my luck there... Even though many electronic repair guys who are familiar with soviet stuff say that this particular amp is pure voodoo, even when having full schematic in hand.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 09, 2024, 09:59:05 am
content: not sure, but i think you need to change him in pair with his brother with one transistor from same batch, otherwise you may get thermal drift