Can anyone help RCA 40458 transistor  - Page 1
Forum section: electronics / beginners

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 01:47:53 am
content: Hi everyone, I have no idea when it comes to SC I’m looking for an RCA 40458 transistorsFrom what I have found it’s;0.7A, 90V, 1W, hFE 40-200@10mA, Ft 100MHz.I’m trying to repair an old 60’s amp and this transistor is making horrible sound on one channel if I swap it out for the one on the other board the horrible crackling follows the transistors..I could do with help to find a good equivalent to it any advice would be appreciated!!!ThanksNick

author: drussell
author_rank: Super Contributor
timestamp: February 20, 2021, 01:59:43 am
content: What model is the amplifier?Where is the transistor being used in the circuit?Do you have a schematic you can post or good photos of the circuitry itself?

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 02:28:33 am
content: Thank you for the reply,The amp is a nas 70 I can’t find a thing on it. Think it’s Nottingham audio service. From searching around the internet think it’s rather close to a harman kardon citation 12.I think the transistor is pre driver what ever that is.I can upload some photos.ThanksNick

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 02:36:17 am
content: Il take some better photos they are just ones I had on my phone. The problem one is 2 nd to bottom on the left of the image

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2021, 08:50:46 am
content: For a low power audio amplifier I would not expect to really need 90 V capability.TO39 case transistors are getting a little rare, but with a bit of pin twisting one should be able to fil somthing like a To126 case BD139/BD140 (NPN or PNP). The voltage rating is a little lower (80 V), but the other specs should be similar.With a change in transistors one would need to re-adjust the biasing current.Predrive should be the first transistor in a darlington stage.

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 12:23:30 pm
content: Thank you for the advice, I’m not sure how I would adjust the voltage is it the little pot on the board?I have uploaded few more photos.

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 12:25:11 pm
content: Few more photos

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2021, 02:56:50 pm
content: There is no need to adjust the votlage. There are mainly 2 function where one may find a trimmer for: the DC offset voltage and the standing bias current in the output stage if build as a class AB amplifier. Unless the resistors are still externally near the power transistors the circuit does not look very much a class AB amplifier. At least I am missing the low value (e.g 0.1 -1 Ohms) resistors usually used to stabilize the standing current. The trimmer is wired as a variable resistor and is rater far from the output stage. So this may be a class B output stage, with no extra need for current adjustment.With the voltage one should check the supply voltage, to check how much voltage rating is needed for the transistor.

author: drussell
author_rank: Super Contributor
timestamp: February 20, 2021, 04:42:41 pm
content: Quote from: Kleinstein on February 20, 2021, 02:56:50 pmAren't the two green guys on the bottom of the board probably the emitter resistors?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2021, 05:09:49 pm
content: Quote from: drussell on February 20, 2021, 04:42:41 pmVery much looks like that, and the inductor + resitor make sense for the output "fitlering".The circuit looks like the trimmer is that to set the volume - not the standing current. So maybe the circuit is relatively tolratant. After a transistor change it may be a good idea to check the voltage over the 2 green resistors on the bottom (with no laod, input shorted).

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 07:21:54 pm
content: Thank you guys I’m sorry don’t want to sound stupid but you have lost me 😂😂😂😂.Would an NTE128 transistor workPart Number: NTE128Manufacturer: NTE ElectronicsPolarity & Material: NPN-SiCase Style: TO39IC: 1ABVCBO: 140VBVCEO: 80VBVEBO: 7VhFE: 90 MinPD: 0.8WfT: 100 MinMost annoying thing is all the other transistors on the board I can get new old stock the one that I need can’t find.. 🙄🙄Thanks nick

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2021, 07:53:23 pm
content: If the RCA transistor is also a NPN the NTE128 should be OK, though the power rating is marginally lower.Ater the change one should still measure the voltage at the large green resistors on the bottom of the board, to check the standing current. A different transistor may change this because of a different BE voltage. With a current far off the distortion goes up and with a rather high current the amplifier would run rather hot.

author: floobydust
author_rank: Super Contributor
timestamp: February 20, 2021, 08:07:43 pm
content: Just a note- if you are rebuilding this board, those 5 blue axial electrolytic capacitors just don't last and are certainly no good. They dry out and go low value for capacitance; I've also had them short.I would outright replace them- at least test them.

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 08:27:44 pm
content: Thank you againSo would you recommend me changing the 40458 transistor on each board so they are the same would this give better results?ThanksNick

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2021, 08:55:11 pm
content: No need to change the still working 40458. The NTE126 would not be an upgrade.With the blue electrolytic caps one could consider changing them, or at least test them. These olf phillips ones have a relatively poor reputation / high failure rate by now. Poor caps could cause extra hum and maybe also missing low frequencies.

author: andy3055
author_rank: Super Contributor
timestamp: February 20, 2021, 09:28:44 pm
content: If you need data on this transistor, check page 308 of this manual:https://worldradiohistory.com/BOOKSHELF-ARH/RCA-Books/RCA-Transistor-Thyristor-&-Diode-Manual-1971.pdf

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 10:04:19 pm
content: Quote from: Kleinstein on February 20, 2021, 08:55:11 pmFunny you say that found the 64uf 64v ones ready to go in, got new old stock as I can’t find anything new about the same,I can’t find2 x 6.4 uf 25v .When diagnosing the issue I messed up a few of the tropical fish caps too so I have a few here, also two of the 40595 as I messed one of the legs up taking it out.. but I know it’s the 40458 that’s the issue and I can’t for the life of me find any.🙄. if the NTE126 is not an upgrade is the any out there that might be classed as one.Long story short the amp has sentimental value to me I know could go out and get a new power amp for a few hundred and would prob sound just as good or better but would like to get the old thing kicking again.. was working fine till one day started to make the horrible sound 😂😂ThanksNick

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 10:05:34 pm
content: Quote from: floobydust on February 20, 2021, 08:07:43 pmThank you

author: Nick1986
author_rank: Contributor
timestamp: February 20, 2021, 10:06:11 pm
content: Thank you andy3055

author: floobydust
author_rank: Super Contributor
timestamp: February 21, 2021, 06:13:18 am
content: If those are NOS new old stock blue electrolytic capacitors, that's quite a find. 64uF 64V closest would be 68uF 63V standardized value.I would still test them, as that phenolic bung seal did not age well and Philips later went with black rubber, which still did not last compared to others.Going from oddball 6.4uF 25V to 6.8uF 25V to 50V (standardized value) would make no audible change (13Hz vs 13.8Hz rolloff). Axial parts are harder to find, I just bend the leads on radial packaged parts.If you need to do up a schematic I can draw it up if I know the part numbers and edge card connector connections.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2021, 09:38:01 am
content: Electrolytic caps have relatively large tolerances. Depending on the use in the circuit, that actual value is not that critical. One of the smaller one may set low lower frequency limit. One of the larger ones looks like some supply filtering - so larger (like 100 µF instead of 64 µF) would not be so bad.It may even be that aging caps can make the circuit more prone to oscialltion, that may lead to audible effects. Some transistors may be more prone to osciallation than others.It is relatively rare that transistors fail gradually (except for germanium ones that tend to show an increasing leakage and gain over time). One could test the suspect transistor separately - at least for the basis function.

author: drussell
author_rank: Super Contributor
timestamp: February 21, 2021, 02:15:55 pm
content: Quote from: Kleinstein on February 21, 2021, 09:38:01 amIndeed!The OP may not realize that typical general purpose electrolytic capacitor tolerance in that era was often-20/+80%They wouldn't be less than 20% below the specified value (which would fall into the realm of some other, lower value) but they might also be up to 80% higher than marked on the can, and I often saw ones whichwereway above spec when new once I actually got a way to easily measure capacitance. People don't realize how spoiled they really are with today's even most entry-level digital multimeters compared to what we typically had to work with back then.Quote from: floobydust on February 21, 2021, 06:13:18 amYes, things like a specified value of 0.5, 5 or 50µF were common component values in the early days of electronic components before E12 values became the norm. These days you would typically choose either 4.7 or 5.6 in place of a 5µF, and likefloobydustsays, using a 68µF would be fine, especially since the original capacitor was quite likely actually more like 70µF anyway.The E12 standard values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, multiplied or divided by factors of 10 for each decade.QuoteYes, using vintage electrolytics (even if they are NOS) can be problematic some may degrade significantly, even if stored properly. Beware when using old electrolytics, even when they are "new". Don't just assume they're good. You don't want to end up replacing functional capacitors with ones that are even worse.QuoteThe OP may not realize that you can always go up in rated maximum voltage, so it is fine to use a 35V or 50V capacitor in place of a 25V one.Anywhere that an electrolytic is used in the signal path could potentially benefit from changing it to a film capacitor or at least an audio-grade electrolytic. Electrolytics in the signal path are a vile enemy of high fidelity sound.QuoteIndeed. The boards are simple enough to trace out, one would just need to know what the various connections go to in order to avoid trying to deduce everything.With a schematic we could more easily tell the OP where various component values might even matter at all.

author: drussell
author_rank: Super Contributor
timestamp: February 21, 2021, 02:45:49 pm
content: Quote from: Nick1986 on February 20, 2021, 10:04:19 pmSomehow there is now talk of both NTE128 and NTE126, I think there is a typo there as NTE126 is certainly not suitable.There are dozens, hundreds of transistors which will function perfectly fine in there. Picking thebestone is a little trickier until we can deduce what the actual operating parameters are as used in the actual circuit. From just a cursory look at the datasheet and punching various specs into the selector to quickly whittle it down to less than hundreds of possibilities,any of these 20 from digikeywould probably work, although I would want to at least see where it was being used in the circuit before I made any kind of choice. I haven't had a chance to look closely enough at your circuit photos yet to even see where they are used let alone if they're ones with heatsinks, etc.Luckily the OP has a working transistor so the actual operating parameters can be measured in circuit in addition to "looking" at how it is used in the circuit on a schematic.QuoteThere isabsolutelynothingwrong with fixing up older gear, even if itdidn'thave sentimental value.

author: Nick1986
author_rank: Contributor
timestamp: February 21, 2021, 09:32:39 pm
content: Guys thank you so much for all this I’m a little overwhelmed to be honest..Right started to replace few bits I have here and tested all the caps that came they all tested ok. I have tried to balance the cap values between each board. Photo Attached.Replaced that transistor that I snapped the leg of and the tropical fish caps too that I damaged.I also took the 40458 out and gave it a clean in some IPA. The is a little sleeve on one leg and had some gunk in so cleaned that out too. Tested it and it seems to be working ran it for about hour with no crackling. I’m not convinced it’s fixed at all but seems to be working.. any ideas?Iv ordered the other two caps I needed I got 47uf 25v x 2 and 6.8uf 35v x2 again.One thing is my soldering station packed in as I was doing it so ended up using my old soldering iron🙄🙄. Hope it’s just the iron that needs replacing.Il put it on in the background tomorrow when I’m working to give it a good test see if what ever I have done is a fix or just a temporary one.One thing about Covid lockdown I have had so much more time to fiddle with things..Thanks again guys.. do you have any idea why it seems to be working ok now?ThanksNick

author: Nick1986
author_rank: Contributor
timestamp: February 21, 2021, 09:42:29 pm
content: Quote from: floobydust on February 21, 2021, 06:13:18 amReally appreciate this how would I go about seeing what the edge connectors go to just using a continuity test for the wires? Besides the two boards the are 4 chunky transistor and 2 caps the power transformer and the IO apart from that nothing else no interface or anything.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 22, 2021, 05:30:27 am
content: You can still get themhttps://www.solidstateinc.com/product/40458/https://www.1sourcecomponents.com/availability/RCA--40458.htm,https://www.silicon-ark.co.uk/rca-40408-transistorand others.ZTX453 is very close

author: floobydust
author_rank: Super Contributor
timestamp: February 22, 2021, 06:17:11 am
content: In your pics, one 270R resistor is discoloured and burnt a bit. It's in-series with a 64uF cap, so the cap must have shorted. I would check the resistor's value.I drew up most of the schematic as a Sunday crossword puzzle. I can't see all the parts though, if you can read some component labels/numbers like the capacitors or diodes.Just a weird archeological dig on the circuit's background. It's got TO-1 diodes (guessing 1N3794) and the design is mostly based on the RCA app note circuits of the early 1970's that were ground breaking it seems. Many parts are the SOA protection for the output transistors. I believe there is another diode trio of some kind located on the heatsinks.I could not see the output transistors on the heatsinks to see if it is quasi-complementary or what. There is 40636 NPN and either two of them, or a PNP part as Q9.The trimpot appears to set gain and not bias current, which seems unusual.I numbered the edge-card fingers 1,2,3.. left to right with double-wide fingers counting as two pins, all from the solder side of the board. I ran out of time to check for errors and give a picture of the locations for reference designators. You can take comparison readings between the two boards with your multimeter as another way to find problems.edit: added pic showing reference designators

author: Nick1986
author_rank: Contributor
timestamp: February 23, 2021, 09:41:18 pm
content: Quote from: floobydust on February 22, 2021, 06:17:11 amThe are a few others on the board that don’t look to good I’ve attached a photo with those highlighted.Big thank you for drawing up the schematic.Iv been manic in work last few days sorry not getting back quicker.Il get tracing out the Connectors soon. Few resistors that can’t be seen il blow up the photo and update values for the components.Few little silver things on the board I’m not sure what they are not sure if it’s a diode or something Iv put a ? By it.Photo attachedThanksNick

author: floobydust
author_rank: Super Contributor
timestamp: February 24, 2021, 12:41:59 am
content: The two (three?) axial silver foil parts are polystyrene capacitors, which are great for audio. One is marked 180, another +/1 and I can't see the one under a heatsink.The horizontal 2-pin TO-1 parts, the diodes, one looks really pitted and oxidized nearby by the 270R resistor. I wonder if it got hot or is a different p/n from the others.Scribbles of connector pinout I used,flipped viewof pcb bottom- but I can change to whatever makes most sense.I think there should be something "special" on the big heatsinks wired between pin 7 and 16, as a diode trio or something.

author: Nick1986
author_rank: Contributor
timestamp: February 24, 2021, 02:08:05 am
content: Yes three of the silver caps one under the heat sink too, il have to take it apart again, still working kind of after I tested during the day and it crackled a few times nothing as Loud as it did but it’s still not right. At one stage it was doing it constantly and at near on full volume.Sorry got to ask how have you managed to work out what you have without knowing the part number the 2 pin diodes are the correct part number on the schematic well from memory they are!! I googled the part number when I was recapping them, as you said the one by the transistor looks in bad condition. Have you got a background in audio?

author: Jwillis
author_rank: Super Contributor
timestamp: February 24, 2021, 06:49:43 am
content: I found the specs for the RCA 40458 herehttps://transistordata.com/bjt/4045840458 BJT DatasheetType of Transistor: BJTType Designator: 40458Material of Transistor: SiPolarity: NPNMaximum Collector Power Dissipation: 0.5 WMaximum Collector-Base Voltage: 60 VMaximum Collector-Emitter Voltage: 40 VMaximum Emitter-Base Voltage: 8 VMaximum Collector Current: 1 AMaximum Operating Junction Temperature: 175 °CTransition Frequency: 150 MHzForward Current Transfer Ratio (hFE Value): 50Collector Capacitance: 20 pFPackage: TO18Noise Figure:Cross equivalents herehttps://alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=0.5&ucb=60&uce=40&ueb=8&ic=1&tj=175&ft=150&cc=20&hfe=50&caps=&r1=0&r2=0&r1r2=0I may have entered some fields incorrectly so you'll have to double check that.