In 2017 I began a thread about this Rolecor RTA-650 receiver which ended late November that year.
The result then was a working receiver, one for which there were no schematics online. But thanks to patience and knowledge of contributors on this forum like Ian Finch, I accomplished a goal.
But here I am again with a new development. I recently moved from a big city and the working Rolecor was packed up, put in storage for 3 or so months then bought with me. Soon after arriving, I hooked it up for listening to FM while in the dining/kitchen area.
Two or three seconds after finding a station (signal being amplified) there was a "snap" and the receiver lost power. The main AC fuse blew.
Replacing the fuse allowed the receiver to power up but very shortly, smoke was drifting out. It was a resistor burning which I replaced. Watching the board closely when switching it back on, a different resistor began smoking which I also replaced. During that process, a third resistor looked discoloured from heat so it was replaced also.
To keep this brief, one of the 3 new resistors, a 330 ohm I believe, began smoking within a very few seconds of power on so I began looking for why.
There are transistors I installed in 2017 - KSA1220 and KSC2690 to replace 2SA497 & 2SC497. One of those KSA1220 & KSC2690 pairs has 53 volts at E-B-C on both. The pair on the other channel is much different and is I believe correct.
Regarding the replacement transistors, I did manipulate the legs to achive the right pinout.
My question now is was the malfunction first caused by the transistors failing, affecting the resistors or was transistor failure caused by one or all of the overheated resistors?
I can replace the resistors with some I have on hand (cheap) and I can also replace KSA1220 - KSC2690 pairs (more expensive) as I have a few of those leftover, purchased as extras.
Any speculation on whether replacing failed transistors and heat damaged resistors is the way to go or should I first be looking elswhere?
I realize that without a schematic, my description probably sounds like gobbledy-gook.
The result then was a working receiver, one for which there were no schematics online. But thanks to patience and knowledge of contributors on this forum like Ian Finch, I accomplished a goal.
But here I am again with a new development. I recently moved from a big city and the working Rolecor was packed up, put in storage for 3 or so months then bought with me. Soon after arriving, I hooked it up for listening to FM while in the dining/kitchen area.
Two or three seconds after finding a station (signal being amplified) there was a "snap" and the receiver lost power. The main AC fuse blew.
Replacing the fuse allowed the receiver to power up but very shortly, smoke was drifting out. It was a resistor burning which I replaced. Watching the board closely when switching it back on, a different resistor began smoking which I also replaced. During that process, a third resistor looked discoloured from heat so it was replaced also.
To keep this brief, one of the 3 new resistors, a 330 ohm I believe, began smoking within a very few seconds of power on so I began looking for why.
There are transistors I installed in 2017 - KSA1220 and KSC2690 to replace 2SA497 & 2SC497. One of those KSA1220 & KSC2690 pairs has 53 volts at E-B-C on both. The pair on the other channel is much different and is I believe correct.
Regarding the replacement transistors, I did manipulate the legs to achive the right pinout.
My question now is was the malfunction first caused by the transistors failing, affecting the resistors or was transistor failure caused by one or all of the overheated resistors?
I can replace the resistors with some I have on hand (cheap) and I can also replace KSA1220 - KSC2690 pairs (more expensive) as I have a few of those leftover, purchased as extras.
Any speculation on whether replacing failed transistors and heat damaged resistors is the way to go or should I first be looking elswhere?
I realize that without a schematic, my description probably sounds like gobbledy-gook.
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This product is apparently older than Rotel, the current brand name of the manufacturer of Rolecor products. No manual is available but it's got to be at least 50 years old and anything fragile or perishable inside has to be a candidate for failure so isolating the actual or original source of the problem may not be helpful, since it will just be one more casualty. I know you replaced some electrolytics previously but was that all of them?
Since you replaced the drivers too, it should be said that those replacements are a lot faster and higher gain than the originals. I suspect the channel that is now burned was oscillating, drawing a lot more than usual current and that caused the resistors to burn, but not before again taking out one or both of those driver transistors and perhaps an output transistor or 2.
IMO, this needs experienced hands and patience - not difficult for a retired service tech, if one is in the area and willing to take a look at it for you. DIY might not be the way to go when we can't help you as there is no manual, schematic or even a spec. sheet to guide us.
I see there is a manual for Rolecor RTA 630 which may or may not be similar but the copy is at site which is just a wall of do's and don'ts about how to buy a copy without annoying the seller who seems difficult to deal with.
Since you replaced the drivers too, it should be said that those replacements are a lot faster and higher gain than the originals. I suspect the channel that is now burned was oscillating, drawing a lot more than usual current and that caused the resistors to burn, but not before again taking out one or both of those driver transistors and perhaps an output transistor or 2.
IMO, this needs experienced hands and patience - not difficult for a retired service tech, if one is in the area and willing to take a look at it for you. DIY might not be the way to go when we can't help you as there is no manual, schematic or even a spec. sheet to guide us.
I see there is a manual for Rolecor RTA 630 which may or may not be similar but the copy is at site which is just a wall of do's and don'ts about how to buy a copy without annoying the seller who seems difficult to deal with.
I would agree with Ian Finch particularly the 2SA1220 which in my equivalents book is recommended for RF power use (20W).
These are not direct replacements for the original BJT,s , different biasing for a start , while it obviously worked it was inherently unstable and something was bound to happen , you cant just think--"now if I fit heavier duty /gain versions it will be more stable " .
Amplifier circuits are made to design specifications if you depart from them by changing active devices that are not direct equivalents then your compensation capacitor (s) will need a change of value plus a need to change the biasing values of the new active devices.
I would look for BJT,s that are nearer the original ones.
These are not direct replacements for the original BJT,s , different biasing for a start , while it obviously worked it was inherently unstable and something was bound to happen , you cant just think--"now if I fit heavier duty /gain versions it will be more stable " .
Amplifier circuits are made to design specifications if you depart from them by changing active devices that are not direct equivalents then your compensation capacitor (s) will need a change of value plus a need to change the biasing values of the new active devices.
I would look for BJT,s that are nearer the original ones.
No, I did not replace all the electrolytics within the receiver. But I did get replacements for all of them. Those I installed where the smallish ones on the amplifier board but those I left alone were larger and on the underside of the chassis. I believe them to be associated with the power supply.
See attached photos.
I thought they might be capable in their as found state to determine functionality of the receiver. My hesitance to putting in new ones was to do with securing them. Except for one, the new ones I bought are radial not axial and physically a lot smaller that what are there.
The thought of damaged output transistors did cross my mind. What the receiver contained when I got it were mixed brands of 2N3055's. I replaced with fresh 2N3055's in 2017 so if need be, I think I can get more.
That would be the extent of what I am willing to spend on trying to fix this thing. I now live far from an urban area and while there is one shop that does repairs, I would anticipate a hefty bill to troubleshoot a piece of gear I am not dependant on.
Thanks for the input. The reason for using the transistors I did was from reading/following a thread in another forum. It was begun by someone wanting to replace the exact same transitors I was dealing with but in a different make/model of receiver.
One of the frequent contributors responded with the recommendation of KSA1220 and KSC2690 in place of 2SA497 & 2SC497. That contributor is held with some regard on that forum so I likewise followed his recommendation.
2SA497 2SC497 replacement | Audiokarma Home Audio Stereo Discussion Forums
Not to sound stubborn, but I am inclined to put new KSA1220 and KSC2690 in. The reason being, they failed on one channel only not both. If they are simply unsuitable to replace 2SA497 & 2SC497, wouldn't those in both channels suffer the same fate?
I am not very adept at interpreting transistor specs plus any I settle on all seem to be obsolete or out of production. I am open to suggestions for currently available transistors that would be better suited.
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I do not like to criticize somebody else,s recommendations particularly if they are "well thought of " on another audio website but I (like you ) am a bit stubborn in my belief on this point and I wont be changing my opinion.
While the 2SA1220 is (much ) heavier duty ) and can be used in audio it is also recommended for RF use by the manufacturers signifying its use in radio circuits .
It has the potential of causing insipid HF oscillation in a purely audio circuit and I have seen this happen before but it depends on the circuit itself .
Have a look at the BC640 while not an exact equivalent its nearer than the 2SA1220 and has a manufacturers recommended matching npn of
BC639 -
datasheet BC640
While the 2SA1220 is (much ) heavier duty ) and can be used in audio it is also recommended for RF use by the manufacturers signifying its use in radio circuits .
It has the potential of causing insipid HF oscillation in a purely audio circuit and I have seen this happen before but it depends on the circuit itself .
Have a look at the BC640 while not an exact equivalent its nearer than the 2SA1220 and has a manufacturers recommended matching npn of
BC639 -
datasheet BC640
Agreed, or you can fit BD139/140 which are the same silicon in the larger package.
We can't assume that the conditions for oscillation in one channel are going remain identical in the other forever. For one thing, components like capacitors don't deteriorate at the same rates and they are a prime factor of oscillation frequency and strength or Q.
Old caps will not only have low capacitance but current leakage and low Q so you don't often notice the likely weak oscillation and the amplifier limps along with low volume, bad hum, noise and sound quality. I've read a thread of yours about this amplifier at AK forum and whilst I also respect the views of the professional repairers there (they have vastly more experience, training and successful outcomes than I) there are are others replying who have only amateur experience like myself and other occasional tinkers. That means we can only speak for the number of similar repairs jobs that we completed successfully. I've seen and caused enough of my own disasters by using suggested modern power transistors in old circuits and learned to look on such advice with suspicion, particularly when you compare the circuits that the different types are found in . Sometimes the new types aren't so different to the specified ones and there is a bit of latitude in the design but 40-50 years on? not likely.
I am quite certain of the eventual disasters that follow wrong replacement semis though - it works but the job soon bounces and you have to fix it again at your own cost and pronto! For your own use, I guess it doesn't matter if it sits in stack of problems to solve or winds up in E-waste but if you're going to invest more time and money in the receiver, make the effort of finding more appropriate semis.
I've repaired a few later Rotel amps with those TO3 output transistors (they are a quasi complementary design, so all same types). They are a lot better than 2N3055 as audio transistors and though near impossible to replace and you may have further problems when you do, whatever you substitute there. Let's hope they don't need replacement and test their functionality before doing anything else.
We can't assume that the conditions for oscillation in one channel are going remain identical in the other forever. For one thing, components like capacitors don't deteriorate at the same rates and they are a prime factor of oscillation frequency and strength or Q.
Old caps will not only have low capacitance but current leakage and low Q so you don't often notice the likely weak oscillation and the amplifier limps along with low volume, bad hum, noise and sound quality. I've read a thread of yours about this amplifier at AK forum and whilst I also respect the views of the professional repairers there (they have vastly more experience, training and successful outcomes than I) there are are others replying who have only amateur experience like myself and other occasional tinkers. That means we can only speak for the number of similar repairs jobs that we completed successfully. I've seen and caused enough of my own disasters by using suggested modern power transistors in old circuits and learned to look on such advice with suspicion, particularly when you compare the circuits that the different types are found in . Sometimes the new types aren't so different to the specified ones and there is a bit of latitude in the design but 40-50 years on? not likely.
I am quite certain of the eventual disasters that follow wrong replacement semis though - it works but the job soon bounces and you have to fix it again at your own cost and pronto! For your own use, I guess it doesn't matter if it sits in stack of problems to solve or winds up in E-waste but if you're going to invest more time and money in the receiver, make the effort of finding more appropriate semis.
I've repaired a few later Rotel amps with those TO3 output transistors (they are a quasi complementary design, so all same types). They are a lot better than 2N3055 as audio transistors and though near impossible to replace and you may have further problems when you do, whatever you substitute there. Let's hope they don't need replacement and test their functionality before doing anything else.
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When this endeavor began and almost right away, I read about someone else's dealings with the same transistors and then what seemed a credible recommendation for replacements, I thought what a good stroke of luck!!
Too good I guess.
I have not made a lot of online parts purchases and I'm in Canada so only know Digikey, Mouser and Newark.
Mouser have kinked leg BC640TA on hand in good quantity but not the complementary BC639. Says obsolete.
Digikey and Mouser have neither.
Does another such pair come to mind? Something from some company that does business in Canada?
EDIT: And FWIW, this afternoon I replaced all the grey Elna capacitors on the underside. Still have the big 63V 2200mF to go. Might install first and figure out how to secure later.
2nd EDIT: Disreagard the question re alternative transistors in Canada. If I can use BD139/BD140, Mouser.ca have those on hand.
I shall now see if I can get hold of more of those 2N3055's. If both in the affected channhes are done, I think I only have one spare.
Too good I guess.
I have not made a lot of online parts purchases and I'm in Canada so only know Digikey, Mouser and Newark.
Mouser have kinked leg BC640TA on hand in good quantity but not the complementary BC639. Says obsolete.
Digikey and Mouser have neither.
Does another such pair come to mind? Something from some company that does business in Canada?
EDIT: And FWIW, this afternoon I replaced all the grey Elna capacitors on the underside. Still have the big 63V 2200mF to go. Might install first and figure out how to secure later.
2nd EDIT: Disreagard the question re alternative transistors in Canada. If I can use BD139/BD140, Mouser.ca have those on hand.
I shall now see if I can get hold of more of those 2N3055's. If both in the affected channhes are done, I think I only have one spare.
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Further to the above,
Mouser indeed has some versions of 2N3055's on hand. At a glance, the difference I can see is gain bandwidth. A straightforward 2N3055 (obsolete) can be 2.5MHz to 4MHz depending on maker. An in stock 3055AG is 6MHz and other variants are 2.5MHz.
Would the 2N3055AG be compatible or would one of the 2.5MHz versions be better suited?
If I am to be placing a Mouser order for more than I wish to, it would be nice to be at least a bit confident of getting the right parts.
Mouser indeed has some versions of 2N3055's on hand. At a glance, the difference I can see is gain bandwidth. A straightforward 2N3055 (obsolete) can be 2.5MHz to 4MHz depending on maker. An in stock 3055AG is 6MHz and other variants are 2.5MHz.
Would the 2N3055AG be compatible or would one of the 2.5MHz versions be better suited?
If I am to be placing a Mouser order for more than I wish to, it would be nice to be at least a bit confident of getting the right parts.
Just a note about Rotel as a reference to the likely design of the Rolecor receiver's power amp. I think this RA610 service manual will help you and others understand the design and possibly gain some better advice. Note the same driver transistors are used and possibly the same original TO3 output transistors too. Hold off on the 2N3055 transistor substitutes - these only look like the originals which are possibly the same as in this Rotel amplifier (i.e. 2SC1030 which has an Ft of 10 MHz - quite a bit more useful gain than typical 2N3055. 2SC1030 Datasheet ETC pdf data sheet FREE from www.radioradar.net
RA610 Manual is free after sign-up here: Rotel RA-610 Stereo Integrated Amplifier Manual | HiFi Engine
RA610 Manual is free after sign-up here: Rotel RA-610 Stereo Integrated Amplifier Manual | HiFi Engine
Thanks for the links. I actually looked at that Rotel manual yesterday. I was searching for the circuit board part number that's in this Rolecor.
It may not mean anything, but this Rolecor had 3055 ouput transistors in place when I got it. Three different makers but they were all 3055's. I assumed one or two were original because they looked like they had some age to them.
I looked up 2SC1030 but they look to be long gone. I suppose it's possible those were what was in this Rolecor at one point but over time, got subbed with 3055's?.
So you feel that the 6MHz 3055 would be insufficient?
There is no doubt I would prefer an easy solution, but might there not be at least a reasonable chance 3055's were installed at the factory? At least those are available.
I can get those BD139/140 and 3055AG from one supplier, in one fell swoop which is very attractive.
No, there is more to the difference between C1030 and 2N3055 than the claimed Ft. There is also the capacitance the driver has to cope with and that impacts on the overall performance too. Also consider that Japanese manufacturers have been way ahead of US consumer audio manufacturers from the beginning and would never fit 115W US industrial kludge type silicon in their 15 - 30W audio amplifiers, even in the 1960s.
These were very popular low power audio transistors. Sansui used them in their still popular 555 and several other models also from that period. So did Lux/Luxman and other big Audio names of the time. There are still several generic product sources for C1030 but you won't find them in Major US distributor listings unless they are certified/qualified products.
One qualified source would likely be Inchange Semiconductor in China but there are more generic part manufacturers and probably more again that I'm unaware of too. In contrast, NTE Electronics list their type NTE280 as a replacement but the brief spec. looks like its not even half-way there.
I'll keep looking but I'm sure there are others on the forum who have faced this longstanding replacement problem too. Some will be happy to even get some sound from the amplifier but others probably want it to sound as good as new, which should be quite possible.
If you have 2N3055 inside though, it's 99% certain that someone has attempted to fix the amplifier section before you and caused the problem you now have to unravel and correct. This is why you don't replace semis without carefully considering the specification of the original part. In this case, the silicon is likely much smaller than in the 2N3055 since power handling is much less and frequency limits higher. This is fine for a small amp as Ft can more easily be made higher, thus HF distortion reduced and the sound quality improved over a similar technology, big chip transistor.
These were very popular low power audio transistors. Sansui used them in their still popular 555 and several other models also from that period. So did Lux/Luxman and other big Audio names of the time. There are still several generic product sources for C1030 but you won't find them in Major US distributor listings unless they are certified/qualified products.
One qualified source would likely be Inchange Semiconductor in China but there are more generic part manufacturers and probably more again that I'm unaware of too. In contrast, NTE Electronics list their type NTE280 as a replacement but the brief spec. looks like its not even half-way there.
I'll keep looking but I'm sure there are others on the forum who have faced this longstanding replacement problem too. Some will be happy to even get some sound from the amplifier but others probably want it to sound as good as new, which should be quite possible.
If you have 2N3055 inside though, it's 99% certain that someone has attempted to fix the amplifier section before you and caused the problem you now have to unravel and correct. This is why you don't replace semis without carefully considering the specification of the original part. In this case, the silicon is likely much smaller than in the 2N3055 since power handling is much less and frequency limits higher. This is fine for a small amp as Ft can more easily be made higher, thus HF distortion reduced and the sound quality improved over a similar technology, big chip transistor.
Thanks for offering assistance for finding a replacement.
After reading your last post, I did some searching for 2SC1030 and found this company:
Home - Hamilton Americas
If I read correctly, they are in Ontario, Canada supposedly with (200) 2SC1030 transistors in stock.
No pricing displayed, only a quote request option. Out of curiosity, I asked the cost for six.
Also saw this page:
2SC1030 npn transistor complementary pnp, replacement, pinout, pin configuration, substitute, marking C1030, equivalent, datasheet
It has 2SC1030 specs and near the bottom are, according to them, substitutes. None of which are available from Mouser, Digikey or Newark.
After reading your last post, I did some searching for 2SC1030 and found this company:
Home - Hamilton Americas
If I read correctly, they are in Ontario, Canada supposedly with (200) 2SC1030 transistors in stock.
No pricing displayed, only a quote request option. Out of curiosity, I asked the cost for six.
Also saw this page:
2SC1030 npn transistor complementary pnp, replacement, pinout, pin configuration, substitute, marking C1030, equivalent, datasheet
It has 2SC1030 specs and near the bottom are, according to them, substitutes. None of which are available from Mouser, Digikey or Newark.
I noticed that in a comment from Echo Wars, actually. I don't doubt his confidence that they will work with the larger drivers but I hesitate to suggest it for use with the original drivers, considering the large MJ21194 die size. If you do fit BD139/140 drivers, be sure to measure the Hfe of them with one of those component tester kit affairs, so there is record of what did or didn't work correctly in any event. It may save some grief to know the facts rather than just criticize a failed plan.
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Currently I do not own a trasistor checker. Not saying I would not like one but I've been leery of any that I could justify - read afford.
Wouldn't mind a capacitor checker too but that's another matter.
I know Hfe is an often cited spec.
Concerning BD139's/140's, is it a matter of equality or near equality? Would it be too much to expect, getting transistors packaged on a strip, that they be nearly equal in Hfe?
Taking my chances
Just submitted an order to Digi-Key.ca. They were a few cents cheper than Mouser.
(4) MJ21194G
(10) BD139
(10) BD140
The last two come in tube packs so unsure if Hfe specs will be near to being close.
I would like to have obtained (8) MJ21194G's just in case but at $10 each, I had to exercise discretion. What they call discretionary income is really thin since my working days ended.
My sincerest thanks again for the input I receieved and my sincerest hope is that nothing more incinerates.
Just submitted an order to Digi-Key.ca. They were a few cents cheper than Mouser.
(4) MJ21194G
(10) BD139
(10) BD140
The last two come in tube packs so unsure if Hfe specs will be near to being close.
I would like to have obtained (8) MJ21194G's just in case but at $10 each, I had to exercise discretion. What they call discretionary income is really thin since my working days ended.
My sincerest thanks again for the input I receieved and my sincerest hope is that nothing more incinerates.
Until a few years ago, small through-hole style TO92 transistors were mostly on paper strip rolls but as they are now legacy product, they're mostly bulk packed. Today, its mostly SMD or surface mount devices that is on plastic rolls or cut strips. The larger T0126, T0247, TO3P, TO264 etc styles are still packed in plastic tubes, likely close to their manufacturing sequence with pretty close matched properties but you may have to buy exact tube quantities to ensure you'll receive them packed that way. It still doesn't 100% guarantee that your shipment is packed in the same sequence of a production run but odds are, they will be close enough. With TO3 style, nothing has changed. It's trays, boxes or loose packed small quantities.
If you're going to tinker with electronics, you must have a component tester of some description and if you only test stuff occasionally, it has to be simple, one button operation, like this cute little multi-function tester - almost identical to my own go-to small parts tester: Electronic Component tester It's about US$22 and is rechargeable via a micro-USB socket like a phone charger, so you don't need a constant backup supply of 9V batteries.
It's fun to use; just plug the 3 transistor leads into any 1,2 and unmarked location in the ZIF socket, clamp it up, hit the go button and bingo! - numbers with descriptions and a simple functional diagram. It tests common small semis and components but no instructions. (you have to experiment a little but its quite obvious once you get the fact that there are a limited number of options for meaningful, likely figures and you can plug in known good parts to check that you have plugged it in properly and the screen says what it should.
There are expensive and better quality versions and heaps of cheaper versions too but the "cheap" bit only lasts until you pay for the first 9V battery or other stable power source.
Power transistors are not it's forte since matching them requires high current and you've probably read that the pros use big, ancient and very expensive semiconductor curve tracers or wing it with about 0.5A collector current and a comparative measurement of the base current required to sustain it. I don't do much more than that because it isn't worth the effort and doesn't do much for small, single transistor pair amplifiers.
If you're going to tinker with electronics, you must have a component tester of some description and if you only test stuff occasionally, it has to be simple, one button operation, like this cute little multi-function tester - almost identical to my own go-to small parts tester: Electronic Component tester It's about US$22 and is rechargeable via a micro-USB socket like a phone charger, so you don't need a constant backup supply of 9V batteries.
It's fun to use; just plug the 3 transistor leads into any 1,2 and unmarked location in the ZIF socket, clamp it up, hit the go button and bingo! - numbers with descriptions and a simple functional diagram. It tests common small semis and components but no instructions. (you have to experiment a little but its quite obvious once you get the fact that there are a limited number of options for meaningful, likely figures and you can plug in known good parts to check that you have plugged it in properly and the screen says what it should.
There are expensive and better quality versions and heaps of cheaper versions too but the "cheap" bit only lasts until you pay for the first 9V battery or other stable power source.
Power transistors are not it's forte since matching them requires high current and you've probably read that the pros use big, ancient and very expensive semiconductor curve tracers or wing it with about 0.5A collector current and a comparative measurement of the base current required to sustain it. I don't do much more than that because it isn't worth the effort and doesn't do much for small, single transistor pair amplifiers.
OK, you talked me into it. I just ordered the same tester on Amazon.ca. $31.99 CAD and free shipping. ETA early/mid October it says.
It will certainly be less tedious than the multi-step transistor checking process I have been doing with a DMM.
Multi-Function Tester, 3.5 inch Colorful Display Pocketable Multifunctional TFT Backlight Transistor LCR-TC1 Tester for Diode Triode Capacitor Resistor Transistor LCR ESR NPN PNP MOSFET LW21: Amazon.ca: Tools & Home Improvement
And it measures Hfe?
Too bad there isn't such a low priced device for measuring capacitors. The DMM I have does it somewhat but only small ones. It's unresponsive with electrolytics beyond a certain size.
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Yes, Hfe is the first thing you see on the OLED screen. Also, like it says - it's a component tester (as in resistor, capacitor, inductor). Look at the details posted on the site I linked. The capabilities and operation details are listed there when you scroll down the pages. I doubt it will cover the full range of possible capacitance or inductance but that's no different to other budget test gear.
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