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Preamp (9VDC): 6 capacitors (4 MMKO box film caps and 2 CAPAR axial 4.7uF 50v electrolytic caps), 1 3-pin junction transistor (M2N5458), 2 unknown transformers, 5 resistors. See attached image.

Problem: The volume when plugged in has reduced over the years to where I can barely hear it when plugged in at full volume. I can't find any OEM preamp replacement (only cheap solid-state knockoffs) so, a repair is in order.

Proposed Solution: The Ovation rep said it most likely needs a cap job because they degrade over time. An electrical engineer friend said doubtful the film caps are bad and to just replace the output jack (done); 2 electrolytic caps and the 1 transistor; "no need to check them to see if they are good because they need replacing anyway due to their old age. Then, call me if that doesn't work."

Parts:

A. Capacitors: I found two Nichicon capacitors (TVX1H4R7MAA) so, it looks that replacement is resolved unless anyone here spots a problem with my choice.

B. Transistor: The only company that still makes the 2N5458 is Central Semiconductors. I've never heard of them but I'm told they're out of India and make decent stuff which I assume is true because Mouser and some other big dealers carry that brand. However, I still like buying made in USA, Japan, or German whenever I can so, buying the Central makes me a little nervous. The main cross is 5457. I found a Vishay-Siliconix 2N5457 which I know to be a "process 55" transistor like the 5458. And, I also found a Motorola 2N5640 which is supposedly is a cross according to the link below: https://www.componentsinfo.com/2n5458-transistor-pinout/

But I can't confirm if it's a "process 55" transistor, which I'm told is important.

Question:

Would you buy the exact match 5458 made by Central or the cross 5457 made by Vishay-Siliconix or the Motorola cross 2N5640 (assuming the link above is correct) even though I'm unable to confirm if the 2N5640 is a process 55 transistor like the 5457 and 5458?

Note: I never buy parts from eBay for obvious reasons, I only buy from reputable dealers.

And, I realize, that most would just buy any of these parts and not worry about it, but this Old Ovation is my pride and joy; I'm trying to keep the sound as authentic as possible.

Thanks in advance for any help, suggestions, or advice you can provide. photo of components and wiring

tayglo
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    I still have my Ovation from 40 years ago, which was my first acoustic guitar. Good luck with the repair. Central Semi is a reputable company, I wouldn't have an issue buying from them, but there are probably lots of BJTs you could use for the repair, maybe with some bias tweaks. Hard to say which would be closest sonically. – John D Jul 22 '22 at 21:17
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    That's a JFET not a bipolar NPN. The Nat Semi Discrete Databook confirms that all 2N5457,5458 and 5459 are Process 55. I'd be reasonably sure the 5457 would be fine. –  Jul 22 '22 at 21:23
  • @user_1818839 Good catch on the "bipolar NPN". That was an inadvertent slipup on my part. Thanks! – tayglo Jul 22 '22 at 21:25
  • @John D Thanks! Ovations take a lot of undeserved bashing. While it may be true for some of the newer models nothing can rival the playability and durability of a made in the USA Ovation; like playing an electric guitar. And, the sound from those old stacked knob preamps is hard to beat which is why I'm taking my time and wanting to get this repair just right. – tayglo Jul 22 '22 at 21:35
  • @user_1818839 Also, thank you for the Vishay Siliconix 2N5457 vote. I've heard that Siliconix is right up there with the best and I'm leaning that way. – tayglo Jul 22 '22 at 21:49
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    The transistor is probably fine; you don't usually see JFETs degrade like that. It's most likely just the capacitors. If you do need to replace it, Central Semiconductor is a perfectly reputable brand--I've used plenty of parts from them and haven't had a single problem. For replacing the capacitors, I'd always recommend Rubycon, Nichicon, or United Chemi-con; they all make pretty reliable electrolytics. Würth Elektronik is also pretty good. – Hearth Jul 22 '22 at 21:50
  • The 2N5457,5458 and 5459 all have different VGS and IDSS, can you substitute one for the other without changing the circuit? I bought some of each about 5 years ago before they went obsolete and tried them all in a simple common source circuit. I'm pretty sure I had to change the resistor values to get each to work. Also Heath is probably correct about it just being the capacitors, I'd try those first. – GodJihyo Jul 22 '22 at 21:52
  • I bet 10 bucks on one or more capacitors loosing capacitance and leaking. – winny Jul 22 '22 at 21:55
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    Have you actually measured the input/output response of the preamp, or observed the waveform on a scope? You should really have the equipment to do so if you want to perform the best repair. And if you have had this device for 44 years, it's quite possible that your hearing has changed. Mine certainly has, starting in my 40s. – PStechPaul Jul 22 '22 at 22:11
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    Whilst I agree that the electrolytics are the prime suspects and should be replaced. check all the other components. resistors can drift with time and the film capacitors might have degraded as well. At least you can replace any of them without affecting the sound of the amp. – Peter Jennings Jul 22 '22 at 23:04
  • caps https://www.tedss.com/Catalog/Browse?searchString=CAPAR&inCategory=ALUMINUM%20ELECTROLYTIC%20%3E%20RADIAL just incase ... FET https://www.tedss.com/2028019691 (ESD sensitive!!) .. schema ,,,,https://www.freestompboxes.org/viewtopic.php?t=7243 – Tony Stewart EE75 Jul 23 '22 at 00:23
  • @winny You're on. I bet on the FET – Tony Stewart EE75 Jul 23 '22 at 00:44
  • Wow, you guys are blowing my mind. I'm just a mechanical engineer who is feeling pretty dumb right now after reading @Tony Stewart EE75 post. I am working all weekend so I won't be able to respond in-depth until Mon. Tony, you should know those two CAPAR e-caps are axial. Talk soon, and THANKS! – tayglo Jul 23 '22 at 18:56

2 Answers2

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My 1st guess was the e-caps but after I saw the FET without ESD protection, if it was plugged into a floating cable, the charge could have progressively damaged the front end softly rather than a big arc.

Any e-caps will do but by luck, I found the obsolete brand of the same name (CAPAR).

Some of these old JFETS run around $22 but I found some pretty close much less. Get a couple , just in case and protect from static and consider adding a Zener or faster a TVS to the input for ESD protection.

ESD awareness didn't become mainstream in design at that time but soon after (circa 80's) unless you had bad experience from CMOS or TTL in the 70's.

https://www.freestompboxes.org/viewtopic.php?t=7243 enter image description here

caps https://www.tedss.com/2028019691

just in case get 2 ... FET https://www.tedss.com/2028019691 (ESD sensitive!!)

Tony Stewart EE75
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  • E-caps: @Tony Stewart EE75 respectfully, the e-caps you posted won't work because they are vertical caps with two leads coming out the bottom in contrast to the original e-caps which are horizontal with leads coming out each end. I've searched high and low and can't find another e-cap with just about the same measurements (original 6x13mm vs the Nichicon replacement 5x12mm) Unless someone here has a better suggestion, my EE friend advises me to stick with these and the votes here concur so decision made on the e-caps: https://www.tedss.com/TVX1H4R7MAA/ Thx anyway for your suggestion – tayglo Jul 25 '22 at 21:59
  • Transistor: @Tony Stewart EE75 respectfully, my EE friend says to stick with the 2N5457 because it is a "process 55" transistor and the one you recommended is a "process 51 transistor." He said always try and keep substitutes with the same process number and, yes, it matters. He said unless I can find the original Moto 2458 that it appears there is no better substitute currently available than the Vishay-Siliconix 2N5457. He went on to say that I am VERY lucky to have found two top brands with such close specs and to buy them before they're gone. https://www.tedss.com/2028002809 – tayglo Jul 25 '22 at 22:08
  • Here is the Nat Semi Discrete Databook that @user_1818839 was referring to. You can find the information on the 2n4856 on pages 432 (8-2) and 450 (9-6). And, the info on the 2n5458 and 2n5457 on pages 437 (8-7) and 455 (9-11). – tayglo Jul 25 '22 at 22:12
  • Now I feel really dumb. I have the book here on my hard drive that my EE friend sent to me and can easily upload it here if I can figure out how to do it. Is it allowed? It's 20.8MB. If so, please advise as I think it will come in handy for all those that seem to overlook the importance of using the same process number when searching for substitutes. – tayglo Jul 25 '22 at 22:16
  • And, one final thought. The reason I am not testing the parts is because I am in LA working and all my tools are in Dallas. Also, because my EE friend said to test a 44-year-old pre is wasted time because you have no doubt that all three parts need to be replaced regardless. So it's pointless to debate whether or not it's the e-caps or JFET and to instead work on process of elimination instead. He concurs with @user_1818839 I want to thank everyone here for your time and advice. This has been an enlightening experience. I'll let ya know when it's working again. Fingers crossed! – tayglo Jul 25 '22 at 22:26
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    I just showed an example where to fish not do the search and find best match – Tony Stewart EE75 Jul 25 '22 at 23:50
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    Process 51 is better for low noise preamps https://www.onsemi.com/pub/collateral/an-6609.pdf while 55 is general purpose. You can always get film axial caps which are even better and last longer. The FETs I found are about 10% disti cost and low noise – Tony Stewart EE75 Jul 25 '22 at 23:57
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    I see. So his goal was to keep it as close to the OEM as possible while your goal was to improve it since I'm having to change out the parts anyway which puts a whole new spin on it. I think it's safe to say, that we didn't think anyone would care enough to go through the trouble to figure this out for a total stranger like you have. Thank you Tony! I'm glad I haven't ordered my parts yet. Something to research and ponder. Hmmm? TBC – tayglo Jul 26 '22 at 00:34
  • @Tony_Stewart EE75 My friend says if I was a tech that I could use replacement parts that may be better than the OEM parts (technically, a redesign), as you suggest, because I could test everything out to make sure it's working properly. He said, but I'm not a tech (I'm an ME not an EE) and I don't have the time or knowledge to do that. He said anytime I replace a part with different specs than the OEM part, I stand a chance of another problem emerging. I also wrote ON/Fairchild and they said even though the 2N5457 is now obsolete, that would be my best bet if I could find one. Thanks – tayglo Jul 27 '22 at 00:00
  • I'm think any low noise JFET will work just fine in that circuit. – danmcb Feb 24 '23 at 07:52
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My apologies. I am just getting around to posting the final solution.

Congrats to those who said that it was just the capacitors and not the transistor. I just received my preamp back (after a four month wait) from a person who was referred to me by Ovation. He said all he did was replace the two caps and the transistor checked out A-okay. Now it sounds as good as new once again.

Thanks to all of you for your input!!!

tayglo
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