Is it worth "Frankensteining" an 80's Panasonic stereo? - Page 1
Forum section: electronics / beginners

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 08, 2017, 01:23:45 am
content: Greetings,I found an 80's Panasonic stereo on the curb, in vary bad shape. It is a combo, radio, turntable, 8 track. The turntable was ripped out and the unit was dead. When I opened it up I discovered that the board for the 8 track was very damaged, so I removed that board and the 8 track deck and decided to troubleshoot the radio/amp that remained.I suspected power failure, so I desoldered the 4 rectifier diodes (1R6J) and discovered one was so overheated that it crumbled apart, and another one was also dead. I tested the other 2 in a component tester and those were the readings.Uf=658mVC=103pFIr=2nAUf=692mVC=50pFIr=12nAI desoldered the big cap and it tested to specs...2200uFESR 0.03Ohms...so I put it back inside.I discovered the board had been bent and lost continuity on 3 traces, so I did a quick solder job (see photo)Then I soldered in 4 diodes that I had on hand (1N5401) and decided to plug it into my variac to see what happens. It was promising, got radio static from the speakers, but after a couple of seconds at full voltage the fuse in the variac blew. So, I did some more testing.I desoldered the input from the transformer and measured 28VAC. There are 3 more wires coming from the transformer, measuring around 12VAC.I discovered that the IC STK435 reads continuity between pin 11 and ground. The only somewhat clear datasheet I can find is the one herehttps://www.digchip.com/datasheets/parts/datasheet/922/STK-435-pdf.php.I'm not sure if there should be continuity between pin 11 and ground, because the datasheet is not clear. It almost looks like there should be continuity between pin 5 and ground, which is the mirror image of where pin 11 would be on the other side, but I'm not sure what the datasheet shows. So, to free up pin 11, I desoldered the 2 caps and 1 resistor, which are the only 2 components connected to it, and still measured continuity between pin 11 and ground.Do you think there's a good change only the IC is bad?Is this thing worth anyone's time fixing?My only goal is, if it's worth the time, to kind of "Frankensten" together a unit with a working radio, CD, turntable and AUX input. It would be more of a conversation piece unit and not at all a restoration attempt, as the unit is simply too beat up.Let me know your thought.Thanks...PS - I do not know how to make my attached images appear inside my text. How can I do that?

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2017, 01:44:50 am
content: You can test the STK435 for shorts, usually the output transistors fail. I've changed 100's of them working in a repair shop. They do not like shorted loudspeakers which causes them to die. From pin 11-7, 11-12 or 5-7, 5-4 should not be shorted or low ohms. I'm not sure how the substrate pin 8 would get to pin 11 unless the transistor melted.You can change the 8-track input to instead RCA jacks for CD/Aux use.I found the STK435 kinda meh for power and fidelity. Popular today is the LM3886 chip amp which might be worth stuffing inside.If this stereo has a dual (verses single) rail power supply, then it's worth it. A single supply is not great.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 08, 2017, 03:05:02 am
content: Quote from: wilfred on December 08, 2017, 01:38:47 amHa, ha... That's funny.Well, my project is more like building a cigar box guitar out of found materials. I'm not looking for fine sound. Just giving it a second life if it doesn't take too much time and having it around as a conversation piece.Quote from: floobydust on December 08, 2017, 01:44:50 amI just found this picture......which explains a bit more.It looks for sure that eh chip is faulty as there shouldn't be a continuity between pin 11 and ground.I don't want to spend too much time trying to figure out how to retrofit the unit with a better chip. But it seems that I can get a STK435 for about $7 on eBay.The question is, would a short between pin 11 to ground cause the symptoms that I experienced? Meaning, it had burned out rectifier diodes and when I changed the diodes it blew the 1.6A fuse on my variac?

author: Awesome14
author_rank: Regular Contributor
timestamp: December 08, 2017, 05:10:52 am
content: Dear OP,Your post demonstrates a condition of sheer boredom. Put a listing on craigslist for electronic equipment recycler, free pick up. Say, "Email me with a description of what you have." Then, if it's something you want, tell them to leave it outside, and pick it up.If you don't want it, don't reply. I guarantee you'll have all the projects you could ever wish for.

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2017, 07:09:27 am
content: A short from pin 11 to GND would end up overloading the DC input - the rectifier would cook.Disconnect STK pin 7 from the PCB (unsolder it) and see if the overload is still there.The power transformers in these usually have a few windings, dial lamps, HV for power amp, low voltage for tuner/preamp, and 12V for tape section.Consider adding a cheap Bluetooth audio module from eBay or Ali, you can stuff one in the unit for a few bucks, in place of the 8-track

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 08, 2017, 01:36:17 pm
content: Quote from: Awesome14 on December 08, 2017, 05:10:52 amAhhh... I have this habit of finding it very difficult to ignore discarded stuff that's half decent. This one I couldn't pass up because it has a dial for the radio tuner. I thought, if nothing else I'll remove the big variable capacitor and make a radio from scratch with my kid. But then I got the Idea to put together a "Frankensonic" also using a curbside CD player and speakers.Quote from: floobydust on December 08, 2017, 07:09:27 amThat's really useful info for me. Thanks...What would be the best way to measure the current? To connect an amp meter on the AC side, or to connect it after the rectifier? Should I use my variac, or just plug it in?Quote from: floobydust on December 08, 2017, 07:09:27 amIt has 2 blue wires carrying 28V into the rectifier for the amp.It has 3 other wires (red, white yellow) carrying lower voltage to another part of the board, but not into a bridge rectifier. I measured some 12VAC between 2 wires, some 11.4VAC between some other 2 wires, and around 1VAC between remaining wires.I can't remember which ones (I think 11V) go to the dial lamps. I did not trace the route of the other 2 wires, but there is no bridge rectifier there. There's just one diode and a bunch of electrolitic caps.Does all that sound about right?Quote from: floobydust on December 08, 2017, 07:09:27 amThat's a very good idea. Thanks for the suggestion.Thanks...

author: CJay
author_rank: Super Contributor
timestamp: December 08, 2017, 02:34:46 pm
content: Depends, do you want to?Will you enjoy it?If you're hoping to make money from it then no, it's probably not worth it, if you want to have some fun and that's your idea of fun then hell yes, go for it.Back in the day when I repaired TV and HiFi gear (as well as pretty much anything else that came across my bench) I Frankensteined a few old HiFis with alternate amplifier modules if they were absolutely destroyed or if parts were obsolete, I enjoyed it and even got paid a salary for it.Your STK hybrid sounds like it's dead BTW.

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2017, 05:36:00 pm
content: If you know there is a short circuit or overload somewhere, it's not good to keep trying to power it up. It stresses the parts and sometimes more parts can fail.Here, the rectifiers and power transformer get overloaded.So you need to try things at less than full current.If you really want proof, I would put an AC ammeter in series on a blue wire, and see how current goes as you dial up a variac. Or use the old "light bulb" test, where you put a 60W lamp in series with AC mains and if it stays lit up, you have a short.I would say the STK435 is bad, shorted internal transistor from pin 11-12 or 8.A second approach is to disconnect the suspect part's power and see if things power up.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 08, 2017, 06:19:24 pm
content: Thanks again. I'm learning a lot.So, I cut off the IC with a Dremmel tool and left all the leads protruding. The IC is bad. There is a continuity between pin 11 and 12.I placed 4 rectifier diodes in place of old one. I used 1N4007. Would you say, are those good replacements for the old 1R6J diodes?I tested the unit after removing the IC and it's all good. There's 36VCD coming out of the refurbished rectifier.Next, I found an IC in my salvage bin. It is a TA8200AH, which can take up to 37VDC, but typically works on 28VDC. So, it should be OK for a test. I'll just connect the TA8200AH with jumpers and breadboard some caps and resistors. That should work, right?But, with my limited knowledge I don't know what is the best way to reduce the 36VDC down to 28VDC. Would I have to install a voltage regulator? Or just build a voltage divider? If so, what value resistors would I use?Thanks again...

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2017, 07:21:44 pm
content: 1N4007's are OK here, 1N5401's better. I can't find a 1JR6 diode datasheet to compare.Because there is no load on the amplifier's power supply (STK is removed) the voltage will go up. Small mains transformers have poor regulation, around 15-30% difference from no-load to full load.Careful the main filter cap may now hold a charge and make sparks soldering... so discharge the cap through a resistor before getting in the circuit. Check with a multimeter all the big caps are around zero volts before going in there, to prevent damage to IC.The TA8200 is a different bird, I'm not sure if you have too much voltage for it. STK435 39V and TA8200AH rated to 37VDC max. and around 28V operating.Running these IC's near their max. voltage made good product marketing for output power ratings, but the IC's were not reliable being pushed to their limit.I'd use a stronger amplifier IC like LM3886 or you could add a simple LM317 voltage regulator circuit as Frankenstein Part 1.TA8200 must be mounted on the heatsink or it will overheat. Power it up at say 1/3 voltage to experiment.

author: ebastler
author_rank: Super Contributor
timestamp: December 08, 2017, 07:27:25 pm
content: Quote from: Adinol on December 08, 2017, 06:19:24 pmProgress on the "Frankenstein" front!

author: jolshefsky
author_rank: Regular Contributor
timestamp: December 08, 2017, 07:47:41 pm
content: I've got a project to do a similar thing. I'm trying to get away from my addiction of making new circuits with new parts and new circuit boards—it's so much fun! ... I'd rather be taking all this junk I find and getting the good bits out and making neat, useful things. Less waste, longer lasting, yada yada.I gutted a busted Sony stereo and got the amplifier chip and its circuitry cut away (then lost it, apparently...) I was looking for a way to get a "good enough" 10-ish watts/channel amplifier. Plenty of support electronics laying around to do it if I could only find that bit of board ...

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 08, 2017, 11:28:21 pm
content: IT"S ALIVE!I gave it a 20,000V boost through a microwave transformer and the lights went off on my block, but the electronic creature came back to life...Seriously, I hooked up the TA8200 with jumpers and it works. I only kept it on less than a minute cause I wasn't sure about the supply voltage. I did all that before I got to your post, floobydust.Quote from: floobydust on December 08, 2017, 07:21:44 pmOK, so I'll put 1N5401's in. I also can't find a datasheet for 1R6J's, but the two surviving ones measure as follows:Uf=658mVC=103pFIr=2nAUf=692mVC=50pFIr=12nAQuote from: floobydust on December 08, 2017, 07:21:44 pmGot it! Thanks...Quote from: floobydust on December 08, 2017, 07:21:44 pmTotally! Thanks for making sure (I did know that but I know there's no way to tell if I know...)Quote from: floobydust on December 08, 2017, 07:21:44 pmI guess I'll get the LM317, to keep things simple. I see that the LM3886 would require a positive and negative voltage. Perhaps I can tackle that at some later time, but for now stick to the TA8200, which I already had in my salvage bin.Thanks again for all the useful information. It really helps.

author: cdev
author_rank: Super Contributor
timestamp: December 08, 2017, 11:57:08 pm
content: If you want to devote time to fixing old audio equipment, you likely could find lots better equipment for free to fix. Old "rack systems" were kind of one of the lower tiers of audio equipment.Panasonic was a step up from the bottom tier of brands and always performed decently, but still they were made to match a price point and so weren't so great. That said, devices like that can often be improved significantly now for almost no cost. With an amp, better capacitors likely would help the transient response, etc.For me the #1 thing would be, is it noisy or quiet, does the amp have decent SN? If so, go for it, but make sure its heat sinked well. For now, it would be smart to point a fan at it.if you don't have a hefty HS you can put it on improvise with the largest piece of aluminum metal you have lying around but don't leave out the fan.A friend of mine likes to collect and fix up old audio gear. He has a bunch of very nice classic old Marantz stuff.My own stereo is built around a 90s vintage NAD receiver. Its 100 wpc and its very solid. Conservatively rated. It weighs a lot. More than my Sorensen power supply, and its smaller. The tuner is very good.You would be surprised at how good some of the modular audio amp ICs are. And very affordable. That would be a better project, in terms of audio quality. The transformer/power supply would likely be the most expensive part.Quote from: Adinol on December 08, 2017, 01:23:45 am

author: Electro Detective
author_rank: Super Contributor
timestamp: December 09, 2017, 12:06:26 am
content: Whatever happens with this unit, Adinol will be better clued and experienced to tackle the next -Frankensonic-Suggest to keep a shovel handy in the car if you need parts in the middle of the night

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 09, 2017, 04:19:14 am
content: Hi again,I found this LM317 calculator...http://www.reuk.co.uk/wordpress/electric-circuit/lm317-voltage-calculator/...and found my resistor values as follows:R1=470 OhmsR2=10 KWill make output 27.85VDCBut how many Watts should these resistors be?Thanks...

author: Brumby
author_rank: Unknown
timestamp: December 09, 2017, 05:22:40 am
content: Quote from: Electro Detective on December 09, 2017, 12:06:26 amI like that. I feel a trademark coming on ......

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 09, 2017, 03:33:05 pm
content: Wow! I love that logo. I just did an image search and I don't see it in the Google Images results.Did you just make that logo?

author: Brumby
author_rank: Unknown
timestamp: December 10, 2017, 12:20:26 pm
content: Yep.That's just a cut down version. I have the high-res original on my computer.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 10, 2017, 05:40:12 pm
content: It's a great logo. Perhaps I'll ask you permission to use this logo on the "creature" if I am able to resolve 2 issues, with everyone's help.So, I now properly breadboarded the IC with it's external components and fed the signal into the bread board (see photo). I also temporarily used a separate 14.5VDC wall wart power supply for the IC, but the rest of the unit is still getting power from it's own power supply. I changed the rectifier diodes to 1N5401, as per floobydust's suggestion.Here are the 2 problems I need help resolving.1 - I get too much distortion from the TA8200, after I turn the volume pot beyond 1/2. What would be the best way to solve this?2 - One channel is not as loud as the other. This is not a problem of the TA8200 (because I changed jumpers around to test) but it's the way the signals read on what would be the original pins 1 and 15 that were originally hooked up to the STK435. I am assuming one of the channels of the pre-amp needs fixing. But I don't know if I should be looking for a faulty transistor or something else. Any guidelines would be appreciated.Thanks...

author: ebastler
author_rank: Super Contributor
timestamp: December 10, 2017, 06:43:33 pm
content: I thoroughly recommend using the breadboard without the Altoids can. Your setup looks like a short waiting to happen!

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 22, 2017, 02:01:58 pm
content: So, when I joined this forum I read the rules for newbies and it says, "show your projects," so this is what I'm doing with this post.I made a home brew PCB (my first ever) and I included a Frankensonic logo. I basically expanded on Brumby's idea and redesigned it a bit, and added a pair of headphones over the head of the adorable Frankie. Here's the PCB, all populated with components that I ripped out of curb side electronic equipment. I ended up using a LA4446 IC, instead of my earlier TA8200.The new power amp PCB will find a new home where the old STK435 used to reside. I will power it with a separate 9VDC power supply.The new LA4446 works fine with the signal coming from unit.I followed the STK435 datasheet and desoldered all the eternal components, to clear the board. I traced the original inputs to a pair of 10k resistors that you can see sticking upwards, on the photo.Now here's my question.The unit is too loud at max volume. This will become my kid's stereo equipment and I want to limit how much he cranks the volume up (because I know he will always crank it to max).The volume pot is a 2-gang 250k pot and (like I said) there are two 10k resistors on the path to the LA4446 board. I guess I can just replace the 10k resistors for a higher value. Would 100k to 150k be a good guess? OR would it be better to just replace the 8ohm speakers with 16ohm?Thanks...

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 22, 2017, 02:19:36 pm
content: Thumbs up for your Frankensonic logo!Seems that you are having lots and lots of fun....this is what the electronics hobbyist experience is all about.Plus, you'll learn a couple of things in the process.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: December 22, 2017, 07:25:38 pm
content: It's good way to learn about electronics, but if you were going to take on a full refurb I'd look for a Technics amp or receiver. That's the more upmarket name used by Matsushita for their better audio gear. The 'music centers' were strictly domestic and not really intended for serious listening.Using a Technics SU-V5 amp on the computer here, was looking a bit sorry for itself when I acquired it from a surplus depot, but didn't need much doing to it electronically. They are quite sturdy and the electrolytics seem to last well too. Main work was removing years of grime from the knobs and panel, and making it look pretty again.It's not strictly Class A, think that's a bit of a sales pitch, but the THD is very low.BTW those 'STK' modules are not too bad soundwise but as said, they pop at the mere mention of a short. Or, maybe even someone thinking about a short.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 23, 2017, 02:09:17 am
content: Quote from: schmitt trigger on December 22, 2017, 02:19:36 pmThanks! It was inspired by Brumby's original idea. I just expanded on the idea.Quote from: schmitt trigger on December 22, 2017, 02:19:36 pmPrecisely what I'm doing.Quote from: IanMacdonald on December 22, 2017, 07:25:38 pmThat looks like a beautiful piece of vintage audio equipment. And thank you very much for this information.At this point I am not making any plans to refurbish vintage electronics. I am simply the kind of person that always notices what people throw out. I did find two very good turntables, a Technics and a Bang & Olufsen. The Technics needed a cleaning and a new belt. The B&O needed nothing.This Frankensonic project is something different. I Fund a broken Panasonic stereo with a missing turntable and damaged PCBs and I just want to "Frankestine" a piece of home brew stereo equipment for fun and for my kid to use. The whole concept of "Frankestining" evolved during this thread and if I keep true to the concept, I would be "bringing back to life" broken equipment, reassembled from from pieces that came from different broken equipment. I have a lot of salvaged electronic components and I have 2 broken turn tables. The LA4446 IC came from another broken stereo I salvaged a long time ago, with the intention to build something some day.So, at this point is just seeing what I can do, for fun and learning, using only broken curbside finds.But that Technics looks awesome and I might just pick one up on eBay, just to have it.Thanks...

author: Brumby
author_rank: Unknown
timestamp: December 23, 2017, 06:38:29 am
content: Quote from: Adinol on December 23, 2017, 02:09:17 amI like the headphones. Nice touch.

author: Electro Detective
author_rank: Super Contributor
timestamp: December 23, 2017, 11:51:58 pm
content: Hey guys, please don't forget impoverished Electro Detectivewhen the  trademark idea/collaboration royalty cheques begin to fly aboutas pigs tend to do sometimesAdinol, that board logo work deserves some serious

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 24, 2017, 02:06:20 pm
content: Quote from: Electro Detective on December 23, 2017, 11:51:58 pmThanks. I feel like I'm getting too much credit for this... Anyway... I agree, it turned out a good logo and when I'm done fixing this unit I'll put the same logo on the front. Will probably just etch it on a PCB.But back to the technical issues....The new amp PCB is picking up quite a bit of noise from the rest of the unit. Interestingly enough, even when the unit is unplugged (and the amp PCB powered from a battery) the noise is coming from the main unit and the noise increases or decreases when I turn the volume pot.I'll do some more testing and post my findings, if I can't get rid of the noise.Again, this is all just learning and fun for me, not a serious project.But first thing's first... Happy holidays to all of you and thanks again...

author: cdev
author_rank: Super Contributor
timestamp: December 24, 2017, 02:12:24 pm
content: What would be the best amp module to buy for a small desktop amp project, I wonder?

author: cdev
author_rank: Super Contributor
timestamp: December 24, 2017, 02:18:44 pm
content: I would take a small magnetic pickup of some kind and try to sniff out the noise sources and re-route them. For example, whatever wires those are going across the front up in the air, you should enclose them and bypass them if possible. Make extensive use of shielded wire, caps, ferrite beads, etc, or similar for noisy signals. Also shield input lines so that they cannot pickup noise. Ground one or both sides of the shield, whatever works the best.Quote from: Adinol on December 24, 2017, 02:06:20 pm

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 24, 2017, 06:31:17 pm
content: Quote from: cdev on December 24, 2017, 02:18:44 pmVery interesting idea. Would this pickup be a coil wrapped around a ferrous core, or wrapped around a magnet (like a guitar pickup)? I'll definitely give this a try.Quote from: cdev on December 24, 2017, 02:18:44 pmWill do. Now I was just using a lot of jumpers, but I understand all those wires are picking up noise. Even when I turned off my soldering iron, the noise changed in intensity.Thanks for the suggestions.

author: floobydust
author_rank: Super Contributor
timestamp: December 24, 2017, 09:25:04 pm
content: Some chip amps have too much gain.The LA4446 is +51 to +46dB, or down to +40dB if you add the two 150R RNF' resistors in the datasheet.The STK435's had about +40dB for a voltage gain of 100.You can also lower the signal level coming out of the volume pot, but it can lessen bass if there are coupling capacitors in circuit.With twice the voltage gain, the IC will pick up more noise.Frankenstein authorMary Shelleyhad a tough life, I had no idea she went through so much... horror...

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 25, 2017, 02:00:20 am
content: Quote from: floobydust on December 24, 2017, 09:25:04 pmAgain, this is very useful information and you gave me a good direction. I learn something new every time there's a post here.Thanks again.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 27, 2017, 03:15:35 am
content: Quote from: Electro Detective on December 23, 2017, 11:51:58 pmIndeed they do! Just look at Pink Floyd's "Animals" album.One of Rock's better and most iconic cover art.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 27, 2017, 05:34:53 am
content: Hello again, friends. Did some experimenting and got results I can definitely live with.Both, cdev and floobydust gave some good advice that resolved the noise issue.Quote from: cdev on December 24, 2017, 02:18:44 pmShielding the input cables made a big difference with the LA4446. Thanks for that advice, cdev.Also, floobydust said something that put an idea in my mind.Quote from: floobydust on December 24, 2017, 09:25:04 pmSo I breadboarded an LM386 (just how it is on the datasheet, with minimal external components) and I got rid of the noise. The LM386 produced good results with a power supply ranging from 5V to 14V. I couldn't hear the difference when changing the voltage.So, thank you floobydust for your tip.So, I will do another PCB with two LM386 ICs and put it in. I'll use the LA4446 PCB for a future project, as I would hate to waste it. I'll post pictures of the progress.Now to my questions:1 - What is my best option to power up the new LM386 board? Should I install another transformer and rectifier for that board? Or should I use a voltage regulator to bring the 37VDC down to LM386 required voltage?2 - Since the LM386 test worked with voltage from 5V to 14V, what voltage should I end up using?Thanks...

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2017, 04:53:05 pm
content: Why is the Frankenstein going down in power?Ugh the LM386 I hate that IC. It's such a dog from the late 1970's. High distortion, oscillates like crazy, loves to pick up AM radio, and a wimp for power output.If you are worried about kids volume/power output, I would add a limiter circuit or just a resistor in series with the speakers, to keep the Justin Bieber volume down.

author: Frankentronics
author_rank: Regular Contributor
timestamp: December 28, 2017, 05:08:40 pm
content: Quote from: floobydust on December 28, 2017, 04:53:05 pmOK, thanks for sharing your thoughts. Since I have quite a few salvaged ICs I'll do some more experimenting and I'll also try to add resistor in series with speakers (or try 16 Ohm speakers) with the LA4446.Something will work.Thanks...

author: Inverted18650
author_rank: Frequent Contributor
timestamp: December 28, 2017, 07:13:43 pm
content: Quote from: wilfred on December 08, 2017, 01:38:47 amOn my! rolling mate, hilarious.