Author Topic: Is it worth "Frankensteining" an 80's Panasonic stereo?  (Read 622 times)

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Online Adinol

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Is it worth "Frankensteining" an 80's Panasonic stereo?
« on: December 08, 2017, 12:23:45 PM »
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=658mV
C=103pF
Ir=2nA

Uf=692mV
C=50pF
Ir=12nA

I desoldered the big cap and it tested to specs...

2200uF
ESR 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 here https://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?

« Last Edit: December 08, 2017, 12:26:28 PM by Adinol »
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Online wilfred

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #1 on: December 08, 2017, 12:38:47 PM »
If you have to ask then I'm going to do you a favour and say no just forget about it. If it wasn't desireable in the '80's then it isn't now.

I haven't finished reading Frankenstein just yet but I don't think it ended well. I decided to wait a few weeks until the 200th anniversary of publication.
 
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Offline floobydust

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #2 on: December 08, 2017, 12:44:50 PM »
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.
 
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #3 on: December 08, 2017, 02:05:02 PM »
I haven't finished reading Frankenstein just yet but I don't think it ended well...
Ha, 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.

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.
I 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?
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Offline Awesome14

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #4 on: December 08, 2017, 04:10:52 PM »
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.
Anything truly new begins as a thought.
 
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Offline floobydust

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #5 on: December 08, 2017, 06:09:27 PM »
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
 
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #6 on: December 09, 2017, 12:36:17 AM »
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.
Ahhh... 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.

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.
That'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?

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.
It 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?

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
That's a very good idea. Thanks for the suggestion.

Thanks...
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Offline CJay

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #7 on: December 09, 2017, 01:34:46 AM »
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.
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Offline floobydust

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #8 on: December 09, 2017, 04:36:00 AM »
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.


 
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #9 on: December 09, 2017, 05:19:24 AM »
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...
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Offline floobydust

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #10 on: December 09, 2017, 06:21:44 AM »
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.
 

Offline ebastler

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #11 on: December 09, 2017, 06:27:25 AM »
So, I cut off the IC with a Dremmel tool and left all the leads protruding.
Progress on the "Frankenstein" front!  ::)
 
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Offline jolshefsky

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #12 on: December 09, 2017, 06:47:41 AM »
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 ...
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #13 on: December 09, 2017, 10:28:21 AM »
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.

1N4007's are OK here, 1N5401's better. I can't find a 1JR6 diode datasheet to compare.
OK, 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=658mV
C=103pF
Ir=2nA

Uf=692mV
C=50pF
Ir=12nA

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.
Got it! Thanks...

Careful the main filter cap...
Totally! Thanks for making sure (I did know that but I know there's no way to tell if I know...)

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.
I 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.
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Online cdev

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #14 on: December 09, 2017, 10:57:08 AM »
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.


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=658mV
C=103pF
Ir=2nA

Uf=692mV
C=50pF
Ir=12nA

I desoldered the big cap and it tested to specs...

2200uF
ESR 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 here https://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?
« Last Edit: December 09, 2017, 11:01:40 AM by cdev »
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Offline Electro Detective

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #15 on: December 09, 2017, 11:06:26 AM »
Whatever happens with this unit, Adinol will be better clued and experienced to tackle the next -Frankensonic-    :clap:

Suggest to keep a shovel handy in the car if you need parts in the middle of the night   >:D

 ;D

 

 

 
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #16 on: December 09, 2017, 03:19:14 PM »
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 Ohms
R2=10 K

Will make output 27.85VDC

But how many Watts should these resistors be?

Thanks...
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Online Brumby

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #17 on: December 09, 2017, 04:22:40 PM »
Frankensonic

I like that.  I feel a trademark coming on ......

 
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Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #18 on: December 10, 2017, 02:33:05 AM »
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?
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Online Brumby

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #19 on: December 10, 2017, 11:20:26 PM »
Yep.   :D

That's just a cut down version.  I have the high-res original on my computer.
« Last Edit: December 10, 2017, 11:22:20 PM by Brumby »
 

Online Adinol

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #20 on: December 11, 2017, 04:40:12 AM »
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...
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Offline ebastler

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Re: Is it worth "Frankensteining" an 80's Panasonic stereo?
« Reply #21 on: December 11, 2017, 05:43:33 AM »
I thoroughly recommend using the breadboard without the Altoids can. Your setup looks like a short waiting to happen!
 
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