I have a couple of these nice 1980 receivers. Not top shelf material, but quite good. They use ICs for L/R voltage amplifiers (STK 3076/3106) and individual ICs for L/R power amps (STK-1080 II), three ICs in total 😕. Both have blown voltage amplifier ICs, but I am finding other surrounding problems such as 4558 op amp blown and zener diode MZ324.
Does anyone have an idea on this MZ324 zener diode and its specs? I need to find a current match.
I am attaching the part in the schematic with the MZ324 zeners highlighted in red. Maybe someone can have an educated guess as to one that will work in both positions.
Thanks in advance.
Does anyone have an idea on this MZ324 zener diode and its specs? I need to find a current match.
I am attaching the part in the schematic with the MZ324 zeners highlighted in red. Maybe someone can have an educated guess as to one that will work in both positions.
Thanks in advance.
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It's a 24V 1/2W 5% Zener diode in a DO-35 case.
Both Zeners seem to be the same type. Should be no problem to find.
Both Zeners seem to be the same type. Should be no problem to find.
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That is nothing in front of the broken STK, which are kind of difficult to find.
And what caused the damage to the pre amp section?
Do a thorough analysis before starting to get parts.
And find all the parts, get them on your bench before starting work.
Personally, I would change the circuit to a Mosfet based kit.
And what caused the damage to the pre amp section?
Do a thorough analysis before starting to get parts.
And find all the parts, get them on your bench before starting work.
Personally, I would change the circuit to a Mosfet based kit.
I think the failure came downstream from the STK1080 II output ICs. Also, the 4558s dual op amps. One has one section passing rail voltage, the other has both sections passing rail voltage.
The preamp section works fine, it is probably the better part of these.
I have checked all voltages on one with all ICs removed and all is within spec.
There are no bleeder resistors anywhere, so the amp is always charged up! Also, one edge or the power PCB has no support. It can flex and short out on the bottom metal plate that has no shielding of any kind!
Probably do not have the knowledge to change the circuit to a Mosfet, but I am all ears.
A few tweaks can make these reliable--I hope.
Anyone have leads on STK3076, STK3106, or STK3156 voltage amplifier ICs? Other than eBay?
The STK series were infamous for going bang without any reason, and sometimes they took speakers and power supply transformers along.
Happened to me, so while I was able to get the transformer repaired, and a good copy STK 4141, you may not be so lucky.
Most of the STK chips are unobtanium, and there have been attempts to emulate them, with varying success.
Salcon and Monty in India sell Mosfet transistor amp kits, I expect there will be suppliers from China as well.
Check the supply rails, and choose.
If suitable, put a Class D chip kit, more efficient.
Also, most Class AB chip amps are on back order, so the suggestion for Mosfet kits.
Happened to me, so while I was able to get the transformer repaired, and a good copy STK 4141, you may not be so lucky.
Most of the STK chips are unobtanium, and there have been attempts to emulate them, with varying success.
Salcon and Monty in India sell Mosfet transistor amp kits, I expect there will be suppliers from China as well.
Check the supply rails, and choose.
If suitable, put a Class D chip kit, more efficient.
Also, most Class AB chip amps are on back order, so the suggestion for Mosfet kits.
Agreed, both units had rail voltage at the speaker outputs, good thing the protection kicked in but probably took a tweeter out on its way. Supply rails are around 52 VDC, hefty transformer, good caps.
Yes, the STK are amp divas and unless I start off with all known good ones, one will take the other out. Not going to be getting into anything with these in the future.
If you have a link for a mosfet transistor kit that you like, share it. I will definitely consider it. I would really like to remove the whole power amp PCB but the protection circuit is on it. It has a very good heat sink.
A fellow member, wg_ski, on another thread has commented that Mosfets are inferior to BJT, some issues with circuit design, so look around on this forum for ideas.
There are plenty of options, and I have given the names of the reputed local suppliers, though I can buy from the market in my city. Just look at what is available from them, and it may help you decide.
Also, check out the amp power supply rating, not just volts, but amps too.
52V is + to -, or 52 - 0 - 52, for a total of 104?
If 52, you might be able to use a 1875 or 3886 kit, both are good.
Circuits for protection can be added, and you are looking at gutting the amp, retaining only the cabinet and power supply, maybe the pre amp section.
Unless it has sentimental value, it is a big effort, you just might get another 80s set for a small amount.
There is a thread here about lunch money amps, look at it as well.
Also, I have no idea about your skills in this field, no offense meant, this job needs some prior experience.
There are plenty of options, and I have given the names of the reputed local suppliers, though I can buy from the market in my city. Just look at what is available from them, and it may help you decide.
Also, check out the amp power supply rating, not just volts, but amps too.
52V is + to -, or 52 - 0 - 52, for a total of 104?
If 52, you might be able to use a 1875 or 3886 kit, both are good.
Circuits for protection can be added, and you are looking at gutting the amp, retaining only the cabinet and power supply, maybe the pre amp section.
Unless it has sentimental value, it is a big effort, you just might get another 80s set for a small amount.
There is a thread here about lunch money amps, look at it as well.
Also, I have no idea about your skills in this field, no offense meant, this job needs some prior experience.
80W Mosfet kits are in $10 range here, four transistor PCB....the heat sink, transformer and so on are extra, those run on 0-30 or 30-0-30 transformers, 10A rated, I forgot which.
Might work on 26-0-26 or 0-52 supplies.
The whole assembled amp, with FM/USB/SD/Bluetooth module and so on, was $80 some time back, before the sudden spike in chip and metal prices, I expect a price increase currently...
Might work on 26-0-26 or 0-52 supplies.
The whole assembled amp, with FM/USB/SD/Bluetooth module and so on, was $80 some time back, before the sudden spike in chip and metal prices, I expect a price increase currently...
https://www.salconelectronics.com/product/4-mosfet-power-stereo/
A sample, they may have Keltron kits, which use better quality Keltron caps in place of the Chinese ones.
Quick conversion is 75 Rupees to one USD.
A sample, they may have Keltron kits, which use better quality Keltron caps in place of the Chinese ones.
Quick conversion is 75 Rupees to one USD.
Thanks for the suggestions, I have looked at them. With 52 VDC +/- rails, most of these will not work. I did find a Class D audio board that will take a max of 52 VDC. CLASS D AUDIO CDA-250 The cost is starting to add up, so I have to think about it. The preamp section and tuner are quite good in this receiver.
I did try to replace all the ICs on the power board, even the op amp. Got the unit turned on and nicely out of protection. Voltages seemed normal, and the unit was producing some nice sound from both channels. As I turned the unit up, one channel went out with a crackle. Then the other went out and triggered the protection. Seems that the NOS STK1080 II that I bought on eBay are probably low quality or fake. I am attaching photos for your opinion. I would advise anyone to stay away from these units...very unreliable. I may change my mind if the transplant to a CDA-250 turns out well.
Pictures of the STK1080 II individual channel power amp modules are attached.
Left side original, right one is the new NOS. Broke open quite easy like an egg. The original was more like a walnut, hard to crack.
I can hardly see them supporting 85 watts at 8 ohms...
I did try to replace all the ICs on the power board, even the op amp. Got the unit turned on and nicely out of protection. Voltages seemed normal, and the unit was producing some nice sound from both channels. As I turned the unit up, one channel went out with a crackle. Then the other went out and triggered the protection. Seems that the NOS STK1080 II that I bought on eBay are probably low quality or fake. I am attaching photos for your opinion. I would advise anyone to stay away from these units...very unreliable. I may change my mind if the transplant to a CDA-250 turns out well.
Pictures of the STK1080 II individual channel power amp modules are attached.
Left side original, right one is the new NOS. Broke open quite easy like an egg. The original was more like a walnut, hard to crack.
I can hardly see them supporting 85 watts at 8 ohms...
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There is a thread here, Fisher CP 7000 distortion, he put MX 50 modules.
He is satisfied, take a look.
He is satisfied, take a look.
Those STK copies are pretty bad.
The cost for MX 50 modules is less than $30 in the USA, off Amazon, with KD series transistors, not the big ear Sankens.
You can search, or post on that thread for help.
The cost for MX 50 modules is less than $30 in the USA, off Amazon, with KD series transistors, not the big ear Sankens.
You can search, or post on that thread for help.
Rails are 52-0-52 VDC.
MX50 modules sound very nice. I have built a few, and they sound great for a while. Then they pop. I am using some inexpensive Chinese source. This is with medium effort, nothing crazy. Still think too much voltage for them as they go up to 45 VDC.
Here is my last fail. I have a bunch of extra transistors, so I may replace this one.
MX50 modules sound very nice. I have built a few, and they sound great for a while. Then they pop. I am using some inexpensive Chinese source. This is with medium effort, nothing crazy. Still think too much voltage for them as they go up to 45 VDC.
Here is my last fail. I have a bunch of extra transistors, so I may replace this one.
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You can select circuits to run at 0-52, on each side of the supply.
At that rating, many BJT and Mosfet circuits should be available.
At that rating, many BJT and Mosfet circuits should be available.
I decided to go with the Wondom amp board 2x125 watts at 8 ohms, just finished early this morning. Had a nasty ground loop to sort out. The amp board is super quiet. Very rich and barely gets warm. It just barely fit, and I had to shave a couple MM on one side of the PCB to clear the large power cap. Except for the ground loop, it was an easy drop in and glue placement after removing all the components that were in the way (in the baggie pic). I did have to add an inrush current limiter, and the speaker protection seemed to like that. It was rushing up to over 100 VDC at power on, triggering the Wondom protection for a second. There are some evident cost-cutting measures on this unit, it could benefit from bleeder resistors and the additional capacitance made the current limiting necessary. These are nice big-box receiver sleeper units worth restoring, specially since they do not fetch in the Sansui 9090DB price, but give you just as good sound. I was able to restore the other 35 to original with spare parts from this one and a new genuine voltage amplifier IC. I will post pics of that later if anyone is interested. Here are pics of this project, and I hope someone finds this info of use in the future. As you will note in the PCB picture, some past repair tech had worn the traces down with heat and I finished them off. No chance it could have remained original.
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Can you please post a DIY walk thru with step by step photos and instructions and where to buy the parts. I not well versed in electronics.