0:15 Intro 2:28 Looking at how it was found 3:27 Amp board after cleaning & Repair 3:38 A look at all the burned components 4:04 Sansui G-7500 Spec Sheet 4:26 Looking at the repaired board installed 6:37 Original output transistors and heatsink 11:12 LAPT transistor vs Standard transistor 15:42 List of replacement transistor types used 16:36 Replacement for the STV-3H stabistor module (original was damaged) 17:08 Can you replace a Ring Emitter Transistor or LAPT transistor with a standard transistor in these amps? 23:05 What is a Stabistor and how can we find a replacement? 30:45 How to make a replacement Stabistor 34:26 Testing the DIY Stabistor out-of-circuit 38:00 Replacement pots for bias and offset 39:30 Replacement relay for the protect circuit 40:22 New output transistor mounting with sockets. LAPT Outputs were replaced with MJ21194/MJ21193 transistors in TO-3 type case, which properly fits the heatsink. 42:39 First Power-up and tests 53:07 Adjusting the BIAS & DC Offset 1:03:50 Testing the amp on a dummy load with signal. The new Stabistors are working Perfectly!!
It must take alot of time to put these presentations together, Thank you for your efforts. 2 years ago I would not have attempted to open up much less try to repair old gear. Times have changed. Fixed and re-capped a couple of NADs, now knee deep in a QRX-9001. No way could I have gone this far without people like you.
Tony your unrehearsed videos that show your troubleshooting process with surprises rather than an edited Hollywood version where everything goes perfectly helps me to justify my own glitches thanks for letting me look over your shoulder.
That was almost certainly the most impressive and dedicated repair I think I have ever seen and a job seriously well done. Also a fantastic level of clearly explained detail, that even as a relative beginner to repairing vintage audio gear, I could easily follow. Nice work on the bias mod too. I really need to try to remember that one.
"Well, that's when I decided to get a little creative." Your extraordinary skill make that sentence extremely entertaining Tony. You made that G-7500 purrrr like a kitten again...at least on your output tester, can't wait for the second part of this one. I have a soft spot for these old G- series Sansui receivers, they were real workhorses in their prime. Time makes us all need a tune up though, this one was run a little too hard and then put away smoking! Thanks for your time and posting this! You and family have a Merry Christmas!
As soon as I saw the heat sink, I was rooting for you to use the T-03's. When you did, I thought yay! I guess just me personally I would rather have robustness with debatable lack of performance than maybe a little better time bomb.
Just an FYI for folks who have issues with a G-7500 doing exactly what this one has done the Bias Pot Tony Mentioned is one of the key failure points for these RCVR's i have repaired 3 or 4 that had that failure mode. Its a chain of events. The pot opens and the Bias voltage goes to the rail. That high base current shorts the drivers and a cascade effect happens . It will open the emitter followers and a bunch of smaller resistors right around the output transistors .
These Sansui DC amps sound fantastic! Superior to all but Spectral for slew rates. IMHO. The downside. These Sansui D.C. amps are finicky and often unstable. Not to mention, difficult to work on. I never turn off my AU-D11ii to minimize power up surge. I only switch out my speakers when not listening.
Wow, I just realized how much equipment you have stacked into such a small workspace. It’s very impressive and I’m sure it has taken you quite some time to assemble all of it and have it all function at the time you need to use it.
Your whole background piece on the different types of high-frequency power transistors: absolutely fascinating. Thanks a million for that little episode of "Transistor Talk with Tony B." I''ll say this regarding higher frequency/ high slew-rate: about the year 2001, I re-chipped an older (circa 1979-81) mixing console called an "APSI 2001" using more modern, higher slew-rate TL072 op-amps. (Just an aside you may appreciate, Tony: the APSI 2001 was used on tour by Rush for front-of-house and stage sound around that time.) Anyway, I submit that the difference was clear, substantial, and a notable and significant improvement. The old Texas Instruments TL072's had a slew rate of 14µV/ sec, while the newer ST Micro TL072's were spec'd at 16µV/ sec. It made, to my ears, a BIG difference. Now, could it have been that it wasn't the slew rate, but simple overall improvements in general op-amp/ chip manufacturing techniques? Could've been. But I don't think the better slew rate played an insignificant role.
Hahaha, your cousin hit the jackpot with this one. Gave you some real work here. Seriously, I don't think for a second that if you use these Sansui pieces and not abuse them, this can not happen. Ever! But that's my opinion anyway. I believe this Sansui was used and abused hard and not taken care of to finish up in this state. As always, great video! Keep them coming! Regards from Zagreb, Croatia (Europe)!
Very nice Tony, just wonder what about stability, phase margin, gain margin, when you change those 70Mhz transistor to slower , do you need to adjust the loop gain too , you test with square wave to see if you have ringing.
I certainly don't understand everything in terms of your strategy, not in the particular but I could follow your step by step process. Very creative indeed!
About making those stabistors. I have a Pioneer SX-838 that has corroded legs right at the base of the stabistors and they both snapped on me as I was removing them to recap the board. My question is, is there certain chips that should be used with each unit depending on each model specifics or is STV-4H stabistors a universal thing? Could you make a video to elaborate on this for dummies as well as making these? I would appreciate the reply as well if you don't have time. Thank you.
Very nice solution for the stabistor replacement. You might want to consider coating that copper with some liquid tinning solution to keep it from corroding over time before you mount/glue the chip, but other than that it looks like it would stand up at least as well as the originals, most likely better. And seeing that you changed output transistor types, I was also wondering if the factory bias specs still apply, or are you going to do some distortion measurements in the next part to verify if tweaking the bias should be done.
I’ve got a Pioneer M22 that is out of service because of a broken “stabistor” that I haven’t been able to find in years. The previous tech was a little ham handed and broke the leads off at the package. Thanks for the tips about the correct IC.
50:40. Your observation is spot on. I have that exact meter and do not like it for the same reasons you have stated. I rarely use it anymore. Also, the Fluke 87V is what I use at work. Had the original 87, then the 87III, now for the past 15 years the 87V. Good all around meters. Also, built safely so there is little worry when I am measuring 480 volts 3 phase high energy circuits.The meter is very unlikely to blow up in my hand as other, more cheaply made meters may do.
Even tube gear can die from a shorted output tube sending sending full on DC though the output transformer. Just replaced one of those expensive and rare nickel cobalt Altec/Peerless output transformers on one of my 2 Heathkit W-5M tube amps this year. Good thing I had a parts donor amp. I am working on a Sansui G-8700DB which required all 8 output devices, but they are around $7 each from Mouser. The Marantz receivers also used direct coupled power amp stages and same devices, as Tony mentioned.
Excelent work, i have a sansui a5 amplifier and the right channel stop working, i don't know nothing about this kind of repairs but seein you i try to get some kind of idea.
Very interesting, especially having to roll your own components. I'm familiar with using a current limiter with tube gear, I never thought about using it with solid state gear. Yeah, I got much to learn.
A current limiter is even more important when working on solid state gear then it is with tube equipment, because tube stuff is much more forgiving and can tolerate brief overloads or slips with your meter probes a lot better than solid state stuff can. Tubes can tolerate red-plate conditions for a half minute or perhaps several minutes, and you will usually have time to observe the fault occurring, or your voltmeter will show the voltages in the circuit getting dragged down by the excess current draw; but if you draw current beyond a transistor or diodes' capability it will self destruct in a fraction of a second, and often cause a daisy-chained meltdown effect on the circuits before and after it. Trust me on this, as I have personally let the magic smoke out of more semiconductor devices than I care to admit!
Tony, perhaps you were also wrong about the protect relay. Maybe it is indeed shorting the speaker outputs when de-activated, since you measured 0.00 volts on each channel!
The normally closed contact terminals are cropped off from the bottom of the relay as there was no mounting holes for them on the board. So they don't connect to anything.
Hi,, I had sansui g-8000 and it in the protection mode that it not go to ready. The output transitor are 2sa7458 and 2sc14038 . You have any recommended for replacing this transitor with different one since I can't find the original one. Thank
Great repair video. I think what will help you is a pc with a capture card then can record directly to pc. Should not loose anymore clips. Just need a webcam or camera connected to capture card
Really enjoying your videos. Your explanations and procedures are the easiest for me to follow of all the channels I subscribe to. Can’t wait for the testing of the big Sansui.
I work on a LOT of Sansui amplifiers, those output transistors you've used change the sound of the amp, they are not good devices to complement the amplifier design. These amplifiers can be a nightmare, the F2980 power supply/Power amp driver board is a particular nightmare, they are in several models of Sansui, I have rebuilt a LOT of these. No reason to not use components which match and complement the original design. However, I respect what you're doing, and its certainly a way to get it running and I know your solution will have it running reliably.
Thanks for the comment! Another viewer and I are discussing some of the reasons why this could be true. It is very interesting. I have always heard that this will change the sound/cause the amp to fail, etc., but I wanted to see this for myself. This was a good opportunity to experiment. I do not doubt that there could be a difference in the sound, but as of the writing of this comment, I'm unable to see it through any of the tests I have done, nor can I hear it (although my hearing isn't what it used to be) by just listening to music through it. I have not done exhaustive testing, nor have I done any listening under the best conditions, so the jury is still certainly out on this one. Looking at the design, you can certainly see how this could affect things, especially in terms of NFB, as I mentioned in the video. Time will tell. I know I would have never tried this on the AU-X1 (waaaaay different design and it was in far better condition than this poor G-7500), but this was a good opportunity to experiment. This is how we learn, and i certainly have a lot to learn myself! Thanks again!
@@xraytonyb Thanks for the reply Tony, its a very complex reason as to why, but I am pretty sure part of it is the very linear phase through the bandwidth of the amplifier, particularly in the top end, there is also something more composed and controlled about the transient response too. I use the same JBL4412 studio monitors when I am assessing amplifiers after service and restoration, and I can absolutely hear the difference. Very basically to me, it also feels like you might be able to hear more detail in the music like for instance a vocal reverb which will appear to have more depth, or perhaps an acoustic guitar buried in the mix in a very electric rock song which might otherwise go unnoticed. Those are just a few things that come to mind. Also the G7500 is an oddball model, its almost like Sansui just threw those together with a bunch of things they had lying around, the tuner board is from one of the Quad receivers, I cant remember what model, the F2980 board (power amp driver) is from other models, the TO3 heatsink and so forth. When I repair these ones, I file off the anodising I believe it helps with the thermal coupling. This design is as you know and have seen, somewhat unstable on a good day!! They do sometimes fail for what seems like absolutely no reason!! I'm enjoying this episode, and as always your work is top notch!!
Hi Ken, I am working on a Sansui G-8700DB and have acquired the ON replacement devices from Mouser. I knew the reputation these G series amps have of self destruction and was hesitant on taking this one on. Kind of like a room full of mouse traps and throwing a ping pong ball into the room. When the owner found out that it may need a dozen transistors and half a dozen resistors on the driver board he said keep it. The Marantz 2275 I worked on was of similar design and used the same T03 devices from Fairchild. The Toshiba outputs that Sansui used are not made anymore, and there are lots of counterfeit ones on eBay. Do you know of a better replacement?
Working on a Sansui G-8700DB. I have acquired all the output transistors from ON semiconductor devices. One channel had mix and match transistors in it and someone had altered resistor values to get it to function, well kinda. On the F-3074 driver boards, 390 ohm resistors which couple the 2SC2071 to the 2SA939 devices got so hot the solder melted and one lead fell out of the eyelet on the board. Many cooked out of tolerance resistors, when these go, they go big! Question: The afore mentioned transistors do not test open or shorted, but since so much current has passed through them I find it hard to trust them. Should I just order the KSA devices and change them out to be on the safe side? Another tech told me to just current limit and bring it up slow, but I don't want all those new T03 devices to go up in smoke in the near future.
If the amp suffered a massive failure, I would replace all the transistors except for the diff pairs (unless they are bad). This is because the cost is not a huge factor, you don't know if any of the old ones were stressed and with new transistors you know the condition and specs of all the transistors at that point. If it is for a customer that wants minimal cost, just test the amp out and verify proper operation. If all is good, leave the old ones in.
Oh, I just also wanted to say, there is a German ebay seller in Germany who has NOS STV3H diodes (tens of thousands), they are still available, I have a decent stock of them. I've used them many times and they seem to be the real deal.
Tony I wished you would spend some more time on how you found the right solution for replacing ( stabistors ) as most people know here these are unubtaininum any more I had to sell my Technics SA 800 for less than half it was worth because I could not find ( your term stabisitor) I had to solder the old brittle wires back onto the resistor where it broke off I was not confident in that repair and that is why I sold it way below what it was worth / I think the pioneer community would like to know where you get these thermistors or a video on a good work around I have heard the three diode and four diode work around but that seems hoky to me. George
Great video, thanks! I was wondering how you set up the power measurement on the Tek scope channels. The scale is VV and the traces appear to be DC level rising with the output level.
The VV is actually the direct power readout of each channel. It reads watts into an 8ohm load, based on the RMS voltage across the 8ohm dummy load. I also have a similar math expression programmed for when I'm using a 4 ohm dummy load. Thanks for the comment!
I think that this is not a stabistor but it is 3 thermal diodes. It control the BIAS voltage depend of the temperature of the heat sink of OPS. It reduce the BIAS when the heatsinc goes warm.
When I was researching these T03 devices from ON Semiconductor, the 93G,94G devices had the same specs as the 95G,96G devices (MJ211xxx). I have used the 93G,94G devices in a Marantz 2275 and they worked just fine. Do you know what the difference is?
Wow my G8000 is completly different inside ? I spent about a week just cleaning inside and out and cleaned all the pots correctly . Its rated at 120watts from 1978 its a Beast at 56 pounds love how all the boards have quick disconnects but Tony my Direct mode switch is acting funny if i wiggle the switch i have the left channel cutting in and out drives me crazy most likely the tone/EQ board needs recapped . Having a hard time finding someone to recapp for me 😥 I work 70hrs a week so no time ! I most likely will sale it and Focus on my Marantz 2270 but vs Sansui g8000 yeah The g8000 on a different level but the Marantz 2300 series is more comparable ! Would you feel like recapping me Marantz or Sansui ? I will take good care of ya ! I want to pass down to my newphew he loves coming to my house and playing with all my amps I have 15 amps Tony !! And I have 14 sets of speakers . Maybe i have something you would like to have 'speaker or amps' I have a VINTAGE Yamaha CA 810 also 😃 Thanks Tony My dream amp is the Sansui 2302b 14k in 1992 !
When I graduated from tech school in 79 my first job was at a Hi-Fi shop first day of the job my boss told me that the guy that just quit left a g7500 on the bench it took me months to get it fixed I learned all about direct couple transistors
I own a full-time business. This is just for fun. I am sick, and possibly a bit crazy. At least I didn't sit on my soldering iron, like one of my co-workers did :) (BTW, dress pants melt when they are made of polyester and you sit on your soldering iron)
@@Homer19521 When we bought our house in the late 90's, it had 1980's pink or fuchsia laminate countertops. My wife hated those, and said to me you can do any work you want to on that counter cuz when we have the money we are gonna replace it. We replaced the counters with granite about 15 years later and now she won't let me leave so much as a loose screw or screw driver on the counter.
Try "the new" uv epoxi for repairing tracks and pad. is working like a charm. no delamination if you clean it well (lead solder tho..) and looks much better.
Interesting stabistor replacement technique, butttttttttt…..how do you know it has the same characteristics of voltage versus temperature as the original? I would assume it has to match those output transistors.
Good question! Even the STV-3h will have slight differences to one another. This is the reason there is a resistor/potentiometer in series with the stabistor. This allows you to adjust the overall voltage in the bias circuit for proper bias. There are two important factors when using a stabistor: First is that the device should maintain a consistent voltage drop as forward current increases. Second, the forward voltage drop should vary in a linear and predictable way with changes in temperature. As a general rule, you want the majority of the voltage drop across the bias circuit to be across the stabistor and the resistor is used to "fine tune" the overall voltage. Any changes in the output transistors due to temperature will also be seen across the stabistor, as long as they are thermally coupled. The stabistor I used had a voltage drop within one diode drop of the voltage drop of the original STV-3h device. The pot can be used to compensate for that slight difference and the circuit should work as originally intended. Sorry for the long answer, but I hope that explains it.
When you get transformer sag (or sag due to those limiting bulbs being in-circuit) you will see the top and bottom of the square wave be sloped, instead of flat and square, due to the voltage drop over time from the transformer or current limiter.
@@xraytonyb Thanks. Testing I almost always see spikes on top and bottom of the square. Not with one amp but many under test. Something wrong with my setup? Cannot figure it out.
Tony just curious since you do a lot of meter measurements why not get a bench meter? I know it is not as portable as the Fluke. Or buy the Bryman 235 meter which is just as accurate as the Fluke.
I currently have 4 of them on the bench; 2 Digital (Keithley 2015/Keithley 197) and two analog (RCA Volt-Ohmyst / HP 410C). Although I use them a lot outside of videos, it's hard to get the bench meter and what I'm doing in-shot. It's much easier to use a handheld meter, so that I can get everything in-frame. As for the Fluke meters, I have other meters, but the Flukes are what I have used at work for the last 32 years. The ones you see on the bench have either been retired from the field or are spares. I am also liking the Amprobe meter I just got, although it needed to have the input jack re-soldered and it has a much higher burden voltage on the milliamp mode than does the fluke. Thanks for the comment!
8 Ohm minimum speaker impedance on the spec. sheet? Is this amp really limited to 8 ohms, most of my speakers are in the 4 to 6 ohm range. When measured with a frequency /impedance sweep, they even have dips into the 3 ohm zone. Is Sansui generalizing or? Would it run hot at the worst.
The unit has outputs for two sets of speakers. If you select both sets at once, it will put speakers A&B in parallel, making the load 4 Ohms (nominal). The amp can handle this OR ONE 4 ohm set of speakers. They just don't explain this in the manual. At 4 ohms, however, the amp will run hotter than with an 8 ohm load due to the added current. In addition, the transistors I used can dissipate 2.5 times the power of the original transistors. Most likely the emitter resistors would open before the transistors failed.
@@xraytonyb Thanks, I thought there was a way the amp was rated for 4 ohms. The 4 ohm question came up on a set of speakers I recently designed and built, (thanks to some great software too) and they have a two woofer MTM setup so 4 ohms is a real number. My question is that I use two integrated amps, an Onkyo and a Yamaha, both less than 5 years old. Each has a switch for 8 or 4 ohm loads output. The manuals dont say much except dire warnings. How do these amps provide two outputs? Pretty sure its not an output transformer as tubes. My limited study and experience hasn't come across a way to perform this. Maybe an additional output stage of power transistors? Thanks a ton, best channel on the tube.
It turned out that there was a cold solder joint on one of the probe sockets, probably due to the use of lead-free solder which has a higher melting temp. Some flux and fresh solder and the meter seems to be working great. It still has an unacceptable burden voltage drop in the milliamp mode. I will use it for everything except current.
@@xraytonyb I guessed that. I usually put some deoxit fader lube on alligator clip hinges, probe connections & the like, to reduce measurement wander. I never checked the burden on mine. The other limitation is 400uF max.
Nope, it was high inflation (like we are now going to experience), the dollar losing it's value, and the carter recession. The japanese tried to keep prices the same as what was replaced from prior years (like this replaced the 7070db). Since 1980, it's been bpc (black plastic crap) in the audio market.
Direct-Coupled. All of the transistor stages in the amplifier board are directly connected to one another, without the use of coupling capacitors. The only bad thing about it is that when one transistor shorts out, it often shorts the other ones in the circuit as well and you end up with multiple fried components.
@@zulumax1 , nobody with a functioning brain or a lick of common sense would try to run a 1970's car with original belts, tires, battery and oil/lubrication; and yet there are audio-fools, and guitar players, who think their amp should be able to run just fine with all original components from 50 or 60 years ago......
shouldn't you use a more powerful relay like 275 volt it was 240V's if that came to the UK that be running at it's maximum voltage limits, it's a switchable universal power supply isn't it? anyway I would do away with it reallys should not be in audio amplifiers they degrade the sound, not sure about solid-state tho! if they are better or not! why don't you use solid-state instead of mechanical relays? isn't it better to have the offset a little bit higher say 10.5V so it's constantly running in class A mode for a higher given output
The relay in question is being used in the speaker protect circuit. It is switched by a 24VDC source and it controls the power going out to the speakers. This relay isn't affected by what mains voltage the amp is wired for. The relay has four sets of contacts which are wired in to parallel sets for right and left channel. They add very little resistance to the overall circuit and while anything can affect the sound, this would be minimal. Hope that helps
@@xraytonyb yeah I didn't think course it is it's not a mains one. depends on the amplifier I'm not sure it's minimal it can affect soundstage ? merry Xmas lol
@@hoobsgroove The G-8700DB has a 3.9 ohm soft start resistor and a timed mains relay that kicks in parallel after the capacitors are charged. Is that the kind of relay you are talking about?
I have this models smaller brother G-5700 on the bench right now. It's the first time I'm repairing a Sansui receiver, and boy am I dissapointed! It's an awful design! It's the worst case I've seen, all burned up inside, half the caps leaking and spilling electrolyte all over. Resistors charred, transistors burned or damaged. Flux crud from the factory hade to be scraped off. I say to myself "what the hell was Sansui thinking?", " why are these so appreciated, it's a bloody failed design". Then some ICs that are unobtainium. And component margins that were too thin. And a design that is a hodgepodge of not very practical or durable solutions. It's a nightmare to work on. Here is where the era of throw-away-consumer-electronics began it seems.. No criticism of Tony's video of course, very helpful and interesting, as usual. And good to see I'm not alone in battling the weirdness of early 80'ies San-sewage. Damn, I so badly want to dump this receiver in the garbage, but it's a customers so I'll struggle a bit more..
