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old news, I know the people that did the test, it got debated on the AK for a year.
The 1980 is an amp topology that is still worlds ahead - even with the built in flaws, but no one is asking any of the makers to do this today. only about 1% of the population listens to music and even then the most go digital because they like crap.
do NOT buy a 1980. Unless it says full rebuild and modernization and the seller has the documentation from the 5 people in the US that can do it, you are going to pay $2000 for one that needs work to run, and you are going to pay at least $2000 and 2 years to get it worked on, from 1 of the 5 people in the USA capable of doing this.
that being said, as the receiver comes on, there is a huge voltage offset across the main amp. This is a 285watt amp so the offset could be a couple volts which will blow the speakers - you never put strong DC across a speaker. They simply use a delay circuit to drive the base of a transistor to trigger a relay that connects the emitter resistors to the speaker terminals and headphone circuit....that is the click you hear. when you tune one of these and it ages, you put a dvm across the speaker outputs and note the reading, if it is still over 100mv, you increase the value of the cap and perhaps beef up the collector current of the drive transistor to handle the increased duration of inrush current.
I'm going through this right now with a Sansui G7500. It was my late fathers receiver, so it has sentimental value. When I got it, and hooked it up, one of the channels wasn't working. I took it to a local shop that had done some repairs on a couple of TV's and a surround sound unit, that claimed they knew vintage audio. They repaired it 4 times before I gave up and found another individual. After spending a couple of hours on the phone with him, talking way over my head about the details, the one thing I did take away is, these aren't "repaired", it's more like a restoration. He's had it now about two years, and just gave me an update last week. He's waiting on a few more parts and then will introduce some power to it with something called a Variac in a couple of weeks.
Hopefully all will go well.
In between one of the first four repairs, I did get a couple of hours of time on it, and was able to A/B test it against a 90's Pioneer VSX-604 and the sound quality difference was definitely noticeable.
He's waiting on a few more parts and then will introduce some power to it with something called a Variac in a couple of weeks.
Hopefully all will go well.
In between one of the first four repairs, I did get a couple of hours of time on it, and was able to A/B test it against a 90's Pioneer VSX-604 and the sound quality difference was definitely noticeable.
A "variac" is an electrical device that contains a rheostat that takes 120V input, and then by turning the dial on top of it's housing, you can "dial in" anywhere from "0" to 120V. It allows you to apply the voltage in incremental doses, rather than slamming the electrical appliance will 100% of the voltage. That way, if something is still amiss, you can both diagnose it, as well as protect the appliance from a full voltage hit.
A "variac" is an electrical device that contains a rheostat that takes 120V input, and then by turning the dial on top of it's housing, you can "dial in" anywhere from "0" to 120V. It allows you to apply the voltage in incremental doses, rather than slamming the electrical appliance will 100% of the voltage. That way, if something is still amiss, you can both diagnose it, as well as protect the appliance from a full voltage hit.
I figured it was something like that, a much more refined and controllable version of the "dim bulb" test I've read about in the hobbyist forums.
the one thing I did take away is, these aren't "repaired", it's more like a restoration.
part of the problem is the silicon is no longer made with THOSE parameters, so the large part of doing a restore is finding modern silicon - which of course may be a lot better, lower noise floor and more robust - that meets the circuit parameters.
When we list trans subs the top values we use are:
Pc - how much power can this think produce or sink. Unless the original was over kill, you go higher. Easy.
Next is Vcb - how much voltage on the 'rail' can this see, before it saturates and fails. Its EASY to go higher today. The ultra low noise signal trans we use today are in the 120v range and the originals generally 30-50v - childs play.
Then Ic - the collector current, this current is what supplies the output - the emitter in an amp circuit. Sometimes, we DO go smaller, same ultra low noise small signal stuff (like the on-semi KSA992F/KSC1845F matched pairs) is 50ma. So in this case you look at the circuit and look for limiting resistors on the collector or emitter...take the rail, divide by the value and get the current. And if the above are too small, we head to KSA1015/KSC1815 which spec 150ma, but not AS much gain and only slightly noisier <-- this is the re-engineering part.
Then we look at Ft - the transition frequency at which point the transistor muddys the waters on vs off, or even responds to level changes within its conductance - like applying a small 10mv signal over a 20v operating region - you need the signal to be faithful in frequency or else...well, it aint music. Series pass transistors for like power supplies can be slow...generally 4-12Mhz, but we like music signal stuff to have a bit more and in the modern non switching amps (NSA) - the really hard stuff - 200Mhz is sometimes specced. Now it gets hard. AND, if you go too fast, it may oscillate in which case you need to determine WHERE the oscillation is and solder 5-100pf across the emitter/collector leads. More engineering on the fly.
Then of course the gain - Hfe. Series pass dont care about the gain. Voltage followers dont care about the gain, but the small signal stuff...you need gain AND for differential inputs, you need to MATCH the gain and Vbe (with a meter, not a datasheet) which is why I buy at least 50 at a time (1000 at a time last buy!) so I can match the gain on my handy dandy meters. Hand me a DMM without Hfe function, and its only good to me for tracing ignition circuits on a mower. (which implies I have a small pile of really good DMMs)
There are other parameters you can worry about but these are the bigguns. Goto a website like the AK and look in 'solid state' and find the 200 page long transistor sub lists we have built over the years. It would be VERY hard for you to bring me say 5 stereos to restore and in the 'birth certificate' I give back with each, not find - for example KSA992F on the inputs and the KSC1845F in the phono board of EACH.
Then the manufacturers for some unknown damn reason drop stuff like the KSA1220AYS PNP which was matched to the KSC2690AYS NPN and we used EXTENSIVELY for drivers...the part of the amp that drives the base current for the outputs. They did this with 60 days notice and you heard a cry an anguish across the web while one of the true smart cookies found that the Toshiba TTA004B/TTC004B subs perfectly, except where the Ic needs to be high.
And dont get me started on the FETs for the FM RF detectors... FM runs 108Mhz so you need over 250Mhz in a FET to START - closer to 400Mhz....not to mention, who the hell does FM RF outside of a sanyo chip anymore? You may not find ANY.
yeah all that was overly technical and of little value to most, but its the 'art form' keeping the vintage silver humming. Hear the radio in the background? MCS 3222 (for sale!) at 22 watts driving HPM100s. (not for sale) Why not my office SX890 driving the carvin PM5 studio monitors? because the damn MPX chip made by pioneer and totally totally unavailable has futzed out on the FM mute circuit. So while I search for a crushed, broken, ugly SX780/880/980 to pirate from, my daily driver is off unless its 'play records while you work day' (generally friday...um TODAY!)
I figured it was something like that, a much more refined and controllable version of the "dim bulb" test I've read about in the hobbyist forums.
yes, the variac is an infinite set of bulbs to go dim, BUT! the dim bulb limits current as well - not just voltage. If an output is shorted, the dim bulb lets it run till you shoot the problem AND, when its time to 'tickle the dragon' (set the idle current, which can be as scary as the ORIGINAL use of the term) the dim bulb lets you assure that the next start full voltage/full current will have the idle current set too low. Its always better to approach that one from the low side and these things go thermo nuclear and to quote one of our master-master techs...its racing to destruction. some say at the speed of light. by the time you see or hear the spark, its long over. The smoke is just the after effect.
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