Hi,
Some days…are wonderful 🙂
I was just gifted a Marantz 7T pre amp.
I have no idea how long this has been sitting, but it’s been at least 2 years. It looks to be all original inside, and is in like new condition in there. So clean!
Now, I don’t have a varistor. Any thoughts on how to go about turning it on safely? I have a dim bulb tester and a light dimmer wired into a short extension cord.
Any thoughts would be greatly appreciated.
Thanks!
P
Some days…are wonderful 🙂
I was just gifted a Marantz 7T pre amp.
I have no idea how long this has been sitting, but it’s been at least 2 years. It looks to be all original inside, and is in like new condition in there. So clean!
Now, I don’t have a varistor. Any thoughts on how to go about turning it on safely? I have a dim bulb tester and a light dimmer wired into a short extension cord.
Any thoughts would be greatly appreciated.
Thanks!
P
A preamp draws constant and very little power from the mains and you may not have a suitably low rated bulb (say, 15W max.) available. There's no point in fitting a larger rating because it can't protect much that is downstream of the power supply. A fuse of the correct rating is generally a better proposition for a constant load. Perhaps though, powering it via a regulated DC bench power supply instead of directly from the AC mains would be an overall, better idea. That would be my choice, anyway.
Wow, this is old - approaching 60 years so it may be wise to routinely replace the electrolytic caps or at least test them for ESR before powering up, whether it was already done at some previous time or not. There will be date codes on larger caps. that may help you decide what's necessary.
Wow, this is old - approaching 60 years so it may be wise to routinely replace the electrolytic caps or at least test them for ESR before powering up, whether it was already done at some previous time or not. There will be date codes on larger caps. that may help you decide what's necessary.
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Thanks Ian!
its all original, so those cans are old 🙂 I suspect I can find a low enough powered bulb for it. But to your point, replace PS caps before powering up seems like a good idea.
i will exercise some patience.
Per
its all original, so those cans are old 🙂 I suspect I can find a low enough powered bulb for it. But to your point, replace PS caps before powering up seems like a good idea.
i will exercise some patience.
Per
Went to the hardware store (which has an electronic component section in the basement with capacitors, resistors, transistors etc) and picked up some super cheap capacitors. The 4 “big electrolytics“ are 2 each of 250uf/80V and 350uf/50V. Closest they had were 220uf/100V, 330uf/100V, 330uf/50V and 470uf/50V
Any suggestions as to what is most appropriate for testing purposes in what spot?
Thanks!
Any suggestions as to what is most appropriate for testing purposes in what spot?
Thanks!
Testing caps for being functional at all after 30 years or more, requires at least a capacitance value measurement, so for electrolytic capacitors, you need a DMM (a multimeter which for this purpose, also needs to be capable of measuring up to at least 10,000 µF (more correctly, 10mF). You need a good, multipurpose DMM for everything you need to look closely at, anyway. Try to find one that doesn't eat or even use 9V batteries though! You will soon tire of having to buy a new battery whenever you need to use the darn thing.
The proper way to look specifically at electrolytic cap. quality though, involves an ESR or "equivalent series resistance" meter which uses a special pulsed signal to evaluate the capacitor's ability to pass AC current - somewhat analogous to a battery's ability to supply DC current because you also want to know how robust the capacitance is when it needs to pass some AC current - just as you may check a used battery that measures 1.5V with a sensitive meter but drops like a stone to O.1V when loaded by a lamp etc. There are are some good videos on the topic of ESR meters and products out there to help understand them.
The absolute values of electrolytic caps aren't critical when the manufacturing tolerance is like +/- 20% so don't be concerned by the 330uF V 350uF etc. figures that can't be precisely controlled. Ballpark values are good enough in >95% of applications anyway. Just keep voltage ratings well above those of the application because there are surges and spikes on power supplies at various times and these will eventually kill closely rated caps and other components that subsequently become exposed.
The proper way to look specifically at electrolytic cap. quality though, involves an ESR or "equivalent series resistance" meter which uses a special pulsed signal to evaluate the capacitor's ability to pass AC current - somewhat analogous to a battery's ability to supply DC current because you also want to know how robust the capacitance is when it needs to pass some AC current - just as you may check a used battery that measures 1.5V with a sensitive meter but drops like a stone to O.1V when loaded by a lamp etc. There are are some good videos on the topic of ESR meters and products out there to help understand them.
The absolute values of electrolytic caps aren't critical when the manufacturing tolerance is like +/- 20% so don't be concerned by the 330uF V 350uF etc. figures that can't be precisely controlled. Ballpark values are good enough in >95% of applications anyway. Just keep voltage ratings well above those of the application because there are surges and spikes on power supplies at various times and these will eventually kill closely rated caps and other components that subsequently become exposed.
Thanks Ian,
i put new caps in, replacing the caps in the power supply section, and it’s alive!
Zero noise at all. And I mean zero! When I had my headphones plugged into it during first power up, without a source feeding it, i could not tell if it was on. Turned it to phone, dialed it up to 3 and then I could hear some noise.Switches and pots, dead silent!
Before I go to town on this and order a lot of other capacitors, I wonder if anyone can tell me the output impedance of this thing? Can not seem to find online.
Per
i put new caps in, replacing the caps in the power supply section, and it’s alive!
Zero noise at all. And I mean zero! When I had my headphones plugged into it during first power up, without a source feeding it, i could not tell if it was on. Turned it to phone, dialed it up to 3 and then I could hear some noise.Switches and pots, dead silent!
Before I go to town on this and order a lot of other capacitors, I wonder if anyone can tell me the output impedance of this thing? Can not seem to find online.
Per
I guess I missed your point about what to measure and where but it's great news that its working fine - congrats!
To check key voltages, these are listed throughout the circuit descriptions in the service manual. Assuming you haven't found this one yet, here's a link to a free service manual download that's detailed enough for almost anyone to follow. https://www.manualslib.com/manual/1020417/Marantz-7t.html?page=10#manual
The main stereo output impedance isn't specified but that wasn't commonly important for line level outputs that were universally 47k ohm back in the day but could actually be lower, according to the intended application. The headphone outputs look to be for 600 ohm cans so I don't think it will suit many modern types.
To check key voltages, these are listed throughout the circuit descriptions in the service manual. Assuming you haven't found this one yet, here's a link to a free service manual download that's detailed enough for almost anyone to follow. https://www.manualslib.com/manual/1020417/Marantz-7t.html?page=10#manual
The main stereo output impedance isn't specified but that wasn't commonly important for line level outputs that were universally 47k ohm back in the day but could actually be lower, according to the intended application. The headphone outputs look to be for 600 ohm cans so I don't think it will suit many modern types.