Replacing electrolytics, Marantz 7T

[mtflycaster]

Hi all -

First post, though I've been lurking a while.

Just successfully replaced all the electrolytic caps in my Yamaha A-1000 amp and cleaned up some corrosion from a leaky cap. Sounds great!!

I'm now on to another project. Recently acquired a Marantz 7T in nice, functional condition. No collectability, unfortunately, since someone replaced the power switch and did a real hack job on it. So, I'm looking at doing a variety of mods on this preamp, starting with replacement of the electrolytics. It only has 9 total, all appear to be original.

I'm good with a soldering iron, but still learning about amps, and interested in only doing straightforward mods. So, pls forgive the perhaps elementary questions.

First, the 7T has what looks like an original 2-in-1 250uF capacitor just after the power supply. I've attached a schematic. Single can, two leads out the positive end, one out the negative. So, my plan is to replace this single cap with two new caps. Am I missing something?

Second, as I did in the Yamaha, I plan to use two capacitors in parallel to achieve the same, or nearly the same, capacitance rating as ones I'm replacing. Any problem with this? The alternative is using single replacement caps that may be 30 to 50 percent greater capacitance, which I fear doing.

Thanks in advance, and I'm sure I'll have more questions. Will also let you all know how it goes.

MTfly

[mtflycaster]

Anyone??

I think at this point, my plan is to replace the electrloytics as follows, rather than use parallel caps:

330 uF/100v for the 250uF/80 volt power supply caps,

and 470uF/50volts for the 330uF/50 volt caps,

and 22uF/50 volt caps for the 15uF/35v output board caps.

and will see how it sounds. Will use Nichicon Muse series and Elna Similic. Hopefully, the higher capacitance values won't be a problem.

While I'm at is, I'm going to replace the heavily oxidized output posts with new gold plated ones.

Any comments on the caps?

Thanks all and best wishes for a fabulous holiday!

MTfly

[fatmarley]

You need to know what the capacitors do (coupling/decoupling etc) before you replace them. If you dont know what they do, let me know and i may be able to help.

[fatmarley]

This is my fourth reply that ive lost. Email if you want help.

[mtflycaster]

Thanks very much for the offer. Yes, I'm sure I could use some help!!

Unfortunately, since I'm new to the board, I can't yet email members.

:-(

Hopefully, that will change soon.

I don't really understand the whole coupling/decoupling thing. I will try to educate myself.

My intention here was to upgrade the original electrolytics (vintage 1965) with more modern, fresh caps, but I know that there is more to caps than capacitance and voltage.

So, I'd welcome some help!!

Best wishes....

MTfly

[fatmarley]

Have a good read here www.acoustica.org.uk and i will get back to you when i have time.

A photo with a discription of the original parts would be very helpfull.

Happy Christmas
Matt

[mtflycaster]

Thanks, Matt!

[MOER]

The 250mfd capacitors in the power supply are the bulk storage pre series pass transistor and then there is a 250mfd capacitor on the base of the series pass device.

The series pass device serves two functions.

1) The zener diode from its base to ground serves as a crude regulator.
2) The capacitor (250mfd) in parallel with the zener has its capacitoance multiplied by the DC current gain of this series pass transistor.

Do this in the power supply and you will improve matters immensly.

1) Replace the four rectifier diodes with faster higher current parts. Since they are hand wires on a tagboard there is no problem of space. HER302 diodes work well.
2) Solder a 0.1mfd 200v polypropylene capacitor directly across the secondary winding.
3) Replace the pass transistor with a darlington type or add in am extra transistor to convert the pass element to be darlington. This will have massive gain and then the base capacitor's value will now be multiplied by a factor of at least 10,000 times. So putting a 1000mfd/63v capacitor to ground will give you an effective capacitance of 10 million microfarads!!! There will be as close to zero ripple on the power supply.
4) To prevent any oscillation, the base of a series pass device should always have a low value of resistance in series. Values from 22 ohms to 200 ohms are fine.

To improve the total performance of the unit, add local +43v decoupling capacitors at each of the preamplifier modules.

Replace all resistors with 1% metal film types.

Parallel all those power supply lytics with 0.1mfd polypropylene types.

If you will use the 7T with vinyl, the best improvement to make to the RIAA preamplifier is too hard wire the RIAA feedback components to the RIAA board. These are on the input selector switch due to the tape head and microphone inputs requiring different eq.

I doubt tat you will have a reel to reel with no preamps and I doubt you will use a microphone.

Convert these inputs to line level types.

Lastly repalce those cheap RCA conncetors with decent types.

Regards


Steve Mantz
Zed Audio Corp. CA

PS I have worked on these Marantz preamps since 1968 and know them like the back of my hand.

[mtflycaster]

Great stuff, Steve!

As soon as I get a chance, I'll follow-up with some questions to clarify a couple of things.

In the meantime, hope you and yours have a great holiday!

Pete

[MOER]

Dear Pete,

And the same to you and your family. Be safe

Feel free to email me directly at ZEDAUDIO@AOL.COM