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7T HOT RODDING !!!

TheLostBrain

TheLostBrain

Active Member
Ok so just as the title says this thread is about modding / Hot-rodding the Marantz 7T!

At this point this thread could just as well be titled "JSTANG's 7T Mods" because just about everything I'm considering doing so far is something that's been posted by him at one point or another. ;)

I've been attempting to build up a collection potential mods gathered from various posts and hope to post them here shortly.

In the mean time if anyone would like to contribute any ideas they might have that would be great! Hey jstang you out there?? :)

In the end I'm hoping the contributions could be used to compile a detailed list of mods that would be beneficial to all.

Thanks!

As an aside... I read a 7T review awhile back that mentioned an Audiophile article (may have been another magazine) that included an interview with one of the original designers of the 7T and that the designer had commented on certain things that he would suggest "upgrading" on the 7T. I'd love to find that article if anyone has any ideas at all.
 
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Here's what I've saved from Zed Audio

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
 
Thanks for responses guys...and thanks bryans12v for the zed audio one - I've included it here as well.

Ok, so..... Step 1: Information Dump!

Below is a chronologically ordered dump of posts on 7t mods / servicing suggestions that I was able to gather. (Note: Some posts may even just be small tidbits of info that *may* be useful) Notice how 99% of it comes from jstang as mentioned in my original post!
The chronological ordering here may be important for determining if certain "earlier" services were replaced with newer approaches, etc.

The next Step will be to go through all these and compose a consolidated list of mods / servicing suggestions...and then go from there.

A quick survey shows a good chunk really is just cap replacements, etc..but there are a few extra goods in there..and MOER's post really adds quite a bit more as well.

If you guys run across anything not here please do post - I'll be on the lookout for more too.

As an aside I was able to locate the magazine with the interview of Sid Smith in it that I was asking about in the original post. I've ordered it and should be here this upcoming week I believe so maybe something interesting in there?


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jstang (2008/04/26)
Ok just replaced the four 15uF coupling caps (L+R,In/Out), these are the only electrolytics in the signal path. Left the power filters alone..... I love this pre-amp.....
All 4 of the caps were reading too high ESR and one was at 12 ohms. They should be below 1.5 or there abouts. The new ones were .56 and I used a film bypass...
http://audiokarma.org/forums/index.php?threads/recaping-a-7t.161532/#post-1816809


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jstang (2008/06/07) - The 7t have very few electrolytics. I replaced 4 or 6 of them....I forget. But the main signal path has only 4, I think....
http://audiokarma.org/forums/index.php?threads/7t-3300-audition.168248/#post-1908416


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jstang (2008/12/19) - So reading the schematic there are four 22uF 4v electrolytic caps. C404,C408 on the Tone Amp board and C206,C205 on the low level amp board.
For the life of me, I can not find any of them in the preamp....
Ok found them and replaced them with an audio grade Electrolytic and Film Bypass. They turned out to be tantalums and very very small in size.
Those 22 uF 4v caps were about the size of a small signal diode. Silver case, but time for them to go after 40 years.
http://audiokarma.org/forums/index.php?threads/model-7t-c404-c408-where-are-they.199411/
http://audiokarma.org/forums/index....c404-c408-where-are-they.199411/#post-2336420


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jstang 2009/06/05)
Very simple mod that will knock your socks off on the 7T....

Bypass the head phone jack for R and L positive wires with a .1uF film cap. Or bypass it totally.... There are switch contacts in the headphone jack that do not like "time" (meaning they lose conductivity) and high frequencies...

The head phone jack is in the signal path to main outputs.

Also, I treated the transistor leads with Silclear. I still have the stock transistors. The sockets are 40 years old at this point. They need some help with conductivity.

Recapped the electrolytics fully, look for the 4 small tantalums and make sure you replace them.

And the complete De-oxit is needed.
http://audiokarma.org/forums/index.php?threads/opinions-on-marantz-7t-preamp.233165/#post-2790347


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jstang (2009/08/27)
Simple tweak for 7T.... that made difference for me.

I de-oxit the transistor sockets and leads of the stock transistors. Also used silver conductive grease on the stock transistors...

http://www.6moons.com/audioreviews/walker3/SST.html

I used the silclear, for 35$ you get more then enough to last for years and years of tweaking...

http://www.vhaudio.com/contacttreatment.html

That made a difference. Those 40 year old sockets need some help...

But I might go for new transistors and do a side by side...or channel by channel.
http://audiokarma.org/forums/index....er-to-16-power-amp.246882/page-2#post-2984228


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jstang (2009/12/31) - My two cents... once recapped and cleaned the 7t is great.... the headphone socket is problem as it has contact that are in the signal path that shut the rear outputs off with headphones plugged in. The contacts should be bypassed for best sound.
http://audiokarma.org/forums/index.php?threads/marantz-7t-and-model-16-amp.272161/#post-3319263



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jstang (2010/10/30) - Clean or bypass the headphone switches in the headphone jack. Also clean the transistor sockets. Just make sure the transistor go back exactly on the original sockets.
http://audiokarma.org/forums/index....-up-one-of-my-7t-preamps.330297/#post-4106714



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jstang (2011/10/21) - Yours (PB_Audio) and mine have most of those mods outlined by MOER plus some others.....but most of the goals/intentions/benefits have been met....or exceeded. (MOER posted here: http://www.diyaudio.com/forums/solid-state/113826-replacing-electrolytics-marantz-7t.html)
The biggest most time consuming mod? Replacement of every resistor with 1% mil films has not been done... The time to do that correctly and cleanly would be days on the 7t. Plus Marantz had already used 1% metal films in some locations. But a few correctly placed 30$ bulk foil resistors might really do wonders....
http://audiokarma.org/forums/index.php?threads/7t-3300-audition.168248/page-2#post-5071108


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jstang (2011/10/21) - Yes different.... I have a very early one # 10430 and very late # 18649. If I remember the serial numbers. One made in '66 and the other in '69. The transistors are different. Some of the caps. Slight difference in the internal shielding. More Wiring insulation on the later. The finish on the face. And slight differences in the schematics by serial number.
http://audiokarma.org/forums/index.php?threads/7t-3300-audition.168248/page-2#post-5071361



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jstang (2011/11/18) - You heard version 2.3 of the 7t.... We are currently at version 3.4. :) Yours is at v3.4 too. :D I am still looking for shavings for the pile. Have plans for v5. But that will be a major change. Need to find a basket case 7t for that one.
http://audiokarma.org/forums/index.php?threads/marantz-33.389583/page-3#post-4983595


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jstang (2012/05/23) - Big fan of Silmic II for the electrolytics
http://audiokarma.org/forums/index.php?threads/best-caps-for-marantz-7t.445465/#post-5721960


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jstang (2012/06/30) - 7t is recapped with Silmic IIs and film bypasses too
http://audiokarma.org/forums/index....g-testing-which-one-wins.453566/#post-5836705


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jstang (2012/07/01) - Also, I added/changed some cap values and many other items on the 7t
http://audiokarma.org/forums/index....g-testing-which-one-wins.453566/#post-5837405


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PB_Audio (2012/07/05) - Here's a hint, specific to the Model 7T: think transistors and silver paste... When's the last time you heard mention of using silver paste?
http://audiokarma.org/forums/index....g-testing-which-one-wins.453566/#post-5849207


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jstang (2012/07/29) - They benefit greatly from new caps because unlike the 7c, the 7t has electrolytic caps in the signal path.
Even has some tantalums that must be upgraded.
http://audiokarma.org/forums/index....ing-which-one-wins.453566/page-2#post-5917843


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jstang (2012/07/29) - (slight paraphrasing here) cleaning of the switches, RCA jack cleaning, cleaning of the internal connections
http://audiokarma.org/forums/index....ing-which-one-wins.453566/page-2#post-5919090


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jstang (2012/07/29) - Nothing that some Silmics and bypass caps can't fix.... And ditching the tantalums is a must these days.... As a matter of fact that is what a lot of people hear in electrolytic caps vs films.... New high grade electrolytics with film bypasses help resolve that issue...they take the harshness out of the sound.
http://audiokarma.org/forums/index....ing-which-one-wins.453566/page-3#post-5920120


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jstang (2012/07/30) - I left the resistors and film caps stock in the 7T and focus on other areas... Some people recommend replacing the resistors in the 7t...
http://audiokarma.org/forums/index....ing-which-one-wins.453566/page-3#post-5921133


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jstang (2014/01/09) - Film caps in place of electrolytics, add caps in some places, bypassing of some old tech, adding of noise filters, ...... The list is long.
http://audiokarma.org/forums/index....ving-up-to-its-potential.567135/#post-7444201








Below are The MOER suggestions plus some people who had 7t work done by him w/ some extra details





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MOER (2007/12/24) - 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.

http://www.diyaudio.com/forums/solid-state/113826-replacing-electrolytics-marantz-7t.html






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naimarantz (2008/04/03)
I recently had Steve Mantz from Zed Audio upgrade my 7T. First off I have been into audio for 38 years and I've seen a lot of claimed upgrades and tweaks that did nothing to enhance the musicality of a system. Too many sounded different, but not better. Therefore I was a little apprehensive about upgrading as I originally sent it to be repaired. Would it change the sound into something I wouldn't like?
Well upon plugging it back in it was immediately apparent the sound was significantly improved and it still sounded like a 7T. Whew!
The improvements were all good and I didn't have to give up something to get the improvements. You really have no idea how good a 7T can sound until you hear this upgrade.

The improvements right out of the box were ;

- more musical
- better rhythm
- more involving
- more coherent
- quieter background
- fuller sounding
- more dynamic
- better balance between channels
- better imaging (solid imaging, not etched)
- better depth
- better soundstage
- more scale

.....and it still retained 100% of the "Marantz sound",

After about 200 hours run in I ran it against my other 7T and confirmed the above.

I would like to publicly thank Steve Mantz for a job well done!

I will be packing up my Model 15 this week and shipping it off Zed Audio for more of the same.

NM

http://www.diyaudio.com/forums/soli...g-electrolytics-marantz-7t-2.html#post1474821


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winters860 (2009/06/05) - Work performed by Stephen Mantz at Zed Audio in December 2008:

1. Power supply has been upgraded and redesigned, down to just the original transformer
2. Additional power regulation has been implemented in the RIAA stage
3. All capacitors in the signal path have been replaced with new film or electrolytic types
4. All transistors have been replaced with new low noise, high Ft, high gain types
5. Faulty volume control potentiometer was replaced
6. RCA connectors have been replaced with new high quality gold-plated ones
7. The scope outputs have been rewired to the main 20db gain stage, bypassing the final high pass/low pass filters
8. The controls have been cleaned and lubricated

At that time, the following options were declined:

1. The addition of a moving coil phono amp
2. Modifying the volume control to function as a variable shunt
3. The replacement of additional passive components

http://audiokarma.org/forums/index.php?threads/opinions-on-marantz-7t-preamp.233165/#post-2789901
 
According to the original designer, Sid Smith : new caps, a befier and regulated power supply, new volume and balance pot will do the job.

You can simplify the signal path by bypassing the tape switch, tone amp, filters and headphones.

But first on the list, the tantalum and electrolytic caps must been change to Silmic II and a good cleaning of the input and output RCA must be done.

Changing the resistors to all metal could be a mistake, resulting in a more dry and hard sound. Allan Bradley carbon comp are more noisy but...they have musical vitue too.
 
I've read most of these from over the years but still have yet to see any actual images of this work performed.

I'm currently in the process of replacing the El caps on my 7T. Someone (before me) replaced all the PS with nice Vishay's but used all 220uf instead of the called for 250uf/350uf's.
 
Since there are no images here of these upgrades I thought I would share some work I did to mine a few months back.

My early 7T serial# 202
20161223_183022.jpg

Wish they were all this easy to work on. The boards are two screws and move out with minimal wire removal. One thing to keep conscious of is that there are a few exposed solid stands that can move onto something they shouldnt when pulling a board out.

The reason I dove into this one is that I lost the left channel, turned out to be one of the 4 15uf Spragues had failed open. At the time I swapped in 2 22uf L+R) silmics and wanted to update the rest. I did a simple bypass on the headphone jack by soldering on some 90° cut off leads which can be reversed in seconds. I also replaced some films and tantalums and gave the transistor leads a dip in silclear.
20161224_194843.jpg

20161224_194854.jpg

20161225_083319.jpg


The rear rca jacks are in rough shape and can't really be polished too well. I have ordered and received all new gold plated ones but am concerned about taking away from its originality. Would you guys do it? I'm thinking of maybe just getting silver ones to keep It from looking horrible.
 
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bryans12v said:
Since there are no images here of these upgrades I thought I would share some work I did to mine a few months back.

My early 7T serial# 202
View attachment 867402

Wish they were all this easy to work on. The boards are two screws and move out with minimal wire removal. One thing to keep conscious of is that there are a few exposed solid stands that can move onto something they shouldnt when pulling a board out.

The reason I dove into this one is that I lost the left channel, turned out to be one of the 4 15uf Spragues had failed open. At the time I swapped in 2 22uf L+R) silmics and wanted to update the rest. I did a simple bypass on the headphone jack by soldering on some 90° cut off leads which can be reversed in seconds. I also replaced some films and tantalums and gave the transistor leads a dip in silclear.
View attachment 867404

View attachment 867405

View attachment 867406


The rear rca jacks are in rough shape and can't really be polished too well. I have ordered and received all new gold plated ones but am concerned about taking away from its originality. Would you guys do it? I'm thinking of maybe just getting silver ones to keep It from looking horrible.
Click to expand...


Hi bryans 12v, nice pictures! I can see that you used some bypass caps in parallel with the Silmics II, could you tell me please the specs of them? Im just trying to restore my newly acquired 7T and would like to put also some bypass caps , cause I heard that they improve the sound on the preamp. Any help would be appreciated.

Thanks in advanced and really nice 7T!

Gonzalo.
 
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