So, after much progress constructing the 7C main board, found another interesting oddity that makes no sense in the wiring from balance/volume pots to grids of tone amp tubes V4 and V5. I have always assumed the large helicoil shielded cables connecting these two points were simply connecting the output of the volume pot to grid 2 of V4 and V5 but such is not the case. This cable connects both grid and cathode to the volume pot but due to the blurred copy of the 7k kit schematic, I can't really pinpoint where the connections happen at the volume pot. Either way, why in the world would Marantz connect the cathode of these sections to the volume pot in the first place? Also included in the images attached is a small detail of the helicoil shielded cable showing a 1k resistor at one end of what I'm assuming is one of the ground leads within the assembly.
Looking for help from anyone with knowledge of this part of the wiring as to what connections actually take place and why?
Looking for help from anyone with knowledge of this part of the wiring as to what connections actually take place and why?
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Howdy,
If I may, let me ask a more defined question since this may be not so easy to figure out except for members who have hands on experience with modding or restoring Marantz 7C's.
Is there any reason at all anyone can guess as to what function or purpose connecting the toneamp first stage cathode to the volume pot since it should only head to the tone selector group and ground via R81A?
Has anyone ever seen anything similar with any other tube phono or line pre's?
If I may, let me ask a more defined question since this may be not so easy to figure out except for members who have hands on experience with modding or restoring Marantz 7C's.
Is there any reason at all anyone can guess as to what function or purpose connecting the toneamp first stage cathode to the volume pot since it should only head to the tone selector group and ground via R81A?
Has anyone ever seen anything similar with any other tube phono or line pre's?
It possible there's two leads in the screen cable one from the grid and one from the cathode. The grid far end goes to the volume pot middle, the cathode far end goes to a resistor next the volume pot, and this resistor goes to the cold end (ground) of the volume pot. The idea is that any differential ground hum between the volume pot cold and the amp is nulled out. That would be my guess.
The second schematic is almost illegible, but it appears the shield is connected to the cathode at the input and ground at the pot. If so this is a mistake.
Possibly they were trying to bootstrap the shield from the input tube cathode to the pot - if this is the case it should not be grounded at the pot. The purpose would be to drive the shield to minimize the effect of shield to center conductor capacitance.
I would recommend using low capacitance cable and not using this technique.
Possibly they were trying to bootstrap the shield from the input tube cathode to the pot - if this is the case it should not be grounded at the pot. The purpose would be to drive the shield to minimize the effect of shield to center conductor capacitance.
I would recommend using low capacitance cable and not using this technique.
let me see if I'm following. The outer helicoil shield connects at one end only, at 1k resistor just prior to vol pot case? The schematic (7k) shows two conductors within the shield, photos clearly show one connected to pin 2 and the other to pin 3 (grid and cathode) , then traveling to the volume pot. To me, having a direct connection from cathode to any terminal at the volume/balance pot would be detrimental to the intended use at the tone amp.
Unfortunately, I don't have a 7C on hand or know anyone who has one to allow me to take photos. All I have to go on visual wise is photos on internet but most of these are not the clearest nor brightest.
It *looks* as if they made an error, but the error wasn't bad enough to keep it from playing sound, so it wasn't caught. It looks like they misunderstood the idea of bootstrapping the shield, which is supposed to work as rayma described it. Better machines of the handbuilt era, including McIntosh, did this too.
You really don't want an extra 1K cathode resistor in parallel with its 4K7.
All good fortune,
Chris
You really don't want an extra 1K cathode resistor in parallel with its 4K7.
All good fortune,
Chris
From my understanding, the 1k resistor terminates the outer shield at one end (at the pot case). Not sure if there is an additional shield surrounding the twisted pair that hooks up to terminals 2 and 3 of V4 and V5, but, to me, would be impossible for there to be a connection with no resistor from cathode to any part of the balance/vol pots but this is what is being presented in Marantz's drawings and photos of the connections verify this. Very weird.
Wow, this is more than I had hoped for! Thanks for going to this effort to verify our assumptions.
Puzzling over this, I guess it's possible that the kit makers don't connect the shield at the cathode end (float it), but carry the cathode connection back to the pot end with a separate wire inside the shield, and connect it to shield through that 1K resistor. Can't understand why, but at least it wouldn't mess up the existing (Marantz) circuit.
Interesting puzzle, thanks!
Chris
Well, these pictures may not really help too much. It's difficult getting close in on the connections. There is only a 1K 1W CC resistor connected to pin 8 and nothing else. Other end to ground buss. There is a coax cable from the volume control to pin 2 (grid). The coax shield is connected to pin 3 (cathode) while the other end is floating. Then there is a separate ground wire from the volume control low end to the ground buss. This wire is inside plastic tubing surrounding the coax cable. One each per channel. There is no other 1K bridging anything.
I'll have the 7 downstairs with the covers off for a few days if you need something else.
I'll have the 7 downstairs with the covers off for a few days if you need something else.
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Hollowstate, these photos are very helpful and answers a lot of questions! Thanks for taking the time to take them. They also bring up a couple more questions as to why certain things are done differently with your particular 7C. One is, why the shield connects to cathode pin 3 with other end floating instead of being tied to ground buss?
From your photos, it shows that there have been various changes in wiring strategies over the years between different 7Cs and how they decided to wire the 7K kit model. An example is the way they wired the connection between cathode (pin 3) and tone amp. Most I've seen have a black coax connecting the two with shield going to ground buss at one end but yours has a single unshielded yellow conductor. Also, I've seen many 7Cs and 7Ks using the large helicoil shielded coax with 1k resistor at volume pot connection which yours does not.
I will construct my 7C build without the more complicated coax and 1k resistor and just use a good shielded twisted pair and wire as your model is done. Would like to know from everyone why the shield connects to the cathode (pin 3) with other end floating?
From your photos, it shows that there have been various changes in wiring strategies over the years between different 7Cs and how they decided to wire the 7K kit model. An example is the way they wired the connection between cathode (pin 3) and tone amp. Most I've seen have a black coax connecting the two with shield going to ground buss at one end but yours has a single unshielded yellow conductor. Also, I've seen many 7Cs and 7Ks using the large helicoil shielded coax with 1k resistor at volume pot connection which yours does not.
I will construct my 7C build without the more complicated coax and 1k resistor and just use a good shielded twisted pair and wire as your model is done. Would like to know from everyone why the shield connects to the cathode (pin 3) with other end floating?
I do not know just why the coax shield is connected to the tube's cathode instead of directly to ground as would seen logical. Technically what it does is to add a tiny amount of capacitance between grid and cathode. Perhaps just a few picofarads worth in six inches of coax, while still providing some shielding against noise pickup. The yellow wire you see from pin three connects to the cathode resistor (4.64K 2%) mounted on the bottom of the terminal board. Why this exact value? No special reason as they probably had plenty of surplus parts available.
What I know nothing about is a 7K (kit?). AFAIK Marantz never sold this as a kit. Has someone marketed an ersatz copy of a model 7 with modifications? I actually have two Marantz 7s. Serial numbers 12798 and 13759. They are both exactly the same inside. And just to be clear, there never was a 7C. Only a model 7. Over the many years the suffix C became added by people thinking it stood for the wooden Cabinet that may have been purchased with it.
What I know nothing about is a 7K (kit?). AFAIK Marantz never sold this as a kit. Has someone marketed an ersatz copy of a model 7 with modifications? I actually have two Marantz 7s. Serial numbers 12798 and 13759. They are both exactly the same inside. And just to be clear, there never was a 7C. Only a model 7. Over the many years the suffix C became added by people thinking it stood for the wooden Cabinet that may have been purchased with it.
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"In 1979 Marantz Japan released the kit form of the Marantz 7 as the Marantz 7K. "
6moons audio reviews: SPEC Corp. REQ-S1 EX
6moons audio reviews: SPEC Corp. REQ-S1 EX
