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I Need Help, Mcintosh/Marantz 7 content

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#25
Does it wreck the value of these things if you cap & res them?

I mean this sillyness of caps aside, cause in some ways I believe some of the cap hooey, it seems a shame that these pieces shouldn't be able to rock and roll as they once did.

:xeye:
 
#26
poobah said:
Does it wreck the value of these things if you cap & res them?

I mean this sillyness of caps aside, cause in some ways I believe some of the cap hooey, it seems a shame that these pieces shouldn't be able to rock and roll as they once did.

:xeye:
Click to expand...

Yep, the market for a recap is about 1/8th - 1/2 of a stock unit, or at least thats what I see on ebay.

This guy has listed his reworked unit 3 times and not sold.
http://cgi.ebay.com/Marantz-7-7C-St...ryZ67807QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Nice job, but one glance shows that its heavily reworked.

Several 100% stock ones have sold during the same period topping $6000 as a high.

To someone like me, If I was keeping it, I have every carbon comp in the thing. Now those tiny wire wounds I have none.

Those Bumble Bees are the Killer if I need them, I may have a handful but not the correct handful.

Preliminary voltage checks are not showing an issue at any of the lytics as best as I can tell.

There's only a few and even the PS voltages check out within a few volts of the schematic.

No odd tube glow,nothing really obvious yet.

The Good channel blew me away, I had no idea these things sounded that good!

I'll hit it harder tomorrow afternoon, If I do not find anything odd, Its going to take a magician lol
Gene
 
#28
Hi Eli,
Thanks for that.

Gene,
Those caps have been good in every single unit I have done. I normally do not have to recap a unit ussing those caps. I'd just relax and do the folowing. Simply measure the plate cathode and grid voltage on each tube in the bad channel. Relax and look them over. This should tell you if the fault is in an active circuit or not. I have seen the odd open pot (rare),

If you do see and odd voltage, hange your meter on it and turn the preamp on. You should get sound and be able to watch the voltage as the sound goes away and troubleshoot from there.

-Chris
 
#29
I'll rat Gene out in front of the whole forum... Gene still has the unterminated cat5 cable in hanging from the ceiling in his shop.

So it's up and down 400 stairs to post after taking a voltage reading...

His wife spent all their money on shoes... so he has to wear her shoes to go up and down the stairs... well, something like that...

:xeye:
 
#33
poobah said:
I'll rat Gene out in front of the whole forum... Gene still has the unterminated cat5 cable in hanging from the ceiling in his shop.

So it's up and down 400 stairs to post after taking a voltage reading...

His wife spent all their money on shoes... so he has to wear her shoes to go up and down the stairs... well, something like that...

:xeye:
Click to expand...


Gene got absolutely nothing down other than final wiring on a Geetar amp that was cluttering up the bench.

Sara on the other hand did manage to sneak out after this mornings estate auction and YES, She bought MORE Shoes!

I think someday she will be the ruler of a third world Island LOL

BTW, 28 steps each way, 56 steps to read a post, check a voltage and reply. I dont need no stinkin stairmaster!!

For sure tomorrow afternoon I will have all the Grid, Plate & Cathode voltages for each tube.





I have seen the odd open pot (rare),
Click to expand...

Interesting, Balance pot possibly?? There was a tiny ink mark on the face plate he used as a balance setting offset towards the dead channel about 30% off center. It does feel a bit easier to rotate than the others.

Trout
 
#34
Hi Gene,
That sounds more like a weak tube or a bad cathode bypass cap somewhere.

I have a Fisher 400 preamp. When the selenium rectifier was busy failing (over the course of several years) the heater voltage dropped. I had gain problems that were predominant on one channel. If you swapped tubes the problem moved with the tube. All the heaters were lit to about the right colour it appeared. So I bought an entire set of tubes. Problem went away for a short while and came back. I was starting to get emotionally involved now so I put it away for a few years. I found the fault by playing dumb to history and checking the basics first. It was hell. Thank goodness it was mine and not a customer unit.

So Gene, no matter what we say, always go back to basics and check everything. No matter how convinced you are that you already checked something.

-Chris
 
#35
OK,

I replaced all the tubes, but the symptom remains, very low (almost none) output on channel B


I looked over the schematic closely and I only see 4 electrolytic caps used as a bypass caps, ( Unless I am missing something)

I circled them on This Schematic

I think that they are the ones in This Photograph

Also, I noted in the picture that 1 cap looks a tiny bit suspicious on the positive end.

I believe it is possibly C62B on the schematic, 200mfd.

Could it actually be that simple?

Trout
 
#36
If I read that old pile of spaghetti correctly those caps are in the low-level source (phone, mic and tape head) section. Two suggestions, check if the channel re-appears when with the mode selector in 'mono' and try feeding the pre-amp from the 'tape playback' inputs. That switching is centre point in the circuit and the results will tell you whether to search upstream (switching) or downstream (electronics).
 
#37
rdf said:
If I read that old pile of spaghetti correctly those caps are in the low-level source (phone, mic and tape head) section. Two suggestions, check if the channel re-appears when with the mode selector in 'mono' and try feeding the pre-amp from the 'tape playback' inputs. That switching is centre point in the circuit and the results will tell you whether to search upstream (switching) or downstream (electronics).
Click to expand...


OK, today is Finally return to the Marantz problem day.

I tryd the tape playback inputs and nothing changed. I tryd the Aux in, The tuner in and TV in, all result in channel B doing little if anything. Channel A remains strong clear and crisp.

Mono still only produces sound from channel A.

However, An observation of the tubes may be a clue.

The Filaments to me look very dim, Especially V4,V5.& V6.
V4 & V6 look like they are not even lit. V5 is very very dim if lit at all.

V1,V2,V3 are lit but in my opinion not what I normally see from a 12AX7 Filament.

I am going to run it in a totally dark room and re-confirm this but, it sure looks

Since testing, Flipping and Replacing all the tubes has resulted in no change, I am beginning to suspect the selenium rect as possibly being week?

I am off to check voltages further. The filament cap voltages are 18.9 - 16.3 - 12.6.
Note: the 18.9V being correct to the schematic.
Trout
 
#38
anatech said:
Hi Gene,

I have a Fisher 400 preamp. When the selenium rectifier was busy failing (over the course of several years) the heater voltage dropped. I had gain problems that were predominant on one channel. If you swapped tubes the problem moved with the tube. All the heaters were lit to about the right colour it appeared. So I bought an entire set of tubes. Problem went away for a short while and came back. I was starting to get emotionally involved now so I put it away for a few years. I found the fault by playing dumb to history and checking the basics first. It was hell. Thank goodness it was mine and not a customer unit.

So Gene, no matter what we say, always go back to basics and check everything. No matter how convinced you are that you already checked something.

-Chris
Click to expand...

You know what Chris? you may have hit it on the head with the selenium rect.

Its appears that it might be failing. The pilot light triggered my thought on this. When I first power up, The Pilot is fairly bright, But by the time the filaments start getting warm, The Pilot dims, and a couple of tubes look like they are not lit up.

I was able to get both channels up briefly by rolling a few different tubes in. But even then, no more than a few minutes of operation before it sounded bad and faded out.

I did take all the telefunkens back through my tester, They checked out fine. I then audio tested each one in my little 5W champ.
No microphonics, Each one sounded as good as the next.

Rule out tubes for sure.

I gotta run out for a few diodes to make up a bridge and maybe I will get lucky!
Gene
 
#39
Hi Gene,
Your voltage readings confirm that the selenium rectifier is failing. It's now toast.

So you need to install a silicon unit and play with a series resistor to get the voltage right. The series resistor will go between teh rectifier and the first cap. I used around 2 ohms (as mentioned in the PM).

I am only posting to make my solution public. This problem can drive you nuts. 😀

-Chris

Edit: spelling
 
#40
anatech said:
Hi Gene,
Your voltage readings confirm that the selenium rectifier is failing. It's now toast.

So you need to install a silicon unit and play with a series resistor to get the voltage right. The series resistor will go between teh rectifier and the first cap. I used around 2 ohms (as mentioned in the PM).

I am only posting to make my solution public. This problem can drive you nuts. 😀

-Chris

Edit: spelling
Click to expand...

Welp,

I had hopes,

I replaced the selenium with a SS Rect. Now at least the pilot & filaments stay apparently working, But still no Channel B.

This morning it worked briefly then faded out so somethings clearly still not correct.

Time to do a full voltage walk thru I guess.
Plates, Grids, Caths & PS again.

Soooo close, yet sooo far lol
Gene
 
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