Two Issues on 800c

marloubow

Best Day Ever
Hey all...
I have to tell you all that I am really enjoying my first tube venture...I couldn't be more pleased with the aux and FM. Both are clear and clean with not a lick of hum.

But I still have two issues on my 800c. (s.n.# 33493D)

1) My AM is still non existent. The tech who corrected my mpx problems, didn't even bother with the AM section??? So before I send more $$$ his way, I was hoping you guys (and gals) might give me a few ideas to try. I've tried to trace the circuitry on the schematics from back to front, but get lost along the way. I don't think I'll ever get the hang of reading schematics.

2) Even though I not really into vinyl, I got excited when I got my Garrard Type A II into working order...it cycles and plays wonderfully thru the 800c.
But my phono stereo, phono mono and tape head all have the same hum.
It's volume sensitive and present with or without source, or dummy plugs for that matter. It's not the Garrard because thru my Yamaha preamp it's totally quiet.

I really think this is all that is left to get my 800c to full working order.
Any and all help is appreciated.
Martin
 
Check the AM Tubes 1st. V7 (6BA6) & V8 (6BE6). As you have FM, None of the other Tuner Tubes need attention. If still nothing, Find another Tech OR locate the local HAM Club and ask a member who's the best guy they have for alignments. Surprisingly most of these guys have tech(OLD TECH classification, not the new tech class. ) licences thru FCC.

Teach it the Lyrics.

More likely you have a lifted or corroded ground lead in the circuit or a FILTER CAP is BAD, as it's volume dependent and both channels. C87 feeds the 12AX7's (V18-19) for B+(there is a M2 & M3 off Pins 1 & 6 of the tubes which is the power lead to M1 @ the CAN CAP). I've seen new caps go bad when new out of the box, or within a couple of hours. Check the Voltage at Pin 1 & 6 of both tubes (DC Voltage and about 208V).
If you have a scope, check for AC Ripple off the can cap.

Larry
 
:banana::banana::banana:WooooHoooo:banana::banana::banana:

One down...one to go!

The lettering on V7 & V8 was getting pretty bad. So in relabeling them I noticed they were swapped...voila, AM is up and running. Needs antenna.

Gee...wonder how they got swapped? :scratch2:

Finally, an easy fix.

Thanks Larry for putting me on the right path. :thmbsp:

Now onto hum issues.

Martin
 
Larry...
I have tried unsuccessfully to decode the cryptic line...

"Teach it the Lyrics"

I'm all ears... :scratch2:

Martin
 
WWWHHHHHOOOOOOSSSSSSSSHHHHHHHHHHH!!

(That was the sound of the Joke going right over your head!))) :D :D :D

GOTCHA!!!

BTW, What did you use to Re-letter the Chassis???



Larry
 
I can only imagine how good your homebrew must taste with the attention to detail you place into this hobby!

:beer:

Kory
 
Larry...
I used a big fat first graders black crayon that was broke in half...yes, I am that good! :thmbsp:


My turn for clarification. I used black sharpie to ID the tube itself, not the chassis.

However I have used a fine point sharpie to repair a few "broken" letters on the chassis with good success.

Kory...
Home-brewing can be as much fun as this stuff is. Combining the two is "twice as nice". Unfortunately I've done it enough times that the front side of the process feels like work...the back side is the enjoyable part. :D

martin
 
Larry...
I opened my 800c back up to check the voltages for V18 and V19, pins 1 and 6. I also checked my power supply voltages and are only a couple volts off at most.

Just to be clear, these are measured with the neg (-) probe attached to chassis and the pos (+) at test point. Correct?

So here are the numbers...
C87 213.8v

V18
pin 1 110.7v
pin 6 93.3v

V19
pin 1 108.2v
pin 6 88.8v

The only thing between the power supply and the tube pins are resistors,
R139-220K / R141-330K (V18) and R140-220K / R142-330K (V19)

All these numbers are correct according to the schematics...so what did I miss?

Thanks for the help.
martin
 
Side note...
I discovered that the 12AX7 that is in V18 is "conducting" noise. What I mean is that if I tap on the tube or the shield or associated components below, the tapping is audible thru the speakers. I rolled a couple of other 12ax7s thru the V18 socket and the issue followed the tube. I don't know what it means, but even rolling the other 12ax7s thru V18 didn't cure the hum. However I may not have rolled it far enough out of the circuit.
It's quite late...does this make any sense?

martin
 
It's called microphonic. Either live with it or don't use the tube. I trash them. The tube is acting as a pickup transducer.
 
Martin; Voltages are fine. And you are correct Red on lead, black on chassis. I'm thinking there is a ground loop somewhere in the phono / tapehead circuit. They are a gold plated bitch to find without a scope at times. Look for crossed or broken wires, etc.

Larry
 
update...

Thanks for the help Larry.
But seeing this only a hobby I have nothing in the way of real test equipment.

I have discovered, ashamedly, that the hum is only in the right channel.
I guess that narrows the search by 50%,
But I'm still 100% clueless. :D Where else should I look?

Prodding around with a chopstick has yielded nothing. :scratch2:

Back to the schematics.

Anyone with a clue or idea is welcome to chime in at anytime.

martin
 
Larry is right, it's tough to trace issues as these even with the right equipment. My 400 had a hum in left channel after it's rebuild. It took my EE friend 2 1/2 hours with a signal generator and scope to find the issue. the problem ended up being a not so good solder joint. He just reflowed the solder joint, instant no more hum.

You could take a look at all the humming channels connections. Maybe start at the selector switch and work your way back. Does your 800 have a ground connection at the phono/tape 12AX7 tube socket? If so, that might be a good place to start.

Good luck!
 
Progress...
I may have this hum on the run.
dsndblm got me looking in the right direction.
As I was looking at pin 8, the ground pin, V19. I noticed that the ground wire for my Individual Bias Pot Mod should have been attached where pin 8 grounds out. I had attached it to the next terminal over and it was going to ground, but thru R132 / 330K to get there. So after securing the IBAM ground wire to the appropriate ground (fingers crossed)...the hum is still there. But it has become less audible and changed frequency, because now the midrange is putting out noise as well and in both channels. Also the pitch of the hum changes (louder) as I hover my hand over V18 and V19 and the selector switch sections. I think I read somewhere that the bottom cover has something to do with grounding hum. Hmmmm?:scratch2:
I've tried soldering the IBAM ground wire to another chassis ground point, not associated with any tube socket (one of the welds that secures metal flex conduit), but still some hum.
And still only in the phono / tape sections, but has a different pitch in each of the three settings.
It has quieted down by about 50%.
Will try again tomorrow.

martin

If all else fails...teach it the lyrics. :D
 
Martin; Make absolutely sure the ground wires on the top of the chassis from the tube socket to the chassis are actually connected to the chassis and the socket. Also the Tube covers need to be on and snapped down.

Different pitch???? Sounds like a ground connection or loop in the preamp section. Time for more study of the schematic vs. the unit. Wanna borrow my Tri-focals??? You may need them eventually. LOL!! Run your hand over the wiring with it running. does the pitch change?

Larry
 
First, my apologies to the group for not being more involved as the designer of the "IBAM". My current schedule only allows me to be an occasional lurker here. I still do intend to produce a document for the mod.

The mod is not sensitive to where where the ground is as long as it is a solid chassis ground. I believe you have fixed that problem.

Did you put the four 33uf caps in? Did you remove the original electrolytic cap? These caps are very important for suppressing the 180hz hum. If you left in the old cap, did not install all four caps, attempted to use one big cap, or put one big cap in the wrong place you will get hum.

How bout some pictures?
 
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