New guy has 500C prepped in the OR.

[TubeDroid]

Hi all,
A few years ago (8 or 10, maybe?) my brother-in-law GAVE (!) me a 500C (S/N 66645U) that he had hauled out of his father's console. I took it home and immediately plugged it in and put my headphones on and I was astounded at the capture and sensitivity of the FM, even in my basement and without any antenna.
I used it with the for a couple of days and considered putting it into my production system until I started to do some internet research and found out that maybe I shouldn't have plugged it in right away and I'm guess I'm lucky that I didn't blow something up.
I packed it up in a box with the thought that someday (not soon) I'd have a chance to do some restoration and mod work when both time and space permitted.
Well, two moves later that time is now, except that my bench is at the cottage and I still work for a living, so I can really only do things on the weekend, but the good thing is that I can spend the week preparing for what I want to accomplish on the weekend!
I started reading this forum a few weeks ago from it's genesis (I'm up to about Nov. 2015 so far), so I'm well versed in what needs to be done and what to avoid doing.

It looks pretty well stock, except that it seems that the Tape Monitor switch buggered up, so someone replaced it with a toggle type of switch instead of a slide switch, so it looks kind of wonky.......and the Low Filter thumb grip has broken off.......cosmetics/ergonomics come later, but I had no problem cleaning the faceplate and the extracting and cleaning the glass was a breeze; is the fact that this is a later model 500C mean that the glass isn't as fragile as the earlier models?

All the original 12AX7's are there, all West German except for two Great Britain's in the Multiplexer (V100 and V101).
Two of the original 7591A's are there (USA), and two have been changed out to a GE and Sylvania. It also came with a spare 7591A, but all it says is ROGERS on the base. New output tubes will come, hopefully after I hook this thing up to my wife's speakers

It's also rather obvious that all the switches and balance and volume tapers need a good cleaning. The power switch seems ok with good closing snap (an immediate 0 ohms when switched on, but that might not mean anything, I guess).

I do have the ubiquitous 30001-49999 Service Manual, and I did manage to get some voltage readings to compare to the schematic so I do have a (somewhat) benchmark to start with, but I don't know what the wall was giving me at the time. I just wrote these down on the schematic at the time, so most measurements at the time were rounded to the nearest volt.

7591
All Pin 6's read -16 VDC (+/- 0.2V)
All Pin 8's read 390 VDC
Pin 3's read 437/435/433/436 VDC
All Pin 5's read (ahem, short to ground for now......)

Output x-formers center tap was 441 VDC

PI
Pin 6's 139/134 VDC
Pin 8's 1.13 VDC
Pin 1's 315/321 VDC
Pin 3's 143/138 VDC

All the other 12AX7's are reading nominally (maybe max +/- 10% of the schematic values, most within 5%).
Again, these measurements were obtained years ago, the receiver's been kept in a cool dry place since.

I managed to get it on (drumroll, please) MY NEW BENCH this past weekend (after I finished assembling a variac station). I gradually turned it up over the Saturday and realized that tree frogs and other insects/birds sound suspiciously like something is going to blow, so after a couple of false alarms it went quite well, and with a jumper wire for an antenna I found that I could pull in a few stations and the stereo beacon worked (click, click) and everything.
WOOHOO!

Nothing but the tubes got overly hot to the touch, and there weren't any creaks or groans (outside of still having to listen to the ball game on AM with the other stereo.....(I know, I know...."Don't talk to me about life").

Couple of questions, nothing really technical, yet:
Am I overly optimistic that this thing will be a charm to work on? (I have the new rectifier and 7591 cathode resistors ready to go, and I'll pick up the screen grid resistors sometime this week and maybe the coupling caps and bias resistors).
This CL80 gizmo everyone talks about: I've been to two different local suppliers and I don't see anything in the thermistor department that's called a CL80. I read somewhere here that it might be referred to as NIC47D-15. Is that a vendor or manufacturers part number? I'm not sure what I should be looking for. I know I can order anything online and I could always ask a store employee, but I'm more of a "it's not the kill, but the thrill of the chase" kind of guy".

What pointers might anyone have to reduce the contortions you have to perform to work on this thing? I have it set up on an IKEA turntable so I can rotate it horizontally at will, but I can see that flipping it over to swap tubes for troubleshooting will get weary after a while, not to mention keeping the meter leads clipped to appropriate places inside. Maybe I can jig up a couple of wooden supports so I can mount it vertically on one side of the chassis; it would give me access to front, back, top, and bottom with just a spin.
Man, this thinking outside of the box is hard work.

After I shut it down yesterday for the week, I took a few pokes with a voltmeter to see what kind of charge was still being held, and after only 30 seconds or so the most I could find was about 8V from a couple of the can caps. Is this normal? I understand that the system should be routinely discharged after power down and before sticking your fingers in there. 26 years ago I built a 100W MOSFET integrated for a college project, and I built in an internal discharge system (SPST/10ohm5W resistor) just for that purpose. I was thinking of modding something similar on this thing but I don't really know the most advantageous point(s) to connect it to, or if it is even necessary.
I've also been thinking of maybe putting in an internal tone control and/or balance control by-pass system (.....'cause you GOT to build bypasses.......). Would anyone know the two most convenient points to link together. I can experiment, I guess.

Sorry if this is a long post, I just thought I'd introduce myself before I actually get down to doing anything. I'm also attaching something to this post: an upside down chassis layout view on PDF for those (like me) who find it confusing to translate the layout in the SM to what is seen when the chassis is upside down. I don't think I've run across anything like this here, and apologies if anyone else has already done this; I'll always give credit when due. Errors are entirely mine, but let me know if there's anything on it needing correction. (Think I've figured out the file upload thing here.)

Who was it that said "The reason I write long letters is because I haven't got the time to write short ones!"?

And then of course I've got this terrible pain in all the diodes down my left side.......

 

[tcdriver]

Amphenol Inrush Current Limiters

Mouser part no: 527-CL80

 

[larryderouin]

The Filter caps circuits are designed to bleed off the Main filter caps fairly quickly. Under a minute isn't unusual. BUT! ALWAYS DOUBLE-CHECK with a METER, before sticking your fingers inside!

TC Driver got it right for the thermistor. That is an AMPHENOL PART #. Otherwise ask the parts shop for a 3amp 47ohm thermistor which are the operating spec's for the CL-80.

Cut a plank long enough for the 500c and the IKEA Lazy Susan, drill a couple holes to be able to attach the chassis to the board, and a couple more to attach to the Lazy Susan. Main Power Transformer on the bottom side for stability.

 

[TubeDroid]

Cut a plank long enough for the 500c and the IKEA Lazy Susan, drill a couple holes to beable to attach the chassis to the board, and a couple more to attach to the Lazy Susan. Main Power Transformer on the bottom side forstability

Sounds perfect. Guess I wasn't thinking far enough outside the box.
Thanks for the tip.
Maybe I'll rig up a coathanger boom to drape the speaker cables from to keep them out of the way as well.

 

[audmod01]

TubeDroid;

I did a similar drawing to help me when I worked on my 500-C. It is an early 10001 to 19999 SN sequence without the nuvistors in the front end. Otherwise it is the same. I also made a drawing of the tone controls mod which Dave developed as I was performing that on my unit.

Looks like you are off to a good start.

Joe

 

[TubeDroid]

Well it's been awhile and I have a couple of questions about the 500C:
1. I'm using SM 30001 to 59999.
According to the Parts Description List, C81 and C83 are 820/1000V caps, and I indeed have 820/1000V caps. Somewhere along the way I found a much more legible Parts Description List from a different and unknown vintage so I printed it off and have been using it too and, yes, found a couple of discrepancies that made sense (reading about them in this forum).
These caps are one of those discrepancies; my more legible Parts List says 1000/1000V. I've also read (elsewhere) that the 1000/1000V caps SHOULD be changed to 820 to either improve, increase, or decrease the lower end ( I can't remember which and they were talking WAY over my head).
Can someone explain in somewhat simpler language just what the change was supposed to accomplish? I'm just curious. I was thinking of maybe changing them myself to 1000/1000V just to hear what someone thought was maybe an inferior sound. Probably a dumb idea but I've solved problems using dumb ideas before........and created problems, too.........
Also, are these caps pretty well ok the way they are? or maybe changed to a more quality cap? I'm thinking that the feedback network is important enough that just refreshing it with new parts wouldn't hurt (but ref. end of previous paragraph......).

2. I'm pretty well resolved to changing out all four faceplate switches. One was changed to a toggle switch (Tape Monitor) and one thumb thingy (Low Filter) was broken what looks like years before I came into possession, and the other two switches feel almost seized; they switch, but I'm afraid that I'm going to break them too. The sort of thing that a good bath in DeOx probably isn't going to help much......... haven't tried, though.
My question is with the High Filter switch wiring:
The center pins are tied together and connected to a local chassis ground lug.
The Right Channel switch "ON" is connected directly to the Right High Filter PIC Pin 2.
The Left Channel switch "ON" terminal is connected to a three or four inch double jacketed wire with what looks like a 'drain' wire inside the outer jacket going to the Left High Filter PIC Pin 2. This 'drain' wire is butted along with the outer jacket near where the signal wire makes the connection to the PIC.
Are you still with me?
There has to be a reason for this 'drain'. The wire runs over top of the balance pots, but can there be that much interference from those pots or something else nearby that the wire requires a drain? This 'drain' doesn't exist on the Right Channel, but it's a short run anyway.
It would be MUCH easier if I installed the new switch without fussing around with the drain wire, but I'd do it if it's somewhat required. I don't want to have to re-do it if the sound goes south.

BTW, I've got the cathode current held steady at about 31 ma (+/- 1.5 ma). Still sounds pretty good, but I may put in an IBBA system to see if I can get it perfectly balanced and level. (I have an integrated that uses individual adjustment so I'm kind of bored with that method.
And I'm still working on finding the right way of lowering the line voltage 7 or 8 volts so I can get this thing off the variac. I have an assortment of wirewound power resistors in the 75 to 200 ohm range that I'm going play with in that area. Wirewound OK? or might there be inductive issues?

'nuff for now, and thanks for watching..........

 

[dcgillespie]

Tube -- To your questions:

1. In your unit, if you have 820 pF caps in your unit at C81 and C83, then that is the value you want to keep. With this value of cap installed, you likely also have typically a 5 pF cap effectively connected between pins 6 and 7 of V12 and V13 -- which may or may not be shown on your schematic. The older 1000 Pf value was used when the caps at V12 and V13 were not included. If these caps are installed but not shown on your schematic, they need to remain as well, and represent a production line change that was not documented. The value and presence of these caps work towards maintaining the HF stability of the power amplifier sections by controlling the supersonic response of the amplifiers (i.e., that above 20 kHz).

2. The wire you're referring to is what is known as shielded cable, with the drain lead as you call it being the connection to the internal shield around the inner wire. The internal shield is actually a carbon impregnated inner jacket that the center wire runs through. The "ground" lead (as it's called) simply runs between the outer and inner jackets thereby making contact with the carbon surface of the inner jacket, and therefore becoming the lead that is "connected" to the shield. This lead will always be connected at least on one end to the chassis, and possibly on both ends, but not always. When used, it is important to keep shielded wiring in place as you perform any restoration work. Shielded cable is used for a variety of reasons, being to:

1. Eliminate the pickup of hum that would otherwise occur with conventional wire if the wiring runs close to the power supply.

2. Eliminate wiring interaction between the two channels to maximize channels separation.

3. Prevent wiring in sensitive circuits (like the phono preamps) from acting like an antenna, that would otherwise allow such circuits to pick up radio stations if the unit is located close to a radio transmission tower.

I hope this helps!

Dave

 

[TubeDroid]

Sure answers my questions, thanks.
Have to check on those 5 pf caps on the weekend. Add them if they're not there?
I have a whole whack of 735a cable that is described as:
26 AWG solid .016" (.40mm) silver-plated copper conductor(s), foam HDPE insulation, Beldfoil® + tinned copper braid shield (95% coverage), PVC jacket.

Would this be suitable? I'd manage the braid with solder to a wire and heat shrink the works. Or maybe the 26 AWG conductor is too flimsy? I'd sure like to find a use for it.
Either that, or hunt down something suitable.

Calling it a drain out of habit. I work in telecom where that kind of ground is called a drain, where there is a foil shield around a multipair cable, and a wire runs the entire length of the cable outside the shield and is attached to ground at one end only (as opposed to a coax, where the ground shield is the return and is attached at both ends to a local ground).

 

[notdigital]

..... and the other two switches feel almost seized; they switch, but I'm afraid that I'm going to break them too. The sort of thing that a good bath in DeOx probably isn't going to help much......... haven't tried, though.

Shooting some cleaner in the switch will cure that seized feel. Stick a tissue below to catch the excess, shoot from the top and work the switch like crazy. You'll be surprised how it'll free up.

.....And I'm still working on finding the right way of lowering the line voltage 7 or 8 volts so I can get this thing off the variac. I have an assortment of wirewound power resistors in the 75 to 200 ohm range that I'm going play with in that area. Wirewound OK? or might there be inductive issues?

Search here for "bucking transformer." Problem solved.

 

[dcgillespie]

Tube -- No criticism meant on use of the term drain. We all come from some background and bring that experience to the table when first approaching this hobby, so the use is perfectly understandable. I was only referring to it in the manner that I did so you would know what I what talking about. Actually, the term "drain" is used in our neck of the woods, but it refers to one of the terminals on a Mosfet component. That is a solid state device, with its name being an acronym for Metal Oxide Semiconductor Field Effect Transistor -- but none of these are used in your Fisher!

If at all possible, I would try to retain the original shielded cable in your receiver, and with the same piece in the same location it was used in. As no doubt you know, all cable has an electrical property of capacitance per foot associated with it, whose value is very depended on the type and construction of the cable design. In your Fisher, there are in fact components used that (in part) act to counteract the natural capacitance effect of the particular shielded cable used, such that if you indiscriminantely change to a different type cable, the balance achieved between the amount and type of cable used would no longer be properly compensated for by the compensation components used. If you have to use new cable in any particular section, you will want to use (typically) a very light gauge single conductor cable (24-26 AWG is fine) so that it will "bed" in nicely with the rest of the wiring where it should, with as low of a capacitance rating as possible -- no more than 50 pF per foot. The original cabling typically represents about 40-45 pf per foot.

If the 5 pF caps are not there, I would not install them. As mentioned, both of the components discussed work to control the supersonic response of the power amplifier circuit -- whose basic characteristic is set by the particular output transformer used. Fisher used different vendors to supply its transformers, and often times, any undocumented production changes can relate to the slightly different characteristics of two different manufacturers building the same component. With an output transformer, wire tension, thickness of insulating paper used, and accuracy of winding count all play heavily into the supersonic characteristics of the transformer, so that theoretically the same transformer can have slightly different characteristics when manufactured by two different manufacturers. That is likely the reasoning for the slight value adjustment and/or inclusion of these components as needed to achieve a common final result in receiver performance when originally produced. Therefore, it is always best to maintain the build of a given receiver regardless of what the schematic for it shows, unless you are well versed in the theory of how the particular circuits in question operate. For those who were authorized to service these units for Fisher back in the day, such adjustments and the reasons for it were understood as common practice to account for production line variables.

Dave

 

[TubeDroid]

Cool. Thanks for the info.
When I started this, it was my intention to leave as much of the original wiring as I could. I think i can suppress my laziness and cope with this, unlike my next project, an AM radio where the wire jacket is rubber and completely disintegrating and the osc. coil looks like Little Orphan Annie, and I have no schematic. Fun, huh?