It Can Happen To Anyone - Bad C4S Voltages - Ok Now And More

[Grainger49]

So... I bought a Full Meal Deal FP 2 a long, long time ago.  I built the C⁴S boards about a year and a half ago and this week decided to put them in.  

After a day and a half rotating the LEDs they all came on.  But no sound came out.  Here are the voltages on the tubes.  The two tubes are the same so here are the bad readings according to the "Anticipation" manual:

Tube Pin     Expected     My Voltage
    1               47V          139.5V
    3               1.7V           5.63V
    7               47V           139.5V
    8               51V           138.7V

See, it can happen to any of us.

[Paul Birkeland]

That's a pretty odd scenario - almost no voltage drop across the C4S boards, yet you do have voltage across the cathode resistors (and too much at that!).

[Paul Joppa]

Check the R1 value - looks like the VA board is sourcing too much current.

[Grainger49]

Thanks for the quick replies guys.

Both R1s, 1k, are parallel to the jumper on the board and measure 995 and 994 ohms.  Pretty good.  

The small transistors are 2N2907As and the large ones are MJE350s.  The other two resistors measure 266.4k and 267.4k.  

The boards are wired to B+, Gnd and to tube pin 1 for each channel.  Edit: See below for a callout of the wiring.

I'm smoked on this one.

[Grainger49]

I will recheck the orientation of the transistors.  The printing on the MJE350 is facing the center of the board but I'll check the 2N2907As tomorrow.

[Paul Joppa]

Is this the old, old board? The resistors are placed kind of strangely on that board, it always puzzled me when trying to use it. Resistance from 2907A emitter to B+ should be 500 ohms (two 1K in parallel) on the VA board.

[Grainger49]

Yes, maybe an 11 year old board.  I'll check this and get back.  I have a hand full of 1k resistors.

[Grainger49]

I'm sorry to have to dig into the collective Bottlehead memory banks for this one.  I have the manual in hand.  But it doesn't show two resistors in the R1 position, the parts list says 2 - 1k ohm resistors, one for each channel.  Both the CF and VA boards show and call for the jumper to be in place.  If you think it will work I'll put another 1k on each board and see if it helps. 

I checked the orientation of the 2N2907A transistor, the base is pulled back under the case and the tab is at a 5:00 position as in the sketch.

Should I have mentioned that this has a PDMPS?  Each VA/CF board is fed from different caps.  But I don't see any difference this would make.

[Grainger49]

Things are getting worse!  I grabbed a pair of CF 1k resistors and soldered them into the open holes.  The combinations measured close to 500 ohms, a little lower and I fired it up.

Tube pin 1 raised to a solid 142V, was 139.5V.  At that point I didn't bother to make the other measurements.  I'm going the wrong way.  

I have done something stupid and I have looked at it so long I can't see it.

How about some pictures?

[Paul Joppa]

Should check the cathode voltage (pin 3?) to confirm that the change did raise the current. Sorry my memory does not give me a solid number for the target current, but it appears the C4A is working correctly - now I'm suspecting the cathode resistor or connection. It will be a few days before I have free time to get into it in detail...

[Grainger49]

Paul, thanks.

The cathode voltage was still 5.6V; rechecked 5.71V.  So higher.  I felt guilty and went back to see if they all were out of spec.

Let's try some pictures.  They were taken on the lowest resolution my camera will do so I could post without hosting.  And, yeah, I'm not good with shrinking pictures in Paint.

The first one (temporarily removed, it was the wrong board) is the trace side of the board, hopefully no bridging, or maybe I hope there is some.  I didn't bother to trim the new resistor, just solder.

The next (temporarily removed, it was the wrong board) is the component side of the board.  I color coded the three wires to help my feeble mind keep them straight.

The third is the wires connecting to the A side tube.  The color coding of the wires will help here.

[Grainger49]

Just to make sure I have this one right on the VA boards:

A is attached to tube pin 1
B is attached to the B+ for the respective channel
C is attached to the ground buss

On the CF boards:

A is attached to the ground buss
B is attached to tube pin 8
C is attached to B+ for that channel

[Paul Joppa]

That's probably the problem then. For the VA, A goes to B+, B goes to tube pin 1 (plate) and C goes to ground. See p.7 of the 1998 C4S manual, if you have one.

I have a 1998 Foreplay manual, but no original Anticipation upgrade manual, which is why I refer to the C4S manual of the same year.

[Grainger49]

Ding, ding, ding, ding, ding!  We have a winner!

See, I knew I had done something dumb! 

No alcohol was consumed in this project, I screwed up all alone.  One of the monkey wrenches I threw in was the PDMPS.  It removed the old terminal numbers as a reference.

I will edit this when I change the wires.

Thanks, Thanks, Thanks!

PJ, do you want me to scan the manual and email it to you as a PDF?

[Grainger49]

Ok, that explains some of the diode direction swapping.  With the in and out of the B+ straightened out I needed to swap back 4 LEDs.

Tube pins 1 are 4 volts high.  Pins 6 are at 160V but I don't think that will hurt.  Pins 3 are 2.0V, close.  So now upstairs to listen to music!