800-B FM Tuning eye circuit

[sloane freeman]

A couple of times the FM tuning eye worked perfectly.  Now it barely reacts.  Brand new Japanese 6E5's.  Schematic says the FM eye adjustment pot should be 1 megohm.  It measures 500K as does the one in another parts receiver.  I tried another 2.2M resistor in parallel with the one in the eye grid circuit and got a little more deflection.  I have tapped, banged, changed the limiter tubes, and still I have not gotten the proper shadow closure as I once had.  Any ideas?

[José Bustos]

Hello everybody,

My Scott 800 B is sounding clear and strong on FM, but I have the same problem  with the magic eye tube on FM.

I am using a very good tested  6E5 tube. On my Hickok 6000A it opens an close very well, but in the radio it  closes just a few. The same problem with others spare good  tubes. 

All the resistors in the radio are new ones, also the 1Mohm inside the socket of the 6E5.

In the grid of the 6E5 I tested just -0.8 Volt DC maximum, with a strong FM station sounding. And in the input grid, pin #4 of V17, FM Limiter I have -1.0 Volt. Are those normal voltajes? What would be the problem?

I will really appreciate any help, please.

Kindest regards,

José 

[sloane freeman]

Hi Jose,

I fixed my problem but I am not sure I still remember what did the trick.  It may have been when I used De-Oxit on the solenoid driven AM/FM band switch and the connecting plugs between the receiver and amp.  Try that and see.  Make sure you use De-Oxit.  I used the DL-100 liquid and a tiny brush but the spray DL-5 is probably OK.  Work the switch back and forth.  Do you have the model with the relays on the motor drive or without?  I have without and made a change in the wiring to allow more voltage to go to the AM/FM bandswitch solenoid because the change over was sluggish and did not always switch completely

[José Bustos]

Hi Sloane,

Thank you for your answer.

I have the model without the relays. I will check following your tips and I let you know.

Kindest regards,

José

[sloane freeman]

Hope it helps, Jose.  There is also an antenna switch on the back panel...it has a screwdriver slot.  Make sure to clean the contacts on it and try both positions.  Alignment may help.  It also seems to need a healthy antenna signal. 

[José Bustos]

Hi Sloane, I will check that also. Thank you very much.

Kindest regards,

José

[sloane freeman]

Hi Jose,

I remember finding a resistor that was way off.  It is on a terminal strip behind one of the solenoid switches and has a red wire from it going into a shielded coil.  I don't have the schematic at hand or I would give you the number.  In my unit it was red yellow red.

sloane

[José Bustos]

Hi Sloane:

I replaced all the resistors, but I will check again that.

Thank you very much for all your help.

Kindest regards,

José

[sloane freeman]

I just took a look at the schematic.  Make sure everything in the 6E5 grid circuit is OK.  Make sure ant. sw.  SW 1 is in correct position. Make sure you have good antenna hooked up properly.

Jose, it seems to me that when I had the problem it was intermittent and the cleaning of the AM/FM changeover switch solved the problem.

I did the tone circuit mods and rewired the volume control to put it between the two 6J5's and was very pleased with the reduction in hum.  I also modified the auto tune auto band select and assigned two push buttons to change from AM to FM.  The changeover was very sluggish because the original wiring has the motor windings in series with the changeover solenoid thus reducing the voltage and current to the solenoid.  I put a small value 10 watt resistor in series for safety..

To be honest, I was not impressed with the performance of the radio on FM and shortwave.  My unit may be in need of an alignment.  I have an older Pioneer SX 780 that blows the doors off the Scott on FM with just the line cord antenna gimmick hooked up versus an amplified dipole on the Scott. Is your 800 B in an original cabinet?  I have mine as the receiver and amp on a two tier swivel top table so it is totally in view.  Makes an interesting show piece!

Good luck and if I can be of any further help let me know.  There is a lot of good info out there and a bunch of helpful fellers willing to help.  Just ask!

sloane

[José Bustos]

Hi Sloane:

I have the radio with the original cabinet. Is really big.

I checked the SW1 and is in the correct position, in order to use an external antenna for AM. The AM magic eye is working fine, opening and closing very well.

For FM-AM, I am using just a 75 feet wire, as is shown in the Fig. 7 of the manual:

With this antenna, FM and AM sound very well, both strong and clear. The only problem is the FM magic eye. It barely moves.

I am checking the FM/AM switch just now. I will let you know the results.

Thank you and kindest regards,

José

[Norman S Braithwaite]

The FM circuits of the 800B are very sensitive and able to receive and reproduce local FM stations loud and clear even if the FM RF and IF circuits are out of alignment to a moderate degree.  These circuits must be correctly aligned for proper operation of the indicator tube.

Norman

[José Bustos]

Hi Norman:

Your information is very useful. The voltage in R69, 2.2 ohm, is -0.5VDC maximum with a strong FM station (not enough to close the magic eye). In the grid of V17, FM Limiter, is just -0.8VDC. All the tubes are tested OK, the resistors and the bathtube capacitors are replaced by new ones. On the past weekend I started reading the service manual in order to try an IF alignment next saturday.

Any hint about the procedure will be very well welcome.

Thank you very very much and kindest regards,

José

[Norman S Braithwaite]

Yes, I do have a couple hints.  The alignment procedure identified by Scott Radio Laboratories (reproduced in Riders and Sams) works very well.  But...  The second limiter filter resistor referenced in the FM alignment setup paragraph is R161, not R61 and the control grid of the mixer tube referenced as the signal generator coupling point for FM IF alignment is pin 1, not pin 4.

Norman