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]

Hi Norman:

You are right about the pin error on the procedure.

I am a bit confused, because the IF Alignment procedure says to connect a high resistance DC voltmeter on R72.(15Kohm) placed on the circuit before V17 (second limiter). I can not find any reference of R61 as you say on the procedure, and the R161 does not exist in the list (the last resistor on the list is R93). 

Here is part of the schematic:

I will really appreciate your help to understand, please.

 

Thank you very much and kindest regards,

José

[Norman S Braithwaite]

R72 is correct on that schematic.  I forgot that Sams Photofact renumbered the components thus renaming R72 to R161 but identifying it as R61 in the edited alignment procedure.  If you are using original Scott service data it is correct in this regard but beware of the other correction I mentioned.  It may have originated in the Scott service data.

Norman

[José Bustos]

Hi Norman:

I understand. Yes, I will take care about the pin.

Next weekend I will try the alignment. and I will let you know.

Thank you and kindest regards,

José

[sloane freeman]

Norman,

What instruments do you use (makes and models) for your FM alignment?  My RF generator is really old and I would like to replace it with something reliable that does not itself need to be aligned.  Something crystal controlled perhaps.  Does the alignment require a scope and what would you recommend if so.  The FM performance is only good on very strong signals.

Thanks!

Sloane
Norman S Braithwaite said:

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

[Norman S Braithwaite]

Slone:

Thanks for your inquiry.  I use a Hewlett Packard 3312A function generator and a Hewlett Packard 410C VTVM but any decent AM RF signal generator and VTVM will be sufficient.  All you really need is a 10.7-mhz signal for peaking the IF circuits and balancing the discriminator circuit plus broadcast stations at both ends of the band for aligning the RF and oscillator coils.  The HP 3312A is not designed for this use but with modification (replacement of frequency and modulation control potentiometers with 10-turn types and addition of a digital frequency meter) it is sufficient.  Although the modified HP 3312A is capable of providing a signal for sweep alignment, I have found the peak alignment procedure recommended in 800B service data to work very well.  The IF does not have to be at exactly 10.7 mhz but it is necessary to make sure the RF signal generator does not drift during the alignment.

Norman

[José Bustos]

Hello Norman, Sloane:

Good news!!

I had some problems with my Heathkit RF Gen. IG-102 and I finally I used the Sencore SG165 to align the IF stages of the Scott 800B.

Following the FM alignment procedure from Scott Service Data, at the begining the DC signal on R72 did not change almost nothing on the VTVM, when I moved the adjustments from E26 to E20, but when I touched the E17, the needle on the instrument went up a lot.  The FM mixer was really missaligned. After that I could apply the IF alignment very well and.....

the magic eye tube is working excellent now.  I got until -6VDC on its grid.when is closed!!!!   Before the procedure I had only  -0.6VDC with minimum movement.

And the quality sound improved also!!!

The next step is repair the automatic tuning. All the mechanism is clean and lubricated but with the pressure of the backplate contacts against the rotor disc the speed goes down and the motor stuck sometimes. Any hints will be very welcome.

Thank you very very very much indeed!!!

Kindest regards,

José from Chile

[sloane freeman]

Bravo Jose!!  You will never go wrong with Norman Braithwaite as your Elmer! He knows his stuff and makes things easy to understand when he explains how to solve your problem.  We are Blessed to have him in our corner!

I am going to try and get my set to spec also.  How much of an overall improvement in operation did you notice, apart from the tuning eye working as it should?

I had a similar problem with the motor and tuning mechanism.  I took apart the assembly and cleaned everything, lubricated the friction points and aligned the mechanism so there was as small amount of friction as possible.  When I finished there was no drag at all.  This I found to be the most important part.  Then I was careful to adjust the motor gear mesh so it engaged deeply but did not bind.  A bit of motor assembly lube or good grease is a help on the motor gear.

After everything was back together the tuning knob could be spun easily turning the dial.  I had a problem with the AM/FM changeover relay hanging up.  The tuning motor winding and the changeover solenoid are in series.  I sort of put them in parallel by taking two of the push buttons and using them to only operate the changeover relay.  One switch to the AM coil, the other to the FM coil.  I put a 2 ohm 10 watt resistor in series with the feed to these two switches to reduce to the voltage a bit, and left the other push buttons to their normal function.  I get a snappy AM/FM change over and the motor moves the dial quickly because of the increased voltage and reduction of drag in the tuning mechanism.

Of course this is not how Scott designed it, but I prefer it this way, along with the tone and volume control mods.

Best Wishes,

sloane

[José Bustos]

Hi Sloane, 

Yes, you are right. This corner has wise and generous people, like Norman and you. Thanks for that!!

My ears are not those of an expert but after the alignment I could note a good difference. I think it was well worth of.

I will check the schematic in order to test some change like yours or a similar solution.

Thank you for your help and kindest regards,

José

[José Bustos]

Hi Norman:

This weekend I made the changes for the tone control using your specifications. The sound really improved!!

Thank you very much for your help and hints.

Kindest regards,

José