800B FM Front End Issues

[Joseph W Strickland]

I have been concentrating on the front end of my 800B serial no. 1035. It is working in so far as tuning in stations without having any erratic tuning problems (which would normally be from dirty ground contacts on the ganged tuning capacitor assembly). The tuning is quite smooth and stations do come in with decent limiting and very little noise - except that during periods of quiet I can hear sputtering noises and occasionally a much louder pop in the detected audio from FM stations the radio is tuned to. I have substituted a complete set of front end tubes from another set and the intermittent noise is still there. Most of the time the noise is so slight that it is not noticed much. However at times it can be rather loud.

I have changed virtually every part in the front end, B+ resistors for both plate and screen grid circuits, the 5 ohm resistors in the signal grid circuit of the mixer and RF amplifier circuit, changed all plate and screen grid silver mica bypass capacitors, changed silver mica capacitors in signal grid circuitry and the noise continues. My much more modern FM tuners do not exhibit this type of noise on the stations I listen to using a signal coming from the same outside antenna. My initial thought was that either a tube or one of the coupling or bypass capacitors was breaking down with voltage applied to it.

I can unplug the IF cable from the FM IF strip and the result is complete silence, so I don't think the FM IF strip has any problem. I may go ahead and try a different set of FM IF tubes too just to see if that makes any difference.

I reached a point of frustration with the radio until I have decided to let it and myself rest before I try again. I have found over the years that when frustration builds up on a radio, it is time to stop and come back to it at a later date and a fresh mind.

The idea of rest is very appealing right now and my cat, Mr. Leo, is anxious for lap time so we may both take a nap.

Joe

[Joseph W Strickland]

Today I brought in an AM/SW/FM radio I have with a digital frequency display. I was able to use it along with my Heathkit SG-8 RF generator and tune the generator to the correct frequency on the low side of the center of the IF frequency, then also to the frequency mentioned on the high side of the 10.7mHz center IF frequency. I was not seeing equal deflection of my VTVM meter no matter what I did. I concluded there was a problem in the discriminator transformer circuitry. I shut off power and began to check the resistor values. The two 100K ohm resistors at the 6H6 detector were way off compared to each other. Even the 56K ohm resistor inside the detector can was measuring nearly 70K ohms. I replaced that 56K with a new one of 1% tolerance and reinstalled the detector shield again. Then I checked the two 100K ohm resistors and the 47K ohm resistor. One 100K ohm resistor measured 116K ohms and the other was 170K ohms. The 47K ohm resistor was measuring 63K ohms. I should have checked these earlier. I replaced the two 100K ohm resistors with new 1% metal film types and the 47K ohm with a new 1% tolerance part and realigned the primary (while the secondary was de-tuned) for peak signal again, then brought the secondary into proper tuning. I could see an equal deflection on either side of center! Then I removed all the connections and put the radio back in operation, tuned to my favorite station again. This time I saw smooth tuning across the station and the sputtering noise I had heard before is gone! I also get a better narrowing of the eye tube as I tune across the stations.

It appears that some of these resistor values were so far off value that mere tuning of the primary and secondary of the detector transformer would not bring the circuits into proper frequency and balance. I had previously failed to check these parts that closely and should have.

Joe

[mike hadley]

Hi Joe,
Yes you're right about resting, I have a Pye console from 1939, only a four tube set but....
I was told by the guy that sold it to me "it works mate", should have walked away then, the tubes are old style side contact, someone had bridged caps across the smoothing caps both sides of the choke filter, all of the original tubes were cooked, the tube coating should have been red paint all of this was black, burnt.
To make matters worse the set had suffered a lightening strike and the antenna coupling transformer was a mess.
after all of this was repaired, the shortwave part of the set still refuses to perform,
To this day the radio sits taunting me at the back of the workshop, ah well.

[Joseph W Strickland]

Mike;

Wow! It certainly has had its host of problems to solve. Some of these old radios develop corroded wires in their antenna/RF and oscillator coils. Having lightning strike the set compounds problems. (Especially if a carbon trace gets established somewhere hidden from view). I have had my share of those over the years. I remember one RCA color TV set that had its shadow mask inside the CRT so badly magnetized that repeated attempts to demagnetize it all had failed. It was due to a lightning strike near the home. I had a new CRT with me to change it in the home if it had to be done. I decided to give it one more try with the degaussing coil (we RCA Service Co. employees were equipped with heavy duty ones compared to others on the market). I gave it a try and succeeded in demagnetizing the CRT. It worked well with its usual yoke positioning and purity ring magnets and had nice solid color red, blue and green fields when I was done. From discussions later with prior technicians who had been there at least 5 attempts had been made to demagnetize the CRT. I just happened to be lucky and succeeded. When I left the customer was happy and had good color displayed on his television again.

Having a nice pet to calm one's nerves after an exasperating time with a radio or other item can be very helpful.

Joe

[Joseph W Strickland]

After some extended listening to the 800B tuned to my favorite station, I am amazed at just how good it sounds. It is hard to believe that it is actually 70 years old! It is a credit to the engineering of the E. H. Scott company that it does such a good job with the signal even at the -59dBm level of this station at my house. An adjacent station normally causes interference with the discriminator or ratio detector in most all tube type FM radios made in the 1950s and 1960s equipped with the typical K-tran IF and detector transformers. Their IF response is broader and they are more susceptible to alternate channel and adjacent channel interference than the 800B and a Silvertone 8127D that I have, both of which were made in the 1940s and do not use naked silver mica capacitors in the base of their transformers with compression contacts. E. H. Scott used fixed ferrites (if any) and air trimmer capacitors to tune the primary and secondary of their IF and detector transformers.

I did have one air trimmer in the RF oscillator circuit that I had to replace with one from a parts unit I had on hand. Somehow it literally came apart at the adjustment shaft and assembly sleeve that surrounds the shaft. It is a pressed on sleeve and it simply came apart. The trimmer I borrowed from my parts tuner chassis worked smoothly once it was installed.

Now I am waiting upon the cabinet I acquired through eBay auction. It is from a Metropolitan 16A model, but the 800B radio tuner will fit as long as I leave the slide rails off the side and modify the faceplate to be narrower than it presently is. It will be in a fixed position in the cabinet but I do not believe there will be any access issue during use. Norman mentioned that some of the late 800B units were done in a similar fashion with no slide rails.

Joe

[Joseph W Strickland]

The above cabinet is shown in the upper left corner on page 109 of the E. H. Scott Radio Collectors Guide, 2nd Edition. This cabinet was offered from 1947-1949 according to the note under the photograph.

Joe