800B FM IF Resistor & Capacitor Issue

[Joseph W Strickland]

Today while working in the FM If strip of my 800B, I looked at R68 marked as a 39K ohm resistor. Its value had increased to almost double its indicated value. It is bypassed by two capacitors, C70 for the 1st FM Limiter screen grid and C71 Bypassing the B+ to the plate of the 1st FM Limiter (V16). The 39K resistor is a 1 Watt device and shows signs of having been overheated. The color bands are changed in color. C71 is one of the .25uF bathtub capacitors. Being a wax paper/foil type capacitor it does need to be replaced.

Years ago I worked on an 800B for a friend. His receiver had no FM. I traced the fault to these same two parts R68 was burned and C71 was shorted. This leads me to believe that this may be a relatively common problem in the 800B power supply chassis where the FM IF strip resides. Two of these same exact parts failing, one a leaky cap and R68 way up in value and another one with C71 shorted and R68 burned.

I will be using my Dremel tool as Dave Poland suggested and open up the two bathtub capacitors (C71 and C79) and install new capacitors inside. I had just finished replacing all the 6200uF bypass capacitors in the FM IF strip.

Joe

[Joseph W Strickland]

C79 is two .25uF capacitors in one bathtub enclosure. The B section takes care of bypassing the B+ feed to the plate of the 2nd Limiter V17) which comes through a 100K ohm 1/2 Watt resistor R75. The A section bypasses the B+ feed to the screen grid of the 2nd Limiter (V17). The B+ to that screen grid arrives through R74 the 39K ohm 1W resistor that has been cooked. It is part of a voltage divider with R73 which is a 15K ohm 1 Watt resistor which has its other lead connected to chassis ground. I measured R74 and it shows to be 66K ohms instead of 39K ohms as marked. So that resistor definitely has to be replaced.

At the first FM IF (V14) I discovered that the 56K ohm resistor R59 that feeds B+ to the screen grid was open. I replaced that resistor. The 2400 ohm B+ feed to the plate of V14 measured good at 2400 ohms.

I replaced the capacitor in the C71 bathtub enclosure. Unlike C79 which was filled with wax, this one was filled with mineral oil. It appears that some of these bathtub capacitors may indeed be paper insulated foil capacitors in an oil bath after all.

Joe

[Norman S Braithwaite]

Joe:

Yes, some were paper in oil capacitors.  Usually these ones were marked "oil" on the top.  Was C71 so marked?  If not marked, they can be identified by a solder plug over the hole used to fill the capacitor with oil.  I leave the oil filled capacitors in the circuit if they test OK.

Norman

[Joseph W Strickland]

Norman;

No, this one was not marked as being an oil type, so I was surprised wnen oil began to drain out after I cut it open. I did not see a solder plug as you mentioned. I will take another look>

Joe

[Joseph W Strickland]

I looked at that C71 capacitor again, and saw no indication of a solder plug to seal it other than the solder that was inside each feed-thru terminal lug. It was quite neatly made and there was no notation as to it being an oil capacitor. It was marked: CP50BIEF254KK; .25 MFD  600 VDC; FAST.

These look so neat when rebuilt with modern sealed capacitors inside and will probably last many years into the future without any problem.

Today I opened up C75 and dug out the old paper capacitors and replaced them with new mylar capacitors inside using .047mFd @630VDC Panasonic capacitors. I also had to replace R71, a 27K 1/2W carbon resistor and used a new metal film 27K 1/2 watt resistor there. Half of C75 had been replaced previously with a small capacitor hanging in the air along with the green wire and the 2.2Meg Ohm resistor. So after I finished rebuilding C75, I put everything back like the original circuit.

I had to order some more 39K ohm 1W resistors to replace those in the FM IF strip. I found some cathode resistors in the first two IF stages that were off value plus their B+ feed resistors, so I have those coming too. This will help assure that the first two IF stages are properly biased for correct gain.

After I finish the FM IF strip I will move on to the audio driver and phase inverter and put new .1mFd caps inside the bathtub capacitors in those stages. The originals were .05mFd so this is part of the tone control and audio frequency response improvements. I will remark those bathtub capacitors throughout the unit as being re-built with the current date and any value changes. That way if someone else works on the unit later on, they will be aware of what has been done in the past.

Joe

[Joseph W Strickland]

Continuing further in the Audio Driver/Phase Inverter and Audio Output stages I ran into these discrepancies:

R84 marked 220K ohms - open, this is the plate load resistor of V19A, 6SL7GT

R83 marked 220K ohms - 246K ohms, the is the plate load resistor of V19B, 6SL7GT

R85 marked 220K ohms - 347K ohms, this is the grid leak resistor of V20, 6L6G

R86 marked 220K ohms - 297K ohms, this is the grid leak resistor of V21, 6L6G

R87 marked 220K ohms - 273K ohms, this is the common grid leak resistor of V20 and V21, 6L6G tubes.

These old resistors definitely have to go. They will be replaced with metal film resistors of 5% or lower tolerance. These are critical to low distortion and correct signal balance to the output tubes plus correct biasing of the output tubes. The bathtub capacitors in this area will also get rebuilt.

Joe

[Joseph W Strickland]

This morning was spent replacing resistors and capacitors in the audio section of the power supply chassis. The R89 negative feedback resistor was completely open on the 20Meg ohm scale of my digital VOM. So all negative feedback was defeated by a part failure.  It turned out that C89 and C90 had already been changed to Sprague orange drop .1@600VDC mylar capacitors in the recent past, so those went back in circuit. in the V19 circuit, the cathode resistor R82 had increased from 1.5K ohms to 2.16K ohms. C88 was swollen, so I replaced it also. R80 cracked apart as I was working in that area, so it was also replaced. All of the 220K ohm resistors were replaced with 1% tolerance parts (not that they have to be - just that they are less expensive than 5% types - go figure). I still have some to do. R81 is also off value (not as bad but still not within 10%).

The audio stages had so much wrong that it likely had considerable distortion and would not have been very decent to listen to.

Joe

[Joseph W Strickland]

I finished rebuilding the bathtub capacitors in the radio-tuner chassis today. Some are difficult to reach for service even though Scott used a large chassis that has lots of space for components. In the process I found it helpful to save some of the wax that came out of the capacitor assemblies and it can be used to stick star-washers and screw heads to the end of a screwdriver while inserting them into tight spots. Works well and saves lots of time and lost pieces (also reduces frustration level).

Tomorrow after checking for any B+ shorts, I will hook everything up and start bringing it up with my variac. This should give the electrolytics time to reform if they need it. They were replaced in the recent past, but have not had power applied in some time now.

I do not have the Scott AM loop antenna but do have a Philco AM loop antenna that I will try to see how it does. This is fun!

Joe