Fisher 800-B AM Z3 and Z5 IF transformer capacitor values

[Weld Engr]

Howdy still working on my 800-B and believe I have some capacitor issues (silver mica disease?) in the IF transformers based on reading posts and other searches. I have not verified voltages V5,V6 and V7 yet and do not have good tubes to swap in for testing at the moment. I do not get a heart beat 400kHz away on another AM radio. If someone has figured out the values on ZZ50210-1 and ZZ2984 I’d appreciate it. I found the procedure for using variable ceramic caps to determine it so I’ll go that route if I get there. Getting the AM alignment done isn’t something I have the tools or knowledge for so I’m going to have to find someone in MI most likely.
I only have static it will vary a little as you go thru the tuner range but just static. Going to work on voltage checks and pulling the ZZ2984 this weekend. Thank you

Wes

 

[Weld Engr]

Howdy still working on my 800-B and believe I have some capacitor issues (silver mica disease?) in the IF transformers based on reading posts and other searches. I have not verified voltages V5,V6 and V7 yet and do not have good tubes to swap in for testing at the moment. I do not get a heart beat 400kHz away on another AM radio. If someone has figured out the values on ZZ50210-1 and ZZ2984 I’d appreciate it. I found the procedure for using variable ceramic caps to determine it so I’ll go that route if I get there. Getting the AM alignment done isn’t something I have the tools or knowledge for so I’m going to have to find someone in MI most likely.
I only have static it will vary a little as you go thru the tuner range but just static. Going to work on voltage checks and pulling the ZZ2984 this weekend. Thank you

Wes

Found it 100pF for ZZ 2984 which I suspect is the issue.
https://audiokarma.org/forums/index.php?threads/another-800c-story.596281/

 

[Weld Engr]

Alright finally home long enough to verify voltages. I do get a faint signal for an AM local in Lansing - AM 1240 tuning eye will vary some but isn’t tuning anything in.

I have new mica caps for the ZZ2984 IF transformer but only checked voltages today I’m gonna need a solid day to work on that if its the issue

Voltage after CL-80 = 116V AC
V5
Pin 5 = 158V should be 164V
Pin 6 = 93V should be 70V
Pin 7 = 0.5V should be 0.8V

V6
Pin 5 = 157.8V should be 164V
Pin 6 = 64.4V should be 67.5V
Pin 1 = -6.4 should be -9V when testing it goes quiet no sound

V7
Pin 3 = 1.72V should be 1.7V
Pin 6 = 96.8 should be 115V
Pin 7 = 151.3V should be 156V

unfortunately my tube tester is still a work in progress
Bad tube?
IF transformer internal cap?
Alignment off?
Something else in the circuit?


Appreciate any advice or assistance.
Wes

 

[larryderouin]

Wes. Voltages are acceptable except V6 pin1 and V5 pin7 which are probably artificially low due to the rest. Schematic voltages can vary as much as 10%-15% per Fisher due to component tolerances. Take ALL of your tuner tubes out one at a time and spritz the tube pins with DeOxit, and insert the tube in the socket. Then roll the tube at the top (one finger) in a circle to get the pins sliding in and out (friction to clean them and the socket leaves.) V1(6AQ8) has a nasty tendency to break contact over the years due to corrosion on the pins and leaves and you lose reception. I suspect that might be the case for V5 & V6 and to a lesser extent the rest of the RF/IF tubes. Also V1 is run pretty hard. Of the 8-9 800-c's and the many console tuners I've rehabbed with 6AQ8's in V1, 90% had a weak or bad 6AQ8. IIRC the tuners in the 800-B are always on, so you might have a couple other tubes weak or bad. A 6DT8 will sub just fine for the 6AQ8. 6AU6, 6BE6, & 6BA6 are fairly cheap.

Check the voltages at the can caps to see if they are within spec. C96B (162v) eventually feeds V6 pin 1. Backtrack all of the B+ supplies. If the Power supply is weak or sloppy, the whole thing will be sloppy. The whole unit is best served with a power supply that is putting out what it's supposed to. If it's only running on 7............................


Update your profile with a location. State or county is fine. If someone is local to you and knows the 800b, you might reach out to them. There is a thread on the Tube forum (it's a sticky at the top) of Local tube testers (guys who have working ones and offer to test them) There might be someone local to you.

 

[Weld Engr]

Larry thanks I did the cleaning today and it improved another local station is clear enough to listen to. I just updated my profile there is a shop not too far from me I’ve been to before but want to narrow it down as much as possible first. I’ve done all the updates to caps it has hayseed hamfest parts. FM and phono work great I have a Dual 721 now so just the AM is of focus at the moment. Probably going to pick up some tubes and throw parts at a little. The Eico 666 I picked up needs more work than was “advertised “. I’ll see if there is anyone local to me thanks again for the advice.

 

[Weld Engr]

Ok I tried NOS tubes in V5, 6 and 7 no change in reception. I did not check voltages yet but with V7 pin 6 nearly 20 volts low and the path on the schematic wouldn’t that be a possible leaky mica cap in the Z5?

edit: actually all the voltages are out of wack that tie to the IF’s. Again I’m not knowledgeable so forgive my possible ignorance.

 

[larryderouin]

Start back at the power supply. Specifically C96B. Check the voltage there. With tubes in and again tubes out. Post the values. Then voltages at following: Tubes in and tubes out. Look for pinched wiring along route, cold solder joints, etc
V5 Pins 6 & 7
V6 Pins 6 & 7
V7 Pins 6 & 7
V8 Pin 6
V9 Pin 6
We're checking with the tubes out the basic circuit to see if it drops voltage somewhere due to a cold solder joint, a pinched wire between chassis and a tiedown nut, cut in the wire from a previous repair, burn on the wire jacket (previous repair and solder pen hit it, exposing wire and it possibly touching something it's not supposed to.)

While you're at it check the voltages on ALL the Main capacitor tabs and post them (if you have a variac, set input voltage to 117vac) Post these also.

 

[larryderouin]

Are the CAN CAPS originals or have they been replaced. If the old ones are leaky (electrically or otherwise) voltages can and will be low or non existant. Best to make sure power supply is operating correctly, before you start chasing your tail.

 

[Weld Engr]

Larry,

all new main caps from hayseed
Tubes in
AC = 116.5 V after the CL-80 using a variac
101B = 191,8
96A = 127.8
96B= 156.6

tubes out

here are some voltage readings got late so I may not have collected them all

No tubes - V5,6,7,9
96B = 168V
V5
pin5 = 167V
Pin6 = 167V
Pin 7 0.000
V6
Pin 5 = 168
Pin 6 = 167
Pin 1 = 0.000
V7
Pin 6= 167
Pin 7 = 167
V8 and V9 pin 6 = 167
Z5 ZZ2984
167; 167; 107mV; 107mV



tubes back in
V8 and 9 pin 6 = 157
V5
Pin 5= 157
Pin 6 = 74
Pin 7 0.66
V6
Pin 5 = 156
Pin 6 = 70
Pin 7= -180 mV
V7
Pin6= 95
Pin7= 150

I don’t see any wiring or solder issues I looked over everything nothing stands out and Deoxit main rotary switch again and cycled it 30 -40 times.
Looks like I get about a 10v drop with all the tubes in everything on FM works - tuning and tuning eye perfect function along with the phono
I took the cover off the AM tuning capacitor and it’s clean with no obvious issues.
Thanks
Wes

 

[larryderouin]

Got another 6BE6? V6 pins 6 and 7 should be around 67.5V and 9V respectively. What looks like a -sign on the Pin 7 voltage is where the page seam is from taping the pages together. It's just the end of the arrow. Otherwise the voltages all look good and are slightly low but within tolerances. I've found over the years that 6BE6 tubes in AM tuners don't last as long as the 6BA6 or the 6AU6 as they are usually doing oscillator and mixer duty.

 

[Weld Engr]

I have one station locally I can get tuned in decent and swapped with a NOS I just picked up they function equally to my ear. Is there a way to test the ferrite antenna? I think I have a loop antenna somewhere.
Good to know on the 9 vs -9. I’ll do some more voltage checks this weekend but probably see if I can get an alignment done on it. I thought Z5 was the oscillator does anyone know the value of Z3 mica caps? I found where Mr. Gillespie calculated Z5 caps to 100pF. I did try another radio close test again and pretty sure I get interaction from the oscillator at the 455hz lower - both tuner scales are not fine by any means but if I move it closer and away I can get the hum on the test radio to reduce/quiet what appears to be the offset. I’ll keep playing with it. Need to finish my cabinet up and glad I made a design where I can easily just remove the lid to access the top hardware. I literally finished typing last night that there were no FM issues and the left speaker just goes to a hum - WTH. The MPX tube must have been wiggled during my tests as I rolled it and it was fine again. The Fisher gods were messing with me. Thanks again

Wes