Fisher 800C Tuner Alignment AM and FM

T

Tomvette

Member
I have a Fisher 800C receiver and the necessary Sencore FM generator, distortion analyzer and scope to align it.

Many service manuals and units label the coils and pots that require adjustment. I don't find a physical drawing that does this. Can anyone provide a labeled photo, drawing or description of the locations of the AM and FM coils (i.e. Z1-Z4, L . . ., C . . ., R . . .)?

Many thanks in advance.

Tom
 
Z1 thru Z4 or Z5 are numbered from the tuner box across the unit. Z1 is closest to the Tuner box with Z4 Next to it. These are the AM Coils. FM Coil Towers are Z2, Z3, Z5,Z6,Z7 laid across the top. The L&C trimmers are inside the tuner box. You'll have to trace out the circuits to mark them up. I haven't done it on a 800c yet. It's not hard, just tedious.
REMOVE THE BOTTOM COVER OF THE BOX. The Coils should be lined up in a group of 3. Trimmers should also be lined up with holes in the cover. Trace out the circuits and mark the cover appropriately.
800C tube layout.JPG
 
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Hi Tom -- If I may, are you well familiar with the alignment process of AM and FM receivers?

Dave
 
Hi Dave, I would say no but I've been successful with alignments provided I have a service manual with clear instructions and all adjustment points well labeled. I've had problems my entire life reading maps and schematics but if forced I can do so. It just takes a lot of time and double checking and sometimes I'm wrong. I'm still hopeful I can get the L and C adjustment points labeled by an audio karma member. This improves my confidence.
 
larryderouin said:
Z1 thru Z4 or Z5 are numbered from the tuner box across the unit. Z1 is closest to the Tuner box with Z4 Next to it. These are the AM Coils. FM Coil Towers are Z2, Z3, Z5,Z6,Z7 laid across the top. The L&C trimmers are inside the tuner box. You'll have to trace out the circuits to mark them up. I haven't done it on a 800c yet. It's not hard, just tedious.
REMOVE THE BOTTOM COVER OF THE BOX. The Coils should be lined up in a group of 3. Trimmers should also be lined up with holes in the cover. Trace out the circuits and mark the cover appropriately.
View attachment 1133176
Click to expand...
Many thanks for the help!
 
It sounds like you are familiar with the process then, which is great! I can't tell you how many times I've received a unit for service however, where well intentioned folks who really had no clue just started to have at it, an who by their own admission, left the unit worse than when they started. May I ask why you think it needs an alignment?

Dave
 
dcgillespie said:
It sounds like you are familiar with the process then, which is great! I can't tell you how many times I've received a unit for service however, where well intentioned folks who really had no clue just started to have at it, an who by their own admission, left the unit worse than when they started. May I ask why you think it needs an alignment?

Dave
Click to expand...
Hi Dave, on stereo channels, the harmonic distortion was audibly high and on all channels, and signal strength was about 1.5 on a scale of 5. That said I replaced several tubes in the tuner section in the last few days and the signal strength is about 4 on strong stations and the distortion is not noticeable now. My interest in an alignment is really based on the fact that I can tell the unit was serviced before I received it by someone with poor soldering skills and my bet is it's been fooled with. To your point, I might push some signals into it and measure distortion and see if it's good enough.
 
The "been fooled with" is always a possibility for sure. Signal strength will be a function of all tubes and coils up to and including Z4. Distortion is largely a product of a misaligned Z5. Distortion can also be a product of an improperly operating diode switching matrix. If the distortion is only in stereo mode, that can also be due to the tuner section proper being misaligned, but usually points to the MPX sub-chassis.

Dave
 
dcgillespie said:
The "been fooled with" is always a possibility for sure. Signal strength will be a function of all tubes and coils up to and including Z4. Distortion is largely a product of a misaligned Z5. Distortion can also be a product of an improperly operating diode switching matrix. If the distortion is only in stereo mode, that can also be due to the tuner section proper being misaligned, but usually points to the MPX sub-chassis.

Sometimes test point 3 does not work as well as the manual states. After doing it the kosher way the signal strength was only a 2 when it started at 3.2 on WMBI at 90.1
Peaked Z2 and Z3 from above and below. Z5 actually turns now from above! Peaked from above and below. Much easier to do with signal meter. Z5 actually peaked from above and below. Z6 peaked from above and below to great effect, bringing the signal meter to a 3 out of 5 on WFMT! Z7 is stereo separation and peaked such on Audio Technica ATH-AD700 headphones. Button her up now. Back in place WFMT is 3 and sounds real real nice, even better with separation control maximized!
Click to expand...
 
Oh just realized that at 10.7Mhz the signal is injected on the UNGROUNDED shield of V2. When the chassis is upside down I may have to use a little tape to keep the tube shield ungrounded. Will try again when the 6HR6 replacement tube is received. The old tube measured 83 but now is down to 67 after just 7 months of use. I could understand if it were a 7591A tube since they get so hot. From now on, I will run the receiver with the cover off.
 
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larryderouin said:
You can use a 6AU6 in place of the 6HR6. https://audiokarma.org/forums/index.php?threads/500c-800c-6hr6-versus-6ah6.975484/ It'll work fine. And you can put the cover on.

If you have a spare Tube cover for the larger tubes, squeeze the open end enough to hold it on the tube. Put it on about 1/2 way down and clip your lead to the top. Works very well. Then you don't have to mess with tape.
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Nice, could one do all the upper transformers, then do all the lower transformers. Or does one have to do the upper and lower then move on to the next transformer?
 
I tried the 6au6 plug and play like a drop in replacement and the signal strength went down. I did not tweak any transformers aka alignment. However I bet that if I were to replace the expensive tube and align it, then the signal strength may have gone up. So it is not a simple plug and play, likely since there are different capacitances within the tubes?
 
But were the transformers properly peaked for the 6HR6 before you tried the 6AU6? Normally, the difference between the two tubes is minimal in a properly aligned receiver.

Dave
 
dcgillespie said:
But were the transformers properly peaked for the 6HR6 before you tried the 6AU6? Normally, the difference between the two tubes is minimal in a properly aligned receiver.

Dave
Click to expand...
I tried to do the proper alignment procedure, but now that I reread the procedure the V2 tube shield was grounded, so likely I did not inject properly since the V2 shield was the input. Could not get it to go smoothly reading at TP3. So adjusted the various transformers reading the signal strength and likely that went well. Then I swapped the known good 6AU6 with a decrease in signal strength. But now I realize that you just tighten the V2 tube shield so that it does not touch ground. I will await arrival of the 6HR6 tube that is supposed to test strong and then retry the correct alignment procedure. It is interesting that the Fisher 800C procedure for 30k serial numbers and above is vastly different than the Scott EZ align for the Scott LT112B which uses the actual signal meter. It is also different than the Dynaco FM3 alignment procedure.
I do not understand how to align the tops and bottoms without flipping the unit over.
 
Set the unit on its side. No doubt the Scott and Dynaco tuners used the signal strength meter because they were kits. So did the Fisher KM60, which was also a kit.

Dave
 
Are these a hex core transformer ? If so, most of the time you can push the alignment tool through the upper core into the lower to make the adjustments rather than flipping the unit over.

if not, just stand it up on its side.


using the signal meter is just monitoring the ALC circuit, which is what the eye tube should be doing in mono mode. Basically you're peaking it for strongest response, which may not always give ideal performance. With FM you're more interested in a symmetrical response than the strongest peak. Symmetry gives lower distortion and widest frequency response.
 
gadget73 said:
Are these a hex core transformer ? If so, most of the time you can push the alignment tool through the upper core into the lower to make the adjustments rather than flipping the unit over.

if not, just stand it up on its side.


using the signal meter is just monitoring the ALC circuit, which is what the eye tube should be doing in mono mode. Basically you're peaking it for strongest response, which may not always give ideal performance. With FM you're more interested in a symmetrical response than the strongest peak. Symmetry gives lower distortion and widest frequency response.
Click to expand...


Once I finish up paperwork, I will put the 6HR6 tube in and align according to the service manual, then do a visual alignment of the IF with the Sencore SG165, assuming I did not blow the Sencore when feeding it into V2 shield that was shorted to ground
 
Yeah, after 8 years my 800c Executive FM multiplex has gone wonky and has trouble staying locked on a station
I cannot do multiplex repairs

Not that there is much to listen to on FM anymore

seems to be a problem with ALL my tube receivers

Best of luck with your 800c
 
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