Temperature of Fisher 800C power transformer

[Tomvette]

My Fisher 800C power transformer measures about 129 degrees F which does seem pretty hot to me. This is not mounted in the wood cabinet.

If you guys have time and a laser temperature probe, could you share your temperature readings. Voltages measure at the correct levels in my unit.

 

[larryderouin]

129*F is fine. Transformers generally do a chernobyl and melt down at about 190*. What's your wall voltage average over a day. If it's over 120 you need to drop it down a few volts either with a variac, a bucking transformer (6v or 12v down) or get the Electric Co to drop it down to 120 (fat chance in hell!). Transformers run hot, fact of life. The problem is, is that ther are so many variables involved, and with todays wall voltages being higher than the 800c was designed for, you'll have some increased temps. My 800c in the Executive Console runs about 138*F-142*F My Standalone 800c runs about 135*F after 2 hours. Wall voltage varies between 118V and 123V, My 400's run about 135*F and my Sansui 1000A runs at 145*F. All units without cases. The coolest of the bunch is a X-101-B integrated, which runs at 115*F. To make sure your unit is running as designed check the heater voltages on the tubes. 6.3V tubes should be between 6.0 and 6.5V No Higher. 12Vtubes (the 4 in parallel/series on the bias circuit) will run between 11.5V to 12.6v depending on the voltages for the bias circuit. This is normal. If the heaters are reading fine, then everything else should/will fall in line.

Moved by ADMIN to the Main Forum. (Thanks Tinkerbelle!)

 

[Tomvette]

Hi Larry, many thanks for sharing the temperature data. To answer your question, I have built a bucking transformer that provides 110 or 117 volts to the fisher 800C. My line voltage is normally about 124 volts. Voltages are all within range and I've made changes to the bias circuit as recommended in these pages to drop the draw to the output tubes to about 22 mv. The selenium bridge has been replaced and a 50ohm 10 watt dropping resistors has been added between the bridge and the first cap.

As a separate matter, I replaced all of the power supply caps twice in an attempt to eliminate low level hum in both channels. (The last set came from Hayseed Hamfest.) I've also swapped the output tubes and swapped all 12ax7's individually to no avail. All coupling caps have been replaced.

 

[larryderouin]

22mv is way too low for the output tubes unless you've incorporated Dave EFB circuits. Minimum normal is around 28-29mv., any lower and distortion rises drastically. Ideal is around 32-33mv without EFB for proper operation of the output tubes on the 800c. Plus the extra distortion from running at 22mv without EFB could add to the load the transformers have already bringing up the temps a bit.

As for the hum, is it in both channels, and or in all modes.....AM/FM/AUX/Phono/Tapehead, etc. You may have a grounding problem.

Would you list all the work done, and any mods done to it so we can see what' going on and make some recommendations.

 

[Tomvette]

Thanks for the insights. The hum was traced to a 50ohm dropping resistor I had added following the bridge rectifier. After removal the hum went away. I replaced the fixed resistor for bias management and adjusted the pot to achieve bias between 28-30mv.

Prior updates include changing the RC network from .047uf/330k to.10uf/200k, adding 10 ohm resistors from pin 5 to ground and adding a 560K resistor across C49 to raise the AM signal strength with the FM scale. All caps were changed to orange drop and Hayseed Hamfest 105 degree C cans.

 

[thornev]

Tom... What does this mean? "adding a 560K resistor across C49 to raise the AM signal strength with the FM scale" I don't understand how raising the AM signal strength has anything to do with the FM section.

 

[larryderouin]

I think he's meaning is that his AM output is now equal to his FM output per the Strength meter. Although I don't understand it either.

 

[dcgillespie]

The 560K resistor is a mod I recommended for the 800C AM section. On AM, the signal strength meter of this receiver reads notoriously low on a signal of significant strength. The addition of the resistor causes the signal strength meter to indicate a similar AM signal strength on strong signals, as it does on strong FM signals.

Dave

 

[Tomvette]

Tom... What does this mean? "adding a 560K resistor across C49 to raise the AM signal strength with the FM scale" I don't understand how raising the AM signal strength has anything to do with the FM section.

Boy, I could have worded that better! Dave who posted the original forum modification explained it very well. The resistor brings the AM signal strength reading on the meter to a level consistent with the FM reading.

I'm old but new to the forum. . . . my apologies for the confusion.

 

[Jailtime]

Just checked my 500C after running it for a couple hours, 115 F. It's completely restored, in the wood cabinet, and has a CL-80 installed. Wall voltage is 123 V.

I'd guess with all things being equal, an 800 would run hotter than a 500 since it has a couple extra tubes for the AM section.

 

[Tomvette]

Just checked my 500C after running it for a couple hours, 115 F. It's completely restored, in the wood cabinet, and has a CL-80 installed. Wall voltage is 123 V.

I'd guess with all things being equal, an 800 would run hotter than a 500 since it has a couple extra tubes for the AM section.

Thanks for sharing. Your's runs much cooler.

 

[Jailtime]

I shot it again with the laser temp gun, it was 127 F after 6 hours. I've used it every day for years, so I'd say yours is normal.