Fisher 400 Wide-Band FM Multiplex Receiver Overhaul

From Rain City Audio:

The Fisher 400 is a very well engineered, and very collectible, vintage stereo receiver with an iconic champagne faceplate and prominent bank of controls. This one came through the shop after being stored in a garage attic on Camano Island for more than a few years, but somehow managed to be in good overall shape with just some minor corrosion.



This one has all the original Fisher tubes still installed.


Underneath, there’s an awful lot going on, but it’s laid out pretty cleanly and accessibly. There’s not a lot of extra room under the chassis, but it doesn’t feel cramped either. Everything has a place.


It doesn’t feel cramped for most of the signal components, but there are quite a few capacitors mounted in large cans and it’s a challenge to fit them all into the available spaces with solid, safe connections. The cans on this chassis are very difficult to remove, unlike the Scott cans from the same era, so it’s more effective to add capacitors below the chassis. In this case all long-life Nichicon electrolytic capacitors.


Replaced the original, leaky rectifier diodes with new 1N4007s.


A printed schematic snip makes a great shop aid. This was taken part way through rebuilding the power supplies.



All in all, five or six new terminal strips were added and capacitors layered across one another at right angles. All of these are Nichicon electrolytic capacitors, with the B+ filter capacitors being 10,000-hour long lifetime models, and the other bulk capacitors being de-rated 5,000- or 8,000-hour lifetime models. It should be quite a while before the power supply section of this amplifier has a failure.


After replacing the electrolytic and paper capacitors, it was time for the first power-up. The 7868 output tubes can be problematic, so for the first power-up the output tubes were removed. Probing for signals at the output, only one side was passing a signal through the full chain. Using the scope to trace the signals, the driver/phase inverter 12AX7 was not lighting. The tube tested fine in an external tester, but the socket had no connectivity. The area around it was corroded, so it was obvious the socket needed to be replaced.





With this resolved, signal showed up properly at the grids of all four output tubes. It was time for some final checks and adjustments. First was to dial them back a little bit and add some more safety with the bias, adding about -1V by changing the 1K divider resistor in the bias supply to an 820 Ohm resistor. Then, double-checking the bias voltage was present on all of the grids. Since this amplifier uses fixed bias, there’s no bias pots to adjust here.



After the first power-up, it definitely did pass a signal all the way through, but it sounded really, really bad. The obvious culprits were the ceramic capacitors which were still a part of the original audio signal chain. Many seemed to have cracked bodies around the leads, and it seemed likely this could lead to internal failures. All the ceramic caps in the audio signal path were replaced with film capacitors.



Much, much better. Sounds great, in fact! Some control cleaner in the pots and switches took out the last bit of crackling on audio inputs.



The dial scale in the Fisher 400 is edge-lit by two 6.3V fuse lamps. One was burnt out, and the other just wasn’t very bright, you couldn’t really tell when it was on. Both were replaced with new LED fuse lamps which should have a very, very long lifespan.


With the audio amplifier sorted, it was time to move on to the FM receiver, which wasn’t receiving anything. Injecting the IF signal into the primary of the first IF transformer did produce a signal, but nothing made it from the antenna through the front end.

DSC_0029 DSC_0028

Both the 6HA5 RF amplifier and 6BA4 oscillator/mixer tubes had no emissions and were replaced, which brought the FM receiver to life immediately, and with the dial scale nearly perfectly tracking already.


Initially received THD on FM was pretty good to start, too. Not too far off from the 0.3% THD over FM.  It was really just a matter of touching up.



With these and many more alignment adjustments, on a strong and clear station, received THD settled at 0.22%. Not bad.


The IF chain, front end, and stereo MPX needed only very minor adjustments. The radio sounds fantastic on strong and clear stations, but weaker stations quickly picked up high frequency noise. There are two separate high filters, one in the FM receiver itself, and one in the audio amplifier’s tone controls to help address this situation but ultimately this is a receiver that benefits strongly from a top quality antenna.


Quite a few parts came out of this Fisher 400 to make it fully functional again, but now it’s in top condition and sounds great!

This entry was posted in Audio, Electronics, Projects, Radios and Tubes, Vintage and tagged , , . Bookmark the permalink.

4 Responses to Fisher 400 Wide-Band FM Multiplex Receiver Overhaul

  1. David Morris says:

    What is the item number for the led replacement lamp you used?

  2. David DeRosier says:

    Nice job but was a lot of work. You didn’the say anything about the tuning eye tube. Was it OK?

    • jwk says:

      The owner requested that I not spend a lot of time on the FM tuner section since it wasn’t a priority to him, so beyond replacing the worn out tube with a good one, I didn’t make any further adjustments to the eye.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s