[MichaelFranks]

So I purchased an allegedly partially-restored fisher 400 on ebay. When I received it I pulled all the tubes and tested them to make sure it was safe to power on. I then tested it out for about 5 minutes bringing it up slowly on the variac, everything worked perfectly. I took it outside to blow it out with compressed air and lube up the knobs and switches. I gave the side of the chassis a brief rub down with ammonia (until I realized how laboorious the full cleaning process was going to be).

I was extremely excited about my new purchase, which should finally be safe for me to use unattended and worry-free, right?

Well once the minimal cleaning was dry and completed I took it inside to give it a more extended listening on the ole workbench. I am always very concerned about the 7868's temperature, on another unit I had taken mkmelts suggestion and hooked up a fan to blow across the 7868's. This time when I listened, I occasionally checked the 7868's with my finger, just to make sure they weren't running any hotter than my other "unrestored" unit.

After about 10 minutes of listening the soft touch turned into a painful burn! I still have a huge blister on one of my fingers because of the extreme temperature, but at least I still have a home since the thing didn't explode on me! Unfortunately the extreme heat took what was practically a NOS 7868 tube down to nothing, I guess the heat blew it out or something. At this point I was extremely discouraged, but curious to see if the problem would persist.

I jerked the remaining good tubes and used a last-leg set from a pal to do some more experiments. Sure enough, the tubes would progressively get hotter and hotter until one of them finally started to glow in a red ball of fire, at which point I would quickly cut the power. I let the unit sit for a night, got up the next morning and unfortunately nothing had changed.

Looking at the under-chassis someone has definitely done some work on this beast, it has the most impressive looking selenium rectifier replacement I have ever seen. Of course various other capacitors have been replaced around the under-chassis and mpx as well. Unfortunately whoever did the work didn't take the time to test it out!

Is there any way (with minimal risk to a rather ignorant electronics enthusiast)I can start to troubleshoot and perhaps rectify this problem? Is it most likely just too much voltage to the power tubes?

The person I got the amp from had great feedback and was kind enough to offer a partial refund (neither of us wanted to pay shipping on the beast again), so I am not disappointed with the transaction overall. I would really like for it to work though! Any tips appreciated, and thanks ahead of time, YOU GUYS ROCK!

 

[MichaelFranks]

mods move this to DIY if you feel it necessary....

 

[Hirsch]

Quote:

Originally posted by MichaelFranks Looking at the under-chassis someone has definitely done some work on this beast, it has the most impressive looking selenium rectifier replacement I have ever seen.

That's the likely culprit right there. The selenium rectifier on old Fisher units is known to go bad, and cause serious damage when it does. Do NOT use your unit until you can replace this with a silicon rectifier. If you need specs, email me.

 

[MichaelFranks]

It has been replaced by some crazzzy looking conglomeration of hardware, perhaps I should take a picture and post it? The restoration work was allegedly done in 99 (the seller included a copy of the receipt) and I know it was not replaced with a selenium version.

 

[Hirsch]

Quote:

Originally posted by MichaelFranks Is there any way (with minimal risk to a rather ignorant electronics enthusiast)I can start to troubleshoot and perhaps rectify this problem? Is it most likely just too much voltage to the power tubes?

A likely problem might be mismatched power tubes. This is a push-pull circuit, and a mismatch could cause the symptoms described. The tubes need to be matched in pairs, that is, the two on the right need to be pretty close, and the two on the left need to be pretty close, but everything should be fine even if the two pairs aren't close to each other.

Do you have the schematics for the amp? It can take awhile to learn how to translate the schematic into a particular place on the amp. It's going to be harder with a modified amp, as you may be missing some reference points. However, they might help you track down something that's clearly not right.

 

[MichaelFranks]

I tried tubes that were relatively closely matched for emissions (it came with a set of NOS tubes, but then again it came restored...). I also checked all tubes for shorts before powering it on.

That is a good idea on trying to track down the mods with a schematic, perhaps the modifications will help give me some inkling of how to read one!

Is there a way to use a multimeter to safely check the voltage at the input to the 7868 tubes? Perhaps I could do a trace back from that point, if in fact the voltage levels are the problem.

 

[AndrewB]

Sounds more like a failure in the output biasing circuit. Perhaps a failing electrolytic cap.

 

[mkmelt]

Your 400 may have been properly restored with the selenium rectifer being replaced with a silicon rectifier. The symptom of the one tube running fine for the first few minutes and then progressively glowing cherry red after 10 minutes may be due to a bad tube socket or solder joint on the pin connnecting to the control grid (pin 2).

Without the correct (negative) bias voltage (-21 volts or even a bit more negative as I recall from the Fisher Doctor's documentation) on the control grid, the tube will start to pass maximum current through the plate and will quickly self destruct. None of the other tubes exhibit this tendency to run away, so you clearly have not lost bias on all of them.

One thing you can try is to resolder all of the connections leading to that tube, but the connection that is suspect is on pin 2, this is Grid #1, the control grid. If you have a bad Novar base tube socket, you will need to replace it.

Be careful if you choose to use a DVM to check the operating voltages while the Fisher 400 is operating. The rated operating voltage on the plate and Grid 2 (pin 7) is 300 volts. This is probably beyond your meter's range. Also, an inexpensive meter will not have a high enough internal operating resistance (at least 10 Megaohm is needed) so as to have a negligible effect on the resistance of the operating curcuit. The proper test instrument to test AC and DC operating voltages on tube audio gear is a Vacuum tube voltmeter (VTVM). These show up all the time on eBay (where else). An inexpensive VTVM made by Eico, Heathkit, or Lafayette may be all you need to get started. The operating theory of the VTVM holds the same for most examples. The best ones were made by HP and these will cost you $100-200. What you pay for is durability and accuracy.