fisher 400 smoke then pop

V

valco51

New Member
I need some help. I installed new electrolytic caps in my fisher 400. I had it playing for a hour or so at low volume and every thing sounded good. I plugged in head phones and turned off the speakers and turned up the volume. For a short time it sounded great then I saw smoke. I turned it off and took off the bottom plate to see the damage. Every thing looked ok, but i could smell something was wrong. So I turned it back on with the same high volume set up and I could see smoke coming out of the wire wound resistor 25ohm@5watt (r114 & r115) going to ground. http://i563.photobucket.com/albums/ss74/dougsgoodies/fisher 400/100_4171.jpg
Then I heard a pop and the 10ohm resistor had blown going to the power tube. http://i563.photobucket.com/albums/ss74/dougsgoodies/fisher 400/100_4154.jpg
Hear are the capacitors I replace the old ones with. 100uf@385v, 40ufx3@525v can, F&T 22uf@500v, 50/50uf@500v can, F&T 220uf@300v.
-17 volts bias with 24.1 to heaters. Power tubes pins 1&7 377vdc, pin 9 @ 416vdc, pins 3 to 10 ohm resistor to ground 283, 287, 333, 248 333+248=581/2=291 The tube that blew the 10 ohm resistor is the 333.
I have more pictures at my photo bucket. http://s563.photobucket.com/albums/ss74/dougsgoodies/fisher 400/
 
It's good to have replaced the caps in the power supply section but I think you really need to replace the coupling caps going to the output tubes as well as most of the other caps.

Were the output tubes glowing brighter then they should have?
 
I have some PIO .1uf coupling caps coming. They are coming from Russia. I have the 220k 1w resistors just waiting. I was going to do a fine tune of the bias when i had them installed. No the power tubes looked fine.
 
Yikes!!

I am not really familiar with that piece specifically but I am *very* leery using these tube amp/reciever things to drive headphones with the speakers switched out due to the small size of the provisional headphone mode loading network which I suspect is now cooked & dead, probably the 25^ WW resistors. It also appears at least one of the post-production added 10^ output protection resistors is blown open. Tube amps *DO NOT* like being driven hard with no loading or light loading, you are risking destroying the output transformers doing this and are doing the tubes no favors.
Playing them thru modern phones loud like this is an invitation to costly disaster, you are almost there. Stop before the unit is toasted and hope for the best with repairs.
If I'm wrong someone tell me.
 
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A pre-emptive thanks for letting me know about the headphone circuit in these amps. I hadn't studied the schematic enough to see where the headphone signal was coming from, and I'm really glad I hadn't tried it out yet.
These output transformers are fantastic, I wouldn't want to risk running them into a dummy load, especially one that's underrated.
 
Ouch.

Hope it all turns out well for you.

I am happy to have read this thread, as I push my phones with my 4oo receiver, and so far no issues, but I am unwilling to take any more chances, as I love both of my 400's.

Hope you post what repairs were necessary.

Cheers
 
Maybe I'm weird but I run often my Grado's on the phone out of my Fisher X-100A. However, the resistors network measure fine (I even replaced the two 220 ohms).

I know for sure that if I crank the volume near max, the OPT's begins to "sing" (you hear the music through them). But you have to put it at insane levels to obtain this effect.

I also run the phones on my HK A500 but I had to add an external switchbox to put a complete dummy load (47 ohms @25w) and really close the speakers outs. No problems here also.

In fact, I built two others external headphone boxes for an Heathkit AA-100 and an AA-32.

Loving to listen with phones, I also implemented an internal circuit into a little Fleetwood console amp (I copied the Fisher circuit) and I plan to do the same on my newly acquired Eico HF-81.

I understand that if the load is sufficient (between 47 and 62 ohms and from 10w to 25w depending), there's no harm done... Correct?

Patrice
 
My early production Sansui 1000 has unmistakable evidence of being cranked hard in headphone mode. Using 'phones on these is OK within sensible limits but that mode does not load the amps optimally for higher level output. I only use 'phones for servicing these things, otherwise for portable devices. My system pleasure listening is always done via speakers so this issue hasn't arisen until this thread.

BTW, how's the Fleetwood?
 
You are dead on the money, Pio. Headphones of the day when the Fisher was manufactured were never meant to be operated at today's "personal" levels, but quietly with the speakers off so as not to disturb others -- man are those days long gone! No doubt the 25 ohm load resistors -- which were fine for the intended application -- are now toast. When that happened, the output transformers revolted, and the resulting back emf can create an arc within the output tubes and take out a cathode sense resistor. Guitar guys know this all too well. The tube with the burned cathode resistor should be carefully checked for stable operation here after, in addition to the more obvious repairs required.

Dave
 
"Can I use these resistors in place of the 25 ohm 3 watt resistors? 1mj25 Mallory they read @ 25 Ohms. "
Thos'l do fine. Replace the 10 ohm resistors with the same type and rating, they are safety devices as well as metering points.

If the basic info in this thread isn't in the essentials of a "sticky", it should be. It is essential info for toob noobz and a vital reminder for golden oldies away for too long.
 
Pio1980 said:
My early production Sansui 1000 has unmistakable evidence of being cranked hard in headphone mode. Using 'phones on these is OK within sensible limits but that mode does not load the amps optimally for higher level output. I only use 'phones for servicing these things, otherwise for portable devices. My system pleasure listening is always done via speakers so this issue hasn't arisen until this thread.

BTW, how's the Fleetwood?
Click to expand...

It's been two weeks since we go for the last time at the cottage. When we was there, I worked mainly on the Nordmende so the Fleetwood is in standby now...

Been busy since one week on my new gem, Eico HF-81.
 
Well I replaced the resistors. I turned it on long enough to get a reading on the bias. The bias is running normal on three tubes but the other tube with the blown resistor is bad. It only reads around -24mv and the other tubes are reading around 270mv. I switch the bad tube to another socket and the problem followed the bad tube. The good news is i had music coming from the receiver. Whats the chance of buying one nos 7868 and get it bias like the other three tubes? :scratch2: It looks like i'll need to buy four eh7868 tubes.
 
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