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Improving the Fisher 400

Dan -- As an easy check to see where the problem lies, use a jumper lead to short the two channels together at different places. Working forwards through the unit from the input, at some point, the sound will even out, at which point, you've found your problem. For example, using (say) a CD player into the Aux inputs, in the following order, short together:

1. The center terminals of the tape monitor switch.
2. The input to the tone control stack/tone control bypass network.
3. The center terminals of the tone control on/off switch.
4. The output of the tone control amps (after .047 coupling cap).
5. The outside terminals of the balance control.
6. The center terminals of the volume control.
7. The jumpers going between the Preamp Output/Power Amp Input jacks.

At some point, the sound will likely even up between the speakers. Whatever portion of the circuit is between the point where the sound evened up, and the previous step before that where it was still uneven is where the problem lies.

I hope this helps!

Dave
 
Dave, that's actually a great trick. thanks for sharing it with me. I'll have to take my unit back apart to do the tests. I'll let everyone know my findings.

Dan
 
dannyr said:
Great work Luis, how's she sounding?
Click to expand...
Danny is working and sounding great!!!:thmbsp:

Was necessary to replace the speakers due to the improved output power!!

dannyr said:
Does anyone know if a balance pot can create misbalance between channels?
Click to expand...


Dan

The issue persists when the tone control and the loudness control is bypassed?

Also do the Dave´s suggestions, is a good point to start!!

Regards

Luis
 
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Luis, good to hear that your unit is paying off.

I have to do some further tests on my balance issue. Believe it or not things may be okay with the loudness enabled but I dislike the sound that way. I have to double check if it is okay with the tone controls bypassed. I like to use them to do a bass cut if i'm listening to FM or late at night. Btw is it me or is the way the Fisher balance control works that it doesn't do much until you get past 3-9 o'clock?
 
No it's just not you, I also noticed the same with my balance control on my 400, not much change until you get past 3-9 o'clock position.

Now when it is at 12 o'clock position I also found it was off from the left channel to the right channel about 25K.
 
ac heater voltage

Hello all. I'm a long time Fisher hobbyist and friends with some of you - I'm catching up on this long thread as well as Dave's and Don's X-1000 thread - a little late to the table, as they say.... Has there been any discussion regarding correcting heater voltage for those 12AX7's which use the "ac" supply such as the phase inverter tubes or tuner tubes? If so can you point me to it? I've generally left my ac heater voltage alone and paid all my attention to the dc string voltages and matching those tubes but I seem to recall my ac tubes run a bit higher than the RCA manual 6.3 volt spec. Dave, how did you deal with this in your 400 mods? I admit I may have missed some discussion on the subject because I'm reading a lot of this all at once, if so I apologize. Thanks, and good work all.
 
Hi Chuck -- I have always used the AC heater voltage -- as measured with the unit fully warmed and operating under normal full quiescent load -- as an indicator for what the intended design center AC line voltage was supposed to be for a given unit. That is, using a variac, set the AC voltage until the farthest output tube from the power transformer is receiving 6.30 vac, and then see what primary voltage produces that voltage -- as measured at the output of the variac that the unit is plugged into. Whatever the B+ voltage is that results from the resulting AC line voltage value is whatever the B+ is going to be.

Personally, as long as the heater voltage is no greater than +5% over the nominal value, then it is a non-issue for me. As the voltage rises over that point however, I typically consider using some sort of external buck transformer arrangement so as to bring all the voltages down to more appropriate values. If the voltage reaches +7%, then I will absolutely adjust it accordingly. High heater voltages cause needless evaporation of the cathode's emissive material, and can also lead to reverse grid current problems as well.

I am fortunate that my line voltage is typically no more than 3-4% over a nominal 117 vac value, so adjustment is really only required for those few (non-Fisher) units designed with a 110 vac power transformer primary winding.

Dave
 
Welcome chuck802.

We're glad to have you along for the ride.
So, are the AC heaters always at 6.3 volts or are they sometimes wired the 12.6 volt way?
 
Almost always, when dual heater voltage tubes are powered by AC, they are configured to operate from 6.3 vac. I can think of only one application where a 12AX7 was wired to operate off of 12.6 vac -- and that is in the Eico HF-89 power amplifier. In that unit, they use a small filament transformer to power the heater of the 12AX7 only, and so chose to operate it in the 12.6 volt configuration. It is typically most desirable to operate 12AX7 type tubes with their heaters wired in parallel for 6.3 volt operation.

Dave
 
ac heater voltage

Thanks Dave, Don. I like your analysis Dave, I see your reasoning. Interesting, years ago I had installed a permanent buck transformer in this 400 receiver. My goal was not just ac heater voltage, it was more to give the unit what it wanted to see overall and start from there. I was unable to conveniently find the right transformer and the one I used knocked the primary down a bit too far, placing me under target. I had used a little Radio Shack 12 volt center tapped transformer from the center tap, and it pulled down 6 volts as I recall. Is there a good source for these at various values? 3 or 4 would have been nice. It's been a while, and for the time being I bagged the idea. I like the buck approach though - it seems most purist to my way of thinking, second only to adjusting the primary voltage like with a variac. I began using thermistors quite a while back, that helps just a little.
Thanks fellas, great to hear from you both.
 
RadioShack still carries the 12V 4A transformer. I use one for a bucking transformer for my tube gear going on 3 years now. The 6 volt setting drops the wall voltage from 123-124 to 116-117 without problems. I also use the 12v tap for older gear that is designed for 110.

I think the part # is listed in the "COMMON PARTS" sticky at the top of the forum listing.

Larry
 
Hi, my 400 has unfortunately developed a intermittent hum in the right channel that I haven't been able to track down so far. It started as a odd very low volume "Thrisp, dit dit dit, Verrppppp" sound that got worse turning into a speaker vibrating hum. It seems to be okay the few times that I've had the unit flipped over with it's bottom off testing things. I was hoping it would still do it so I could at least poke around with a plastic sharpy looking for a short. So far I've swapped tubes and resolded a lot of the connections in the right channel. The controls don't seem to effect the problem at all so I'm assuming that it's most likely the volume pot on towards the outputs. Would something odd with the opamp board cause this? Any thoughts on what to check are appreciated. Thanks.

Dan
 
Hi Danny -- Your diagnosis is well thought out. If the volume control does not affect it, then the source of the noise is after the control. And, with it only being in the right channel, then that eliminates anything with the power supply or EFB control circuits.

It is certainly possible that one side of the op-amp has developed a fault that's creating the noise. One very easy way to determine if that is the problem is to remove the straps connecting the Preamp Output and the Power Amp Input jacks together. With the straps removed, then use a conventional stereo interconnect cable to reconnect the Preamp Outputs to the Power Amp Inputs -- except reverse the channels when you do. In this scenario, if the noise changes channels, then the fault must be in the op-amp buffer. If it does not change channels, then the fault lies in the right channel power amplifier circuits.

Good luck with it, and let us know what you find!

Dave
 
Danny bypass the preamp board and let us know

Regards

Luis

Enviado desde mi GT-S7562L mediante Tapatalk
 
Thanks guys for the replies, ideas. I greatly appreciate it.

I unfortunately don't have the jumpers installed atm:(. I figured I'd never use the fisher as a preamp if I it was aging powertubes to do so with dummy loads etc. Maybe I should have asked about this ahead of time. I'm probably misinformed. I can always rewire it if so. What I can do is swap channels going out of the buffer to see if the noise moves or not.

Now if I can just get the noise to show up without putting the unit in it's shelf it would make things a lot easier.

Dan
 
Dan

Disconect the input in the board and then use a shielded wire to conect in the output of the board, don't forget to make the conection out of the board to warn any damage in the op- amp, that's is the easiest way to descart the hum in the board. If persist let us know to defeat this issue.

Regards!

Luis
 
Okay. I flipped the outputs of the opamp buffer and the hum remained in the right speaker so I think it's in the driver/output stage. Hmm. Does anyone have any guesses on what I should concentrate on looking for? Could the IBAM be at fault even if the bias is holding? Thanks

Dan
 
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