Fisher 400 ringing transformer, output voltage difference between channels

[Speedbump71]

Hello all....

I have a Fisher 400 that I purchased locally. We have a very well thought of tube gear repairman here in Knoxville, and I was told he went through the unit by the seller.

Upon inspection, it does seem to have been recapped with decent caps (Nichicon Muse/Poly) in all of the expected places. I just finished up some new 8 Ohm dummy loads, so I have been going through and "checking" actual power output on some of my collection.

The unit begins to clip at about 13Vrms, which seems correct into an 8 ohm load (approximately 22Wrms) but there is a pretty decent sized crossover distortion blip and one side has a ring in the transformer as power levels get towards clipping. The biggest problem seems to be a discrepancy in the output voltage between right and left channels. If i move the balance pot so that both voltages are equal they both seem to track fairly well, and begin to clip at the same time.

I could spend my time going through things again, but thought a quick post might get me on the right track sooner.

Thanks in advance...

Speed

 

[Speedbump71]

Voltage before clipping with adjusted balance control:



Voltage before clipping with balance centered:



You can see that channel 2 is significantly lower. Output tubes are new Electro Harmonix. I switched the 12AX7s side to side and the problem remains.

 

[dcgillespie]

That display does not represent classic crossover distortion due to an over-biased output stage. That almost appears to be part of the test signal used to drive the Fisher. Is the test signal presented to the receiver in fact clean? And if so, at what power level does the waveform deviation first start to show up? You might also check to see if the deviation is present at the output tube control grids as well. Let us know!

Dave

 

[Speedbump71]

It is certainly possible that it is coming from the source, as I am using a tone generator on a tablet, but I didn't see it while testing some of the other amps. I will verify a few things and post when I know.

 

[Speedbump71]

Dave - Thanks for the sanity check.

Update: It was the source. Evidently the tablet must be low on juice or something after running it for a while. I used my phone to double check and all problems went away. Serves me right for being too lazy to drag in my function gen. Here we are with the balance knob centered with the better source. Still a bit weaker on one side, and the ringing is still there, but at least I can explain the really odd stuff. My guess is with a decent generator on it, instead of a stereo output from a smartphone, the output voltages will be closer as well.

 

[dcgillespie]

Glad you found it! Make sure you do your channel balance tests with the volume control up full -- to eliminate any influence it might have on the balance performance of the active circuitry. Once you eliminate any imbalance out of the active circuitry, then you can start checking how the volume control affects the balance at different settings.

Those last wave forms look great!

Dave

 

[Speedbump71]

The ringing output transformer seems to be slightly different than the other one. Power transformer and one output transformer have bolted on endbells, while one seems to be riveted? Possible replacement?

 

[dcgillespie]

When you say ringing -- do you mean ringing as in actually hearing the sound of the test signal coming out of the transformer itself that the other transformer does not produce? Or, do you mean the classic understanding of "ringing", where the tops and bottoms of a square wave presentation are not smooth, but riddled with wiggles and squiggles?

The two transformers look to be the original authentic pieces. The wiring under the chassis suggests this as well. Early Fisher transformers used bolts (of various lengths) to secure the transformer laminations together, while later ones used rivets. Your unit is an early version 400 receiver, that was almost certainly produced at the time when the transformer manufacturer was switching over from bolts to rivets, with your unit receiving one of both examples.

Dave

 

[Speedbump71]

By ringing, I mean the transformer makes an audible sound equal to the input frequency. It becomes audible at approximately 50% output. The riveted transformer is the one that makes the noise. It is quite noticeable.

 

[gadget73]

mechanical noise then. Might just be the rivets aren't clamping the lam stack as well and its buzzing. I've had some output transformers that "sang" a bit but it never caused any problem.

Are the part numbers the same on those two?

 

[dcgillespie]

Maybe -- but a push-pull output transformer will also sing if the amplitude of the two push-pull sides is not reasonable equal as well. Try swapping the driver and output tubes from one channel to the other, and see how that affects the problem. If the problem moves with the tubes, then that's telling you that one of your output tubes is pretty weak causing a significant imbalance to produce the sound from the transformer. If the problem stays with the one transformer, then check that the tubes are getting equal drive from the phase inverter. If all is good, then the lam stack may simply be singing for its supper.......

Dave

 

[Speedbump71]

mechanical noise then. Might just be the rivets aren't clamping the lam stack as well and its buzzing. I've had some output transformers that "sang" a bit but it never caused any problem.

Are the part numbers the same on those two?

My guess is I wouldn't hear it much if I were playing music, since the speaker output would be louder than the transformer. I may try to apply some sort of clamping force to the transformer to see if the noise diminishes. If so I will source some bolts of the proper length and clamp things a little tighter.

 

[larryderouin]

What are the numbers on the endbells of the transformers. The 400 used 2 different suffixes to denote Early and late models.
We use the 48001 serial # as the demarkation to denote early from late.
EARLY VERSION T1020-116-1 or 2 (according to the 2001-29999 manual, all manuals below 48001 show same part #)6.5K ohms
LATE VERSION T-1020-116-1AX or 2AX (according to the 48001 and up manual) 10K ohms

 

[RayWu]

What are the numbers on the endbells of the transformers. The 400 used 2 different suffixes to denote Early and late models.
We use the 48001 serial # as the demarkation to denote early from late.
EARLY VERSION T1020-116-1 or 2 (according to the 2001-29999 manual, all manuals below 48001 show same part #)6.5K ohms
LATE VERSION T-1020-116-1AX or 2AX (according to the 48001 and up manual) 10K ohms

Transformer are never riveted, put the screws back it, is will , stop the ringing.
had a repair like this, i put the screws back in, stopped buzzing.

 

[larryderouin]

RayWu; What does my last post have to do with your statement re: "non riveted transformers"? FISHER DID use riveted transformers in some models in the 60's. The reason I asked for the numbers off the endbells is that the early transformers have an impedance of 6.5K, and the late transformers have an impedance of 10K. It's possible that one of the transformers were changed and the NFB and accompanying circuits were not changed to reflect this.

 

[dcgillespie]

All three transformers in my 800C are very definitely riveted together. They're quiet, and riveted.

Dave

 

[Lo-Fidelity]

That display does not represent classic crossover distortion due to an over-biased output stage. That almost appears to be part of the test signal used to drive the Fisher. Is the test signal presented to the receiver in fact clean? And if so, at what power level does the waveform deviation first start to show up? You might also check to see if the deviation is present at the output tube control grids as well. Let us know!

Dave

Dave,
You continue to amaze me with your ability to diagnose problems and generate solutions!
Rick

 

[Speedbump71]

Will finally get back around to this tonight. Curious to see if the transformer part numbers are correct.......