Newly-Acquired Fisher 400

[Dave451]

RATIO DETECTOR/LIMITER ISSUE RESOLVED--BAD TRANSFORMER

OK, after returning I continued work on the 400 and finally resolved the ratio detector/limiter issues outlined exhaustively above. The transplanted radio detector/limiter transformer from the 500B into the Z5 position of the 400 cured the 'no resonance' issue with the 400 and all is as it should be now. The tuning eye is closing very nicely on strong stereo signals (picture) and distortion is gone. Completed a full alignment, including a re-alignment of the MPX detector and everything seems to be working properly. The only bad news: there a hole in my 500B where the RD/Limiter transformer should be (picture) and I'm not sure I can repair the one from the 400! Well, we'll cross that bridge later.

So Dave, as usual you were spot on with your advice. I measured the voltage on either side of the secondary of the original Z5 and got 5+ VDC on one side and zero on the other. Recall that the radio was displaying tuning eye symptoms right after I got it and I found a cracked and stuck slug in Z5. I think that a prior owner damaged the transformer by cranking on a stuck slug. Interestingly, I compared the DC resistance of the all the windings between the two transformers and they were identical. No shorts or absolute open windings that I could see.

ANOTHER MPX DECODER LEARNING: After I aligned the MPX decoder before, I noticed the left and right channel were reversed, while the other inputs (Aux/Tape, etc) were still correct. I scratched my head and figured it must be due to the alignment I did. I remembered that the Lissajous figure would "flop" during adjustment of Z101 and the figure could be made stable on either side of the 'flop'. I guess I picked the wrong side! When re-aligning, I went to the other side and it was just as stable and proper L and R output were restored. Sound reasonable? It's working correctly now-Left is Left and Right is Right.

Any advice on how/where to get a replacement transformer to use in my 500B?

As always, thanks to all for the support and guidance. The 400 is going back to the listening room and seems to be performing well in all ways at this time!

Dave

 

[dcgillespie]

Dave -- It is very possible to align the MPX adapter to "work" properly, but have the channel identities reversed. This is a product of you getting used to using your Sencore mpx generator in conjunction with the Fisher MPX circuit. No doubt, your generator likely reverses the phase of either the pilot or composit signals (but not both) before making them available for connection to the decoder -- versus that of the signals made available from the Fisher Model 300 Multiplex Generator, whose use the alignment instructions are based on. Just something to realize going forward.

Regarding your previous question, I have seen the IF transformer of every stage in a Fisher tuner/receiver fail over the years. That's hardly meant as a slam against Fisher. It's invariably a direct result of how a given unit was stored unused over many decades (the environment it was in). I'm not saying it will be a piece of cake to find another Ratio Detector transformer -- but I do think you'll be able to get one ultimately. Try putting a WTB (want to buy) in Bartertown. It may surprise you!

Congrats on getting it going!

Dave

 

[larryderouin]

I would check the 500/800C and 500/800b, and the TA600/TA800 parts lists and see if they use the same part#. Also check the Tuner's for that part. Then when you do post a Bartertown WTB you can ask for the part and note that it is used in these units. Larger parts base, and greater possibility of getting one.

Larry

 

[Dave451]

Thanks, guys. I have a line on a replacement for Z5 in the 500B, courtesy of a kindly AK member (whom I'll credit during the 500B restore thread). You have to love such a supportive community.

Yes, Dave, I think the "flop" was a phase reversal during the adjustment, which I finally got to the good side. The learning curve is pretty steep when you're learning about the rig, the theory, and your test equipment at the same time!

The 400 really sounds superb. I can certainly see (hear!) the benefit of your mods, Dave, to the power amp and the op amp in the sound. That in addition to the benefits to IBAM/EFB. The original 7868's are really singing into my AR-4x speakers (and they are not very efficient at all--85 dB--be the amp is driving them very nicely).

On to the 500B in the near future!

Dave

 

[Dave451]

Brief refresh on this Fisher 400. Purchased a wood cab from a fellow AK'er to have it 'properly clothed' now--nice custom-made cab in walnut veneer and very well made. Took the opportunity to check the bias on the Fisher label and GE 7868 output tubes (recall this uses Dave G's EFB circuit combined with IBAM) and they were 2-3 mA low after about 2 years of intermittent operation. Re-biased to 21 mA per tube, per Dave's original recommendation, and she's back in the rotation.

I did all the mods to this 400, as per above, but did not bother with re-checking the 'noose' and driver stage mods made, since I believe Dave said that these mods should be OK notwithstanding his recent posting on revised noose approach. Still sounds marvelous!

Dave

 

[onemug]

Still sounds marvelous!

Looks marvelous too. Nicely done.

It's pix like yours that made me want to own a Fisher receiver some day. That day came a few weeks ago with a 'properly clothed' 500c. I am enjoying it a lot.

 

[Dave451]

Thanks! Now if I can just find a cab for my Sherwood S8000 IV?! Congrats on the 500C--absolutely great receivers!