adjusting bandpass on FT 101 filter board

[N3DRB The Derb]

I put in the new AM filter board that Jack sent me into #1, and was surprised to find that it sounded cleaner and 'flatter' than the old board, a noticeable improvement on the ears. On the new board, there's no "bandpass" swish when tuning past a AM signal, sounds like a faint hi hiss in the background which gets lower as you get on the frequency nose, then repeats on the other side going lo to high. I was getting that effect on the old board, but I had nothing to compare it to, so I didnt know anything was off.

I'm assuming that the bandpass adjustments in the old board are off or the filter is not set dead on freq. I dont have the gear to set it right, but I know it would be <30 minutes of work for someone that does. Assuming it's a simple tweaking needed with a sweep gen, can anyone here volunteer to sweep the board and reset the adjustments on it for me? I'm pretty sure it just needs tweaked a bit, per the Yaesu service manual.

I'll send ya the board and a check to cover return shipping cost and some time. I'd like for #2 to sound as good as #1 does now, very clean sharp skirts, and no hiss noise as the signal moves across the passband. very noticeably cleaner tuning moving across a carrier signal.

I should mention that Jack's board has a International filter in it where the old board has an original Yaesu AM filter in it...hmmm XF30B I think? Anyways, am I on the right track with this     Nothing on the old board seems amiss with a good looking over. 

[ke7trp]

I found this to be true also. I have 3 or 4 AM filter boards. They all sound a bit different. I am lucky enough to have the original Riser cards so you can install each board up and out of the radio. Then, its easy to align and or test components.

Clark

[N3DRB The Derb]

wow r they still available from Yaesu or did you just find em somewhere else? I'm going to be looking for a set myself.

the new board has a flat s meter reading tuning across a carrier, old board has some peaks, like 3 of them. It's got to be misadjusted.

[ke7trp]

It is.. I have never seen another set of these boards.. I got them years back.  Its neat.. You can install the filter board on the riser card and then adjust it while the radio is on.. Same for all the boards. 

Clark

[KA2QFX]

The filter board in the FT-101 has no adjustments that would affect the sound, passband or performance of the crystal filters. The tunable 'cans' at the input and output of the CA3053 amplifier provide impedance matching to make optimal use of the amplifier's I/O characterisitics. Their frequency response is FAR wider than the response of the crystal filters and has no affect in terms of their function.
Further, both the input and output of the crystal filters are buffered and further isolated.  The signal into the filters is driven by an FET source follower to allow for the widely varying input impedance of the crystal filter. Similarly the filter's outputs are isolated by a bipolar voltage amplifier.

So unfortunately, no adjustments you might make will alter the sound of the filters response. Rather, that is a function of the filter's skirt slope and the swish you hear is largely a function of the phase shifts exhibited at the filter's band edges where it's impedance becomes very high and very reactive.

T108 and T109 should simply be adjusted for maximum signal at 3.180 MHz regardless of what filters are used.

Regards,
Mark

[ke7trp]

Thanks for the lesson in theory Mark. However, If you adjust those two cans while the radio is receiving you will hear a difference in the sound.  Since I have the riser card, its easy to adjust. I do agree that the filter slope is probably the largest factor. Out of the 6 or 7 AM filterboards I have had over the years, Everyone has bit a bit different in tone and width!

Clark

[KA2QFX]

Point taken, I agree you can probably make changes to the sound of the boards response by adjusting the cans. But we need to be clear on what we are doing and why we are doing it.

The original and later entries on this thread suggested that the "swish" sound was an indicator that the tuning must be off. And further comments mentioned ripple in the passband. Both statments, knowingly or not, referred to the characteristics of the crystal filters which are NOT "adjustable". That misinterpretation is what prompted my (perhaps overly detailed) response.  I hope my theory lesson did not come off offensive.
 
As for changing the perceived sound changes by retuning the cans; shifting them off resonance would similalrly move their phase shift above and below the center of the passband to be more leading or lagging overall rather than 50/50.  While this may "sound" different,  I don't think it would necesarrilly sound "better" and certainly the response would be less than optimum if you're shooting for best signal to noise and sensitivity.  It must be kept in mind as well that the bandwidth of the cans is so many times wider than that of the filters, that a significant detuning would largely affect the overall board response uniformly across the narrow bandwidth of the filters. That is to say, passband ripple would not be affected by tuning the cans.

 I stand by my assertion that the cans should be tuned for peak response, or better yet, zero phase shift at center passband, the two are not always coincident and the latter is usually preferable for lowest noise and IMD response.

Regards,
Mark

[N3DRB The Derb]

There's a procedure for adjusting the filter passband for a desired slope in the factory service manual. I haven't looked at it, but I noticed it's there. Shows a response curve that looks like a 2 humped camel back with a small dip in the middle as the desired response. I'll bring it in here and scan it and post it.

hope my theory lesson did not come off offensive.

not to me, but you're missing the point. I dont need the theory lesson, I need the older board to sound as nice as the one Jack gave me.

think practical, not technical.  Knowing what's the real theorem deal isnt all that important to me.


 

[KA2QFX]

The bandpass filter adjustment to which you refer is for the HF mixer unit where the VFO enters the IF chain. It covers 800 KHz. It is not related to the bandpass of the filter board you would like to sound better.   Page 7-17~18 in the manual.

"Knowing what's the real theorem deal isnt all that important to me."

Apparently so. I've learned a valuable lesson. Thank you.

Very 73,
Mark

[ke7trp]

Mark. You where not offensive to me or anyone. More of a chuckle... But what is interesting is that you came on here and posted how there is no way adjusting the cans can effect the sound of the receiver. You did that without even trying it yourself and said in certain words that we where wrong.  This is confusing to me.  I have been adjusting those cans and working on FT101s for about 10 years. I guess in all those 10 years I simply had a case of Phycosis?  At any rate.. Thanks for participating.

Back at the issue at hand.  Various brands and types of filters are going to sound different. That was the point I was making, however, you can adjust them to your liking. You need to adjust them while using the proper tools. If yours is off, you wont be happy with the radio. Even when adjusted properly, the filter you have will determin how the rx sounds but 90% of the time, I can tweak them in by hand to get the sound and tone I desire.

Clark

[N3DRB The Derb]

He's a PC and I'm a Mac. 

[KA2QFX]

Gentlemen,

Well, at the risk of being proven wrong, and learning something new, I threw my FT-101s on the bench. I have a “daily driver” and a spare.  Here’s what I found.  T108, the input tank to the IF functions for all modes. T109  primarily feeds the product detector for SSB/CW, but does affect the levels into the AM and AGC detector.   
In order to achieve any appreciable reduction in “Hiss” at the edges of the filters the tank(s) needed to be detuned about –8dB on the high side and –10dB on the lower band edge.  Since this affects the AGC voltage directly an additional 12dB of signal was required to restore the level in the pass band to the same signal/noise ratio.  So signal to noise deteriorated a few dB, no surprise there.  Listening to signals on the air I heard no appreciable difference in audio quality.  While the background noise did seem to diminish obviously, so did the signals themselves, requiring that much stronger stations to be heard.  Weaker signals that I was clearly able to copy before the detuning were simply gone or in the noise.

Phase shift through the detuned tanks was surprisingly slight, which is good. I attribute this to their input and output windings being rather tightly coupled, which makes sense since these tanks aren’t needed to be highly selective.   
The shape factor and response of the crystal filters was completely unchanged by detuning. I was able to skew the passband (AM filter) by 1.3dB when detuning either way, not a significant difference but clearly measurable.

When testing for the affect of nearby interference, using both steady test signals and actual stations there was a slight increase in “trash” depending on which way the tanks were detuned. This was a very subjective observation but clearly improved when retuned on center.  Again, the signals had to be close enough (or strong enough) to be a problem regardless of tuning.  I attribute that predominately to the characteristics of the filters simply being imutable.

Overall, I have to say that given the loss in sensitivity and measurable degradation of signal to noise detuning these tanks to achieve any change in perceived audio is ill advised. Workable signals were lost and no appreciable sonic changes were evident.  While I didn’t measure it directly it can be readily surmised that the dynamic range of the receiver (front end overload level) would be reduced by approximately the same 10dB due to the loss of gain in the AGC loop. 

So I stand by my statements.  These tanks should be tuned for peak, flat IF response to achieve the best performance of the receiver under any conditions.  The characteristics of the filters are steadfast and  detuning peripheral circuitry is ill advised.

If you’d be so kind, I’d be very curious to know by what amounts (relative dB) you change the response of this circuit, which direction you tune them and which tanks largely create the effect you desire.   

Oh, and not that it matters, but I’ve probably serviced between 75 and 100 FT-101s over the years.  My professional work has been primarily in electronic design and laboratory instrumentation work.  I try to share my knowledge freely (and thoroughly) and pay particular attention to instances where I feel people have misinterpreted findings or been misled by ambiguous observations.  This is what prompted my replies.  I never said you were wrong, but I suppose that was the implication. My apologies again if I’ve offended, it was not my intent. How pointless would that be?!  It’s been my pleasure to discuss the matter and investigate your assertions for myself.

Very 73,
Mark

[N3DRB The Derb]

dont worry, I'm ok fine. I just glanced at the page in question in the manual, saw it had a bandpass slope, and assumed it had something to do with the filters. I just started reading it in earnest today.

I went out there and subbed my 2 boards in and out - and also noticed that the new board gives about 2 s units more on receive on a steady ground wave signal - ( local am BCL ) so I think the board still needs adjusted to be on the nose.  New board still sounds noticeably cleaner than the old.

I might get around to finally fixing my EE today. needs he power distro board replaced. r1 burned up so bad it burned a hole thru the old board!

Ed sent me another one and the R1 in that one go so hot you can see the wire. replacing with 3 W metal oxide, that should hold it after I replace all the disgusting elderly caps.