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RichP714

Overpriced 'deals':

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$515.00 for this TX-11b. msp_thumbdn.gif
 
$_57.JPG 
 
Well, it *is* the most famous TX-11b! And it *has* been improved in many ways and fully documented.
 
Besides, TX-11b's have a rare feature: stereo AM reception. If somebody somewhere is fanatical enough about stereo AM, they're probably crazy enough to pay that much for a tuner that does it. 

 
This unit was modified, tweaked and tuned by master DX'er Brian Beezley of www.Ham-Radio,com as noted below:
 

Modifications

fltrd.jpg

To improve stereo S/N and to eliminate HD Radio self-noise, I installed a postdetection filter on the underside of the main PCB. The filter reduces noise without incurring the artifacts of the Carver noise reduction system.

I added a .0013-µF phase compensation capacitor from LA3401 pin 3 to ground to improve stereo separation. Holes and pads exist for such a part, but neither the schematic nor silkscreen show it. To bring the separation trimpots within range with the postdetection filter installed, I added 3kΩ across R113 and 3.9kΩ across R114.

I could not quite tune the 7.2-MHz PLL reference oscillator on frequency. TC501 wound up set all the way to its lowest capacitance. I tried unsoldering one lead of C318, 2.2 pF across TC501, but it didn't help much. I lifted one lead of C319, 47 pF on the other side of the crystal, and added 47 pF in series. That worked. The crystal is marked and specified as 7.199 kHz, but generating correct LO frequencies requires oscillation at 7.200 MHz. Perhaps 7.199 kHz is the series-resonant frequency and a parallel-resonant crystal was mistakenly supplied.

Adjacent-channel selectivity for the original 180-kHz narrow filters was 24 dB. While this was a great improvement over the wide filters, it still wasn't enough for the crowded FM band in my area. I replaced the filters with two 110s and a 150. I selected the individual filters for symmetrical modulation-induced noise to minimize sensitivity degradation. Adjacent-channel selectivity improved to 52 dB, which is the highest figure I've ever measured for an analog IF strip.

tx11-mn.jpg

The TX-11b provides no way to force monophonic reception. Mono reduces noise without adding artifacts. It's particularly useful with stations that retain the stereo pilot during monophonic programs. You can add a resistor from the AM deemphasis switch to the stereo decoder to force mono when the pushbutton is out. Normally you'll leave the button in for FM stereo and AM deemphasis. The modification stops the 38-kHz VCO by applying +12 V through 39kΩ to LA3401 pin 17. A half-watt resistor has leads long enough to install directly between the switch PCB and the main PCB. Solder it to the front left deemphasis switch terminal as shown.

I discovered that a strong local AM signal caused spurs in the FM band. Grounding the insulated RCA audio output jacks to the rear panel eliminated the interference.

The original AM circuit used a fixed 120-pf capacitor across the antenna transformer secondary. Resonance was much too narrow to cover the entire band and off-resonance loss directly degraded the receiver noise figure. To improve sensitivity, I removed the capacitor and added a varicap diode between the tuning voltage at C002 and the transformer secondary. Tracking was not that good for the junkbox varicap I used, but sensitivity still improved over much of the band. I replaced C002 with a high-quality ceramic with lower impedance in the AM band to eliminate some spurs coupled by the varicap. With this modification I can receive signals on any AM channel at night with the stock loop.

Pressing NOISE REDUCTION or MULTIPATH REDUCTION blanks the audio for about 1.4 seconds. I found this delay annoying during rapid A/B tests (see Sound Samples). I tried reducing the blanking interval by changing the RC value on the switch PCB, but occasionally a loud pop occurred. I reluctantly restored the original blanking interval.

The unbuffered audio output impedance is rather high and rises with frequency. To minimize high-frequency roll-off, avoid high-capacitance output cable and equipment with excessive RFI input filtering. To eliminate the problem altogether, add op-amp output buffers. You can use a relay or JFETs to increase the buffer gain in narrow IF mode to compensate for the lower output level (3 dB lower for the narrow filters I used).

Alignment

TX-11a AM and FM alignment instructions are here, with changes for the TX-11b here.

Align the main PCB with the noise reduction board unmounted and folded over to the left. Run a ground lead to the unmounted board to align AM, which passes signals through it. The board is grounded only through one of its standoffs.

Instead of alignment steps 7 and 8, I swept the FM front-end and greatly improved the RF passband shape. Input return loss was remarkably constant at 9 dB across the entire FM band.

The alignment instructions do not mention T601. This tuned circuit on the front-end board in parallel with the IF signal path can help minimize stereo THD (I used it mainly to suppress the third harmonic). The mixer transformer and two IF transformers on the main PCB also affect stereo THD. These adjustments provide so many degrees of freedom that I was able to make the distortion products disappear in wide IF mode. (Later they rose somewhat when the temperature changed. I report that figure below.)

Noise reduction alignment instructions are here. Though not explicitly stated, they assume that the signal generator uses preemphasis. To align without it, alter the audio levels in steps 8, 13, 27, 30, and 32. Use the deemphasis calculator in this set of tuner measurement utilities. Step 34 specifies an absolute RF signal level. Check for the expected noise behavior somewhat above and below the specified level since the front-end alignment and noise figure affect where it occurs.

A 1987 service bulletin applies when “customer complains of too much echo with the ACCD circuit engaged”: align to 75 mV in step 19. This lowers the delayed L+R component 9.5 dB.

tx11-de.gif

A 1989 TX-11a application bulletin changed the AM deemphasis to the new NRSC standard: R403 and R404 should be 3.3kΩ and C161 and C162 should be .0033 µF. These components are in the upper left corner of the noise reduction PCB. This change was supposed to have been incorporated in the TX-11b, but mine (S/N A90700999) had the old values. The circuit simulation curves above compare the two responses. The blue curve is the new deemphasis and the red curve is the old.

tx11-6.jpg

When I first aligned the 10-kHz AM notch filters, I neglected to preset RP23 and RP24 as instructed. I simply adjusted all trimpots for the deepest notch at 10 kHz. I got the response shown above for the two channels. The frequency span is 5–15 kHz at 1 dB/div.

tx11-7.jpg

After presetting the trimpots and realigning the filter, I got this response. Notch depth was > 45 dB.

tx11-4.gif

Instead of peaking the IF transformers in steps 2 and 3, I adjusted them for the most symmetrical response while sweeping the IF. I noticed that the IF passband was somewhat rounded, which made me wonder how the audio response could be so flat. I hadn't noticed the stereo equalizers in the AM audio circuit. The circuit simulation curve above shows their response with the single adjustment at midsetting. Instead of step 15, I swept the modulation and set the adjustment for the flattest tuner frequency response from 500 Hz to where the notch filter takes over above 9 kHz. Then to compensate for any small channel difference in deemphasis, I enabled it and slightly readjusted one channel so that the response curves coincided over most of the frequency range (at 1 dB/div they were indistinguishable to 8 kHz).

Measurements

For the following measurements I used IEEE 185-1975, updated as described here. I used the test equipment listed here. The tuner had all of the modifications described above except output buffers and shorted noise limiter resistors. I did not implement the 1987 service bulletin.

50-dB quieting sensitivity, mono W 14.4 dBf, N 15.2 dBf  
50-dB quieting sensitivity, stereo W 36.0 dBf, N 36.0 dBf
THD, 1 kHz, stereo W < 0.03%, N 0.3%
THD, 1 kHz, stereo NR W 0.11%
THD, 1 kHz, stereo MR W 1.1%
Stereo separation, 1 kHz W 48 dB, N 44 dB
S/N, 65 dBf, mono 83 dB
S/N, 65 dBf, stereo 75 dB
S/N, 85 dBf, stereo 77.5 dB
Capture ratio W 0.7 dB, N -0.4 dB
Capture ratio, stereo W 13.5 dB, N 14.0 dB
Capture ratio, stereo NR W 7.5 dB, N 7.0 dB
Adjacent-channel selectivity N 52 dB
RF intermod 66 dBf (roughly 20 dB worse than for other tuners)
RF spur 99 dBf
RF image 91 dBf
RF AGC threshold 72 dBf
RF mismatch loss 0.6 dB
Noise figure, 96.9 MHz 3.9 dB
Modulation acceptance W 280%, N 160%
Minimum stereo pilot injection 2%
Treble response W +0.1/-2.2 dB (-0.3 dB to 14 kHz)
Bass respnse, -1 dB < 10 Hz
Output level W 0.57 V, N 0.40 V
Output impedance, 15 kHz 5kΩ
AM THD, 1 kHz, 95% AM, deemphasis W < 1%, N < 3.5%
AM deemphasis error to 9.0 kHz L +0.8/-0.5 dB, R +0.5/-0.5 dB
AM deemphasis error to 9.5 kHz L +0.8/-0.6 dB, R +0.5/-1.0 dB
 
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Here's a 'deal'!!! C-16/TFM-42
 
s-l1600.jpg 

 

That individual must have been having heat stroke in Hot Springs when they set that price

 
And he has zero feedback, what a first item to try to sell.  Listing ended with no bids.
 
Mark 

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notice the face has that rose colored hue?  In the last year, there's been 3 or 4 like that.

That's usually the result of cleaning with a product containing ammonia.

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notice the face has that rose colored hue?  In the last year, there's been 3 or 4 like that.

That's usually the result of cleaning with a product containing ammonia.
 
Or, cheap dye/poor process in the anodizing. read: Finishing.Com  Apparently it's a common problem. I'd blame it on the anodizer, not so much cleaning products, because there are so many causes. If done properly, color-fastness should not be an issue.

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notice the face has that rose colored hue?  In the last year, there's been 3 or 4 like that.

That's usually the result of cleaning with a product containing ammonia.
 
Or, cheap dye/poor process in the anodizing. read: Finishing.Com  Apparently it's a common problem. I'd blame it on the anodizer, not so much cleaning products, because there are so many causes. If done properly, color-fastness should not be an issue.
 
I'll change "usually" to "sometimes" if you change "if done properly" to "if cost were no object". eusa_dance.gif 
 
Light-fastness is related to the size and type of dye particles (smaller and more opaque is "better" and more expensive), the depth of the anodization (more time/cycles in the bath) and the quality of the seal coat.  In fact one of the responses on the page you linked to states that "All dyed anodize will fade if exposed to constant sunlight and certain fluorescent lights." It also states that "Some lab cleaners can be a kiss of death for some parts".

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notice the face has that rose colored hue?  In the last year, there's been 3 or 4 like that.

That's usually the result of cleaning with a product containing ammonia.
 
Or, cheap dye/poor process in the anodizing. read: Finishing.Com  Apparently it's a common problem. I'd blame it on the anodizer, not so much cleaning products, because there are so many causes. If done properly, color-fastness should not be an issue.
 
I'll change "usually" to "sometimes" if you change "if done properly" to "if cost were no object". eusa_dance.gif 
 
Light-fastness is related to the size and type of dye particles (smaller and more opaque is "better" and more expensive), the depth of the anodization (more time/cycles in the bath) and the quality of the seal coat.  In fact one of the responses on the page you linked to states that "All dyed anodize will fade if exposed to constant sunlight and certain fluorescent lights." It also states that "Some lab cleaners can be a kiss of death for some parts".
 
I'll concede to multiple causes. Compromises are part of the business process. No one knows for sure what happened, it's just curious how most of the C-19's are affected, but few of the C-11's or TFM amps are. Variations in the anthracite color of MXR-130's alone indicates a less-than-perfect process. Who would have known this stuff would still be around for 25-30 years? We have a unique view on a product history way beyond it's life expectancy. More than enough examples still exist that we can scrutinize things like cleaning habits and dyes. I thought only car guys went this far!

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Someone must have developed a solution....
 
The easy fix is to paint rather than anodize.
 
Making a black anodized surface that is UV resistant requires a deep etch, completely filling the etched surface with dye, and topping that with a UV-resistant sealer. This approach probably doubles or triples the cost per faceplate.

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Notice the crack in the case at the bottom edge, and the appearance of swollen particle board along the same edge in the amp plate photo.  Looks strange at the lower edge on last photo too. It's got issues that are not being disclosed as well as being overpriced to begin with:
 

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The link goes to something that was removed
 
Highlight all three lines of the link and right click go to...
 
When I copy and past a link on this site, the link gets clipped when the post is saved. So I copy/paste then hit return to split the link into multiple lines that do not get clipped when saved. Site buffer problem?

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The link goes to something that was removed
 
Highlight all three lines of the link and right click go to...
 
When I copy and past a link on this site, the link gets clipped when the post is saved. So I copy/paste then hit return to split the link into multiple lines that do not get clipped when saved. Site buffer problem?
 
Using the site's 'Insert Link' icon, THIS should work for the $199K EV. 

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The link goes to something that was removed
 
Highlight all three lines of the link and right click go to...
 
When I copy and past a link on this site, the link gets clipped when the post is saved. So I copy/paste then hit return to split the link into multiple lines that do not get clipped when saved. Site buffer problem?
 
I fixed the original link.
 
Note that only the part of the link prior to the question mark is required. 
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Aaaaah "The insert hyperlink" third from the end vs copy/paste eusa_doh.gif
 
Now, I just have to remember to not hit the "Buy It Now" button yikes.gif 
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Now, I just have to remember to not hit the "Buy It Now" button yikes.gif 
 That could be pricey Big Grin
And thanks for the link fix Dom 

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I like how all the photos of his 'rarities' are in front of the storage locker doors. Wouldn't be trying to flip stuff from locker auctions, would he? The asking prices are hilarious. Are these quasi-ransom notes to the owners?

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