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Hallicrafters SX17 modifier noise limiter
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I have been working on a hallicrafters SX17 receiver and so far work is progressing (audio and detector circuit restored and interstage transformer replaced) although a bit slowly and cautiously. Yesterday, when I removed it for the first time, I discovered that in the position of the 6J5 'silencer' triode, was actually a 6H6 double diode. This puzzled me initially until I Google and looked at the datasheets and discovered that the 6H6 can be used for the noise limiter function. Both the early and late versions of the circuit diagrams I have so far found both show the 6J5 in that position. It is no wonder that the voltages don't match the table in the manual! However, was this modded by someone, or was it a factory design? I couldn't see any obvious indication of changes to the wiring, only that some components were different. However, someone gave me a hint to have a look at the valve base, which I did this morning and it is stamped 6H6! This does suggest that the modification was added at the factory.
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I wonder, does anyone have or come across a circuit diagram showing this modification, i.e. a 6H6 rather than a 6J5 in the noise limiter circuit?
Has anyone already reverse engineered it perhaps? I am minded to reverse engineer it anyway, but it might help if there is already an updated circuit in existence somewhere.
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Hi gang, The 6H6 dual diode was used in many Hallicrafter rx's, as noise tube. Check Bama for: S85, S40, and other series rx's schematics. I have an S85, that is wired as an late S40B, for the noise limiter ckt., and not as shown for S85 schematic. The factory did many wonderful running changes,( my sarcasm) and using?up of?parts. They are a good receiver. Good Luck, Dale, N2DM.?? On Thu, Aug 1, 2024 at 8:00?AM John via <siloam=[email protected]> wrote:
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The mod to the SX-28 is a major one. The SX-28 was built with a Lamb
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noise blanker, AFAIK the first application of Lamb's patent. Unfortunately, it didn't work very well so, eventually, an extensive mod was published in a service note to remove it and replace it with a conventional series diode clipper. The SX-28 is a classic antique and I would never mod one like that. The later, cheaper, version, the SX-32 was essentially the same arrangement as the the modified version of the 28, i.e. it had a conventional noise clipper and much simpler IF. I have not looked at the SX-17 circuit. It is too early to have the Lamb silencer (silence of the Lambs?) but triode clippers were used in some receivers, for instance, the Super-Pro. Also, a triode could be connected as a diode and used as a clipper. As far as circuits for a series diode clipper they were used very widely after about 1945. See for instance, the S-40A. Earlier receivers often used shunt diode clippers, they have some advantage for CW but the series diode is biased by the carrier which sets the clipping level automatically for AM. Some earlier receivers have a NL clipping control to set the bias manually. I think the S-20R has a shunt diode but not an adjustable one. The 6J5 is a medium mu triode, very commonly used for oscillators and audio amplifiers. BTW, if you use a 6H6 or the miniature equivalent the 6AL5 as a series limiter its usual practice to reduce the heater voltage by means of a resistor across it. The purpose is to eliminate hum. This became almost universal for these tubes. On 8/1/2024 12:50 AM, John via groups.io wrote: PS, I am aware that the 6H6 was used as a modification in the SX28 and I --
Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
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If I recall properly, the SX-17 had variations with time of build, and eventually became an sx-18. I wonder if it is more like an SX-18? ??? ? august 3? sorry all this^ is total hogwash, I never learn not to shoot from the hip. I should have at least looked at Dachis before shooting. -- don??? va3drl |
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I pulled down the user manual for the SX18 and had a look at the circuit. It looks to be much simplified to the SX17. Only one RF stage, no noise limiter. It uses a 6Q7 rather than 6R7 in the last stage before the audio amp and one 6F6 rather than two 6V^s in the audio output stage. The overall design philosophy seems quite similar though, including the external standby switch connector, dual impedance matching connectors, crystal etc.
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The 6H6 in the SX17 has both diodes wired in parrallel, which is quite different and simpler than another circuit I have seen where each was used individually. |
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You may want to look at "The Radiotron Designer's Handbook" 4th
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edition, chapter 16, section 7, p.694 ff. which has a fairly comprehensive survey of noise limiters as used in AM receivers. The Lamb noise blanker is also treated in another section. There are essentially two forms of noise limiters, series and shunt. Shunt limiters appeared first but were mostly supplanted by series limiters. Series limiters can be fairly effective although simple. There are also balanced limiters. An interesting form of adjustable threshold limiter is used in the RCA AR-88, it was patented by Lester Fowler, the designer of the radio. Limiters with two diodes are often of the balanced type with the second diode compensating for the space charge effect in the first. It is also very common for the heater of a noise limiter to be run on reduced voltage to reduce hum also by reducing the space charge. For a 6H6 for example, a series resistor of about 5 ohms is right. Some receivers, for instance the Collins 75A2, have two noise limiters, one for AM and a second for CW. I believe in this receiver the AM limiter is a series diode while the CW limiter is an adjustable threshold shunt limiter. In any case the Big Red book covers most and has lots of citations. The SX-16 and 17 are very similar, the S/SX-18 appears to be a lower cost version. On 8/3/2024 10:11 AM, John via groups.io wrote: I pulled down the user manual for the SX18 and had a look at the --
Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
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I meant to say that the Radiotron 4th edition is available as a PDF
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from several sources on the web, all free. For instance tubebooks.org Despite its age this continues to be a very useful and interesting book. IMO a must have. On 8/3/2024 10:11 AM, John via groups.io wrote:
I pulled down the user manual for the SX18 and had a look at the --
Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
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I altered? my previous post? but you might not notice it if you are not on email If I recall properly, the SX-17 had variations with time of build, and eventually became an sx-18. I wonder if it is more like an SX-18? ??? ?this^ is total hogwash, I never learn not to shoot from the hip. I should have at least looked at Dachis before shooting. i see he mentions an SX-17F built for the FCC, but you would have seen only one output tube then. --
don??? va3drl |
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Richard, thank you for the reference to the "Radiotron Designers Handbook". I found it online and it is indeed an excellent reference, well worth a look. Not seen this one before. I had a look at the section 7 as well as 6, however I am not sure that the design here matches any of the included examples. Below is the circuit for reference.
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Apologies for the rather grainy image but that is unfortunately the quality of the original. I had been considering re-drawing the circuit and including the modifications to component values that I have found.
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The good news is that having rebuilt the audio stage and replaced leaky capacitors in the detector and two IF stages, the audio stage is now working and the receiver is able to pick up stations on some bands. The Noise Limiter valve also now seems to have sensible voltages around it. The primary culprit for weird voltages around the 6H6 circuit seems to have been C41. Leaky capacitors (C42, C79) at the second IF amp caused the signal to be drained to ground and consequently only faint distorted audio could be barely heard on one or two strong stations.
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Don, it was interesting to have a look at that SX18 circuit anyway to compare. The SX17 though, with its two output tubes in a push-pull configuration and 18W output seems to have very powerful audio output stage. I get the sense that this receiver ought to be a LOT louder than it actually is. I mean, a single output tube can be pretty loud. I am using it at somewhere around half to three quarters volume to get a comfortable level, but I would expect it to be ear splitting by that point. At full volume it ought to destroy the speaker I am using, but it is just rather loud, although it has to be said clear as well. That being said, my temporary working setup is a speaker transformer from an Ekco eternal speaker and a 4in 8 ohm speaker from an old doorbell. Also I am still some way off from looking at the IF and RF alignment.? The original Ekco speaker is 3ohm so once I get that restored, it should sound a lot better and probably louder. I am curious though, since this is a communications receiver, why such a powerful audio output stage?
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Now that the repair of the radio is complete I can post an update. I worked through the remaining IF and RF stages replacing waxies and resistors that had gone substantially high. After component replacements had been completed, I went over everything to check all functions and confirm nothing had been missed. A couple of errors were corrected including adding a missed capacitor.
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An EF39 had been used in one of the IF amplifier stages and although this does work, it was replaced with a NOS 6K7 as per circuit diagram, it does seem to have slightly more gain than the EF39.
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One of the bands (band 2) did not work and the culprit turned out to be the antenna input coil for that band. After repairing the coil, the receiver was ready for alignment. IF alignment was fairly straightforward. RF alignment was not quite so straightforward and quite a way out on some bands. On band 4 a 10pF capacitor had to be taken out of circuit because the tuning had shifted (I understand that the coild former inductance can change over time) and there was no longer enough adjustment range. Band 2 is still a bit quiet compared to the other bands, but all have been aligned successfully. Band 6 is kind of interesting as the alignment points cover only part of the range of the dial and the tracking does go adrift outside of the bottom marker. Nevertheless all bands are now aligned to the dial.
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All functions of the radio were tested before and again after alignment and all work just fine, including the noise limiter which, to my surprise, actually works and does considerably reduce pulsing noise. The broad and narrow filter work nicely as well and after alignment function exactly as the manual described. Prior to alignment this did not work correctly.
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I have also acquired an external speaker that has the capability of having its impedance tuned in several steps to match that of the radio. The quality of the audio from my temporary speaker was clear but a bit weak and tinny. However, from this Stentorian, the sound is positively booming. The tonal quality is very good and the audio is nice clear.
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I am currently in the process of re-spraying the case and cleaning up the trim, but am wondering how I might go about rejuvenating the front fascia? I have cleaned it as much as possible and its appearance is much improved, but it will likely look dirty next to the re-sprayed case. Can the white lettering be rejuvenated with something? Or could it be re-created somehow if the panel was re-sprayed?
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don Root
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