|
Re: S-20R mixer cct.
Tom Jacques is right on the money, with a positive triode grid voltage on pin 5 of the 6K8, just about all of the other voltage readings will be off except for perhaps the heater voltage.? It is worth noting that the 117 volt convention for the power transformer primary voltage is actually an average expected voltage.? The tolerance at a specified 117 line volts is about +/- 7.5%.? For 115 line volts convention, the tolerance is approximately +/- 10% and the modern convention for a 120 volt line voltage tolerance is +/- 5%.? In all of the above conventions, the maximum line voltage is about 126 volts.? The only voltage that has actually changed over all these years is the lower voltage limit. The ohms-per-volt loading of a circuit by a VOM is essentially how much current is required to deflect the meter pointer to full scale deflection.? The 1kohm-per-volt rating implies that the meter movement is 1mA since 1 volt / 1kohm is 1mA.? Early VOMs were inexpensive so they used less sensitive meter movements.? In like manner, 20kohms-per-volt has a meter movement that requires 50 microamps for a full scale deflection.? I have seen VOMs with 50kohm-per-volt ratings. Most VOMs and some VTVMs have an accuracy of +/- 3% at full scale deflection.? This implies that the meter accuracy is worse (actually twice the FSD) when the meter pointer is below mid range.? At the same time, the ohms-per-volt loading of the VOM on the circuit under test is dependent on the range multiplier.? For instance, the Simpson 260 has a 20kohms-per-volt rating so on the 250 volt range, the shunt resistance of the meter on the circuit is 5 megohms.?? This is sufficiently close to the reported shunt impedance of a VTVM or DVM of approximately 10 megohms.? This shunt impedance is independent of the range switch and is only dependent on the voltage amplifier in the VTVM and the range multiplier circuit or the method of conversion used in the DVM.? Some VTVMs have even higher loading resistances such as the Keithley 601 which is 100 trillion ohms with an accuracy of 1%.? Truly amazing for a mechanical meter movement. Getting back to the line voltage measurement, if the 117 volt primary sees 120 volts, the error is ((120v - 117v) / 117v) =? +/- 2.6%.? Since the VOM or VTVM has a +/- 3% full scale error, the combined tolerance is ((3^2) + (2.6^2)) ^1/2 = +/- 4%.? When you add in the line voltage tolerance of +/- 7.5% the measurement error is increased to +/- 8.5%.?? When the increased error caused by less than half scale deflection of a meter movement is added to the mix, the reading error can exceed +/- 10%.? So when comparing the manufacturers voltage measurement with your meter reading, if you are below a 10% disagreement then you are golden.?? The remaining issue is that hallicrafters didn't include an expected voltage reading at the tube sockets in the S-20R manual which is at the heart of this problem.? In this case, the tube manual will give you reasonable voltage levels for the tube in question. But remember, the tube manual ratings are bogey numbers that are, in fact, more average values. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, March 4, 2025 at 02:41:02 PM CST, Allthumbs via groups.io <btse1@...> wrote:
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty—the LO is not oscillating. ? I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I’m posting the readings in the chart below. FYI—I took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM. ? ? If it helps, here's the mixer section: If you’re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I’m posting will point to a solution that would mean I won’t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-) ? Any thoughts? Thank you, ? Tom |
|
Re: S-20R mixer cct.
开云体育Richard ?do your DVM’s have an isolation probe for capacitance decoupling back to the DOT? ??? Resistance of DVMs can be? very high, but ?test wires have capacitance that will ?mess with tuned circuits as the LO.? Halli sometimes indicated which pins can be measured with only a VTVM , but the S-20R on Bama has no chart and the schematic I see is mostly not readable. ? _ -- don??? va3drl |
|
Re: S-20R mixer cct.
Thank you, Don. No, the set was not operating when I acquired it last year. It is dead on all bands. Over the winter I through it and replaced all resistors and capacitors, upgraded the power cord to 3-conductor, and replaced the speaker with a PM type plus a choke. I have a spare, 4" electrodynamic type of speaker like the original but have not yet tried it. Yes, I tried cleaning the 6K8 socket pins with De-Oxit, and I tried a second tube. If that was the only problem, I don't think the voltages on the tube socket would still be "off."
?
Tom
|
|
Re: S-20R mixer cct.
Thank you, Jacques. The problem is on all bands. I have replaced all the resistors and capacitors in this set except for a large mica cap buried under the oscillator coil assembly. I was hoping not to have to lift that assembly.
?
Tom
|
|
Re: S-20R mixer cct.
开云体育Hello Tom Around the HF, especially the LO, I wonder if you can trust a DMM having only wires {no isolation probe } at some pins. It never did work while you have had it? ? Assume, you tried another tube ? ?and since you did not say..it ?must be dead on all bands? What all has been changed {or not] in that area… ?? De-Oxit on socket ? Do you have other test equipment?
-- don??? va3drl |
|
Re: S-20R mixer cct.
开云体育Hi Tom, The triode section of the 6K8 is effectively not oscillating, and that explain all the voltage readings you got. There should be an open coil somewhere, or an open capacitor in the oscillator path. Are all the bands defective (aka not working) ? If all the coils windings are OK when checked with an ohmmeter, either C33 or C37 (or both !) can be “open”. ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de Allthumbs via groups.io ? Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty—the LO is not oscillating. ? I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I’m posting the readings in the chart below. FYI—I took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM. ? ? If it helps, here's the mixer section: If you’re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I’m posting will point to a solution that would mean I won’t face the daunting task of partially disconnecting and? lifting the whole coil assembly.? :-) ? Any thoughts? Thank you, ? Tom |
|
S-20R mixer cct.
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty—the LO is not oscillating. ? I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I’m posting the readings in the chart below. FYI—I took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM. ? ? If it helps, here's the mixer section: If you’re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I’m posting will point to a solution that would mean I won’t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-) ? Any thoughts? Thank you, ? Tom |
|
Re: S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1?
开云体育Hi Jim , and all; ?revising numbering ?makes it tough to compare things !!!! So the Halli C62 function remains? ?despite our communication inability. ? I guess that it one reason I often take a snip of what I? See on the drawing and circle it?? etc. ? From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Thursday, February 27, 2025 5:14 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1? ? The S-40B Photofact Folder that I copied calls it C7 but it is still a 2.2 pF cap just as is C62.? Sam's has a format that they follow so their component designations are not the same as hallicrafters, especially after several hallicrafters S-40 revisions. See:?? Jim ? ?? -- don??? va3drl |
|
Re: S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1?
开云体育Maynard FYI ??S-40 [no suffix]?? This seems to have no C62 and looks like “your” 40A at first glance? ..SO? maybe you have an early run manual of the “40A” ? ? FYI ??S-40A I was using= ??? ? Another thing, the bias/AVC line to V1 changed when going to the 40B, so it needed that C33? because L3 was not a transformer. ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Maynard Wright via groups.io
Sent: Thursday, February 27, 2025 3:51 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1? ? And my S-40A schematic is from Sam's Photofacts (via the Web) and it -- don??? va3drl |
|
Re: S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1?
The S-40B Photofact Folder that I copied calls it C7 but it is still a 2.2 pF cap just as is C62.? Sam's has a format that they follow so their component designations are not the same as hallicrafters, especially after several hallicrafters S-40 revisions. See:?? Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Thursday, February 27, 2025 at 02:52:05 PM CST, Maynard Wright via groups.io <m-wright@...> wrote:
And my S-40A schematic is from Sam's Photofacts (via the Web) and it doesn't have C62 as in your schematic nor does it use independent designation of each component, but labels them with numbers referred to the table of components. So more than one component can have the same number. So I suspect that your schematic excerpt is from the S-40A Hallicrafters manual, which looks (in your excerpt) more like the S-40B and S-85 schematics I have from Hallicrafters manuals. But I don't have that manual. Since those two schematics don't agree, I wonder whether there was a revision during production of the S-40A. 73, Maynard W6PAP On 2/27/25 12:38, Maynard Wright wrote: Hi, Don, |
|
Re: S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1?
开云体育Maynard, yes wording is tough.? “C62”? had been started..? then it moved ? To me it showed that it was thought to be needed? for? a simple coil, but not for a transformer? .. A version Then when the B came along? with two coils, they moved it ?a bit. From: [email protected] [mailto:[email protected]] On Behalf Of Maynard Wright via groups.io
Sent: Thursday, February 27, 2025 3:38 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1? ? Hi, Don, Maynard on the S-40A,? C62 is ??in use on band 2 only but connects V1 -- don??? va3drl |
Jim Whartenby
don Root
Allthumbs
Jacques_VE2JFE