06-29-2019, 02:10 PM
New member--trying to post for first time. Attempted half hour ago, but either I made an error in selecting proper forum section, or Admin permission is needed for first post. Will try again. If permission needed for first post, Admin please delete duplicate post!
I am a new member to this forum who restored a model 89 some six years ago for my brother-in-law. At that time the restoration appeared complete. It worked well then. Today, after removal from my closet to return, following case restoration, it has a big problem.
My restoration steps are described, in hopes that some of you can provide useful feedback.
As many of you can attest, "war stories" about old radios are in no short supply. My first peek inside the chassis in 2013 revealed a cache of hickory nut shells stored by an industrious and efficient squirrel. As well, moths had consumed almost every fiber of cotton wire insulation with the exception of the transformer leads. Sound familiar? J
All wiring, capacitors, and resistors were replaced with modern devices of equal or higher power/voltage rating; the original Bakelite housings were retained. The exceptions were: mica trimmers, the 232 W/32 W wire wound bias resistors in power supply circuit, and of course, ganged tuning caps. The later were cleaned with compressed air.
The primary winding (1-3) of coil T-6 was found to be open. It was re-wound with the same number of turns of enameled wire. Both the original and replacement were 0.006” diameter, including enamel, implying #35 wire. Its new resistance was 1.8 W compared to the value 1.68 W given on the schematic. I think I baked the coil with a hair drier before rewinding—it’s been a few years. Bill Legall, Millersound in Landsdale, PA, re-coned the badly damaged woofer cone, and a spare piece of Acoustic Research AR-3 gill cloth will make the speaker look like new when re-installed in cabinet.
Fast forward to a few weeks ago, when it was dusted off for return to its owner, and found to misbehave. After some seconds, I would hear a click, and loose audio. Turn it off, and then on, and repeat. Nothing would make it continue to play.
After reading the many #36 oscillator tube instability discussions, I replaced it with Type 77 and new 6-pin ceramic socket, as recommended. The 77 is basically a 36 with a suppressor grid. Its behavior did not change. Next, I removed coupling transformer T-14 and replaced its feedback winding (1-2). That winding has been the subject of much discussion. It was replaced, even though its resistance was 5.5 W (schematic 5.25 W). Its resistance should have been fine, but perhaps a problem, when high voltage is applied to plate via winding (4-5)? As with T-6, new polycarbonate plastic insulation of 0.010-in thickness (same as original cellulose acetate) was used. Twenty-seven turns of same 0.006-in-diam enameled wire. Its resistance now measures 3.5 W, instead of original 5.25 W. A 2-W resistor was added in series so as to maintain both turns ratio for positive AC feedback, and the cathode‘s DC bias voltage.
The capacitor between pin 2 of T-14 and ground was increased to 1500 pf, and the resistor reduced first to 7.5 kW, and even 3.5 kW, with no difference in oscillator behavior; changing R did change DC bias on cathode of 77.
Its behavior now is much better but still has a problem. When powering the radio, it plays for a few seconds, then click, and dies. But now, when tapping the #75 dual-detector/triode amp tube, it springs to life and stays on for days, until turned off. And then we have a repeat of above. On, off, tap 75, on for good. Many local stations are heard loudly and clearly, both low-, mid-, and high-end of dial. A crude alignment of IF stages was done by peaking audio output with trimmer caps. The 75 does show microphonic behavior when tapped. Additionally, I realized too late that I did not bake T-14 before replacing its feedback winding. Aging brain! Perhaps I should remove coil and do this? I presume the purpose is to outgas any residual acetic acid that had been absorbed in both windings—under and over—the original cellulose acetate insulation in the decades since manufacture.
Basic issue: why does the oscillator need to be shocked into starting a second time?
Any and all ideas/suggestions welcome!
I am a new member to this forum who restored a model 89 some six years ago for my brother-in-law. At that time the restoration appeared complete. It worked well then. Today, after removal from my closet to return, following case restoration, it has a big problem.
My restoration steps are described, in hopes that some of you can provide useful feedback.
As many of you can attest, "war stories" about old radios are in no short supply. My first peek inside the chassis in 2013 revealed a cache of hickory nut shells stored by an industrious and efficient squirrel. As well, moths had consumed almost every fiber of cotton wire insulation with the exception of the transformer leads. Sound familiar? J
All wiring, capacitors, and resistors were replaced with modern devices of equal or higher power/voltage rating; the original Bakelite housings were retained. The exceptions were: mica trimmers, the 232 W/32 W wire wound bias resistors in power supply circuit, and of course, ganged tuning caps. The later were cleaned with compressed air.
The primary winding (1-3) of coil T-6 was found to be open. It was re-wound with the same number of turns of enameled wire. Both the original and replacement were 0.006” diameter, including enamel, implying #35 wire. Its new resistance was 1.8 W compared to the value 1.68 W given on the schematic. I think I baked the coil with a hair drier before rewinding—it’s been a few years. Bill Legall, Millersound in Landsdale, PA, re-coned the badly damaged woofer cone, and a spare piece of Acoustic Research AR-3 gill cloth will make the speaker look like new when re-installed in cabinet.
Fast forward to a few weeks ago, when it was dusted off for return to its owner, and found to misbehave. After some seconds, I would hear a click, and loose audio. Turn it off, and then on, and repeat. Nothing would make it continue to play.
After reading the many #36 oscillator tube instability discussions, I replaced it with Type 77 and new 6-pin ceramic socket, as recommended. The 77 is basically a 36 with a suppressor grid. Its behavior did not change. Next, I removed coupling transformer T-14 and replaced its feedback winding (1-2). That winding has been the subject of much discussion. It was replaced, even though its resistance was 5.5 W (schematic 5.25 W). Its resistance should have been fine, but perhaps a problem, when high voltage is applied to plate via winding (4-5)? As with T-6, new polycarbonate plastic insulation of 0.010-in thickness (same as original cellulose acetate) was used. Twenty-seven turns of same 0.006-in-diam enameled wire. Its resistance now measures 3.5 W, instead of original 5.25 W. A 2-W resistor was added in series so as to maintain both turns ratio for positive AC feedback, and the cathode‘s DC bias voltage.
The capacitor between pin 2 of T-14 and ground was increased to 1500 pf, and the resistor reduced first to 7.5 kW, and even 3.5 kW, with no difference in oscillator behavior; changing R did change DC bias on cathode of 77.
Its behavior now is much better but still has a problem. When powering the radio, it plays for a few seconds, then click, and dies. But now, when tapping the #75 dual-detector/triode amp tube, it springs to life and stays on for days, until turned off. And then we have a repeat of above. On, off, tap 75, on for good. Many local stations are heard loudly and clearly, both low-, mid-, and high-end of dial. A crude alignment of IF stages was done by peaking audio output with trimmer caps. The 75 does show microphonic behavior when tapped. Additionally, I realized too late that I did not bake T-14 before replacing its feedback winding. Aging brain! Perhaps I should remove coil and do this? I presume the purpose is to outgas any residual acetic acid that had been absorbed in both windings—under and over—the original cellulose acetate insulation in the decades since manufacture.
Basic issue: why does the oscillator need to be shocked into starting a second time?
Any and all ideas/suggestions welcome!
for the sheer dumb luck of having found a 75 in that bin of old tubes, thereby solving problem quickly. Next to the bin, an RCA-67A signal generator for $17; now to find a manual!
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