Model 89 - Printable Version
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Model 89 - vacuum - 06-29-2019
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!
RE: Model 89 - klondike98 - 06-29-2019
Welcome to the Phorum!
Yep, the first two posts by new members are reviewed by moderators. There are only a few of us so it can take some time before one of us logs in and does the review. This helps weed out folks who are simply trying to post spam.
RE: Model 89 - David - 06-29-2019
Welcome to the phorum. Lots of good folks here to help you out with the model 89.
RE: Model 89 - Steve Davis - 06-29-2019
The best solution to this problem is Ron’s SUPER 89 modification where the 36 oscillator tube is replaced with a 6A7. Take a look here. https://philcoradio.com/phorum/showthread.php?tid=14919
I had a Philco 19 that the oscillator would drop out. I tried every trick that I knew and could never make it work. I did the SUPER mod and it now works perfectly
Steve
RE: Model 89 - Ron Ramirez - 06-29-2019
Hold the phone, folks. (And welcome here vacuum, by the way.)
You say that you have to tap on the 75 tube to restore proper operation. The 75 tube is the second detector and 1st audio tube; it has nothing to do with the oscillator circuit.
A possible fix for your 75: unsolder the solder which is on top of its grid cap, then apply fresh solder. This sometimes fixes 75 tubes with intermittent issues as yours appears to have.
RE: Model 89 - Radioroslyn - 06-29-2019
+1
I would take a listen for the osc signal in another set and hear if it dies out when the 89 does. Just because there is some history w/ the osc section I wouldn't make the leap that that's the issue before doing some troubleshooting.
GL
RE: Model 89 - Ed Locker - 06-30-2019
+1 on what Ron said.
Or replace the 75.
RE: Model 89 - Steve Davis - 06-30-2019
I had a Philco 18 one time that would come and go. When troubleshooting I noticed the 75 tube was going dim when the sound went away, then would get brighter and the sound come back. I resoldered the pins and the problem went away.
Steve
RE: Model 89 - vacuum - 07-01-2019
Gentlemen:
Thanks to each of you for taking time to reply; much appreciated!
Ron, I agree. The 75 does not influence oscillator operation. I shall re-solder its pins and grid cap when I have time in next day or so. For a period in my working life, I was involved in making picture tubes so am familiar with how pins and caps are physically attached and soldered.
This set does have an AGC, but it does not connect to the oscillator. The 75's dual diodes rectify the signal in the secondary of T-23. A portion of that is returned to the first grid's of both 44 (RF amp) and 39 (IF amp). Thus, if 75 were not working properly, it would for sure cause problems.
Also, I realized after submitting the first post, that I was not paying attention to the dial setting when measuring some DC voltages. AND I did not have the tube shields in place until the end. Six years ago, I recorded a DC voltage of about +15.7 V on top of the RC connected between (pin-2 of T-14) and ground. I then discovered that this voltage varies considerably (15.7 V -> 3.5 V) with both dial setting and presence of shields! That || RC (1500 pf -- 7,500 Ohm) has a time constant of 11 useconds. The oscillator frequency ranges from ~290 -> 1340 kHz (260 kHz IF). Over this range, the AC reactance of that RC ranges from (18 -j365) Ohms to (1 -j80) Ohms meaning it is for practical purposes, a capacitor.
The AC voltage between oscillator cathode and ground would determine the magnitude of the positive feedback, while the DC voltage between cathode and ground would determine the bias point and gain.
I started with 7500, then tried 4000. Will revisit this with smaller incremental changes.
That said, first thing will be to re-solder #75 and re-attach and re-solder loose connection on GC of #77.
Thanks again for your suggestions!!
John
RE: Model 89 - vacuum - 07-01-2019
We an operational Model 89!
Here is the short version:
*Exhibited classic oscillator instability when powered again for first time in 6 years after rebuild.
* First, replaced the #36 oscillator tube & socket with #77. Result: same unstable behavior, lots of microphonics when tapping on #75.
* Second, re-wound the feedback coil of cellulose acetate insulated oscillator transformer T-14. Result: same.
* Reduced 7k3 Ohm resistor between T-14 (pin 2) and ground to 6k6 Ohm. Result: osc starts, otherwise same.
* At Ron's suggestion, re-soldered all pins and grid cap on 75. Result, no change. Conclude that 75 is remaining culprit, but the intermittent is a bad spot weld inside the 75.
* Drove to Elliott Electronics and found the only #75 in a bin of about 200 used tubes--5$. What is the probability finding one? And finding it to be good? Changed tube: EUREKA! ... a stable, beautifully sounding, smoothly tuning Model 89
Conclusion: More than one thing had gone south since this unit was first refurbished--as a minimum: #75, feedback winding, and incorrect bias resistor.
Even though it is 108F today, I am feeling pretty cool
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!To this Philco we now say: “stay tuned.”
Thanks all, your help was great.
John