My Dad told me that he got this in about '56 or '57 from Watt and Shand's department store in downtown Lancaster, Pa. He had his own tube tabletop radio (a hand me down) which gave up the ghost, and I guess since he was old enough (he would have been ten) my grandpa decided it was time that he have his very own. I think he said it was the first all transistor anything in his house.
Well, it's dead now-when I got it, I couldn't pull any audio off of it, although when I turned it upside down, and fiddled with the volume, it did begin to motorboat for about thirty seconds. I haven't been able to reproduce those results since. I replaced the electrolytics, but without a schematic, or really any clue on what to do with this newfangled fad called transistors, I'm more or less at an impasse. Hope someone can help with this one.
thanks,
Wade
NO CLUE?. . . .O.K. then, lets see if this gives enough clues for you to be able to enact a successful repair.
Should be an electrolytic problem, considering that you might have overlooked one of the 4 or possibly mis polarized 1 on installation, or possibly an onset of
cancer of the battery terminals if there is nothing at all.
At least, that unit is so early in the game, that the foil traces are quite easy to trace and work with. Couldn't quite make out the polarity of C2 to red line its polarity on the pictorial, but the + is probably to the left, and I additionally referred to possibly
using an ohmmeter to confirm, that its C1 mate beside it, shares the same negative connections.
Technical & Schematicumas Referencing:
73's de Edd
Your advice is great but remember these early radios are notorious for defective volume and switch controls. The switch often develops a high resistance between contact (oxydation) and creates strange problem often assimilated to bad capacitors (motorboating).
Syl
after its swim in contact cleaner, the volume control tests very nicely on my multimeter.
I replaced all of Edd's delinquent caps EXCEPT for C3. C3 was not labeled with a value that I could find, but now thanks to that lovely schematic, I can swap that one out too.
I'll report back after that cap swap.
Wade
:Edd,
:
:Your advice is great but remember these early radios are notorious for defective volume and switch controls. The switch often develops a high resistance between contact (oxydation) and creates strange problem often assimilated to bad capacitors (motorboating).
:
:Syl
the following are voltage readings off of transistors that tested abnormal to Edd's chart:
all abnormals marked '!'
conv
t1033:
c: 0v!
B:1.9v
E: 2v
1st If
t1233:
c:0v!
B:3.29v!
E: 4.91v!
Det
t1033:
c:4.93v!
B:4.7v!
E:4.8v!
to me, it looked like the center of the trouble was more around the 1st IF, so I replaced c7 and c8. I'm out of .01 caps for now, or I'd have replaced the linking capacitor on the collector and emitter of the Detector as well. The audio section from the driver to the speaker tests great. The 2nd IF measurements are also all right on the button.
Hopefully we are getting close, yes?
No?
Wade
:
:the following are voltage readings off of transistors that tested abnormal to Edd's chart:
:all abnormals marked '!'
:
:conv
:t1033:
:c: 0v!
:B:1.9v
:E: 2v
:
:1st If
:t1233:
:c:0v!
:B:3.29v!
:E: 4.91v!
:
:Det
:t1033:
:c:4.93v!
:B:4.7v!
:E:4.8v!
:
:to me, it looked like the center of the trouble was more around the 1st IF, so I replaced c7 and c8. I'm out of .01 caps for now, or I'd have replaced the linking capacitor on the collector and emitter of the Detector as well. The audio section from the driver to the speaker tests great. The 2nd IF measurements are also all right on the button.
:
:Hopefully we are getting close, yes?
:
:No?
:
:Wade
:t1233:
:c:0v!
:B:3.29v!
:E: 4.91v!
:
:Det
:t1033:
:c:4.93v!
:B:4.7v!
:E:4.8v!
From these readinsg, are you sure C2 isn't shorted or inserted backwards ? It's almost as if R14 was shorted or Vcc was going right thru C2 to R5 and R13.
Syl
That is a start. Now you know this part is covered.
Before taking reading here and there, save time and use the sig gen and inject a signal starting from the audio section and working your way to the antenna. You'll quickly find the defective block and fix it. It could be a broken connection or an open resistor...
First things first, at least verify if the oscillator is...oscillating. Turn radio on and tune at around 900Khz, use another radio tuned at around 455Khz higher (rock back and forth the tuning to find the "sweet spot") brought close to the defective radio and listen for heterodyning (squealing). If it happens then go to next steps:
Start by injecting audio at R15, then base of DET.
This will cover the audio section.
Next inject a signal to base of 2ND IF then 1ST IF. By now you should know where the problem lies.
Syl
With the numeros majicas supplied, are we to assume that the 2nd IF is so close to normal that its voltages were not supplied ? The converter stage also looks right on, with the odd ones being the 1st I.F. stage and the detector. Now, in concentrating on those stages initially: Touching on your disc ceramic replacements, they are almost trouble free, especially at low voltage uses, but if there was one point to suspect in this sets circuitry, that would be C11's position, but usually, disc ceramics are my last components to check out. Check out the mentioned suspect areas and give us some feedback. . . . .
From the readings given, I think that the first thing that I would check
in a power off condition, would be an ohmic test to confirm the resistance from the collector of X4 det xstr to ground, in order to confirm the integrity of the Collector-R15-R1-Gnd loop.
Expecting its series ~ 4K plus 330 ohms.
If that flies, then I would pull X4---T-1033 out of circuit to confirm ohmmically, that its C-E junction has not crashed / fused or become SOMEWHAT leaky, as that is an old PNP item, tracing back to its past germanium mesa ancestry.
The overall secondary effect of a collector voltage error in that detector stage can result in the R13 AVC current feedback loop to the frontal 1st I.F. not getting its AVC shifted base bias, which bucks against against its quiescent biasing level as is being acquired from resistor R14.
You are currently placing a high + value on that AVC bus, so the bias voltage presented to the 1st I.F. is not being
swung negatively enough to enact conduction and gain in that stage. [The Green Buss markup. . . Schema.]
The 2nd I.F. stage runs with a fixed base bias and is not tied in to the AVC control buss.
73's de Edd
With the numeros majicas supplied, are we to assume that the 2nd IF is so close to normal that its voltages were not supplied ? The converter stage also looks right on, with the odd ones being the 1st I.F. stage and the detector. Now, in concentrating on those stages initially: Touching on your disc ceramic replacements, they are almost trouble free, especially at low voltage uses, but if there was one point to suspect in this sets circuitry, that would be C11's position, but usually, disc ceramics are my last components to check out. Check out the mentioned suspect areas and give us some feedback. . . . .
From the readings given, I think that the first thing that I would check
in a power off condition, would be an ohmic test to confirm the resistance from the collector of X4 det xstr to ground, in order to confirm the integrity of the Collector-R15-R1-Gnd loop.
Expecting its series ~ 4K plus 330 ohms.
If that flies, then I would pull X4---T-1033 out of circuit to confirm ohmmically, that its C-E junction has not crashed / fused or become SOMEWHAT leaky, as that is an old PNP item, tracing back to its past germanium mesa ancestry.
The overall secondary effect of a collector voltage error in that detector stage can result in the R13 AVC current feedback loop to the frontal 1st I.F. not getting its AVC shifted base bias, which bucks against against its quiescent biasing level as is being acquired from resistor R14.
You are currently placing a high + value on that AVC bus, so the bias voltage presented to the 1st I.F. is not being
swung negatively enough to enact conduction and gain in that stage. [The Green Buss markup. . . Schema.]
The 2nd I.F. stage runs with a fixed base bias and is not tied in to the AVC control buss.
THE INFAMOUS. . . . GREEN BUSS MARKUP SCHEMA:
73's de Edd
the audio seems to still check out fine-voltages fine from the detector on down...injected audio into R15 and the base of the detector and got a lovely audible signal.
However, no Rf could be picked up at any of the signal injection sites suggested by Z. I tried the oscillating 'dueling radios' technique from Z's post, but got nothing-however, since I have never done this before, I could just be suffering from severe ignorance how how to do this properly. For instance, 900 plus 455 in my locale hits a lot of strong local signals which could be swamping me out. Also, where would I hear oscillation from-the radio tuned at 900 khz or 1355?
Okay, and Edd-I got about 5000 ohms from my resistance test. I did not take the detector out of service we seem to be getting closer to confirming that this is an RF issue.
thanks so much guys,
Wade
wade
Ok, try it htis way then:
Find a silent spot on the [b]good[/b] radio between 1000 and 1500, turn the volume half way up. All you should hear now is a faint hiss.
Now, turn on the defective radio and tune it about 500Khz [b]below[/b] the good radio's frequency. Say you found a silent spot at 1200Khz. 1200-500=700Khz...
Start at that frequency on the defective radio's dial and slowly go up in frequency. You should hear a squealing on the good radio once you hit the right spot if the oscillator is working (which should be around 455Khz below the good radio's dialed frequency. Radios must be as close as possible. If you get no squeal then this means the oscillator is dead.
Come back with the results and we'll work from there.
Syl
Ok, try it htis way then:
Find a silent spot on the good radio between 1000 and 1500, turn the volume half way up. All you should hear now is a faint hiss.
Now, turn on the defective radio and tune it about 500Khz below the good radio's frequency. Say you found a silent spot at 1200Khz. 1200-500=700Khz...
Start at that frequency on the defective radio's dial and slowly go up in frequency. You should hear a squealing on the good radio once you hit the right spot if the oscillator is working (which should be around 455Khz below the good radio's dialed frequency. Radios must be as close as possible. If you get no squeal then this means the oscillator is dead.
Come back with the results and we'll work from there.
Syl
Methinks. . . that the powers that be. . . . seem to be pointing to that Detector / AVC transistor being at fault.
ASIDE:
On your marking of the collector voltages being in error, this is a set using germanium PNP transistors
And they are placed in what I call a stacked upside down circuit where they draw the schematic in the normal
fashion, with them showing the collector at the top and then routed over to connect to the low Z impedance
of a coil or I.F. transformer, and THEN that coils supply line gets connected to “ground” which actually is the +
connection of the power supply and a + voltage is what the collector is supposed to be receiving. I think that you would be hard pressed to read that voltage across that coil with even a 5 digit DVM.
Now let’s go back to that 1st I.F. stage where the voltages INITIALLY show up as being in error from the supplied schematics data..
I can now give you a simple test to evaluate that 1st I.F. transistor with the mere utilization of but a variable
potentiometer and a series fixed resistor to act as a “stop” on its lowest adjustment capability.
That pot can be a 5,10,20,50,100K in value or possibly even a 1/2meg—2 meg volume control pot, in case you are control poor. But if the latter, since that is such a high value and with it being a log taper type of unit, lets use its portion that is resistively spread out, rather than its compressed up portion , which if you were viewing the control shaft end, that would be the center tap and the terminal just to its left.
Now we need to add a series resistor 470 ohm (common ½watt ) that is then tacked onto either one of those
pots 2 terminals. That way then, we now have a ‘lebenteen kazillion ohm variable pot with a protective current limiting stop of 470 ohms on its minimum value setting.
Referencing schematic now, we see that the loop of AVC feedback from the detector stage that is fed thru R13 6.8 k is so excessive such that the base bias of the 1St I.F. is now sitting at 3.29 V and that stage is not conducting and amplifying.
Here is what you do now, monitor the emitter voltage of that 1st I.F. xstr and be expecting your same ~4.91v , but you now go over and connect onto the lower side of that that R13 with one of your special pot connections and the other lead of the special pot goes to ground (+6vdc) .
Then, considering that you initially had the pot set for maximum resistance position, you start decreasing the pots resistance and monitor the metering on the emitter circuit and confirm that voltage is gradually starting to decline with a lower value of resistance being trimmed into, and should finally get down onto the specified ~2.4 VDC range as the variable base bias that you are applying with your pot, makes that xstr go further into conduction.
If this procedure works for you without a snag, you are then assured that there is minimal case of fault with that stages xstr and that the stage has current gain. Time to go investigate where I think that the trouble is, and that would be the X4 stage.
Referencing:
1st If
t1233:
c:0v!
B:3.29v!
E: 4.91v!
ZUJ’ing. . . . .
73's de Edd