Hello Everyone, I picked up an old Magnavox am/fm phono tube radio last summer. I plugged it in and turned it on (I’ve since learned this was a no-no) and was only able to get one am station to work and intermittent record playing. Also, the power switch didn’t work all the time.

Admittedly, I am a newbie to electronic repairs, but I had a can of deoxidit (for cleaning pots in my guitar amps) and much gusto, so I went to town on it.

Over the course of the last several months, I’ve taken my electronic (cleaning) repair abilities to another level and learned to solder, I’ve since switched out the capacitors (save for two) in the amp and pre-amp and got the power switch to work consistently and the phono is working well, but I’m unhappy to report that I haven’t noticed any improvement to the am/fm radio, in fact it has gotten worse, as I no longer have my one working am station. Any tips or advice would be greatly appreciated, thank you!

Made around 1956. The radio and amplifier will also need all the wax and paper capacitors replaced too. Tubes tested. Resistors test. Do the capacitors first one at a time. Avoid an error.

Hello Thunderbird281 and thanks for the reply,

I have replaced all the capacitors except for two of them, one in the amp section and one in the pre-amp section. I have not addressed any resistors yet.

Is it possible to assume that my lack of radio is due to failing tubes in the pre-amp section? If so, I have recently picked up a tube tester, but have found it hard to test some of the tubes where I can’t read their numbers. In this situation, can I assume the tubes next to the tuner section are related to the radio and should I just try to replace them? There are two tubes next to the radio tuner and they are both glowing when the unit is powered on.

Also, I wanted to provide pictures of the two caps I haven’t yet replaced; these are color-coded types.
I’ve tried to figure out their values by using charts I’ve found online, but I have gotten different values each time, so I’m not confident in my ability to identify their correct value. Any help from the experts would be appreciated! Thanks!

A chart can be seen at the second link, here...
viewtopic.php?p=3173979#p3173979

If you are unsure, remove them and upload a photo.

Greg.
oh... look for chassis numbers, rubber stamped or pasted to paper tags.

Thanks Greg, I will try to remove the caps in the next day or two and upload a close-up of them.

As far as the model #'s:

The amp is a 142AA, and I have been unsuccessful at finding a schematic for it. I've read somewhere the magnavox yahoo group is a great resource, but I believe it's shutdown now, or not taking new members?

The pre-amp is a CR 178AA, and no schematic found yet for this either.

Magnavox 142 - rebuild/modification thread...
https://www.audiokarma.org/forums/index ... ad.330832/

▼ Sams Photofact Set-311 Folder-9 MAGNAVOX AMP142AB

Thanks Greg, those schematics are helpful! Of course, I'm not good at reading them, but they are helpful nonetheless. In fact, you inspired me to go deep into the interweb this last weekend to look for a schematic for my pre-amp (CR 718AA).

I wasn't able to come up with the exact one, but I did find a CR 718, and most identifiable landmarks made it seem like a close replica to my pre-amp. From this, I was able to determine what the values were on all of my tubes, and in testing them, I found one bad one in the pair of 6BA6's. I purchased replacements and seated them today, and I'm happy to say I'm getting both AM and FM reception! It's not great reception, and not every station is coming in, but it's definitely working!

During my research of tubes and am/fm reception, I found someone talking about silver mica disease and the need to replace the IF cans. I have no idea what these are and am hoping someone "can" assist (pun intended). I'm attaching a picture of my pre-amp, I'm hoping someone can tell me "if" the IF can is on top and which one it is.

Thanks!

Hello, I'm finally uploading pictures of the the two black beauties capacitors that I need help identifying. I have tried to read the chart, but I just don't know where to start or end on the bands. I'm also attaching what I think one of the capacitors is from the schematic you provided. It's a capacitor connected to the speaker output, and I believe the schematic says it's .22mf, that said, what is the voltage rating and percentage of tolerance? The other capacitor is located in my pre-amp. I'm not sure what it's connected to but a pink wire runs from its soldered point to a pink wire that runs to the base of one of the cans (again, I don't know what these cans are). Also, to remind everyone, I'm a newbie to all of this, and I'm not good at understanding schematics, so I really appreciate your time and expertise

Bueller? Bueller? Bueller?...

you wrote: ...and I believe the schematic says it's .22mf
This is incorrect - - - - it's .0022μF (microfarad)

Red Red Red Green Blue is 2.2nF (nanofarad)
2.2 nF ±5% (min:2.09, max:2.31 nF) 600V

2.2nF equals .0022μF (microfarad) as shown on your schematic as C7
(see link to conversion chart below).

to convert from μF to pF (and nF)
Here's a handy capacitor conversion chart.
via... viewtopic.php?p=3178963#p3178963


Your second capacitor's fifth-color... Red, denotes a capacitor rated at 250V

Apologies for the French calculator, google for others.
Greg.

So awesome! Thanks Greg!

All of the tube's (pinouts) pin numbers are identified on the schematic above for...
MAGNAVOX AMP142AB
The corresponding leads are numbered one to nine around the tube's circular diagram perimeters.

pins 2 and 7 are the filament/heaters connections - - Y and Z on this schematic.
Their leads/schematic lines are omitted from the drawing for clarity. Their origin Y and Z can be seen at the power transformer T1

They also connect to the AM-FM Tuner Power Socket - - pins 2 and 4

The schematic representation of 12AX7 is "split-in-two" on the schematic, again for clarity.


From the tube pin numbers...
You will now be able to identify/follow their pathways into the circuit, and the components that are connected to them.

Viewed from below - - bottom view of sockets aka tube bases. lol








Large image→ https://a4.pbase.com/o10/56/215056/1/16 ... titled.png

A few different (tube bases) examples are shown above, but... no doubt you get the idea...


Greg.
oh... label the wires (both ends) with marked-up masking tape flags, as you familiarize yourself with the circuit.

Edit: 12AX7 has a center tapped filament
pins 4 - 9 - 5

CR-718 is a Tuner/radio.
https://www.radiomuseum.org/r/magnavox_cr_718.html

Sams Photofact (pretty pictures) service manual Set-344 Folder-9
Original copy $6.25 shipped eBay.
https://www.ebay.com/itm/382137479857




Edit: the Photofact pages can be previewed here...
https://www.radiomuseum.org/r/magnavox_cr_718.html

A real pain in the derriere to download!

Some years ago I restored a 252MX, a similar model to yours. Poor FM reception turned out to be silver mica disease. I’ve forgotten now what exactly I did, but here’s the thread. You might find some helpful information there. I did work on the FM IF cans, and I think I actually found a NOS discriminator online that I replaced the original with. I did finally get fair FM reception, but it was never as good as AM.

https://www.antiqueradios.com/forums/viewtopic.php?f=6&t=208803&start=0

Greg! I can't thank you enough for this most excellent and simplified explanation for understanding schematics! I'm looking at connecting the aux line to the volume knob, since this was never done in the factory, and this information will definitely come in handy!

Thank you for this also! I purchased the first capacitor as you advised. I ran the 2nd capacitor through this chart calculator (super cool btw) and I thought it was this:
1st color: Brown
2nd color: Black (is this what black stripes look like a on black cap, or is this blue? I'm a little colorblind with dark blue)
3rd color: Yellow
4th color Black (again, is this black or blue?)
5th color Red

So based upon my translation, I ordered a 100 nF +/-20% at 250V. Do you agree with this? I'm a little concerned because it was not an easy capacitor to find...

Thanks so much!

Thank you Ed for providing this information. I've read through your topic a couple of times now. I haven't yet tried to dig into the IF cans yet on my console as I'm trying to decide if I want to satisfy my OCD for a perfect functioning console or not, but I do have one question, you mentioned a pic or two in this thread as showing what SMD looks like, can you please explain what a tell tale sign of SMD is? thanks so much!

It will look kind of like black mold on the mica. You can clean it off with alcohol or a pencil eraser. Be gentle, though.

Service literature for your amp and tuner are mentioned above.
This should enable you to identify all your component values.

From the tube pin numbers...
You will now be able to identify/follow their pathways into the circuit, and the components that are connected to them.

google tube numbers for pinouts.

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you wrote: So based upon my translation, I ordered a 100 nF +/-20% at 250V. Do you agree with this?

Nanofards are not used too much around these parts, that why I apologized for the French on-line calculator.
Stick to microfarads and picofarads.

call it 0.1μF (microfarad)
To answer your question... I have no idea...
is it from your amp?
or... is it from your tuner?
what was it connected to?

The Voltage rating of a capacitor is how much Juice that it can handle - this can be increased.
Some of the old μF (microfard) values are no longer common... so choose the next nearest capacitance value, up or down.

Sal has 0.01μF axial film capacitors for peanuts.
http://www.tuberadios.com/capacitors
.01uf @ 630v.........25/$6.25.....10/$2.75.....1 for $0.30


He is a forum sponsor, and can be found on the right/hand side of this very page → → →

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My main monitor is on the bench and this one is not color calibrated.
If you have trouble identifying it ask a woman, only 2% of women suffer from color vision deficiencies.


Greg.