Low amplification

[Gomek]

Hello, and greetings as a new member to the group. I have an OLD weather radio from 1976 and I am trying to get it working again. I installed a fresh battery and I'm able to get a strong yet faint audio signal from the speaker. To clarify, the actual RF signal is fine but I can not amplify the audio portion of the circuit. I bought some contact cleaner today and sprayed a little on the rheostat and the crackling disappeared but the audio is still too low. I'm going to try and attach a photo of the circuit board in question. Thanks in advance!

PICT4644 on Flickr - Photo Sharing!

 

[unclejed613]

with something that old, begin by replacing electrolytic caps in and around the signal path. if they're used as coupling caps and are dried out, they won't pass much signal. if they're used as emitter bypass or collector load bypass caps and are dried out, they reduce the gain of the amplifier stage. i have a friend who collects transistor radios like that, and he goes through a lot of caps when he refurbs them.

 

[Gomek]

with something that old, begin by replacing electrolytic caps in and around the signal path. if they're used as coupling caps and are dried out, they won't pass much signal. if they're used as emitter bypass or collector load bypass caps and are dried out, they reduce the gain of the amplifier stage. i have a friend who collects transistor radios like that, and he goes through a lot of caps when he refurbs them.

Thank you very much for the reply! I thought about that, but figured right off the bat that if the caps were bad there would be some leakage or signs of the dielectric oozing out,etc. I'll get around to changing out the caps and see what happens, thanks again for the advice!

 

[unclejed613]

electrolytic caps are more likely to dry out than leak. especially when the device has been sitting for years. caps sitting for long periods can also lose their oxide layer, reducing the capacitance and working voltage (the effect has also been called "de-polarization"). electrolyte leakage is more often caused by overvoltage, or reverse voltage.

 

[Gomek]

Thank you again for all your help! I'll replace the caps and let you know how it goes. Also, I tried adjusting the variable caps I think they are, they're the components with the slotted screw heads.

 

[unclejed613]

please don't adjust those, those are your RF and IF transformers and they are tuned (or were....) and you will have a hard time getting them tuned right once you get the radio working again

 

[Sceadwian]

You have to use a long handled plastic screw driver to adjust trimmer caps and in order to be properly tuned they have to be done in a specific order. If you've moved them out of their original position you're gonna have a heck of a time getting the circuit to tune properly again.

 

[Gomek]

After mucking with the var transformers I still have a signal, but not the volume as mentioned. To be a total geek I went ahead and made a video of the operation of the radio just to show exactly how it's operating. We had this radio when I was a kid and tornado's moved through our area in Michigan, fortuently we were spared except the metal shed in the back yard being thrown into the neighbours yard. Anyhow, here's the video.

YouTube - PICT4645.MOV

 

[unclejed613]

you need a signal generator that can supply 10.7Mhz for the IF chain and 150-170Mhz for the RF front end, and a frequency counter for the local oscillator. the local oscillator needs to be tuned 10.7Mhz above or below the frequency of the station you're listening to you then inject 10.7Mhz at the first mixer transformer and tune the IF's for maximum audio (with a modulated signal). then inject a signal at the frequency of interest (162.550Mhz for instance) at the antenna and tune the RF stage and first mixer. then remove the direct connection to the signal generator and retune the IF chain (leave the LO alone) , the first mixer and the RF chain for maximum audio with the least amount of hiss (maximum quieting). to know which transformer is which, you will likely need a manual or schematic of the radio (along with a layout diagram). a lot of stuff made for radio shack was made very cheaply, and you might only have one RF stage, then the first mixer, one IF stage, then the demodulator. the demodulator gets tuned for maximum signal without distortion (kind of a trade-off). i think the weather stations use 25 or 50khz deviation for their audio (the amount the carrier changes from the center frequency when fully modulated).

 

[transistor495]

Hello, and greetings as a new member to the group. I have an OLD weather radio from 1976 and I am trying to get it working again. I installed a fresh battery and I'm able to get a strong yet faint audio signal from the speaker. To clarify, the actual RF signal is fine but I can not amplify the audio portion of the circuit. I bought some contact cleaner today and sprayed a little on the rheostat and the crackling disappeared but the audio is still too low. I'm going to try and attach a photo of the circuit board in question. Thanks in advance!

PICT4644 on Flickr - Photo Sharing!

First you need to clean the pcb from track-side to remove oxides and other dirts using isopropyl alcohol.

You stated that your RF section is working fine. So do not touch or adjust any IF cans. My assumption is it is the problem with the audio section.

One question: Are you able to raise/lower the volume or the volume remains constant? -if the volume is constant then it can be a problem with the control potentiometer.

If you're able to raise/lower the volume(even though at lower volume), then the problem will be with the audio amplifier. There you need to check:
1. Electrolytic caps(big ones at final stage)
2. Power transistors blown out( rare chance as the set kept idle for long)
3. Two audio transformers -they're impedance may change after long long years.

All these audio related problems can be confirmed by a simple yet practical method. Take a connection from the volume control pot center pin and ground terminal. Feed this audio to your computer speaker by some wires...check the relative volume level. If you get a wide range of volume control through the speakers then you need to check the above mentioned audio section and it's parts. You may replace the entire audio section using an IC.

 

[unclejed613]

he also later said he messed with the IF and RF adjustments...

Also, I tried adjusting the variable caps I think they are, they're the components with the slotted screw heads.

 

[transistor495]

If any noobs open a solid state radio, there's a first tendancy to adjust those IF and RF cans...natural

 

[unclejed613]

the worst part of it being that the RF and IF sections are not laid out in a linear fashion as they were with "glass and steel" radios, so it's difficult to align by guessing which transformer is what...

i did find a listing of the color code for IF cans
455khz (AM radio):
red oscillator
yel 1st IF
wht 2nd IF
blk 3rd IF/det

10.7Mhz (FM radio):
grn 1st IF

orn,brn, or blk 2nd IF or demod


IF transformers are pretty much a thing of the past with the availability of cheap ceramic resonators

 

[transistor495]

That's a great bit of information unclejed, however I feel they are not standardized and sometimes green will come as 3rd IF in AM, and what about pink cans?

I think orange, brown and pink are specially for FM part. Is that right?

 

[unclejed613]

no they're not well standardized..... i've seen yellow and orange and blue in the stereo decoders for older FM receivers there are also a few different methods of demodulating FM. the most common up until about 1980 was a discriminator. discriminators used either two identical IF cans, or a single can with two cores and the two cores or cans were stagger tuned 50 or 75 khz from the center frequency. around 1980 or so, PLL demods began to appear. they used a 10.7Mhz VCO, a phase detector, and a loop filter.. the 10.7Mhz VCO had a single can with an adjustable core. the loop filter was disconnected and a frequency counter connected to the VCO output, and the coil (or in cheaper versions, variable cap) adjusted for 10.7Mhz, then the loop filter was reconnected. the control voltage coming out of the loop filter was the source of demodulated audio. by this time stereo decoders had already become exclusively PLL operated de-muxers with inductor-less oscillators, so coils were no longer required for them. a weather radio doesn't have a stereo decoder, but can have an audio PLL for tone encoded aquelch for emergency alerts. this PLL probably has an adjustment pot, and if the 10.7Mhz demod is a PLL type, can have an adjustable coil or a variable cap

 

[unclejed613]

another "standard" IFT color code list

this one's a little more detailed, but as you can see, even transformers from the same manufacturer can have a variety, even with transformers with the same color marking on the core... hardly a standard....

 

[unclejed613]

and another manufacturer uses the color to represent the last digit of their part number

 

[Gomek]

Thanks, great info!

Uncle Jed wrote:

"a weather radio doesn't have a stereo decoder, but can have an audio PLL for tone encoded aquelch for emergency alerts."


Yes, I remember when it worked that if you had the power on but the volume all the down, an alert tone would sound.
It's real nice to have a group like this that still deals with older electronics, etc.

 

[Gomek]

I really appreciate the help from you and others. I'll eventually get around to trying out these options. Thanks again what a great source of knowledge. And yes I will update my progress when I get around to tinkering.

 

[spicydeath82]

i noticed in the video that the indicator light(LED?) on the radio doesn't seem to work. could be an idea while your restoring it to replace that.
also i'd just try another un amplified speaker on it. could be the voice coil on the old one is badly oxidized or restricted in it's movement. just an idea.