My first thought was the electrolytic capacitors on the circuit board. I replaced all three, but the problem remains. The only one left I haven't replaced is the big multisection electrolytic (I can't tell all the values of it except for one section that's 850 ufd).
I was able to set the collector bias on the output transistor to 1.2 Volts as suggested in the reply to my previous posting. I have continuity through the antenna inductor.
I double checked the solder connections I made and they're all good. I am using a bench power supply that can supply more than enough current. What else can I check? Thanks!
If nothing, put a high impedance meter across the Base and Emitter of the Oscillator transistor and measure voltage then do noise injection into the imediate area such as touching your finger to the oscillator coil itself and see if the voltage changes. If no change, look in that area. If it does change, suspect an IF, cold/bad solder joint, or broken solder track.
Have you peaked the IF's? They should be about 262Khz. Most of the time car radios were never peaked up at the factory. Also too, make sure the antenna has a good ground plane (counterpoise).
Also the antenna trimmer has a significant impact on performance in car radios. If you ahve not hooked up a signal generator yet to this one, now is the best time because it can save you a lot of work.
If you want to do the radio for long term reliability, replace the big can cap. It might be leaky enough to be bleading off current from the B voltage buss.
:I am currently working on a 1965 Chevelle Malibu AM radio. The radio does not pick up any stations. I can detect the volume level as it increases, but no stations come in.
:
:My first thought was the electrolytic capacitors on the circuit board. I replaced all three, but the problem remains. The only one left I haven't replaced is the big multisection electrolytic (I can't tell all the values of it except for one section that's 850 ufd).
:
:I was able to set the collector bias on the output transistor to 1.2 Volts as suggested in the reply to my previous posting. I have continuity through the antenna inductor.
:
:I double checked the solder connections I made and they're all good. I am using a bench power supply that can supply more than enough current. What else can I check? Thanks!
:Hi,
:Check for broken solder tracks around the IF's and the mounting pads. Also check the condition of the detector diode and the volume control wiper.
:
:If nothing, put a high impedance meter across the Base and Emitter of the Oscillator transistor and measure voltage then do noise injection into the imediate area such as touching your finger to the oscillator coil itself and see if the voltage changes. If no change, look in that area. If it does change, suspect an IF, cold/bad solder joint, or broken solder track.
:
:Have you peaked the IF's? They should be about 262Khz. Most of the time car radios were never peaked up at the factory. Also too, make sure the antenna has a good ground plane (counterpoise).
:
:Also the antenna trimmer has a significant impact on performance in car radios. If you ahve not hooked up a signal generator yet to this one, now is the best time because it can save you a lot of work.
:
:If you want to do the radio for long term reliability, replace the big can cap. It might be leaky enough to be bleading off current from the B voltage buss.
:
:
::I am currently working on a 1965 Chevelle Malibu AM radio. The radio does not pick up any stations. I can detect the volume level as it increases, but no stations come in.
::
::My first thought was the electrolytic capacitors on the circuit board. I replaced all three, but the problem remains. The only one left I haven't replaced is the big multisection electrolytic (I can't tell all the values of it except for one section that's 850 ufd).
::
::I was able to set the collector bias on the output transistor to 1.2 Volts as suggested in the reply to my previous posting. I have continuity through the antenna inductor.
::
::I double checked the solder connections I made and they're all good. I am using a bench power supply that can supply more than enough current. What else can I check? Thanks!
:John, what do you mean by counterpoise? I am working inside a lab on a workbench with a metal top. The antenna I'm using is one provided by the owner of the radio; it looks brand new.
:
::Hi,
::Check for broken solder tracks around the IF's and the mounting pads. Also check the condition of the detector diode and the volume control wiper.
::
::If nothing, put a high impedance meter across the Base and Emitter of the Oscillator transistor and measure voltage then do noise injection into the imediate area such as touching your finger to the oscillator coil itself and see if the voltage changes. If no change, look in that area. If it does change, suspect an IF, cold/bad solder joint, or broken solder track.
::
::Have you peaked the IF's? They should be about 262Khz. Most of the time car radios were never peaked up at the factory. Also too, make sure the antenna has a good ground plane (counterpoise).
::
::Also the antenna trimmer has a significant impact on performance in car radios. If you ahve not hooked up a signal generator yet to this one, now is the best time because it can save you a lot of work.
::
::If you want to do the radio for long term reliability, replace the big can cap. It might be leaky enough to be bleading off current from the B voltage buss.
::
::
:::I am currently working on a 1965 Chevelle Malibu AM radio. The radio does not pick up any stations. I can detect the volume level as it increases, but no stations come in.
:::
:::My first thought was the electrolytic capacitors on the circuit board. I replaced all three, but the problem remains. The only one left I haven't replaced is the big multisection electrolytic (I can't tell all the values of it except for one section that's 850 ufd).
:::
:::I was able to set the collector bias on the output transistor to 1.2 Volts as suggested in the reply to my previous posting. I have continuity through the antenna inductor.
:::
:::I double checked the solder connections I made and they're all good. I am using a bench power supply that can supply more than enough current. What else can I check? Thanks!
Good suggestions from John. The detector diode in these radios is sometimes "lazy". If probing around the diode causes the radio to come to life, you should probably change the diode. It is the one (of three) all by itself, closest to the output I.F. transformer. To eliminate the owner's antenna as a possible problem, un-plug it and just use a length of wire clipped to the antenna input. If working properly, the radio should pick-up stations with just the wire. If you need a schematic for this radio, I would be glad to send you a photo-copy of the Delco service pages for it. I don't have a scanner, so it would have to be by mail.
The radio (model 986202) is also covered in Sams Auto Radio Series AR-31. Maybe someone with this information and a scanner can email it to you.
The large electrolytic can is: 400 at 16 volts, 850 at 16 volts, 4 at 11.5 RMS, and later production also had a 100 at 4 volts section (Delco part No. 7292515).
Meade
:
:
:
::John, what do you mean by counterpoise? I am working inside a lab on a workbench with a metal top. The antenna I'm using is one provided by the owner of the radio; it looks brand new.
::
:::Hi,
:::Check for broken solder tracks around the IF's and the mounting pads. Also check the condition of the detector diode and the volume control wiper.
:::
:::If nothing, put a high impedance meter across the Base and Emitter of the Oscillator transistor and measure voltage then do noise injection into the imediate area such as touching your finger to the oscillator coil itself and see if the voltage changes. If no change, look in that area. If it does change, suspect an IF, cold/bad solder joint, or broken solder track.
:::
:::Have you peaked the IF's? They should be about 262Khz. Most of the time car radios were never peaked up at the factory. Also too, make sure the antenna has a good ground plane (counterpoise).
:::
:::Also the antenna trimmer has a significant impact on performance in car radios. If you ahve not hooked up a signal generator yet to this one, now is the best time because it can save you a lot of work.
:::
:::If you want to do the radio for long term reliability, replace the big can cap. It might be leaky enough to be bleading off current from the B voltage buss.
:::
:::
::::I am currently working on a 1965 Chevelle Malibu AM radio. The radio does not pick up any stations. I can detect the volume level as it increases, but no stations come in.
::::
::::My first thought was the electrolytic capacitors on the circuit board. I replaced all three, but the problem remains. The only one left I haven't replaced is the big multisection electrolytic (I can't tell all the values of it except for one section that's 850 ufd).
::::
::::I was able to set the collector bias on the output transistor to 1.2 Volts as suggested in the reply to my previous posting. I have continuity through the antenna inductor.
::::
::::I double checked the solder connections I made and they're all good. I am using a bench power supply that can supply more than enough current. What else can I check? Thanks!
Hope that clarifies the situation, though sometimes when I explain things I make things even more complicated.
Thomas
On an automobile, the whip antenna is part of a "Ground plane" antenna system. Compared to a dipole antenna system as in use in post 1970 GM cars with the antenna leads (1 being "ANT" the other being "GND") go to 2 wires in opposite directions in the laminate glass, that wire that ties to the shield braid is the counterpoise, as is the automobile's body in the older style "classic" automobile radio setup.
The single wire idea will work, and will help you trouble shoot the owners' antenna and base mount (they can be deffective, or simply corroded real bad). The metal work bench, will make for an excellent counterpoise for future car radio work, or for working with radios that have a jack for an external antenna lead you can set up one permanent whip antenna that has the grounding clamp sinking into the metal top of your work bench.
Also, if performance of the radio is poor when installed in the fellow;s Malibu, make sure the antenna base is making good electrical contact with the fender, but be careful not to tighten the base too much as the clamp will distort the fender sheet metal.
:When people wire up a home radio so that one lead goes to an antenna and one goes to ground, one tends to think of ground as some shielding or base source or something like that. The truth is that you could hook the ground connection to another wire spanning 180 degrees opposite the wire you already have up, and this would work well. Wiring home radios to ground uses the earth as the other side of the antenna. Signals alternate through the antenna wire to ground and then back the other way. In your car, your car radio uses the whip antenna to feed one side of the antenna coil and uses the car chassis to feed the other side. This is what counterpoise is. It's the other side of the antenna circuit. If the coil lacks one side, the radio will not receive stations properly. When wiring the radio to a power source on your bench, there is a slight chance that you will not have this "ground" to complete the antenna circuit. With my experience with tube type auto radios, though, the chassis of the radio is enough for strong stations, and when powering the radio from an AC source (battery charger for instance), the wiring in the home, which couples magnetically and capacitively (if that is a word) through the transformer, is a more than ample ground source. Furthermore, if you happen to have the radio chassis sitting on a large metal bench, this will also serve as a good "other side" of the antenna circuit, otherwise known as ground. At any rate, should the slight possibility of a lack of ground exist, you want to be sure that you are able to correct this situation. In my experience this is rare, but you never know.
:
:Hope that clarifies the situation, though sometimes when I explain things I make things even more complicated.
:
:Thomas