# Automotive Stereo Repair amp popping

#### MattStrike

##### New Member
I have a question related to repairing an amplifier in a 1986 GM Delco stereo. I am using a 300w pc power supply (12v @ 15a) to power the stereo, which powers the speakers for my PC and doubles as a radio (I removed the broken tape deck and hard wired the input signal from a 3.5mm audio jack to the audio signal that normally comes from the tape deck).

Everytime the amp switches on and off (changing stations, turning radio on/off, switching between radio and tape deck) it makes a loud, annoying pop through the speakers. Also the amp has a 'loud' feature that boosts the bass frequencies. It pops when loud is turned on but not when it is turned off.

Okay, now I can get as technical as I can about this (I'm an ME not an EE). The radio has 3 separate major PCBs inside. The first contains the input from the antenna and as far as I know is the tuning portion. The second contains a processor (nothing fancy) and the memory and appears to be the basic control of the stereo. The third is the amp PCB. It contains mostly caps, but has a pot, one each Delco 205 and 213 chips, a chip labeled (l1)dm171 (l2)ad8634m, and two amplifier chips.
The power input is filtered using a 4000µf cap and a choke. The power is distributed from the amp board to the other two boards. The amps are turned on and off by transistors (can't tell if they are npn or pnp nor narrowed down which of the 12 are the ones i need...). There is a 2.15v signal that turns the amps on; the voltage measures 2.15v when the amp is on and 30mv when the amp is off, and it is the only change I can measure.
The signal that control the loud feature is grounded to enable it. The Delco 205 chip controls this.

In my feeble attempt to repair the amp (make the popping noise vanish) I found about 15 caps that had been leaking out the bottom (all the ones that were leaking were the same brand - brandless. I've replaced all leaking caps with quality solid aluminum cased caps (I believe they are called solid state caps). There is a reduction in hissing noises (at max volume with no input), but the amp still pops with no reduction in intensity of the popping.

I understand that this is a common problem for these stereos, but I am looking to fix it myself as a hobby rather than spend $15 at a junkyard to get a different one or send it in to one of those online companies that will fix it for$50 plus shipping and handling.

I am limited to finding stuff with a $30 Radio Shack DMM and a$10 Craftsman pocket DMM that can measure up to 100µf capacitance.
I have built circuits before, but haven't designed my own just to indicate my skill level.

#### Menticol

##### Active Member
Hey I know its a old thread, but I hope you still have your radio, I have exactly your same arrangement in my room with a Pioneer car stereo. First, make sure the PSU is grounded, and connect all the (-) leads in your system to the same point. Then run a wire from the (-) rail of your PSU to the negative of the 3.5 mm jack (the base). This eliminated the noise in my system, hope it woks in yours too

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#### MattStrike

##### New Member
Yes, I still have the stereo and it still pops.

The popping noise isn't coming from the input signal unfortunately. It is a problem with the stereo itself. It doesn't appear to be a grounding issue.

Thanks for the reply though.

#### audioguru

##### Well-Known Member
Most Helpful Member
The very old car radio should be buried. It was designed very cheaply and so it makes popping sounds.
New car radios do not make popping sounds.

#### rjvh

##### New Member
Without a schematic and a scope it's just guesing what you do

if the device is 20+ years old (only if you have emotional value atached to this object) why do you spend money on it

Changing all the capacitors is the first thing to do, cleaning all (mechanical) switches and pot meters the second thing.

that's al about you can do with only a multi meter and no schematic

you might check just other components but without a schematic or a good understanding how the device is build up it is just guesing

schematic an scope availible will give you all the tools to find the cause of the plop

still than I think it's cheaper to buy a complete new car radio from china

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