Hi, folks. First post, but I'll try to make this make sense. I have some electronics repair experience but need a bit of help with this.
I've had substantial hum any time the incandescent headlights or ignition are on and static/clipping above a certain volume level especially on one side on a factory amplifier in a 1990 Volvo. Some people claim they were Alpine units. It is listed as a 20w x4 channel amplifier with Volvo part number 353004. The front speakers are 4 ohm and the rear are 8 ohm, which seems interesting.
The board is marked 55 11511-02 front and rear (front says Art No. in front of it). The rear is also marked 94V0 (looks like the board material) and w8939-Le and has 2 symbols. One is an F with ELEC above the center line and 3330B below, and the other is a mirror images LR with the vertical line of the R touching the bottom line of the L.
There is one 14-pin IC marked Portugal 8926BR and TL074IN. Looks like a 4-channel amplifier from TI (https://www.ti.com/product/TL074) but I don't understand the info on that page very well.
I took it apart and snapped some photos. I checked the glass case SMD diodes inside because 2 of 3 glass cases on surface mount diodes are cracked. Two test ok with a diode mode test reading of 538 and 539, but one of those has a cracked case. The third one tests with a value of 1850. None are shorted or open on a 2k-ohm resistance check. I know these aren't really meaningful values, but the big mismatch and very cracked case makes me think this diode could be blown. Then again, these readings have all been done with the parts installed.
Where I need help is I would like to replace these 2 or 3 diodes but I'm not sure what they are. The glass case makes me think Zener diodes. They have no numbers or identifiers on them even with a 20x magnifier loupe. I see only a black stripe on the end and a silver stripe in the center, which I don't see on any diode color code charts. In fact, I've seen several charts that list silver and say N/A. I've looked at some sites like Mouser but am not sure what voltage rating I should be looking for.
I found forum website from the BCAE1 website, and it has several photos of diodes exactly like this in amplifier circuits. Is this some sort of generic noise filter diode or does the info on the OP-AMP page give enough info to pick a voltage limit for replacement parts? Does anyone else have ideas on repairing this? I've tried to keep this car a budget driver while I finish my master's degree, and I'd love to not buy more than a few small components to keep the stereo working. I have another of the same amp like this, but I have little faith in it being in better condition than this one.
The suspect diodes on the back correspond to the front area with C03 T01 and T02 in the top left of the image I posted.
Thanks very much in advance!
William
I've had substantial hum any time the incandescent headlights or ignition are on and static/clipping above a certain volume level especially on one side on a factory amplifier in a 1990 Volvo. Some people claim they were Alpine units. It is listed as a 20w x4 channel amplifier with Volvo part number 353004. The front speakers are 4 ohm and the rear are 8 ohm, which seems interesting.
The board is marked 55 11511-02 front and rear (front says Art No. in front of it). The rear is also marked 94V0 (looks like the board material) and w8939-Le and has 2 symbols. One is an F with ELEC above the center line and 3330B below, and the other is a mirror images LR with the vertical line of the R touching the bottom line of the L.
There is one 14-pin IC marked Portugal 8926BR and TL074IN. Looks like a 4-channel amplifier from TI (https://www.ti.com/product/TL074) but I don't understand the info on that page very well.
I took it apart and snapped some photos. I checked the glass case SMD diodes inside because 2 of 3 glass cases on surface mount diodes are cracked. Two test ok with a diode mode test reading of 538 and 539, but one of those has a cracked case. The third one tests with a value of 1850. None are shorted or open on a 2k-ohm resistance check. I know these aren't really meaningful values, but the big mismatch and very cracked case makes me think this diode could be blown. Then again, these readings have all been done with the parts installed.
Where I need help is I would like to replace these 2 or 3 diodes but I'm not sure what they are. The glass case makes me think Zener diodes. They have no numbers or identifiers on them even with a 20x magnifier loupe. I see only a black stripe on the end and a silver stripe in the center, which I don't see on any diode color code charts. In fact, I've seen several charts that list silver and say N/A. I've looked at some sites like Mouser but am not sure what voltage rating I should be looking for.
I found forum website from the BCAE1 website, and it has several photos of diodes exactly like this in amplifier circuits. Is this some sort of generic noise filter diode or does the info on the OP-AMP page give enough info to pick a voltage limit for replacement parts? Does anyone else have ideas on repairing this? I've tried to keep this car a budget driver while I finish my master's degree, and I'd love to not buy more than a few small components to keep the stereo working. I have another of the same amp like this, but I have little faith in it being in better condition than this one.
The suspect diodes on the back correspond to the front area with C03 T01 and T02 in the top left of the image I posted.
Thanks very much in advance!
William
Attachments
Could be zener or small signal (1N4148), really need the schematic to figure that out. The tiny MELF package either doesn't have markings or the marking is facing the PCB, so you'd have to desolder the device to look for it. 94V0 is a UL flammability rating.
I suppose if you can find a known good copy of this unit you could use it to figure out the diodes and fix this one (alternatively trace out the schematic although that's not my idea of fun!)
I suppose if you can find a known good copy of this unit you could use it to figure out the diodes and fix this one (alternatively trace out the schematic although that's not my idea of fun!)
Zeners typically have a blue band but that's not definitive. The sample photo is from an Alpine amp. You can see the difference between switching and Zener diodes.
Go to the following site and punch in every number you can find on the board or housing to see if anything similar shows up.
Go to the following site and punch in every number you can find on the board or housing to see if anything similar shows up.
Attachments
Thanks for chiming in, Perry. I don't see any website linked in your comment. Web search engines have mostly given me Yamaha parts or nothing at all. I do see the blue stripe you mean and the difference in the markings.
I think I am going to try replacing the diodes under the belief they are fast switching type. It looks like they're held to the board with some red adhesive. I wonder how to remove that and also if that helped lead to the case failure by restricting expansion on one side of the case and not the other.
I think I am going to try replacing the diodes under the belief they are fast switching type. It looks like they're held to the board with some red adhesive. I wonder how to remove that and also if that helped lead to the case failure by restricting expansion on one side of the case and not the other.
https://elektrotanya.com/?q=keres
It's a brittle epoxy.. It will break off relatively easily.
I don't think it's a problem. Alpine used it for many years. That diode should never have been passing enough current for it to heat up.
It's a brittle epoxy.. It will break off relatively easily.
I don't think it's a problem. Alpine used it for many years. That diode should never have been passing enough current for it to heat up.
Thanks everyone for the help so far.
I received the diodes and replaced them today as well as the 14-pin amplifier IC TL1074IN just for kicks. One of the cracked ones came off in pieces. All 3 now read about the same on the diode test, but I can't say this fixed all the issues. I also refreshed some solder on the small transformer on the board that showed some corrosion.
I'm not sure if the hum is gone or not yet, but the audio overall has serious distortion issues any time I turn the head up over 30% volume or so. I may disassemble the head to see what I can see wrong with it, but that will need another thread. I think the first thing I need to check is that the incoming voltage is at least 12V. It seems like it's under driving the speakers pretty badly.
I received the diodes and replaced them today as well as the 14-pin amplifier IC TL1074IN just for kicks. One of the cracked ones came off in pieces. All 3 now read about the same on the diode test, but I can't say this fixed all the issues. I also refreshed some solder on the small transformer on the board that showed some corrosion.
I'm not sure if the hum is gone or not yet, but the audio overall has serious distortion issues any time I turn the head up over 30% volume or so. I may disassemble the head to see what I can see wrong with it, but that will need another thread. I think the first thing I need to check is that the incoming voltage is at least 12V. It seems like it's under driving the speakers pretty badly.
Attachments
I'm back and with a new photo of a non-failed set of diodes.
It appears that the previously most-crispy mini-MELF device was a 4v7ph with a yellow band that turned black. This is the device that came apart as soon as the solder melted. According to smdmark.com (interesting site), it's a bzd23 series c4v7 . Some poking around shows it is a "voltage regulator diode" from Philips and is rated 4.7V and 1W.
I'm going to replace it with the correct part. Good thing I had a spare amplifier to look at!
The other two diodes do seem to match what I replaced visually.
It appears that the previously most-crispy mini-MELF device was a 4v7ph with a yellow band that turned black. This is the device that came apart as soon as the solder melted. According to smdmark.com (interesting site), it's a bzd23 series c4v7 . Some poking around shows it is a "voltage regulator diode" from Philips and is rated 4.7V and 1W.
I'm going to replace it with the correct part. Good thing I had a spare amplifier to look at!
The other two diodes do seem to match what I replaced visually.