@Nelson Pass
I own a beautifully kept Adcom GFP-750 (blue PCB) that is the backbone of my stereo system and has been for many years. It's developed a problem lately and I'm not sure what to do about it. When I use the remote to adjust the volume up or down, sometimes it "sticks" and the motor runs the volume ALL the way up or ALL the way down, and it keeps going in that direction until I physically stand up and go stop the knob from trying to go in whatever direction it's going.
Any ideas on what part(s) need to be replaced or repaired to stop this? Everything else works great.
Thanks!
I own a beautifully kept Adcom GFP-750 (blue PCB) that is the backbone of my stereo system and has been for many years. It's developed a problem lately and I'm not sure what to do about it. When I use the remote to adjust the volume up or down, sometimes it "sticks" and the motor runs the volume ALL the way up or ALL the way down, and it keeps going in that direction until I physically stand up and go stop the knob from trying to go in whatever direction it's going.
Any ideas on what part(s) need to be replaced or repaired to stop this? Everything else works great.
Thanks!
did you tried shading remote (hand or any other barrier), to determine is remote sticking or it's something inside?
agree with Zen Mod; definitely should investigate the possibility of a button sticking on the remote
Thanks for the replies guys. I know it’s not the remote because I ordered a brand new replacement remote and it does the same thing.
There's an 87C750 microcontroller in there... that sounds to me like a digital glitch, possibly bad caps in the 5V power supply, or the bypass caps near the micro?
@Nelson Pass
Hi Everyone,
I have some frustrating updates, and am hoping that the community or the man himself (Nelson) can bail me out here. My tech has scoped the chip output, after replacing the inverter chip and BJT's in that circuit. The output from pins 7 & 8 of the microprocessor are erratic when receiving remote signals. So everything works, except the volume control. Input selection, balance control, etc all work perfectly.
There are a few problems preventing me from fixing this.
Paul at EBC Electronics used to stock and sell these pre-programmed microprocessors, but I spoke with him and they are completely out:
https://www.adcomparts.com/IC P87C750EBPN Microcontroller-item-21008775.html
If you google the part number, it looks like they are still made by other manufacturers (Phillips for example), but they're blanks, it would need to be programmed.
https://www.digipart.com/part/P87C750EBPN
Which leads to yet another problem, the chip itself is encrypted, even if I had the ability to read/write to the blank, I don't have the software that Adcom used.
I'm really hoping that someone in the community can bail me out, because right now my whole pre is basically dead in the water, because if I hit the up volume button, I don't know if it's going to get stuck in that mode and run the volume all the way to max until I jump over my coffee table and power the unit down.
Hi Everyone,
I have some frustrating updates, and am hoping that the community or the man himself (Nelson) can bail me out here. My tech has scoped the chip output, after replacing the inverter chip and BJT's in that circuit. The output from pins 7 & 8 of the microprocessor are erratic when receiving remote signals. So everything works, except the volume control. Input selection, balance control, etc all work perfectly.
There are a few problems preventing me from fixing this.
Paul at EBC Electronics used to stock and sell these pre-programmed microprocessors, but I spoke with him and they are completely out:
https://www.adcomparts.com/IC P87C750EBPN Microcontroller-item-21008775.html
If you google the part number, it looks like they are still made by other manufacturers (Phillips for example), but they're blanks, it would need to be programmed.
https://www.digipart.com/part/P87C750EBPN
Which leads to yet another problem, the chip itself is encrypted, even if I had the ability to read/write to the blank, I don't have the software that Adcom used.
I'm really hoping that someone in the community can bail me out, because right now my whole pre is basically dead in the water, because if I hit the up volume button, I don't know if it's going to get stuck in that mode and run the volume all the way to max until I jump over my coffee table and power the unit down.
So motorized balance does not misbehave but motorized volume does?
Assuming I've found the right schematic... Consider maybe the volume drive motor is wearing and has become electrically noisy. Looking at the schematic I see almost no attempt to suppress motor noise. Might want to add some and see what happens - it's easy enough to try. Good luck getting it fixed.
And correct me if I'm wrong but wasn't it Walt Jung that was involved with the earliest Adcom products?
Assuming I've found the right schematic... Consider maybe the volume drive motor is wearing and has become electrically noisy. Looking at the schematic I see almost no attempt to suppress motor noise. Might want to add some and see what happens - it's easy enough to try. Good luck getting it fixed.
And correct me if I'm wrong but wasn't it Walt Jung that was involved with the earliest Adcom products?
Thanks for the feedback, I’ll see what my tech says.
Jung worked on later Adcom designs, the GFP-750 is a Nelson Pass design and very similar to the Pass Labs Aleph P.
https://www.stereophile.com/solidpreamps/133/index.html
Jung worked on later Adcom designs, the GFP-750 is a Nelson Pass design and very similar to the Pass Labs Aleph P.
https://www.stereophile.com/solidpreamps/133/index.html
I had the same problem with my Adcom GTP-500II. Likely an Adcom-wide problem with the gear of that vintage and not specific to the GTP-750.
I wrote the initial software for the GFP-750, Adcom made some hardware and software changes before production.
I find it difficult to believe the microcontroller has failed they rarely go bad especially if most of it is working.
When the volume is stuck what are the pins 15 and 16 doing on the 87C750 doing?
I find it difficult to believe the microcontroller has failed they rarely go bad especially if most of it is working.
When the volume is stuck what are the pins 15 and 16 doing on the 87C750 doing?
Hi Wayne,
Nice to be speaking with you again, you helped me repair my Pass Labs x5!
Regarding the GFP.
Pins 15&16 for the balance motor worked correctly, flipping between 5v and 0 quickly and as commanded by the remote. When we repeated the same test with pins 7&8 for the volume control, the signal would drop to 0 when the remote was pressed and despite letting off the remote, the output of the pin remained at 0 for a varying amount of time. Sometimes it would work as designed, other times remaining at 0v for up to 5-10 seconds. It's worth noting that while this was happening, the unit becomes unresponsive to all other remote commands, i.e. you couldn't mute, vol up/down or even power off when it was running away.
Nice to be speaking with you again, you helped me repair my Pass Labs x5!
Regarding the GFP.
Pins 15&16 for the balance motor worked correctly, flipping between 5v and 0 quickly and as commanded by the remote. When we repeated the same test with pins 7&8 for the volume control, the signal would drop to 0 when the remote was pressed and despite letting off the remote, the output of the pin remained at 0 for a varying amount of time. Sometimes it would work as designed, other times remaining at 0v for up to 5-10 seconds. It's worth noting that while this was happening, the unit becomes unresponsive to all other remote commands, i.e. you couldn't mute, vol up/down or even power off when it was running away.
I found some software for this unit that may work. It looks like the correct pins on the micro and I think there are some 87C750 stashed away.
Is the micro in a socket?
Is the micro in a socket?
Wow! Awesome!
I love mine, particularly after re-capping, replacing some MOSFETs that had drifted over the years, and a few other minor updates. Until last week, it was used in my system powering our SLOB speakers (thanks to Nelson), and is now in one of our systems in the living room.
Was this preamp basically designed by both of you, Nelson and Wayne? I've never known the history of the design, and have read that Nelson designed it, then was told that Wayne designed it. Sounds like you both did, each working on certain aspects.
I love mine, particularly after re-capping, replacing some MOSFETs that had drifted over the years, and a few other minor updates. Until last week, it was used in my system powering our SLOB speakers (thanks to Nelson), and is now in one of our systems in the living room.
Was this preamp basically designed by both of you, Nelson and Wayne? I've never known the history of the design, and have read that Nelson designed it, then was told that Wayne designed it. Sounds like you both did, each working on certain aspects.
Sounds like a collaboration, but they explain the products genesis in the original Stereophile review from 1999:
https://www.stereophile.com/solidpreamps/133/index.html
Stereophile Class A, for what it’s worth. Mine is also recapped and upgraded in various ways, hence my great interest in keeping it healthy.
From the review:
“The preamplifier that most reminded me of the GFP-750 was the Mark Levinson No.380S, which costs $6495. The two had similarly open, grainless characters. In direct comparisons I had an extremely difficult time discerning differences between them—and that was a sighted comparison! Blindfold me and ask me to identify which one was playing and I'd probably have to flip a coin.”
This has been my experience. I haven’t found anything worth replacing it, ever.
https://www.stereophile.com/solidpreamps/133/index.html
Stereophile Class A, for what it’s worth. Mine is also recapped and upgraded in various ways, hence my great interest in keeping it healthy.
From the review:
“The preamplifier that most reminded me of the GFP-750 was the Mark Levinson No.380S, which costs $6495. The two had similarly open, grainless characters. In direct comparisons I had an extremely difficult time discerning differences between them—and that was a sighted comparison! Blindfold me and ask me to identify which one was playing and I'd probably have to flip a coin.”
This has been my experience. I haven’t found anything worth replacing it, ever.