Repairing old Hifi Amplifier - (when) can I trust ebay? - Page 1
Forum section: electronics / repair

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 09, 2017, 08:08:13 am
content: Hi guys!So I'm doing my first proper repair ever. I'm repairing an old Philips FA775 amplifier, that was died a year or two ago, but up to recently I didn't know how to get started on a repair.I bit the bullet yesterday, after finding a service manual with schematic (and a somewhat good schematic for service at that! hundreds of nodes have their expected DC voltage labeled!).I think I've ruled down the problem to a driver IC (an NEC uPC1270H). It's supposed to drive the final Class AB stage, but it would seem that it's broken - half of the pins on the package output -47V DC! Obviously, this chip is no longer built, but there are a few sellers on Ebay that sell it in reasonable prices (4-5 bucks, even with shipping that ends up around 10-15 euros, which is something that I feel this amplifier could be worth - if not just for the joy of having fixed it!). However, how trustworthy is this? I'm not expecting people to give me a yes-no answer, but I was hoping you could give some pointers as to what to look out for. And even if it is not sure it's a real part, I might actually risk it anyways, just to give it a shot.Unfortunatly I don't have a proper way of verifying it didn't break the output transistors. They still measure correctly when I just probe the two diodes and such, but as all I have to work with is an old oscilloscope and a multimeter, I can't see if it isn't damaged and will die as soon as they are powered up properly.

author: ap
author_rank: Frequent Contributor
timestamp: April 09, 2017, 08:13:48 am
content: Well, what you could try, and this depends on the actual schamtics, is to isolate the circuitry behind it from the driver. You need to be sure no feedback loop or so is open, that may make it worse. If you do not find any other failed part, often the best way is to just give it a try. Not sure how valuable the broken amp would be in good condition, but maybe buy two chips, just in case it dies again, then you have one spare for that case. I always do that if the price is ok. The likelness that chip is a fake is probably low, based on assumption that it is exotoc. Also, there are some knwon good chip vendors out there also in CN.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 09, 2017, 08:33:37 am
content: Quote from: ap on April 09, 2017, 08:13:48 amCould you elaborate on the "isolate" part? I don't know how I can isolate it further. From what I can tell, the only way the amplifier section is connected to all the other stuff (input swtiching, digital control, ...) is though the audio in and ground, and a relay that is mounted on the digital board for controlling the ouput channels (1 set of speakers or 2 sets), but there is no actuall electrical connection there. I've desolderd the ouput transistor pair, to verify they werent damaged and shorting the +-46v rails to the driver. I've then done the same on the working channel to verify that what I'm seeing isn't simply due to cutting the feedback loop, but it seems like it's not the case. I will doublecheck this later before ordering.