Hi everyone. I wanted to post on this forum as it will get "out there" on the web, and this is probably the best forum of the lot! This is my first post (I've been lurking with an account for a while), so thanks for having me.
I've had an old Hitachi HA-7700 up in the attic on the "fix or dump?" pile and finally got around to looking at it for a weekend project. The symptoms were originally that the protection relay wouldn't pull over at switch-on. When it occurred about 8 years ago, I did a quick measure of the MOSFET voltages, and one channel was clearly broken - with the sources sitting at something like -20V.
When I did the same test yesterday, both channels were displaying odd, but different, MOSFET voltages. I trust my younger self enough to believe that in those 8 years in the attic, the other channel seems to have gone wrong too.
A look online and I noted that a number of German forums mentioned that the "fuse resistors" aged badly - so I tested them. "Aging badly" is something of an understatement! All but two in the Main Amp section had gone open-circuit and the other two had gone high in value. So I had to discharge the reservoir capacitors, disconnect and extract the whole Main Amp section. (After carefully photographing and documenting the connections.) This wasn't too difficult to do. I had to "unwrap" the wrapped wire connections, then remove a few strategic screws and the whole Main Amp unit could be taken out of the case, along with the heatsink. Desoldered the MOSFETS carefully and the Main Amp PCB could then come away to allow me access to the underside of the PCB.
The MOSFETS all checked fine. Phew! I replaced all the "fuse resistors" with 0.25W metal film units - checking beforehand that none were carrying heavy current. With the PCB on the bench, all the electrolytics on the board were also replaced. I checked all the semiconductors in-circuit and they seemed fine. (There is only one IC in the unit, see later...)
Weirdly, one of the fuse resistors was actually incorrectly wired, which still has me beat - it connected to a pad which went nowhere, which would have impaired the function of the protection circuitry, but clearly it must have still worked "well enough" to have passed original Quality Control - I put my replacement in the correct holes. Then I remounted the PCB on the heatsink, resoldered the MOSFETS, remounted the whole Main Amp block in the case and reconnected all the wires to the correct tags, soldering them this time, as I don't do wire-wrap.
Before applying power, I then took another look at the schematic/circuit diagram and started to wonder about the Regulator/Protection IC board. Since I'd already gone to a fair bit of trouble, I decided to give that a once-over, in case there were any poorly-functioning electrolytics there. A few more hours were spent replacing all the electrolytics on that board and checking the semiconductors (all checked out OK). Obviously the one thing I couldn't check was the IC - an HA12002 8-pin SIL unit, almost certainly now unobtainable, or at a high price. "Well, let's hope..." Given that nothing else on the board seemed to be malfunctioning, my efforts were simply being cautious about old electrolytics.
So came the big power-on. I removed the links to the pre-amps, put a voltmeter on the MOSFET sources (which are the output of the amp) - held my breath and switched on. No bang! Then, after a few seconds I heard the delightful sound of the protection relay, which was the first time it had pulled in at least 8 years. The source voltages were each about 100mV off zero, which is about right. I didn't bother checking the idle current, as I hadn't replaced any semiconductors and the resistors I had replaced were all the same as new.
Next I checked the Amp on load, it works fine.
I have decided now that I have a working Main Amp again, I am going to service the pre-amp (including doing the requisite electrolytic replacements). Given the high quality nature of the unit, I think it will be worth the time.
Hope that's a help to someone in the future with problems similar to mine - bottom line, replace all those "fuse resistors" and be very careful that you document your dismantling and be very careful to preserve circuit integrity. I'm experienced in electronics build and repair, and it was still quite a big job. But worthwhile...
Paul Roberts (UK)
I've had an old Hitachi HA-7700 up in the attic on the "fix or dump?" pile and finally got around to looking at it for a weekend project. The symptoms were originally that the protection relay wouldn't pull over at switch-on. When it occurred about 8 years ago, I did a quick measure of the MOSFET voltages, and one channel was clearly broken - with the sources sitting at something like -20V.
When I did the same test yesterday, both channels were displaying odd, but different, MOSFET voltages. I trust my younger self enough to believe that in those 8 years in the attic, the other channel seems to have gone wrong too.
A look online and I noted that a number of German forums mentioned that the "fuse resistors" aged badly - so I tested them. "Aging badly" is something of an understatement! All but two in the Main Amp section had gone open-circuit and the other two had gone high in value. So I had to discharge the reservoir capacitors, disconnect and extract the whole Main Amp section. (After carefully photographing and documenting the connections.) This wasn't too difficult to do. I had to "unwrap" the wrapped wire connections, then remove a few strategic screws and the whole Main Amp unit could be taken out of the case, along with the heatsink. Desoldered the MOSFETS carefully and the Main Amp PCB could then come away to allow me access to the underside of the PCB.
The MOSFETS all checked fine. Phew! I replaced all the "fuse resistors" with 0.25W metal film units - checking beforehand that none were carrying heavy current. With the PCB on the bench, all the electrolytics on the board were also replaced. I checked all the semiconductors in-circuit and they seemed fine. (There is only one IC in the unit, see later...)
Weirdly, one of the fuse resistors was actually incorrectly wired, which still has me beat - it connected to a pad which went nowhere, which would have impaired the function of the protection circuitry, but clearly it must have still worked "well enough" to have passed original Quality Control - I put my replacement in the correct holes. Then I remounted the PCB on the heatsink, resoldered the MOSFETS, remounted the whole Main Amp block in the case and reconnected all the wires to the correct tags, soldering them this time, as I don't do wire-wrap.
Before applying power, I then took another look at the schematic/circuit diagram and started to wonder about the Regulator/Protection IC board. Since I'd already gone to a fair bit of trouble, I decided to give that a once-over, in case there were any poorly-functioning electrolytics there. A few more hours were spent replacing all the electrolytics on that board and checking the semiconductors (all checked out OK). Obviously the one thing I couldn't check was the IC - an HA12002 8-pin SIL unit, almost certainly now unobtainable, or at a high price. "Well, let's hope..." Given that nothing else on the board seemed to be malfunctioning, my efforts were simply being cautious about old electrolytics.
So came the big power-on. I removed the links to the pre-amps, put a voltmeter on the MOSFET sources (which are the output of the amp) - held my breath and switched on. No bang! Then, after a few seconds I heard the delightful sound of the protection relay, which was the first time it had pulled in at least 8 years. The source voltages were each about 100mV off zero, which is about right. I didn't bother checking the idle current, as I hadn't replaced any semiconductors and the resistors I had replaced were all the same as new.
Next I checked the Amp on load, it works fine.
I have decided now that I have a working Main Amp again, I am going to service the pre-amp (including doing the requisite electrolytic replacements). Given the high quality nature of the unit, I think it will be worth the time.
Hope that's a help to someone in the future with problems similar to mine - bottom line, replace all those "fuse resistors" and be very careful that you document your dismantling and be very careful to preserve circuit integrity. I'm experienced in electronics build and repair, and it was still quite a big job. But worthwhile...
Paul Roberts (UK)
Good job! Success is always nice. Might be nice to check the idle current anyways, just in case things have aged (you kept the same semis and the resistors you didn't replace are carbon film, right?) or pots got "bumped" during service ... 👍
Nice work and the steps are practically same for any troubleshooting. May be a good idea to replace all Old electrolytic capacitors and it will be reliable Amp.