Help with Marantz 2325 Amp Rebuild

[Ray122]

Hey everyone. Been grappling with the amplifier section of amplifier section of Marantz Model 2325. When I received it, the left channel amplifier was keeping the unit in protection mode. When I disconnected that section and just ran it with the right channel amp, the unit comes out of protection mode and works fine.

Initially looking at the left channel amp I think someone attempted to replace the output transistors, but did it with some parts that didn't look correct, and I also spotted R742 had cracked and broken. After replacing the output transistors with MJ21196Gs and 1195Gs, along with that resistors I had intermittent results, working fine one minute, then lighting up the DBT the next. Clearly the outputs weren't the only problem at large, so to play it safe, I replaced all of the semiconductors in the amp - unfortunately that hasn't seemed to solve the problem either.

Currently Im checking everything with the power transistors out, and what I'm finding is that with all of the new semiconductors I've put in, and while the unit is on the DBT, I'm getting 40V from the output of the amp. Basically +VCC which I can trace all the way back to C704. Otherwise, the voltages everywhere else actually seem to be in line (low because I'm on the DBT, but not above, and in line with what I'd expect.) I believe the issue lies around H704/705 or H706/707. The biggest clue I have is that when I removes H706 and 707, I have 18V on the collectors of H704 and H705, which should only be around 1V. What I can't seem to figure out is why that would be the case when everything else is checking okay.

Anyone have any thoughts on what components I should check next? I'm at a little bit of a loss.

Heres my list of components I've replaced.
H701/702: 2SA722 > KSA992
H703: 2SA722 > KSA992
H704: 2SA720 > KSA1013
H705: 2SC1318 > KSC2690
H706: 2SA818 > KSA1318 (Crossed the collector and base legs to match the pinout of the original)
H707: 2SC1628 > KSC3503 (Crossed the collector and base legs to match the pinout of the original)
H708: 2SC1669 > MJE15032
H709: 2SA839 > MJE15033
H710/713: 2SC945 > KSC945 (Crossed the collector and base legs to match the pinout of the original)
H711/712: 2SA733 > KSA 992
H714/715: 2SC5960 > MJE2436
H716/717: BZ-210 > 1N4747A
H718/H719: VD1122 > 1N4148
H720/H721: 1S2471 > 1N4148

The past couple nights I've tried measuring resistance to ground on all the transistors, and although I'm finding some discrepancies between the good channel and the bad channel on the collector of H706, base of H708, emitter of H712, collector/base of H714 and collector of H715 - however I've checked everything around that area and still can't seem to find a smoking gun.

(I Originally posted this in the DIY sub section - but was advised to move it over
here https://audiokarma.org/forums/index.php?threads/help-with-marantz-2325-amp-rebuild.1029827/)

 

[jmhanna]

Hi Ray! It's been a while now, but I wonder if you ever got anywhere with this problem? I'm starting a battle with a similar issue here, just curious...

 

[Ray122]

Hi Ray! It's been a while now, but I wonder if you ever got anywhere with this problem? I'm starting a battle with a similar issue here, just curious...

Hi! Yes, after letting this one sit for a while... probably almost a year, I did discover pretty quickly that one of the leads on H701 or H702 (I don't recall which one) had a bad solder joint. After reflowing that one connection, everything worked great.

Basically it was an issue with the differential pair at the input stage. I've found a few times since this project that if all your transistors upstream are good, but for some reason the amp wants to output the supply voltage, theres something wrong with the input stage.

 

[jmhanna]

Hi! Yes, after letting this one sit for a while... probably almost a year, I did discover pretty quickly that one of the leads on H701 or H702 (I don't recall which one) had a bad solder joint. After reflowing that one connection, everything worked great.

Basically it was an issue with the differential pair at the input stage. I've found a few times since this project that if all your transistors upstream are good, but for some reason the amp wants to output the supply voltage, theres something wrong with the input stage.

Thanks! I'll have to spend some time on the foil side of the "bad" P700 board and look closely at the joints. I may just get the full rebuild kits from peace.love.music on the 'bay. A good way to get rid of potentially problematic transistors and reflow at the same time. Glad you sorted yours out!

 

[dankik]

I would suggest fixing the problem first, BEFORE you shotgun replace all caps and transistors. Compare measured voltages between your good channel and bad channel and see where they differ. That should help isolate the problem. Use your kit for the parts that you need until you find the problem, get it working, then upgrade the other caps and transistors testing along the way.

 

[ocdecio]

I fully agree with @dankik especially when it comes to the pre-drivers and drivers which are extremely hard to find and subs are not as good. They can work but if originals are OK I'd rather keep them.