Need a little help with an old amp

tbone8

tbone8

Active Member
I have this Marantz PM700 I essentially got for free a few years ago. One channel was hosed on it, put new output and driver transistors in it. R766 &Q726 was shorted out and Q806 on the power board was dead. I recapped the power board, amps and tone boards. I have desoldered and checked darn near everything in this unit over the past couple weeks, cannot find the culprit. When you power it up, it draws excessive current and Q726 gets very hot. There must be a short somewhere as the rail voltages are about 8 volts low. It is actually good from a learning standpoint that I have been messing with this but I think I am at my wits end. I linked some hi res pics and posted a low res pic. If anyone has the inclination to lend some support, it would be much appreciated. I would have done more voltage measurements but that output transistor gets so hot so fast I don't want to fry it. So, I will do any requested measurements as to limit the time its on. Thanks!


700 amp voltages 2.png

https://ibb.co/bVocwm <- recorded voltages

https://ibb.co/c7J5hR <- full schematic
 
first pull both both outputs to test them . then test all other transistors .
also test the emitter resistors .
it looks like at least one driver shorted and one output . quite possibly more damage too .
also check orientation of transistors .
dont forget to test the diodes too
 
Remove the output Xters Q726, Q728, Q722 , Q724 and report what the rail voltage is.... Then replace Q724 and Q722 ( without the output xters ) and report what the rail voltage is....

Even remove Q718 & Q720 if you have too....

Like Pete is suggesting something is shorted somewhat around there....

Never under estimate the value of a very detailed visual inspection.... Might be a solder bridge or other short....

Low rail voltage is bad....

johnk
 
Last edited:
Thanks for the test tips very much. I removed Q726, tests good. Checked the rail voltages with it out and they return to good voltages now. The dimm bulb tester shows no excessive current draw now also. Wasn't sure if you all wanted the driver transistors still removed since the rail voltages are back up to specs? The voltage at L002 is still high at -51v as compared to L001 which is .08v on the good channel. I forgot to mention in the opening post that the protection relay won't click on and the voltage at pin 6 on QN07 is still high and I swapped a new QN07 with no change. It's that -51v coming from somewhere that I assume is heating the Q726 and not allowing the protection relay to click on? I really appreciate you guys lending a hand, feels like we are 1 click closer to the problem.
 
what is now removed ? if just outputs the best way forwards is get all marked voltages that are on the schematic for the bad channel .
 
petehall347 said:
q712 may be in trouble .
Click to expand...

Q712 was originally a 2SA970 that I replaced with a KSA992FBU and Q714 was a 2SC2240 that I replaced with a KSC1845F. The originals tested good with a Peak Atlas DCA Pro but since they were on a heat sink I replaced with the subs. I pulled Q712 and it still tests good.
 
need to figure out why its gone negative . need more positive .
q704 also wacky voltages .
its never easy considering the feedback circuit .
 
I desoldered and checked every transistor on that bad channel with the Peak tester, all checked good. That's why I am completely at a loss to find the problem, everything checks good.
 
Don't take this the wrong way....

But can you change your avatar....? Makes me think you are in a panic all the time.... :)
 
You must log in or register to reply here.

Similar threads

N
R2R Teac A-6300MKII Control Board - Need Help!!
Replies
2
Views
1K
NG TRI DUY
N
B
1090 -- Need a little help
Replies
8
Views
439
Boomer543
B
Steven Tate
A little scope help
2
Replies
29
Views
624
Steven Tate
Steven Tate
trainbuftony
Burning Amp has inspired me to pick up an old project
Replies
5
Views
194
trainbuftony
trainbuftony
rooster2001
Seeking help with Marantz 2240b
Replies
2
Views
173
rooster2001
rooster2001
Back
Top Bottom