Marantz 18 receiver needs help

[nickb]

Frankly Dude, I can never tell myself which drawing I'm referencing at any given moment because I seem to have to jump around to verify which part I'm working on.
But ya, its the rheostat R27. Sorry bout that.
Again turns out you were right because Q2 is bad and I need to find a replacement.

Just about any small low voltage signal si NPN transistor will work.

[bigdrums925]

Great! I'll see what I can find

[bigdrums925]

Crap, now I see with a diode meter test that CR18 is open in both directions too.
How can I find a workable replacement?
Help!

Oh wait! my meter wasn't energizing the Vf=2.
So yea, the diode CR18 is good. put in the new transistor Put the whole thing back together and the bias is holding steady!
Turned on the unit and no excessive current draw. Doing the listening test now.
All is well in the neighborhood
Thanks guys!

[nickb]

Crap, now I see with a diode meter test that CR18 is open in both directions too.
How can I find a workable replacement?
Help!

Try measuring the voltage across it by forward biasing using a 9v battery and 10K resistor in series. Your meter is expecting an ordinary diode and might be giving you misleading readings. Expect about 1.95v or so.

[tonebells]

I have two of these from a parts unit they both diode test on my fluke 117 at 1.66 and 1.69. They test 2.02 and 2.15 on my Peak DCA55. I have seen where someone used three 1N4148s as a replacement.

Tony

[olddawg]

Are you dealing with a "stacked" diode in the bias circuit. Usually the schematic symbol looks like a series of diodes with 2, 3, and even 4 junctions in a single package. Normally it's 3 junctions with a foreword voltage drop of about 2v. I've used 3 rectifier diodes in series many times to replace these. They usually attach to the heat sink and are the first thing novice techs seemed to break back in the day.

[bigdrums925]

Yes ~ 2 V. I compared the new drawing and the original. At first I didn't know what Vf meant. Looked at the original and could see there 3 stacked diodes. My regular meter didn't read them, but tried the new better one and it wasn't bad after all. Everything seems to be working fine now and test ran it medium hard for an hour. Putting it back in it's pretty wood case now. Thanks again. If this forum was on Yelp I'd give 5 stars!
Set the bias at 25mV

[DGSanders]

I am new to this forum but have been an original owner of the Marantz 18 since 1967. I recently acquired a used one in fair condition and it has similar problems to this discussion. The right channel seems to working correclty but I can't get the DC balance control to meet the minimum of +/-80mv on the left channel. In comparing the outputs of the two driver boards for the +/- 1.4 v the right channel measures +1.27 and -1.24 for the left channel it measures +0.796 and -1.78. Adjusting the DC bal control (R1016) puts it at max CW rotation and the the voltage is - 500mv. Resistor values for R1016,1013 & 1017 check OK Any ides where to start to troubleshoot this would be appreciated.