Marantz 1090 protection cct PUBLIC

Sometimes the simplest thing can cause a lot of grief. Marantz 1090 stereo console amplifier protection circuit failure had distorted the audio from the pre-amp to the main amp. All fixed now.

I had been working on a friend's Marantz 1090 stereo console amp. He hadn't used it for quite some time & asked me to look at. I put some music through it & found that the volume was very low for a 45 watt x 2 amp & when I cranked it up it was full of distortion banging in my speakers. Then I put a 1KHz sine wave on it, scoped the pre-amp outputs & the peaks on 1 end were flattened. I had <50 mv AC going to the main amp which is kind of low for a pre-amp output. I did have issues with the Hi Filter switch being erratic as all signal leave the pre-amp through this. The switch was replaced which took care of that secondary problem but now back to the distortion. I had the same problem despite changing the FET & 3 transistors & it turned out the the diode to the FET's gate was not quite a diode anymore & was keeping the FET gate slightly positive keeping the protection circuit on. A new diode fixed everything here! Marantz used this scheme to ground out the pre-amp signals to the main amp if the main amp's power supplies were weak or lost. The pre-amp uses a transformer secondary, which is rectified & then regulated to +38 VDC to power the phono pre-amp as well as the 'main' pre-amp & tone components. It supplies +54 VDC (pre-regulation) to power this protection circuit on PCB PV00 mounted behind the rear panel with all of the RCA jacks on it. The main amp power is bi-polar (~ +/- 45 VDC) & a wire from 1 end of the center tapped secondary for this supply provides ~ 90 VAC peak to peak ( 33 VAC rms) to RV09 (220K) to keep this circuit 'happy'. When I ordered replacement semiconductors for this, I bread boarded the circuit to see how it worked & how it failed. I found that when the AC signal fell <60 volts p-p QV02 turned off allowing current limited voltage via the zener QV07 to turn QV03 & QV04 on which allowed continuity between both collectors to ground. In the faulted amplifier's case those 2 transistors had a very low impedance to ground (not a dead short) which allowed a very small amount (<50 mv) of audio but with distorted peaks. Diodes listed in this schematic are Japanese W06B & the zener is WZ-052. The failed diode was replaced with a 1N4005 & if needed the zener could be replaced with a 5.1 volt/ 1 W North American version. The FET & other transistors were replaced with the same component numbers through Ebay as my 3 North American suppliers listed them as 'obsolete' & therefore unavailable. NOTE: if you have the almost 30 page service manual, be aware that there are a few connector mis-labellings & wires landed in wrong places on the main amplifier schematic.