Another 2285B, this one has problems.

Discussion in 'Marantz Audio' started by 1978_DREAMER, Jan 29, 2018.

  1. Jailtime

    Jailtime Standin' on a corner Subscriber

    Messages:
    11,378
    Location:
    Velva ND
    So that 1.08V is your DC offset, and you can't adjust that? Waaaay too high.

    Have you checked the power amp connectors to make sure they're plugged in right? Sounds silly, but back when I did an 1180DC, I managed to get the connectors offset by a pin so one of the pins wasn't getting power at all. Took me a while to figure out why my rebuilt 1180DC was only making 72 WPC. :idea:
     

     

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  2. 1978_DREAMER

    1978_DREAMER AK Subscriber Subscriber

    OK, I put the main amp back into the circuit and made sure the pins connected correctly. The right channel (which plays), DC offset floats from zero +/- 0.010V, Idle Current does not register on the meter. The left channel (doesn't play), DC offset floats worse at zero +/-0.100 but the Idle Current can be set to 30mV. After I messed with this for a while I hooked up some test speakers and gave it a music feed through the AUX terminals. The right channel plays the left is dead for both 1&2 speaker outputs. Jumping the Pre-In and Main-Outs yielded no difference no left channel. The right channel had some static. I pulled the jumper back out. I tested the filter switches, the 9KHz didn't seem to cause anything but the 15KHz forced the speaker relay to kick out with a snap and the dim bulb tester lit up. All push switches have been cleaned twice.

    Tonight the speaker relay kicks in and out about every 10 seconds, with no speaker connections and volume all the way down and no push switches engaged. When I test for sound the left channel is dead and the right channel is strictl

    This unit looks like it has been messed with by possibly more than one person.

    Its serial number has an XP in it and what I was able to find was that this is a military unit. It has a voltage selector but it also has the switched and un-switched receptacles. The input voltage is currently set at 110V and my available house voltage is about 119V. The other input options are 120V, 220V & 240V. I'm wondering if my low voltage readings can be caused by this setting. I pulled the selector and tried to set it to 120V but the pin connections were rattle loose in this position. That's probably why it's set to 110V.

    I'm going to keep exploring, there are probably multiple gremlins. I may put the original 2SA798s and trimmers back in to see if I can get back to some output. Any tips are welcome.

    Jerry
     
  3. Mike Sweeney

    Mike Sweeney AK Subscriber Subscriber

    Messages:
    269
    Location:
    Orange CA
    I just finished my 2285B which came to me after sitting outside or in barn for a long time :/ The main caps must be replaced. You say they had been replaced but did you run a leak test on them? Anything you didn't personally do is suspect and just the ESR test isn't enough on these caps. Make sure they are wired up correctly. I actually removed my OEMs then using gator clips, patched in a pair for each OEM so I could easily swap caps in and out and check my voltages. Both rails should be within 1 volt. The power supply board is crucial. I replaced the relay and the diode arrays ( Q816,817,818). I also replaced Q808 on the PS with a new TIP 32B.. the OEM power transistor was really corroded under the mounting screw and it was adding noise to the system. All electrolytic caps were replaced. Between this and the new filter caps, clear most of my issues. I had bad pots on the main amp board for the idle adjustment and DC bias. I was able to save two of the four with removing them, serious soak in DeOxit and then faderlube. Two were replaced with some perboard and new pots. I ended up replacing ALL the power transistors due to silicone migration and as it turns out, that cleared a ghosting issue on the bias. Not that it was bad, it just would not stablize on one channel.. floating about +- 1 volt when it should be zero. I also pulled all the heatsinks and redid all the goop :) All pots got cleaned using D100 DeOxit in the small squeeze bottle. I also used that on the switches which wicked into them nicely. The speaker switches were pulled and taken apart then cleaned and lubed for reassembly. The power switch was taken apart, cleaned and lubed. The 1345 transistors on the tone board + caps were replaced. There are three caps hidden on the switch board under the tone board. All caps replaced on the tuner board. All electrolytic caps on the phono/selector board were replaced.

    One place to pay attention to for crappy connections is the main amp connectors on the board. They are pressed on connectors but can stand some cleaning. Or mine did :) I also gave a squirt of the DeOxit grease that I use on PS switches. All my diodes on the amp checked good. I did have corrosion on the wirewrap speaker connections so those were cleaned and then soldered
    PS- I added a spray of DeOxit Shield to the panel pots ( vol, balance, tone etc)
    PPS :) Total current draw is about 42 watts at .5 amps at zero volume. The power transistor hit about 90 degrees at the same point. At 3/4 volume dumping into a load bank, they hit 190 degrees.
     
    Last edited: Apr 27, 2018
  4. MBuras

    MBuras Restoration is an Addiction Subscriber

    Messages:
    572
    Location:
    Champaign, IL
    Been following along. Interesting rebuild. I have two questions based on the following
    What is the perceived or actual benefit of mounting the new trimmers on perf board? Were they originally the large metal trimmers like on 2325?

    Do you have any links to write ups on silicon migration? This is a new term to me and I’d be interested in researching it further. Google wasn’t especially helpful. Thx!

    Mike
     
  5. Mike Sweeney

    Mike Sweeney AK Subscriber Subscriber

    Messages:
    269
    Location:
    Orange CA
    Yep... monster pots ;) They are about twice to three times the size of the new gen trimmer pots. The ones on the 2285B are about double this size.. .610 compared to .345. The pin spacing is also off, .350 to .197.

    You can see in the upper left one of the perfboard replacements and the OEM that was salvaged side by side.

    IMG_3445.jpg
    I've got a fair number of images and notes of my 2285B in this thread

    The silicon migration is where the thermal compound breaks down a bit the silicon moves or migrates to some other place other than being under the transistor. In my case, it went down the legs and screws to the bottom connector and covered it with just the clear silicon while the white stayed under the transistors, Also, every locking washer was bad..aka.. they stayed flat when I pulled them out. I ended up replacing the transistors AND the hardware. I had a thread about that too. Some good info from other members when we went down the rat hole of how compound really works, why you need just a thin coat, why use lock washers and so on :D Learned more than I wanted to know about mounting transistors but it's all good
     

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