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2285 mystery

G

Gtown

Active Member
Having an issue with my Marantz 2285 bought new in 1977. Dial light went out and decided to send the unit off to be reconditioned as well as LED dial lights (everything worked fine on it except one dial bulb). Sent it to well known technician who knows his stuff around Marantz receivers. Got it back and had an issue with phono. Have a Marantz 6300 turntable. When I plug the jacks into the phono slot of the 2285 (with everything else plugged as original before I sent it off) and turn it to phono, get a loud oscillating humming noise (even with the turntable off). However, when I plug the turntable into my preamp (1300dc), it works fine. Shipped it back and he replaced the phono board and it worked fine for him (sent me a video playing a record...however, he did not have any other equipment plugged in). Nevertheless, when I got it back I had the same issue. And yes ,everything is grounded properly. Now, have spent a number of hours trouble shooting. Unplugged all RCA jacks plugged into the 2285 (Reverb, CD, Aux (Auris bluetooth), TEAC A3340S reel to reel... except for phono. Phono works fine by itself with the 2285. Slowly started adding back components...CD, Reverb, Aux...phono still works. It's only when I plug in the R2R with 4 RCA plugs (play and record in either Tape 1 or Tape 2) does the oscillation happens. Thought it might be the R2R, so I unplugged the R2R and plugged in the 5220 Cassette player, same thing...loud oscillation noise. Both the 5220 and R2R work fine on the 1300dc AS WELL as the Phono! CD works fine in either the Tape 1 or Tape 2 IN of the 2285. Therefore my non-technical conclusion is that there must be some type of interference or problem with the Tape 1 and/or Tape 2 OUT plugs with the Phono. Hope all this makes sense....any ideas or suggestions? The tech just says it is a mystery to him, so am at a loss. Thanks!
 

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Just a WAG, you say there's a hum...could it be a grounding issue between the Tape 1/2 RCA OUT and the Phono RCA plugs? I assume you're grounding the TT to the 2285?
 
eiraved said:
Just a WAG, you say there's a hum...could it be a grounding issue between the Tape 1/2 RCA OUT and the Phono RCA plugs? I assume you're grounding the TT to the 2285?
Click to expand...
Grounding is fine and familar with a grounding hum. This is a loud oscillation noise
 
Gtown said:
When I plug the jacks into the phono slot of the 2285 (with everything else plugged as original before I sent it off) and turn it to phono, get a loud oscillating humming noise (even with the turntable off).
Click to expand...
Gtown said:
Grounding is fine and familar with a grounding hum. This is a loud oscillation noise
Click to expand...
Improper grounding can cause oscillation. Since you've verified grounding is not the issue we can move on.
 
Many Marantz "rebuilt" phono preamp boards will oscillate when the input is shorted (like the 6300 does when in stop or performing auto return etc.) It typically is because the wrong transistors were used to rebuild the phono preamp. I use 2SC1815GR's and don't have the issue. Check to see if the oscillation goes away when the record is actually playing. If the issue does go away when the record is playing I would suspect the the transistors used on the phono preamp board, as this is the typical symptom of the wrong transistor substitution. The issue will not occur with most turntables, as most do not short the input to mute the signal when off or cycling.

There are many posts about this issue on the forum.
 
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AE Bob said:
Many Marantz "rebuilt" phono preamp boards will oscillate when the input is shorted (like the 6300 does when in stop or performing auto return etc.) It typically is because the wrong transistors were used to rebuild the phono preamp. I use 2SC1815GR's and don't have the issue. Check to see if the oscillation goes away when the record is actually playing. If the issue does go away when the record is playing I would suspect the the transistors used on the phono preamp board, as this is the typical symptom of the wrong transistor substitution. The issue will not occur with most turntables, as most do not short the input to mute the signal when off or cycling.

There are many posts about this issue on the forum.
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Yes...the loud oscillation does go away when the record is playing and I did discuss with the tech. He says he used the 1815Gr's when rebuilding the phono board, so we both are at a loss
 
I suspect the issue has to due with a common mode grounding issue between the tape record output RCA ground and the phono input RCA ground. One thing that may have to do with it is that when folks repair the phono preamp they "recap" the PC board. I don't replace the caps unless I find one bad, and then only it and the same one in the other channel. Folks typically use film caps to replace the low value ones, and it may be that the change in type (lower ESR or something) promotes the oscillation. I don't know how the circuit is configured (never checked), but I would look to see if the phono jack grounds are only connected to the preamp board. If they receive their grounds from some other connection at the rear panel, I would modify the circuit so they only connect to the preamp (to eliminate any common mode ground connection shared with the tape output).
 
Last edited:
AE Bob said:
I suspect the issue has to due with a common mode grounding issue between the tape record output RCA ground and the phono input RCA ground. One thing that may have to do with it is that when folks repair the phono preamp they "recap" the PC board. I don't replace the caps unless I find one bad, and then only it and the same one in the other channel. Folks typically use film caps to replace the low value ones, and it may be that the change in type (lower ESR or something) promotes the oscillation. I don't know how the circuit is configured (never checked), but I would look to see if the phono jack grounds are only connected to the preamp board. If they receive their grounds from some other connection at the rear panel, I would modify the circuit so they only connect to the preamp (to eliminate any common mode ground connection shared with the tape output).
Click to expand...
Thanks! I'll pass on to the tech to see what he thinks, but it makes sense to me!
 
I used 1815s in my 2245 and I ended up having to add two 10pf caps across the base and collector of Hx01 and Hx02. (The x is whatever the first number of your preamp is.)

There is a thread on this as mentioned above.
 
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