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RCA MI-9257C

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Posted on October 2, 2009 at 18:05:39
dynacopas
Audiophile

Posts: 45
Location: Central AL
Joined: September 26, 2009



Hi got an amp that I don't seem to be able to troubleshoot. It is an RCA MI-9257C. It blows fuses repeatedly. I checked the output transformer and it has resistance. Any ideas where to start. I am new to tubes and still getting the hang of it. I changed out two of the caps that looked to be in bad shape. I am thinking the PS caps need replaced before going further but I want to check the amp out to make sure its not junk.
Allen 75's,Eico HF60's Dyanco PAS3x, Orcale Delphi Mark 2, RCA MI-9358A, MI-9257C
RE: RCA MI-9257C, posted on November 3, 2009 at 07:08:42
dynacopas
Audiophile

Posts: 45
Location: Central AL
Joined: September 26, 2009
 Well just an update took things a lil further and did some further testing. I checked more caps with my CAP tester. I have a cap that is marginal reads like 28uF and its a 40uF cap. I have voltage in the other output tube now but its on the high side. 364 and 395 at the plates. Now there only suppose to have 310 but the tap on the tranny is only 110 and my wall voltage is beyond that. Is this higher plate still acceptible. The tubes are okay I know but what about everything else. Curious to hear ideas. The link is a link to the schematic. Mine is a 9257C so the only place the voltage regulator appeares is in the wiring diagram on the last page.
Allen 75's,Eico HF60's Dyanco PAS3x, Orcale Delphi Mark 2, RCA MI-9358A, MI-9257C
RE: RCA MI-9257C, posted on October 2, 2009 at 19:01:15
Michael Samra
Dealer

Posts: 13111
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since: June 24, 2005
 Lets start by taking all the tubes out and plugging it back in and see if it still blows fuses. If it does,you may have a bad power transformer but usually thats unlikely as it would have to have been under water the way these amps are built.Get back to us.

If the power supply waveform isn't pretty,neither is the sound in most cases.
RE: RCA MI-9257C, posted on November 2, 2009 at 20:20:06
dynacopas
Audiophile

Posts: 45
Location: Central AL
Joined: September 26, 2009
 Hi guys just an update. It was a bad PS cap (well a couple). I did replace them and powered back up. I do have sound but very faint and noisy. Not sure where to go from here as my voltages are all over the place. The 991 tubes that are part of the voltage regulator one seems to be bad. They look like regular light bulbs BUT there posts inside the chassis have 330 volts. I don't know if this would cause the issue with voltages being erratic. I have 364 volts at the first output tube but no plate at the 2nd one. The heater are lit. My guess would be to disconnec the voltage regulator as those 991 tubes seem to be tough to come by. Any ideas or HELP appreciated.
Allen 75's,Eico HF60's Dyanco PAS3x, Orcale Delphi Mark 2, RCA MI-9358A, MI-9257C
RE: RCA MI-9257C, posted on October 2, 2009 at 19:47:29
dynacopas
Audiophile

Posts: 45
Location: Central AL
Joined: September 26, 2009
 Hi got all the tubes out and it doesn't blow the fuse. Put the tubes in with the exception of the rectifier and the filaments are lit. Put another rectifier tube in and it arc'd and sparked in the tube. Is that a Power supply cap gone bad? The output tranny has resistance. This is a monitor amp and I don't have specs but I would think this amp might be better sounding then the other as it has no input transformer.
Allen 75's,Eico HF60's Dyanco PAS3x, Orcale Delphi Mark 2, RCA MI-9358A, MI-9257C
RE: RCA MI-9257C, posted on October 3, 2009 at 19:40:31
Michael Samra
Dealer

Posts: 13111
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since: June 24, 2005
 Thats a power supply cap gone bad,
If the power supply waveform isn't pretty,neither is the sound in most cases.
RE: RCA MI-9257C, posted on October 4, 2009 at 06:13:32
dynacopas
Audiophile

Posts: 45
Location: Central AL
Joined: September 26, 2009
 It will undergo a recap of the power supply caps as I know there is hope for it now. The 9358C seems to be okay as I just need more input voltage so I have a preamp on the way that has more output voltage.
Allen 75's,Eico HF60's Dyanco PAS3x, Orcale Delphi Mark 2, RCA MI-9358A, MI-9257C

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