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Old 3rd February 2009, 11:47 AM   #1
john65b is offline john65b  United States
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Default Adcom GTP-740 Preamp Problems

I know some of you are well versed in the Adcom Lineup, (especially Anantec??), so I thought I would start off with my question here...

I have purchased an Adcom GTP-740 Preamp/Processor. I actually thought I was getting the GTP-750 preamp that is very popular among audiophiles...anyway, I knew it had some minor issues, but the price was cheap enough that I bought it anyway thinking I could at least use parts if I couldn't get her up and running.

Anyway, all inputs including digital all work... sounds nice, but blue display is very dim and eventually the unit just "reboots". It slowly comes back and repeats. I noticed if I unplug it for a minute, she is good to go for a little while and then issues return.

Did a little googling and found this preamp had many issues. A very high return rate. One persone said he reoplaced a couple components on power supply and it was fine. Namely the rectifier, voltage regs and caps.

Anyway, before I tear into it, anyone have something to add??
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Old 4th February 2009, 02:00 AM   #2
john65b is offline john65b  United States
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I was poking around some parts of the power supply while the ground was connected to the chassis. I put the positive lead voltmeter on dc volts) and for the hell of it I touched the top of a capacitor - the metal part of the body that has the cross where they rupture - I measured +18V. I noticed there were three others near this cap that all read 5V to 18VDC. all others were zero or just below 1 volt and dropping.

The four caps show no bulging or anything. Could these four caps be bad? Why would the cap body be reading voltage?

Sorry if a lame question.
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Old 5th February 2009, 12:08 PM   #3
john65b is offline john65b  United States
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Well I replaced the four caps and no difference. Same issues.

It seems to "reboot" less often when I have the cover off, so I think it is heat related...maybe the relay on power supply? I don't think its a protection circuit, as all leds flicker and go out...

Wierd thing is I still have 5vdc - 18vdc on the new cap casings....what gives here??
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Old 9th February 2009, 12:41 PM   #4
john65b is offline john65b  United States
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The Audio Gods have smiled upon me once again.

Well, I have found if I plug the preamp in and leave it plugged in overnight in "standby mode", the unit does not reboot or have any issues...

Maybe there is a battery in the unit?

Anyway, all good now. Sounds very nice through the digital inputs.
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Old 11th July 2009, 12:59 PM   #5
john65b is offline john65b  United States
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Well, that worked for about 4 hours...put it aside and forgot about it...

Last week pulled it back on the bench. I have read that these run hot and have PS issues...

This time I replaced all four rectifiers.

Now she works perfect... four days running.

As I said earlier, I bought this thinking I was getting a GFP-750.

Ooops.
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Old 15th January 2014, 12:35 PM   #6
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Default Adcom 740 rectifier replacement

I wonder if you can post part numbers for rectifier replacement. I just acquired the unit and is showing same symptoms. If you have pictures or more detailed instructions, that will help greatly. Thanks very much.
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