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Post subject: 58 Harvard over voltage
Posted: Mon May 30, 2011 7:40 am
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I have decided to keep this amp,pics posted awhile ago. I replaced the selenium rectifier but while doing so I tested voltages based on the schematic almost anyone I check is over by far more than the 20% listed on the schematic. What would cause this? Ithink I am quite capable of performing any work myself I just need troubleshooting help. I dont want to wholesale just replace parts.


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Post subject: Re: 58 Harvard over voltage
Posted: Mon May 30, 2011 9:25 am
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Joined: Wed Mar 31, 2010 12:48 am
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Your DC voltage readings measure high because wall-socket currents of today runs in excess of 120VAC -- typically higher than 125VAC. The Harvard's comfort zone (in stock trim) is 110-112 volts. There are some minor P/S mods that can easily be performed on these old tone monsters to drop the input voltage to more comfortable (and safe) levels. A google search to find them will be productive.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 58 Harvard over voltage
Posted: Mon May 30, 2011 10:04 am
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Did you use a UF4004 diode to replace the selenium rectifier? Or a UF4007 (or equivalent). You only need the UF4004. Or the PIV will be too large. Even a UF4003 might do fine. We are only talking 100 volts max, plus a little leeway for reliability.

http://www.datasheetcatalog.org/datashe ... UF4004.pdf

Check the schematic for the Princeton (6G2 circuit) using a similar bias power supply setup. My guess is you'll prolly be around 68k-ohm for that dropping resistor to get -26 VDC.

I may have mislead you in some of the emails, but what you need to do is play with the dropping resistor that goes from the PT to the diode. Right after that electrolytic cap in the bias power supply (note polarity), you should be getting -26 VDC. Which is what you want.

http://www.ampwares.com/schematics/princeton_6g2.pdf

Be sure that diode isn't too big for the job. And follow the Princeton 6G2 circuit. You should get -35VDC. Then start subbing resistors until you hit -26VDC. Or you can sub in a 250k-ohm linear pot for the 100k-ohm dropping resistor and have an adjustable bias pot.

I'd choose 250k-ohm, so that the ideal setting would be around half-way on the dial.

Good luck. Sorry for the mix-up.


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Post subject: Re: 58 Harvard over voltage
Posted: Mon May 30, 2011 10:53 am
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I used a UF 4007 ,it is not just the negative voltage that concerns me where the schematic says +302 I have +406 wher it says I should have +250 I have 300.

Thanks in advance for what I am sure will be more questions.


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