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Post subject: 1973 Princeton Reverb squealing
Posted: Wed May 22, 2013 6:48 pm
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Hi Fellas. My buddy at work has been taking guitar lessons and his instructor gave him his PR to try to fix. I was able to get a date of '73 from the serial number and I think it's an AA1164 circuit. Oddly the power cord is grounded as well as the courtesy outlet which isn't like that on the schematic but the cord has a Fender tag still hanging off it. The inside of the head is clean and looks untouched. We vacuumed out the dust balls from the cabinet but did little more.

Here's the problem. When turned on with no input at all and the volume turned up about halfway we hear a motor boating squeal. We swapped tubes 1-4 and no difference. We have no rectifier or output tubes to try. Tomorrow my bud will bring in his guitar and cord (don't know why he didn't to begin with) and I may have more information to give you as we tinker around with it some more.

My question to you gents is what should I look for/at and be checking to start this repair?

Thanks

Arc

PS: one of the pots for the reverb seems busted since it turns and turns and turns.I forget which one. Ill get back to you on that.

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Wed May 22, 2013 6:52 pm
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Arc-n-spark wrote:
I was able to get a date of '73 from the serial number and I think it's an AA1164 circuit. Oddly the power cord is grounded as well as the courtesy outlet which isn't like that on the schematic but the cord has a Fender tag still hanging off it.


That's because the amp is likely the B1270 circuit revision......

http://web.archive.org/web/200805160944 ... _schem.pdf

Arjay

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Wed May 22, 2013 7:19 pm
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Arc, the first suspect for oscillation would be tubes. Then the filtercap can. Good Luck!! Art

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu May 23, 2013 12:46 am
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Spot on Arjay. Which tubes Art? Are you talking about the big can cap sticking out on the transformer side of the chassis? 20 mfd I think?

Thanks again fellas

Arc

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rob


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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu May 23, 2013 3:16 am
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The filter can actually contains three 20ufd caps rated for 450 VDC. Onset failure of any one or all three usually produces a 120 Hz hum that is audible regardless of the volume control's setting. If these electrolytics go titz up altogether the result can be catastrophic, turning a routine $90 cap job into a total rebuild of the power supply (including the tranny) that might cost upwards of $200. A word to the wise......

Arjay

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu May 23, 2013 4:38 am
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Sounds like you are getting coupling across the sections in the cap can. This is giving you that horrific "motorboating." Sections are positive feedbacking onto each other.

You need a new can cap (many good ones are avaiable, now). And I'd replace the resistors in the power rail, too. I like low inductance Mills wire-wound, 5 watt types for this use.


Good luck!


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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu May 23, 2013 9:06 am
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Retroverbial wrote:
The filter can actually contains three 20ufd caps rated for 450 VDC. Onset failure of any one or all three usually produces a 120 Hz hum that is audible regardless of the volume control's setting. If these electrolytics go titz up altogether the result can be catastrophic, turning a routine $90 cap job into a total rebuild of the power supply (including the tranny) that might cost upwards of $200. A word to the wise......

Arjay


There are 4 20ufd caps below the 6V6 tubes in the schematic. Is the another that's not in the can that I need to replace also?

Arc

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu May 23, 2013 10:13 am
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My bad, a typo.

There are indeed four caps within the can.

Lo siento, señor.

:oops:

Arjay

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu Jun 27, 2013 6:14 pm
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Good news fellas. My buddy at work and I fixed this amp. We bought a cap kit which included the big can, the bias cap and 6 25-25s. First we replaced the big caps and no go so we put the rest in and badda bing she worked but for some reason the reverb stopped working. We knew it was working before because even with the squealing we could tell it had reverb. We checked the cables and one was open so we fixed that but still no reverb. We looked inside and saw the cap hanging off one reverb connector had one lead touching ground. We must have bumped it when handling it. Easy peasy.

As a side note, the owner of this Princeton has been in possession of it ever since his dad bought it for him just over 30 years ago. He brought the amp to a shop that repairs amps but said they couldn't fix it. I don't know what they DO fix if they couldn't fix this one. I hope the dude is happy now and I hope my buddy gets a few free guitar lessons for our trouble... which was kinda fun for us really.

Arc

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
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rob


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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu Jun 27, 2013 6:45 pm
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Congrats!

Rawk on.

8)

Arjay

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Thu Jun 27, 2013 7:18 pm
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"Which was kinda fun for us really" That says it all!!!! Art

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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Fri Jun 28, 2013 11:54 am
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DOH!... I forgot to thank you guys. Your guidance is always appreciated.

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: 1973 Princeton Reverb squealing
Posted: Tue Jul 02, 2013 6:02 pm
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¡Por nada, Señor!

Arjay

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