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Post subject: Re: Vintage Circuit?
Posted: Sun May 14, 2017 1:26 pm
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Here is the amp wares schematic/layout if you need it Mbeaubs.
http://ampwares.com/schematics/princeton_5f2a.pdf


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Post subject: Re: Vintage Circuit?
Posted: Tue May 16, 2017 9:10 am
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Mbeaubs wrote:
Wow, lots of information for a rookie like me. I will enlist a professional to assist in service, although I may get brave, buy a soldering iron, and give a go with all of your assistance! .


If you decide to tackle this yourself, PLEASE make up one of these for draining the 300+ DC volts that are hangin' around in those filter caps. I've used this one for years. Two sticks, some wire, a couple of alligator clips, a little tape and a 10K resistor. It's a good idea to mark the two alligator clips with green and red tape. Then you can proceed in the following order.

FIRST - Hook the clip that's NOT taped to the stick to a good ground (chassis, etc.)

SECOND - DO NOT USE YOUR BARE FINGERS TO ATTACH THIS CLIP! Using the spare stick to open the other clip, attach it to pin one of V1. This will drain the caps in a few minutes (in your case) and make the amp safe to work on.

BUT, and this is a big but, (no pun intended) always check the voltages with a meter prior to sticking your hands in there. If you're not comfy with this, bring it in to someone who is.

Also, remember to remove this prior to plugging the amp back in and turning it on.

UN

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Post subject: Re: Vintage Circuit?
Posted: Wed May 17, 2017 5:59 am
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This is what I use to drain the caps. A 5-watt, 7k-ohm resistor inline with the clips. With the resistor, you don't get a spark, from sudden discharge. The more resistance the slower the drain time. I've used anything from 1K-ohms to 10k-ohm bricks. :)


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Post subject: Re: Vintage Circuit?
Posted: Wed May 17, 2017 10:15 am
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Discharging is imperative prior to working inside the chassis.
I keep a ground strap installed at all times after discharging unless I'm taking live voltage/current readings to prevent caps from re-forming charges(usually near the capacitance value in volts over time).
I prefer Mueller #2 clips w/ rubber boots connected to a spaghetti type 18 gauge wire w/o any resistance for the ground strap.
I use a 3watt 470k in-line resistor for a quick discharge.
Safety first has been imprinted in my cranial circuit.


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Post subject: Re: Vintage Circuit?
Posted: Fri May 19, 2017 4:59 am
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Hey guys, after further thought I think I am going to bring it in to a local amp tech. Dammit Jim, I'm a plumber not an electrical engineer! :shock:

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Post subject: Re: Vintage Circuit?
Posted: Fri May 19, 2017 8:12 am
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Mbeaubs wrote:
Dammit Jim, I'm a plumber not an electrical engineer! :shock:


You KNOW what he's gonna tell you.

Scotty......I need more power!

:mrgreen:

Arjay

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Post subject: Re: Vintage Circuit?
Posted: Fri May 19, 2017 10:53 am
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sfceric64 wrote:
Discharging is imperative prior to working inside the chassis.
I keep a ground strap installed at all times after discharging unless I'm taking live voltage/current readings to prevent caps from re-forming charges(usually near the capacitance value in volts over time).
...


In my 40 some odd years experience, I've only observed bounceback in filter capacitors ranging in millivolts. No need to keep a ground strap installed.


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Post subject: Re: Vintage Circuit?
Posted: Fri May 19, 2017 1:46 pm
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Quote:
In my 40 some odd years experience, I've only observed bounceback in filter capacitors ranging in millivolts. No need to keep a ground strap installed.
Whatever works best for you, I've seen 14-16 volts w/in a few minutes. I prefer the ground.


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Post subject: Re: Vintage Circuit?
Posted: Fri May 19, 2017 8:26 pm
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sfceric64 wrote:
I prefer the ground.


+1

That's what the Air Force teaches in tech school and the Army in AIT.

Arjay

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Post subject: Re: Vintage Circuit?
Posted: Sat Jul 22, 2017 6:34 am
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So, I had time to pull the chassis this morning and I looked at all the wires. They all seem to be soldered solidly but some of the points seem to be thin. I did see that the circuit board itself, attached to the chassis is actually touching the chassis on one side. That seems odd to me. It would seem that there needs to be some sort of stand off between the circuit board and the chassis? I have not touched anything inside, just reviewed - I don't want to die. I'd send pics but I don't want to suffer through photo bucket. I prolly will bring in to a tech.

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Post subject: Re: Vintage Circuit?
Posted: Sat Jul 22, 2017 9:49 am
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Mbeaubs, without the original schematic or converting the amplifier to a Fender spec amplifier schematic and having little experience. I would recommend finding a tech to look it over for you.
As far as the circuit board touching the chassis, that may be ok. Most have some standoff or backing under the board. As long as it has a three prong power cord properly grounded, you should be ok. When probing around use a chopstick, plastic pen case, etc(no pencils-lead is a good conductor of electricity) unless you know there is no charge.


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Post subject: Re: Vintage Circuit?
Posted: Sat Jul 22, 2017 3:20 pm
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I may be missing something here but is the cathode bypass cap for V1 even an electrolytic in the photo?

UN


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Post subject: Re: Vintage Circuit?
Posted: Sat Jul 22, 2017 3:26 pm
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The hum you describe is likely coming from the preamp circuit. The changing volume with more hum nails it.
First thing is to try a different preamp tube.
One thing to check is that the ground on the rightmost filter cap is not connected to the ground with the rest of them. It should be grounded to the brass strip close to the volume control or input jack.

The grounds of the main caps should have a wire to where the red/yello wire is grounded. Don't just use the chassis for ground paths. The brass strip should have a wire to ground as well.

The cathode cap does appear to be a poly. It will limit bass, but is more articulate

For best tone and more power, use a genuine Fender output transformer. They are rated for 8 watts, not 5.


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Post subject: Re: Vintage Circuit?
Posted: Sun Jul 23, 2017 6:46 am
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Poked around with a stick. Also put in a 5751 in place of the 12ax7. No change in hum, no sparks, no audible changes, scratchiness, weird sounds or changes. I am still alive.

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Post subject: Re: Vintage Circuit?
Posted: Mon Jul 24, 2017 4:53 am
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Chassis is going in today to be serviced. I'll update on the issues / outcome.

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