Princeton 65: what to make of TP values

[op-amp]

I have a Princeton 65 guitar amp (designed 1999, not DSP, not reverb, is solid state [no tubes/vales]).

I get distortion in the clean channel. it should as if you are using the drive channel at 2 or 3 but the distortion is not a "good" distortion. i noticed this after not using the amp for about a year.

I am new to amp repair, I have done my best to follow the rules (on the schematic) for testing the amp. I used a fluke 115 DMM, homemade resistive load to replace the speaker (schematic was from Aiken Amp site) and a Heathkit AG-9A audio generator.

The expected 1kHz input was 969.7Hz (old heathkit AG-9A), unfortunatly this is as close as i can get to 1kHz. Also i did not attempt the Output Power test (notes in schematic by the SPKR -).

listed below are the values i got vs the values listed for the test points (TP) on the attached schematic. There is some obvious voltage differences at some TP.

This is as far as i dare fret with out advice. i hate blue smoke, i hate AC electrocution, (yet to discover DC electrocution), and as cool as it sounds i never want to experience a cap explosion.

I can read a schematic, i understand very basic circuit design, I'm not to sure on the rules for testing individual parts (what parts can be tested in circuit vs. one end disconnected).

TP | schematic value | read value (all in Volts)
---------------------------------
1  31.4VAC    31.34
2  31.4VAC    31.40
3  +41VDC     40.00
4  -41VDC     40.03
5  +16.5VDC  +16.35
6  -17VDC    -16.43
7  5mVAC       0.500
8  19mVAC      1.970 <------- where they start getting bad
9  92mVAC     10.54  <-------
10 650mVAC     2.176
11 270mVAC     0.919
12 100mVAC     0.407
13 90mVAC      0.360
14 3mVAC       0.272
15 20mVAC      0.078
16 300mVAC     1.616
17 >5mVAC      0.060
18 40mVAC      0.163
19 40mVAC      0.161
20 250mVAC     0.57
21 3VAC       11.09    <---- to speaker (SPKR +)
22 +15VDC    +14.62
23 -31VDC    -29.85
24 -15.5VDC  -14.81

Any help is appreciated, I'm doing this mostly to learn and secondly to restore my amp.

[Euthymia]

You need to turn down your test signal.

Schematic calls for 5mV; you are (according to your own measurements) feeding it 500mV.

Sometimes that can be tough to do with an older generator that puts out a minimum half a volt signal. Just pad it down with a voltage divider if you can't get the knob to go any lower.

Anyway, you won't get useful numbers until your TP7=5mVAC.

[oc disorder]

Hi opamp and welcome to the forum.
My tupence worth is that usually these amps are more likely to break down in the power amp area than the pre-amp.
Another point is it possible the distortion channel is masking the fault ?
Eg An older amp was brought to me once and I was told that the owner couldn't get a decent clean sound out of it.
Cutting a long story short half the power amp was faulty thus was only getting half the waveform at the output.
I'm just pointing this out as sometimes one can assume (I have) the wrong diagnosis which can lead to a lot of head scratching...
I would first remove and inspect the circuit board (can usually just do it leaving the transformer attached) especially the solder joints around the main output transistors and emitter resistors Q18,19 and R103 ,105.

Perhaps it is oscillating check R106 and C56.

Ahh just noticed it has a pre-amp out and a power amp in.

If you can get hold of another amplifier similar try driving an external pre-amp output into the power amp in.
This will determine if the power amp is alright.
Otherwise use a cd player or a mp3 player with a volume control to go into the power amp and see if it still sounds distorted.
You need a volume control on the external player otherwise it will be full volume as there is no master on the Pwr Amp in.

If that appears ok its possible one of the opamps is faulty.
Sometimes DC appears on the audio pins which indicates a fault.
Its best to fit a good IC socket so you only have to unsolder/solder once.
Then its just a matter of plugging in the IC.

Hope that was of some use and take care there are lethal voltages where the power connects.

[Enzo]

These are guitar amps, not laboratory equipment. ANything remotely close to 1kHz will be fine. Fender usually says somewhere on their schematics that voltages are approximate average readings and may vary 20%.

First thing I would do is check the solder under the main filter caps. Then I would verify the high voltage supplies - whatever the power amp runs on - are both ABOUT the same voltage, one positive adn one negative, and both relatively free of ripple. Then i would find the positive and negative 15 volt power supplies the preamp uses and check them similarly.

No need to get fancy, just plug your guitar into the power amp in jack. Does it sound clear and halfway strong? And as suggested above, runa cord from the preamp out jack to the inout of some o0ther amp for a liswten. How does that preamp- out sound?

[op-amp]

Opps and thanks

first thanks (Euthymia) for the obvious catch (which i didn't), once i got home i plugged in my guitar and tried out the amp to make sure i didn't cook it. I scared myself until i realized the guitar volume was at 1, lucky it still sounds just as shitty, no less, no more.

also Euthymia your right on the heathkit, i can only get a min of 14mVAC out of it. I will have to make up a voltage divider, only issue there is how good of a voltage divider would i need? would a few resistors work (by trying it out and changing the resistors until i get 5mVAC with what should be a constant load. or would i need to use a voltage divider with an op-amp with feedback to compensate for the load?

I also want to thank Enzo and oc disorder; I will try your ideas out once the the weekend rolls around or if i find time in the week.

Later this week i will attach an MP3 (once i have time) of what the amp sounds like in a few different situations, just for reference.

also just want to note that the values for TP1-6, TP23 are still valid (no input AC) but i will recheck and re-post all the values once i can come up with a voltage divider.

----------------------------------------------------
update:
I did both methods with the PWR IN, and both sounded crappy, how ever i have to say the guitar sounds exactly as the clear channel does when used (like crap) and a little easier than the other method but way not try both so, as for using a mp3 player, I cobbed up an adapter to use it and i noticed that vocals and higher (tones/freq) sounds where better/ok, on the other hand lower sound like crap and the song shadow of the day by Linkin Park proved this the best (i may use this as my mp3 sample, i'd do it now but no mic).

knowing this is there anything i can rule out? next best thing to look at? (still have to look at soder joints or oscillation (related to r106,C56).

[Euthymia]

The power supply rail stuff that Enzo suggests looks good from the numbers you already put up.

Simple resistor voltage divider should be fine for this application. You could even use a 10K pot with the high side to the signal generator output, the other side to ground, and the output to the UUT (the amp, in this case) coming from the wiper. Just turn the pot down until you get the 5mV at TP7.

So it looks like your problem, whatever it is, is downstream from the preamp.

I'll go for another obvious one here and ask if you have tried connecting the amp to a different speaker.

[op-amp]

new TP values (better results)

OK, this is an updated values list (note that 1-6, 23 are still the old values but valid)

TP | schematic value | read value (all in Volts, unless said else wise)
---------------------------------
1  31.4VAC    31.34
2  31.4VAC    31.40
3  +41VDC     40.00
4  -41VDC     40.03
5  +16.5VDC  +16.35
6  -17VDC    -16.43
7  5mVAC       5.0mVAC (actually fluctuated between 5.0 and 5.2mVAC)
8  19mVAC      16.6mVAC
9  92mVAC     85.5mVAC
10 650mVAC     0.641
11 270mVAC     245mVAC
12 100mVAC     108.4mVAC
13 90mVAC      101.3mVAC
14 3mVAC       4.4mVAC
15 20mVAC      22.6mVAC
16 300mVAC     313.2mVAC
17 >5mVAC      440mVAC (note states >5mV upto 0.5VAC)
18 40mVAC      46.5mVAC
19 40mVAC      46.2mVAC
20 250mVAC     0.248VAC (mVAC setting causes "sound" so done in VAC)
21 3VAC       3.20VAC    <---- to speaker (SPKR +)
22 +15VDC    +14.79
23 -31VDC    -29.85
24 -15.5VDC  -15.55 (but slow goes to -15.00VDC and stabilizes)

some of the numbers seem to be off a bit but not sure of the except able deviation. If anyone can make since of what i have here let me know.

and Euthymia, I thought about the speaker, i had had this amp repaired when i got it about 5 - 8 years ago, they said the speak mag was shifted (it was dmg during shipping i believe). so i have a harder time believing its the speaker. I will attempt to use another speaker if i can find one. i guess i could also test the speaker by hooking it up to my stereo if it has an 8Ohms hookup. i will try that out later this week.

any more ideas, help, what you think of number is appreciated. so far i haven't blow anything up or gotten electrocuted so hopefully this will be a successful learning experience.

[Euthymia]

Not sure I understand about the speaker. The repair people told you that the magnet shifted in shipping? If that is so, it may have finally shifted enough to rub the voice coil.

Anyway, those voltages look good to me.

Find a way to test that speaker. Hooking it up to your stereo should be fine, as long as the stereo has enough power to exercise it.

[op-amp]

problem found

well an expensive DMM and 15 dollars in resistors later i found that my old stereo (free none-the-less) is all that i really needed. Many thanks Euthymia. unfortunately it was not my first thought (actually it was yours).

after i realized the problem i inspected the speaker, i see some hot melt glue from the fender certified amp repair guy!! looks like he basically fixed the speaker and took the warranty replacement speaker for his own. kinda amazed it worked for this long and at the same time I'd like to pay him a little visit, but its been to many years since the "repair". i tried both a different speaker on the amp (sub woofer) and the amp speaker on another stereo to confirm. now i just have to find a replacement speaker...

any one know a good place to look for a replacement speaker?

I like to thanks to all the ppl here and this is one of the best forum experiences i have had. (software forums not so fun).

what i leave with:
top line DMM (fluke 115, well worth the money)
resistor block capable of testing 4 or 8 Ohm 100w systems.
new testing procedure:
1) check the pre-amp and power-amp first with guitar (easy, free)
2) check the speaker on another system (semi-easy, usually free)
3) try another speaker on your amp (best to know someone else that plays at this point)
4) follow the schematics to a T to get TP values (right input voltages, one hand in your pocket, etc... this is when i should have bought the DMM and resistors).
5) if you get to this point come back to music-electronics-forum for useful help.