Crate FXT120 Test Point Questions From a Newbie

[Crawl0816]

So I'm fairly new to electronics repair all together. I own a machine shop and had quite a few power supplies in my CNC machines go out that cost an arm and a leg to replace, so I figured I would start watching "Learn Electronics" videos and figure out how to repair them to save some money... Now here I am, full on immersed in the electronics hobby and trying to fix amps for some friends of mine. I am very open to constructive criticism and love to learn, so if there's something obviously ignorant that I'm doing, feel free to call me out!

On to my question. I am on my 4th Crate amp repair for a friend of mine, and all of the repairs have been replacing components that kept the amp from switching channels, replacing some of the 5 watt resistors because the legs broke, etc... But once the amp is operational, I have noticed that every one of them that I have fixed has some weird stuff going on at the test points. The schematics give a detailed set of conditions for the test signal input and a detailed list of readings that you should be getting at the test points. But when I set everything up, some of the test points will be correct and some of them will be off by half value or more, but the amp still plays and sounds good. Should I be chasing components around the Op-Amps to try and get the test point values reading correctly? Is this just something that happens with amps? Is my setup somehow incorrect, causing faulty readings?

I'm also confused by some of the values being incorrect, but the next value down the line being correct. For example, on the amp that I'm currently working on, in condition C (clean channel) TP1 reads very low, but TP2 is very close to what its supposed to be. TP1 feeds the input of the op-amp that TP2 is on... so how can TP2 be correct if its incoming signal is low?

The amp is plugged into an Isolation Transformer. Speaker jack is hooked to a 4 ohm resistor. I have a signal generator feeding the required signal from the "Test Point Conditions" listed on the schematic into the Input Jack. I have my oscilloscope lead grounded to chassis ground and I am probing the Test Points on the legs of the Op-Amp IC on the board. +/-16 V for Pins 4 and 8 of op-amps checks good. Here's a list of the test point measurements that I took. Compare to attached schematic. All waveforms looked correct, values were just off.

CONDITION A - Voltage at speaker jack during test was 15 Vrms
TP1 = 200 mVpp
TP2 = 1.8 Vpp
TP3 = 6.24 Vpp
TP4 = 4.88 Vpp
TP5 = 10.8 Vpp
TP6 = FLAT LINE
TP7 = 3.3 Vpp
TP8 = 1.62 Vpp

CONDITION B - Voltage at speaker jack during test was 20 Vrms
TP1 = 220 mVpp
TP2 = 5.9 Vpp
TP3 = 7.4 Vpp
TP4 = 5.08 Vpp
TP5 = 13.8 Vpp
TP6 = FLAT LINE
TP7 = 7.6 Vpp
TP8 = 3.76 Vpp

CONDITION C - Voltage at speaker jack during test was 8.6 Vrms
TP1 = 520 mVpp
TP2 = 5.12 Vpp
TP3 = 216 mVpp
TP4 = FLAT LINE
TP5 = FLAT LINE
TP6 = 2.4 Vpp
TP7 = 2.06 Vpp
TP8 = 1.0 Vpp

POWER AMP
TP9 = 1.0 Vac / 0 Vdc
TP10 = .65 Vac / .6 Vdc
TP11 = 28.8 Vac / 1.25 Vdc
TP12 = 28.8 Vac / -.588 Vdc
TP13 = 26.6 Vac / .028 Vdc
TP14 = 1.68 Vpp ripple / 32.7 Vdc
TP15 = 1.68 Vpp ripple / -32.7 Vdc
TP16 = .8 Vpp ripple / 10.06 Vdc

If you were working on this amp, would these readings lead you to dig farther into it, or would you send it out as complete since it plays, operates and sounds good? This is just a hobby for me, and my friend isn't relying on these amps for live shows and stuff, but I still don't want to be sending out junk because I don't know what to do with test point readings.

[Helmholtz]

What were your input signal levels?
Should be 40mVrms (= 113mVpp) for conditions A and B and 130mVrms (= 368mVpp) for condition C.

Did you adjust the controls as advised?

[Crawl0816]

Helmholtz,

You may have just solved my issues! I had my input signal levels set at Vpp, not Vrms...

I guess I just assumed that it would be peak to peak instead of RMS. And since 40 is close to 1/3 of 113, it makes sense that TP1 is measuring about 1/3 of what it should measure.

When reading schematics, is it standard to assume that voltages given without an RMS or PP specification will be RMS?

And I did adjust the controls as advised. Gain @ 10, High & Low @ 5, etc...

I will redo the test when I get home this evening and see if that was the issue. Thank you for the quick response!

[Enzo]

Aha, you seem to have two threads going.

[Crawl0816]

Enzo,

Yeah, when I hit post on the thread my internet browser shut down. When I opened it back up, it was still on the post creation page, so I hit post again and it doubled up on me. I poked around a good bit and couldn't find out how to delete a post.

In response to your reply on the other thread, I think Helmholtz's answer is spot on. I was putting in a peak to peak valued signal and it should have been an RMS value.

Thanks for your response!

[Helmholtz]

When reading schematics, is it standard to assume that voltages given without an RMS or PP specification will be RMS?

Yes, if not otherwise noted, AC voltages are given as RMS.
To check, I divided the TP1 voltages by the gain of the input stage of 4.

Test point voltages are useful to find the voltage gain of an amp stage.