Amp makes high picth sound but not feedback

[52 Bill]

So I hooked it up the to the light bulb limiter and it was weird it keeped going bright then dim then bright so I was checking voltage on the 6 + and -rails seem like I was getting the - side but the + side was reading very low and seem like my reading were up and down

What are the 6 + and - rails?

All of the voltages will be lower than expected with the limiter in-line, but they will allow you to see what and where any problems are. If the + voltage is low, then something is drawing too much current and pulling it down. Check for something shorted like D68.

[Jam]

stage 160

The 6 + and - rails are the transistors Q14,Q16,Q18,Q15,Q17,Q19

The D68 on the strip side it says O to 1 V and the other side is O

[Jam]

Stage 160

I also wanted to add I pulled one of the emitter resistors out to test they are really hard to get a good reading but with my meter set at 200 OHM. I get 00.6 is that good or bad ?

[52 Bill]

OK, when you test a diode or a transistor, set your meter to diode test. When you place the leads across the diode or the transistor junction that you are testing, you are reading the voltage at which the junction conducts. With the leads attached in one direction you will get a low reading. When you reverse the leads you will get open reading.

With a diode, depending upon the type, you will normally find that in one direction the meter will read somewhere around 0.6 (volts). When you reverse the leads you will get an open reading.

If by O you mean the meter readout is the same as when the leads are unattached, then that is an open reading. If by O to 1v you mean the reading gradually slides from O to 1v and settles at 1v, then you are reading the voltage as your meter charges or discharges the filter cap in the power supply. If this is the case then that diode is OK.

In order to get an accurate reading of a resistance lower that 1 ohm, you need a special ohmmeter, or at least a really special multimeter. The reading of 00.6 ohms on one of the 0.47 ohm resistors is fine for what you need to know. As long as one doesn't read open or something like 10 ohms is good enough for now.

[Jam]

Stage 160

Ok one thing I did was I was change the three - power transistors Q14,Q16,Q18 because two of them were a different brand. So hooked it up to the light deal and now Iam getting - + 37.00 V on the six power transistors does that seem little low?

[Jam]

Stage 160

I just asked a dumb question you said All of the voltages will be lower than expected with the limiter in-line. Is their anything to check before I try it normal?

[Jam]

Stage 160

Whats the best way to test or see if the dual op-amps are good or bad ?

Do each dual op-amps do different jobs like is one for clean one for dirty

[52 Bill]

Does the amp pass signal now? Is the light bulb just dimly lit? If the amp is passing signal now with the limiter and the bulb is barely on, you can move on to plugging it in straight to the ac.

Whats the best way to test or see if the dual op-amps are good or bad ?

Do each dual op-amps do different jobs like is one for clean one for dirty

The op-amps are just little amplifier circuits built into an IC chip, so they can be used for any purpose. The fact that there are 2 built into one case is just convenience.

Each IC has terminals for plus and minus voltages and terminals for the inputs and the outputs. In your amp all of the chips are 8-pin DIPs. The pins are numbered 1-8 in a counter-clockwise direction, starting with the upper left hand pin. Pin number 1 will be marked with a small dot or will be on the end of the chip that has a molded in edge notch. The PC board will also be marked with a dot locating pin #1.

If you read the voltage to ground at pin #4 you should get -16 vdc. At pin #8 you should get +16 vdc. The inputs are located at pins 2 and 3 for section A and at pins 5 and 6 for section B. The output for section A is on pin 1 and the output for section B is on pin 7.

If you read the voltages on any input or output pin on the signal path op-amps, you should only get a reading of a few millivolts of dc voltage. If there is a volt or more the IC is suspect and should be changed. This only applies to the signal path ICs and not the switching section chips, as there will often be a dc voltage applied to the ins and out of these chips.

Be careful when taking these readings, as it is easy to slip with a meter probe and short two pins together, sometimes causing a chip to fail. If you do find a bad chip, I'd suggest installing a socket in that spot, so you can just plug in the replacement. Sockets are fairly cheap and this will make any future repairs easier. Make sure that the new chip is installed correctly, again noting the location of pin #1.

OK with all that being said, why do you want to check the op-amps?

[Jam]

stage160

The light bulb is dimly lit and so I hooked the speakers up and pluged it in and it turns and it sound like its working I didnt jam out on if for a long time but it didnt blow a fuse so thats good.

I wanted to check the want the op-amps because I wanted to make sure everthing tested good before I put it back together plus I dont know if I solved my first problem with the high picth noise I had before . Every time I test op-amps I dont know how to test them or know when they are bad? So when like you check pin 1 and 7 do you the 200 DCV on the meter? so if you dont get a reading on those two its good? So why does the service diagram how different reading then I get on the test point.

[52 Bill]

The light bulb is dimly lit and so I hooked the speakers up and pluged it in and it turns and it sound like its working I didnt jam out on if for a long time but it didnt blow a fuse so thats good.

OK good, you're back to where you were before you started blowing fuses.

I wanted to check the want the op-amps because I wanted to make sure everthing tested good before I put it back together plus I dont know if I solved my first problem with the high picth noise I had before.

I've found a tech note that may apply to your amp. Look and see if your amp has a wire jumper W10 located near IC U5. Is it still there or has it been cut out? Is there an S-shaped jumper attached to the top of the circuit board connecting C73 and R23?

So when like you check pin 1 and 7 do you the 200 DCV on the meter? so if you dont get a reading on those two its good? So why does the service diagram how different reading then I get on the test point.

If your meter has a 20v scale then use that, as it will be easier to read the smaller voltages that you are working with. The supply voltages are + and - 15 vdc, so you should not have a problem with the 20V scale.

Even though you may not find any stray voltages on the inputs or outputs of a chip, that doesn't mean that the chip is ok. That being said, usually this test will find the bad ones.

What voltages and what test points are you talking about?

[Jam]

stage 160

Ok I found W10 but nothing their but it does have a green wire from C73 to R23 what does it do? I have another amp just like this and it don have the green wire or W10 Is good to have or not.

I notice the test point like TP 21 say >5mV or Tp1 says 66.7 VAC I alway get o volts on most of the Test points. Is it because some say VAC,VDC,mVAC,mV, so do you use different setting on the meter?

[52 Bill]

Ok I found W10 but nothing their but it does have a green wire from C73 to R23 what does it do? I have another amp just like this and it don have the green wire or W10 Is good to have or not.

There was a problem with some of these amps, so the fix was to remove W10 and re-route the signal away from that section of the PC board with a jumper wire. Your second amp is probably a newer version that doesn't need the fix.

I notice the test point like TP 21 say >5mV or Tp1 says 66.7 VAC I alway get o volts on most of the Test points. Is it because some say VAC,VDC,mVAC,mV, so do you use different setting on the meter?

I hope that you're not kidding me here.

Each test point is listed with a value that you should find there either at all times or when the specific test parameters have been applied. The value is specified so that you know how to set your meter to read it. If you need to measure 5mV (millivolts) and you have your meter set to 200V scale, the 5mV will not show up as a valid reading (0.005V), while on the 200mV scale it will read as 5.0.

So set your meter to match whatever the listed value is before you try and take any test point readings.

[Jam]

stage 160

I will play my amp to see if I can get it to act up again with the high picth sound. If it comes back I will let you know . Sorry for all all the dumb questions and thank you so much Thanks

[Jam]

Same issue Fender stage 160

Ok I just got time to the amp and still makes the high pich noise. I did notice that when you turn up the reverb it would take off? I dont know where to start

[52 Bill]

Ok I just got time to the amp and still makes the high pich noise. I did notice that when you turn up the reverb it would take off? I dont know where to start

Ok, let's recap here.
Amp is making a high pitch noise when the distortion channel is selected.

You've checked and the factory mod has been performed, removing a jumper and re-routing the signal via a wire mounted on top of the board.

You've checked for broken feedback caps and for any loose parts in the pre-amp.

You've moved all loose signal wires around to eliminate any chance of feedback.

Now you say that the reverb control effects the noise. What does it do when you turn up the reverb?