Fender Roc Pro 1000 - Output is SUPER low

[g1]

Are all the boards etc. assembled? If not, your chassis ground may be intermittent or non-existent connection to circuit ground, which would explain the inconsistent measurements.

[nickb]

Its a Cen-tech 95670. Its certainly not a top of the line meter or anything. The battery is good afaik, though I will change it and check the voltages again. Negative lead is to the chassis just above the power switch. I am not using clips though, I'm using probes if that matters.

I can't find the all important impedance specification in the data sheet I found for that. I would not be surprised if it was lower than usual and that will skew some measurements. It does not explain the inconsistencies.

If the board is installed and the pot nuts are tight then there should only be 4.7 ohms from chassis to ground but I would not count on it. Better to connect directly to the real ground. The sleeve of the input jack (or footswitch) is a handy place.

So, with the new ground, plug in and measure the DCV on either side of CR1 ( same as R3 but the colors tell me nothing) , either side of CR20, the gate of Q7 and the source of Q7.

Note:
The anode CR1 should be the same as the cathode of CR20
The anode CR20 should be the same as the gate of Q7

If still not consistent then put your meter under the rear wheel of your truck and back over it

[glebert]

OK, someone correct my thinking on this if I am wrong. The voltage at U8 pin 7 is +15V for clean channel, but the cathode (stripe) of CR21 is about +8V, right? The only thing between those points is R152. This is DC so the capacitor C68 should be blocking it from ground (but we should not assume this is working correctly). There has to be current flowing through R152 for there to be this voltage difference. Current should not be able to flow through CR21 as it is reversed biased. So if the voltage measurements are correct, it seems like either C68 is leaking, CR21 is faulty, or there is some other current leak on that net.

[nickb]

OK, someone correct my thinking on this if I am wrong. The voltage at U8 pin 7 is +15V for clean channel, but the cathode (stripe) of CR21 is about +8V, right? The only thing between those points is R152. This is DC so the capacitor C68 should be blocking it from ground (but we should not assume this is working correctly). There has to be current flowing through R152 for there to be this voltage difference. Current should not be able to flow through CR21 as it is reversed biased. So if the voltage measurements are correct, it seems like either C68 is leaking, CR21 is faulty, or there is some other current leak on that net.

I think the thing you're overlooking is the meter input impedance. That is why I tried (and failed) to find that info. Generally, if crucial info is missing I'm suspicious that it was left out on purpose.

I'm focusing on the inhibit path since we know for certain that bypassing the FET makes it work.

[glebert]

I have and have used similar Harbor Freight meters and never had them pull down that much. Low battery will give wrong values though.

[g1]

I think the thing you're overlooking is the meter input impedance. That is why I tried (and failed) to find that info. Generally, if crucial info is missing I'm suspicious that it was left out on purpose.

A bit of an oddball meter having a tach function, but pg.7 of manual (attached) says input impedance 1Meg for all DC voltage ranges.
Having said that, if this is a gate reading, I've had my share of situations where you can't measure DC at the gate due to loading of high impedance circuit, even with pro meter.

[nickb]

A bit of an oddball meter having a tach function, but pg.7 of manual (attached) says input impedance 1Meg for all DC voltage ranges.
Having said that, if this is a gate reading, I've had my share of situations where you can't measure DC at the gate due to loading of high impedance circuit, even with pro meter.

Thanks G1 - that was the manual I found but missed the impedance. I was guessing 1meg based on the numbers measured. I also have a cen-tech meter (different model) I got as a disposable one and it is 1meg also. A good meter will be at least 10 meg. I miss the "like" button,

Now that we know what we are dealing with we can correct for it.

PS: slow6i, can you add measuring the DCV from source to gate of Q7 when plugged in please?

PPS: g1, I guess the tach function is handy for shredders

[glebert]

Now that we know what we are dealing with we can correct for it.

You know, without really thinking about it my initial response was "1 MEG input resistance should be good enough" but in this case it really will hugely impact the measurements. For the voltage at CR21 cathode it makes sense he is getting about 1/2 of the 15V from the U8 pin 7, because R152 is also 1 MEG. Guess that is my "new" thing I learned for today

[g1]

PPS: g1, I guess the tach function is handy for shredders

lol. 'How many NPM are you up to?'

[slow6i]

So, with the new ground, plug in and measure the DCV on either side of CR1 ( same as R3 but the colors tell me nothing) , either side of CR20, the gate of Q7 and the source of Q7.

Note:
The anode CR1 should be the same as the cathode of CR20
The anode CR20 should be the same as the gate of Q7

If still not consistent then put your meter under the rear wheel of your truck and back over it

I hope I have the anode / cathode terminology right so its not confusing. (Im taking Stripe side of CR1 as Cathode)

CR1 Cathode: 12.2->8.54 dcV
Anode side: 5.68 dcV

Anode CR1 - Cath CR20 are equal.
Anode CR20 - Cath Q7-Gate are equal.

Source to Gate Q7: -7.83 dcV

Thanks!

[nickb]

I hope I have the anode / cathode terminology right so its not confusing. (Im taking Stripe side of CR1 as Cathode)


Source to Gate Q7: -7.83 dcV

Thanks!

If the negative lead was on the source for this measurement.then this is wrong. I should have specified to put negative on the source (sorry!)

If you switch off and wait a few mins for the voltages to fall you should measure < 1k ohm between source and drain of Q7. Otherwise there is a problem with new FET - wrong type, wired in wrong, busted

The results for CR20 are missing (mainly sanity check as I'm questioning my own right now).

[slow6i]

The results for CR20 are missing (mainly sanity check as I'm questioning my own right now).

Sorry! I missed that part of the comment. Newly measured values are below.

Im assuming you wanted the measurement on clean channel, so this is done on clean with everything at 10 except effects mix at 0:

Q7-2-3: -32.5 mV
between Q7 1(d)-2(s): 24.5 ohm
CR20 Cath: 5.69 V
CR20 An: 6.9 mV

[nickb]

The set up is correct. We'll keep it like that.

Now we have a new contradiction. The FET is low resistance as it should be and the voltages are correct. Bridging the (low resistance) FET made it work yet remove the bridge and it doesn't. That makes no sense.

Do you still get a noise when touching:

pin 6 of U9?

Source of Q7?

Drain of Q7?

R23? One side is the same as the drain while the other is rather low impedance so the effect will be small so it's not a very good test.

Pin 5 of U1?

[slow6i]

The set up is correct. We'll keep it like that.

Now we have a new contradiction. The FET is low resistance as it should be and the voltages are correct. Bridging the (low resistance) FET made it work yet remove the bridge and it doesn't. That makes no sense.

Do you still get a noise when touching:

pin 6 of U9?

Source of Q7?

Drain of Q7?

R23? One side is the same as the drain while the other is rather low impedance so the effect will be small so it's not a very good test.

Pin 5 of U1?

U9-6: no
Q7S: no
Q7D: no
R23: no
U1-5: no

Im going to redo all of the measurements through this thread to confirm them.
I will update the google sheet previously listed.

[slow6i]

https://docs.google.com/spreadsheets...it?usp=sharing

Sorry that took so long.

Voltages have all been checked and verified with settings shown on sheet, and guitar at mid pickup and tone / volume all the way up.