Fender Roc Pro 1000 - Output is SUPER low

[nickb]

I'll look these over...

[nickb]

On the spreadsheet the pin on Q7 that behaves as the gate i.e varies from 17mV to -15.24 when you unplug is given as pin 2. The Onsemi datasheet says this the gate is pin 3. Therefore there is some confusion over the pin numbering or the orientation of the FET. Using the schematic, figure out which hole of the FET is connected to what. From that you can identify which hole is gate, source and drain. Then use the datasheet to check you have the FET in right.

[slow6i]

On the spreadsheet the pin on Q7 that behaves as the gate i.e varies from 17mV to -15.24 when you unplug is given as pin 2. The Onsemi datasheet says this the gate is pin 3. Therefore there is some confusion over the pin numbering or the orientation of the FET. Using the schematic, figure out which hole of the FET is connected to what. From that you can identify which hole is gate, source and drain. Then use the datasheet to check you have the FET in right.

These are the values that I just measured. The pin 1 / 2 naming was in place before i knew that there were actually 3 pins on the FET, that was my mistake.

Q7 test

Pin1	Pin2	Pin3
-.37 dcV	-.39 dcv	0 dcV	Plugged
-.56 dcV	-.59 dcv	-2.93 dcV	unplugged

[nickb]

Again, the data is not consistent. We see the anode of CR20 (non-stripe) as 0.22V/-15.4V yet pin 3 of Q7 as 0V/-2.93V (plugged / unplugged). Either the measurement is wrong or the gate of Q7 (pin 3) is not actually connected to the anode of CR20 - I think we tested this for continuity once before

[slow6i]

Again, the data is not consistent. We see the anode of CR20 (non-stripe) as 0.22V/-15.4V yet pin 3 of Q7 as 0V/-2.93V (plugged / unplugged). Either the measurement is wrong or the gate of Q7 (pin 3) is not actually connected to the anode of CR20 - I think we tested this for continuity once before

I've just gone through everything again, and with two different meters, I get two different readings. I dont have a meter that goes into the mVAC with any resolution other than 10's so I cant get a fully accurate reading with those anyhow (I started checking all the TP's.)

When we bridged over Q7 and Q6, what exactly was that doing? Why did that make this amp behave normally?

Im at the point now where ill probably just order all new op amps, jfets, replace them, see if that works, then scrap this thing and realize that I made a $100+parts dollar mistake by trying to fix something that I had no business fixing.

[glebert]

Bridging past Q6 and Q7 was getting past the muting circuit. IIRC the muting circuit should silence the unused channel during normal operation and both channels with nothing plugged in.

[slow6i]

Bridging past Q6 and Q7 was getting past the muting circuit. IIRC the muting circuit should silence the unused channel during normal operation and both channels with nothing plugged in.

So, by bridging over that, I effectively had both channels active at the same time?

I did some more poking around, this time reading the actual test procedure for ACV on the schematic, results are listed on the "Test Points" tab at https://docs.google.com/spreadsheets...it?usp=sharing

I used a tone app on my phone as an input at 1kHz as thats the best I can do with the equipment I have.

Those MC1436 buggers are expensive! (Probably because they dont make them anymore.)

[nickb]

I've just gone through everything again, and with two different meters, I get two different readings. I dont have a meter that goes into the mVAC with any resolution other than 10's so I cant get a fully accurate reading with those anyhow (I started checking all the TP's.)

When we bridged over Q7 and Q6, what exactly was that doing? Why did that make this amp behave normally?

Im at the point now where ill probably just order all new op amps, jfets, replace them, see if that works, then scrap this thing and realize that I made a $100+parts dollar mistake by trying to fix something that I had no business fixing.

The idea was to bridge just one of them, not both. Replacing stuff at random is a horrible idea and will almost certainly be an expensive failure. I'll bet it's a simple fault to do with the muting.

It's time to hand the amp to someone with the right skills. It'll be fixed in in hour at minimal cost.

[glebert]

It's time to hand the amp to someone with the right skills. It'll be fixed in in hour at minimal cost.

Considering how much this amp has been tinkered with I wouldn't assume that this can be fixed for minimal cost. Was the original problem (foot controller switching) ever fixed? Local guitar store is $70/hr. and this amp is maybe worth $150 around here on a good day. I'm not taking in outside work at this time BTW.

If bypassing the mute FETs worked you could permanently jump it. It would be fixed in one channel. I don't love the drive on this amp, and I have had three of them (still have one) so I would do it clean and add a overdrive pedal.

[slow6i]

NickB - I think you are right. Probably better not to just start replacing things. I am just going to bypass the muting circuit as glebert suggested and run a pedal setup.

If that fails, I will save the speaker, and scrap the rest of it, or break it down for components for other projects.

Thank you g1, glebert, and NickB for your responses and attempting to help me get this thing fixed, but unfortunately, I must admit defeat on this one.

I now turn my attention to a Baringher Powered PA speaker! Yay!