Stiff 23kHz Oscillation on GK 1001RB U7 in the preamp....Tweeter Circuit

[nevetslab]

In slowly restoring an odd failure of a Gallien-Kruger 1001RB, where I had failure of Q14 in the power amp circuit.....the upper voltage gain stage xstr, with the emitter resistors of that string burning open (R33, R26, along with the previous stage's emitter resistors R31 & R20), I got that circuit back together, but also found on the Power Amp PCB complete failure of the bipolar adjustable Regulator IC's U4 & U5 (LM337T, LM317T), which, just like Q14, were a 'thrill' to slip back in under the heat sink while having to fold their leads up with adequate length so they could then be pushed back down into the solder pad holes and pulled thru. Then, when I checked the power amp module for restored DC levels, the Tweeter Amp IC LM3886T was still going south on it's output. So, had to remove it....also a PITA to extract.

Finally got the Power Amp PCB back together, and moved on to see if the amp works. Not a chance. I had a wide variety of DC levels appearing on the output of most of the Preamp's Op Amps U1 thru U9....all TL072's. Initially, it looked like was just a few causing the problem, with those following the bad op amps just passing that DC level on down the line. I finally got U2, U6, U7, U8 & U9 replaced and had to do some selecting of U7 to get the lowest DC level on it as well as on U9, which is the output stages to drive the power amps.

When I patched into the Return Jack on the front panel to see if the LF amp was working, it was working fine. The Tweeter Amp appeared to be working, only I discovered that wasn't changing in frequency when I retuned the oscillator. Removed the input signal, and still had a nice looking 23kHz Sine Wave sitting on the output of the amp. Unplugged the input to the Tweeter Amp on the Power Amp PCB, and the oscillation signal was gone. Probing thru the signal path following the Return Jack, I found IC 7A was in hard oscillation, rail to rail, which further down the stages got smoothed out.

This one I've never seen before on the GK 1001RB.

While that was one problem yet to solve, I moved my 400Hz test signal to the input jack, and found a horribly distorted signal on the LF output. Further probing, the FET input amp was fine, as was U2A, but crapped out at U3, where it was asymmetrically clipping, and further clamped at U4, and nothing out of U5. Looks like when the bipolar supplies failed, it took all of the op amps out in the preamp board.

Now, looking at this oscillation on U7A.....first stage of a 3-stage state-variable LP Filter (HI Cut), I also notice there is NO power supply bypass caps anywhere on the board, other than C13/C14....3.3uF/50V Electrolytics at the input to the Preamp Board. Nothing locally on any of the op amps.

I'm just about to power the preamp board up external to the chassis off a lab supply to see if I still have that oscillation, now that I've removed the last four op amps from the PCB. I was thinking of putting 0.1uF Mono Ceramic caps bypassing the supply terminals across U7 & U8, both of which are the gain stages of the state-variable filter.

This is DAY 3 of surgery on this stupid amp. I've NO IDEA how this amp failed. I grabbed it off our inventory shelf last week for use in checking the latest of the Bass Cabinets in doing preventative maintenance on our Loudspeaker Inventory. It didn't power up at all (no Pilot Light), so obviously whoever checked it in from the last rental didn't bother to see if it still worked!! Such is life at the moment in our Rental Dept.

700RB-II_1001RB-II_PREAMP_PN206-0251.pdf
1001RB-II_POWER_AMP_PN_206-0260.pdf

[nevetslab]

After replacing the rest of the TL072 IC's, and added 0.1uF Monolythic ceramic bypass caps across U7 and U2, that made no difference. Powering the Preamp PCB externally, looking at U7A, that oscillation is STIFF. I've swapped op amps, changed types (tried NJR 4560D's in U7, U8 & U9 with no difference. I ran out of time while digging for replacement 3.3nF caps and 10.0k resistors to swap out those in the State Variable Filter that lives in U7/U8 stages. Removing U8, as one might expect, eliminated the oscillation at U7A. I didn't have any 3.3nF, so will use 2.2nF and 15.0k to set it to the same frequency when I get in tomorrow morning.

[drewl]

Working on a fried one the latest prints show a few EC's for limiting overvoltage and inrush current damage to the tweeter amp.

Yours have those ?

[nevetslab]

Working on a fried one the latest prints show a few EC's for limiting overvoltage and inrush current damage to the tweeter amp.

Yours have those ?

This one is another Rev B2, and has the in-line 1.5A bridge rectifiers wired in series (four parallel-diodes in series from the LV tiers to the LM3886T Tweeter Amp. Also has the in-rush NTC Thermistor on the AC mains input following the mains switch/fuse. I hadn't seen anything on limiting overvoltage, though I haven't looked for that either. I need to call GK and get their latest documentation anyway.

[nevetslab]

While pulling the three 3.3nF caps (C34, C35, C36) from the HPF for the Tweeter channel, I came upon a resistor adjacent to one of the caps that didn't look like it had been soldered in. I soldered it back in, but still went and installed the three 2.2nF caps, all I had on hand in that 5mm L/S size, and, their body was thicker than the original parts, so put them on the foil side. I powered the preamp board back up, just to see if the new caps and that resistor changed anything. YES....oscillation gone. But, not having replaced the resistors in the circuit to a higher value to bring the corner frequency back down, its' half an octave higher than it was. Hell with it......this has been dragging on for too long, now DAY 4 on the stupid thing. Leave it.

Checked to see if the Preamp circuits were now working, having replaced the rest of the op amps. I got NO signal thru the preamp. Then, remembered....the discrete FET input stage runs off of +/- 35VDC, which I didn't have applied. I had already seen it work, so I injected a signal into the top of the Volume pot, and found the rest of the circuit was working again. Finally!

Put it all back together, again having to fight the large Toroidal Pwr Xfmr's obround shape to get it to sit in the space provided. All back together, works fine again. I just can't bill for all the hours it took to find all the problems it had. I would have swapped that preamp board with the previous GK 1001RB that had high random noise problems, but, it had already been sent back to GK. That would have made this a lot easier, and let them deal with it.

I later checked those three 3.3nF HPF caps I had removed on my bridge. All three were ok. Figures!