Bugera 1990 Channel Switching Problem

[TomCarlos]

Team MEF....

Yes, another Bugera amp (with DSP for Reverb) with the infamous channel switching problem. I am not 100% sure if this uses the same BDM board as the 333xl model. Apparently that board has been problematic.

The problem starts when the amp has been played around 15 to 20 minutes at a moderate volume level. I previously checked the tubes and bias on this amp - it's fine. It is running Tung Sol 5881 tubes. So it must be something else. You can definitely hear the relay going bonkers causing the switching.

I have reviewed the following MEF Threads -

Bugera 6262 problem please help !! Desperate

Bugera 1990 Channel Switching Problem

And I have seen lots of videos on the Bugera channel switching issue. But before I get the amp from my friend to take a closer look, I wanted to pose some scenarios here and see if you can help me eliminate going down the wrong path.

One guy was saying check the 7815 positive regulator and add a heatsink. Ok, maybe the positive regulator is getting hot. But if the regulator is overheating and causing this problem, wouldn't this have showed up on day one? Why now. unless the 7815 is damaged?

Another guy mentions a bad switch on the front panel. But if this were true, why wouldn't the problem show up as soon as you turn the amp on?

The second thread mentions the DSP board, to check the solder connections and the MPSA06 transistor- I'll do that.

The first thread mentions a board where the footswitch connects, to check transistors- I'll do that.

Since this seems to be a common problem, I am hoping someone can clue me into the place to focus on.

More on this when I get the amp. But for now, I am making up my checklist.


Bugera_1990_Schematic.pdf

Bugera_ 333xl_BDM1.pdf

[The Dude]

As you mentioned and already know, start with power supplies and make sure they stay up and consistent. Intermittent channel switching problems would not likely be tube related. I'll add: Since this appears to be a heat related/warmup problem, I'd look for warm components and use some freeze spray to help narrow down the culprit.

[Enzo]

Switching problems are common in most any amp with switching. I think it is a mistake to assume if your amp has the issues that it will be the exact same thing as someone else's amp. Note how in the examples you report, that "common problem" has several different things might be causing it.

Tubes won't cause channels switching problems. A bad tube in a channel can make the channel dead whenever you switch to that channel. But that is different.

Heat issues are "common" in solid state circuits. Why not from day one? PArts age. Why would a switch fail only after a warm up? Switches are mechanical, and as the chassis warms up, tiny dimensions can shift.

This is why electronics shops keep freeze spray on the bench.

[TomCarlos]

Points well taken. I guess my challenge will be to replicate the problem with the chassis outside the cabinet. I will ask the owner to test that out before I tackle this one.

[Delta362]

Several years ago, I fixed a Carvin Legacy that would switch channels after about 45 minutes of playing. For that particular amp, it ended up being the 15V regulator eventually dropping out causing the channel switching relay to drop out.

[TomCarlos]

Thank you.... When I get my hands on the amp, I will first let it sit, turned on, no input, and see if the problem occurs. At that point, I am hoping to measure critical points, including the 7815 to see if it becomes unstable.

[TomCarlos]

Ok.... I have the amp once again.

I can reproduce the problem. Turn the amp on, either with Standby on or tubes off, no difference. After 5 minutes, give or take, the channel indicator lamp shuts off, turns on, shuts off. In other words, the Green LED (for the clean channel) will shut off momentarily, then turn back on, then repeat. If I repeat the test and start with the Red LED (the overdrive channel), at the 5 minute mark, the LED will switch over from Red to Green then will shut off, turn on, shut off, etc. At that point, the owner tells me the volume will also diminish. At this point in time, I am not testing for output. I am testing to resolve the channel switching on/off problem.

Since the regulator has been mentioned as a culprit in many other threads for Bugera amps, I ran a series of tests and repeated those test, same results.

I am monitoring the temp on the two regulators using my Infrared Thremometer. At turn on, the Pos Reg is around 41 degrees C, the Neg Reg is around 36 degrees C. For the next five minutes, the temps start to increase gradually. At around the 5 minute mark, when the LED shuts off, the Pos Reg hovers around 78 degrees C, the Neg Reg is around 54 degrees C.

The second test is to monitor the output voltage of the Regulators. Even when hot, the Neg Reg remains at -14.5 volts. The Pos Reg starts at 12.45 volts when cool (already low) and when it heats up, the output volts drops to less than 10 volts. At 10 volts, the LED remains lit. At 9.6 volts, the LED will shut off. When the LED turns on again, the Pos Reg is once again at 10 volts.

I am monitoring the incoming voltage on the Pos Reg. I am reading across R97, the 27 ohm resistor. At turn on, I get 25 volts at one end and 21 volts where it feeds the Regulator. It remains at 21 volts consistently.

When the LED shuts off, meaning the Pos Reg is hot and putting out 9.6 volts, I hit it with a blast of freeze spray. The LED immediately stayed lit until minutes later when the problem reoccurred (as the Regulator heats up once again).

There may be other problems upstream or downstream. But by virtue of cooling the Pos Reg, and knowing it only starts at 12.45 volts, I am thinking I have a regulator problem. So now, with such little space, how to replace the regulator and install something with a heatsink. As you can see, there is little space where the Pos Reg sits. The Neg Reg is to the right and a big resistor to the left. The only thing I might be able to do is to bend a regulator under the PC board sitting in front of it and mounting a flat heatsink.

See Post 1 for the schematic.

As mentioned in my other thread JRC 7815 Regulator, Helmholtz informs us that these two are "TO-220F or TO-220FP package, F meaning fully isolated.." So adding a heatsink might not be effective, even if the parts are new. I know, at one point in time, this amp was new and the regulators were working fine. But that was then, this is now.

Is there anything else I should consider before yanking the Pos Reg and replacing it? Yes, I will check C66, C67, and C68. Opinions always welcomed!!

[The Dude]

Yes, do check the caps on that supply, also. Other than that, it seems you've at least found the cause of the issue.

[Enzo]

If the 15v regulator only produces 12v even when starting off, that is a problem. Replace the thing.

[Jazz P Bass]

Those KIA brand regulators have a history of being problematic.

[TomCarlos]

Update... making progress !!!

I will soon add another post showing the regulator replacement and how I added a heatsink. But before doing that, I want to ask about the "twin tower" resistors that are part of the plus and minus 24 filament supply. I realize that this +24v feeds the tube filaments, but one of these resistors is running hot. Maybe that is the way it is designed. At least for now, the amp is working and perhaps it has been this way since day 1.

As you can see in the schematic (lower left) and photo, there are a pair of Ceramic 51 ohm, 5 watt resistors in this circuit. R46 runs very hot. I am trying to get a consistent temp measurement with my infrared thermometer. For R45, I am seeing around 85 degrees C, which should translate to 190 degrees F. R46 is running cooler, perhaps around 60 degrees C.

Do we call it good and move on or should I look into this?

Thanks.

[Delta362]

Check the caps C27 and C6. If R46 is running hotter, I'd suspect there may be some excessive current pull across C27. But, I'd also check the DC and AC voltages at D14K/D17K, and D13A/D16A points, and then voltages at V5-4 and V7 pin 9 to get the +/-FIL voltage readings to see how much drop is across those two resistors since it doesn't' look like you'll be able to easily measure them directly.

[TomCarlos]

Thanks Delta....

To retest the temp reading, I am using a different infrared with a new battery. Depending on where you point the laser onto the resistor, you get a different reading. When I take the temp readings on the back side, I get 40 and 45 degrees C respectively. So perhaps we are ok.

But for good measure, I wanted to check what is happening across R45 and R46 as you suggested.

All measurements taken with respect to chassis ground and amp in Standby position.

D14/17 Cathode: + 25.8 vdc
D13/16 Anode: -26.3 vdc

V 5, Pin 4: + 17.63 vdc
V 5, Pin 5: + 6.16 vdc
V 9, Pin 5: + 11.99 vdc

V 6, Pin 4: + 6.15 vdc (coming from V5)
V 6, Pin 5: - 6.24 vdc
V 6, Pin 5: + 0 vdc

V 7, Pin 4: - 6.24 vdc (coming from V6)
V 7, Pin 5: - 18.134 vdc
V 7, Pin 5: - 12.14 vdc

So if I am doing this correctly, it appears we have the same voltage drop across R45 and R 46

Vr45: 25.8 - 17.63 = 8.17 vdc
Vr46: -26.3 - (-) 18.13 = 8.17 vdc
Current: 8.17v / 51 ohms = .160 amps

P=V*V / R

(8.17*8.17) / 51 = 1.3 watts (and we have a 5 watt ceramic resistor).

So I "think" I am ok.

[g1]

Thermometers don't differentiate other nearby heat sources. So they can give you some ballpark data, but the voltages across the resistors have the final say. As you found.
There is probably another heat source closer to one of those resistors, so it gets hotter even though it is doing the same work.
I wasted a bunch of time chasing some 'overheating' caps in a boogie. It turned out to be due to the ridiculous placement of other high heat components (resistors), and basically using the board as a heatsink.

[TomCarlos]

And here is what I did with the new regulators.... I know, probably an overkill.

For the 7815, I added a winged heatsink and extra piece of flat aluminum and bent the regulator leads so that it would fit under the Input / Controls PC board. You can see that in pic 2. That gave room to add a smaller heatsink to a replacement 7915, where I left it standing as was the previous 7915.

When I removed the PC board to make this fix, I noticed one of the standoffs became detached from the chassis. It is hard to tell how those things are affixed - solder, spot weld, glue? Since I did not see that it made a "connection" to the PC board, I used JB Weld to fasten it back to the chassis. I think there is one PC board connection to the chassis, and that is the screw that is located near the large C24 C25 caps.

Anyway, I conducted another 1.5 hour test by running some music as a source. No problems. So it is time to button this up and give back to the owner. And let's hope nothing else goes wrong.

Thanks again everyone.