Ampeg B5R: Overdrive channel always on

[decades]

Hi, this is my first post but have been lurking since 2 months ago, when I bought a non-working Ampeg BA115. After reading through several posts about the amp, I manage to resurrect it and am now enjoying it. I'm not a tech and I don't really know anything about electronics or its theory. Every mention of a component or a type of circuitry in those threads, I have to do a search in google or youtube. I can solder, kind of...and know some components and how to use a multimeter.

I had fun fixing the BA115 that I bought another project: an Ampeg B5R. This is where I definitely bit off more than I can chew.

The amp has a short according to the light bulb limiter...But before I dig deep into the power amp section, the first thing I'm trying to tackle though is the overdrive channel that is always on. The LED indicator for the overdrive channel is also always on. The channel select button doesn't switch the channels. The combine button, on the other hand, turns on the clean channel.

To eliminate that it's a light indicator problem, I've tried connecting the preamp out to a mixer and I could hear that the output is overdriven. I've meter checked four JFETs while on the board and thought they were shorted. I'm now thinking that meter checking these while on the board probably doesn't work. I've replaced those four JFETs: Q6, Q7, Q10, Q11....but the overdrive is still on. Can someone explain how the channel select circuit works on this amp? I've stared at the diagram trying to grasp what is happening but as soon as I think I know how the circuit works, then something else doesn't make sense.

What do I need to check next?

Thanks!

[52 Bill]

The amp has a short according to the light bulb limiter...But before I dig deep into the power amp section, the first thing I'm trying to tackle though is the overdrive channel that is always on. The LED indicator for the overdrive channel is also always on. The channel select button doesn't switch the channels. The combine button, on the other hand, turns on the clean channel.

I would suggest that you first try and remove the short from the power amp or power supply as it is probably causing a problem with the preamp power supply, which in turn may be the cause of all of your preamp and switching problems.

Oh yeah, and welcome to the place.

[decades]

Thanks Bill,

I should've posted first before I jumped into replacing parts.

I went through some of the test points in the power supply and none of the voltages are in spec. Is the transformer bad?

Here are the voltages I got (without signal)...

TP13 n/a
----- 0 vac (DB1 pin 1)
TP14 17 vac (DB1 pin 2)
TP15 17 vac (DB1 pin 3)
----- 29 vac (DB1 pin 4)
TP 16 2.7 vdc
TP17 inaccessible
TP18 6.9 vac
TP19 6.9 vac
TP20 18 vdc
TP21 18.6 vdc
TP22 18.6 vdc
TP23 16.4 vdc
TP24 0.2+- (there’s a 22k resistor piggybacked on the existing 10k resistor)
TP25 not used
TP26 4.8 vdc
TP27 5.5 vdc
TP28 4 vdc

So from the get-go, straight out of the transformer, test points 14 and 15 are measuring 17 vac instead of the specified 61 vac. Test points 18 and 19 are measuring 6.9 vac instead of 20 vac.

[52 Bill]

So from the get-go, straight out of the transformer, test points 14 and 15 are measuring 17 vac instead of the specified 61 vac. Test points 18 and 19 are measuring 6.9 vac instead of 20 vac.

Is this with the transformer connected to the board? Is this with the light bulb limiter in the primary circuit?

[Enzo]

A bulb limiter will drop al your voltages, especially if it is lighting up.

Make sure the transformer is not wired for 240v. If you have it wired 240 and put 120 into it, all the voltages will be half what they should be.

[decades]

Yea, the transformer is on the board and the amp is plugged in to the limiter. How do I check voltages without the limiter? I'm afraid to remove the limiter and blow fuses, etc. Where do I even begin to find the short?

[52 Bill]

I'm afraid to remove the limiter and blow fuses, etc. Where do I even begin to find the short?

Try this for a first step. Unplug the high voltage transformer plug from the board and power up the amp. Does the light still light up bright? Next remove the low voltage transformer plug from the board and turn on the amp. Does the light brightness change?

If either test stops the light from glowing brightly, you have a direction to search for which circuit has the short in it. If the light never gets dimmer, then the transformer and the primary wiring are suspect.

Tell us what you find out.

[decades]

I think I found the culprit (or at least know where to look now) with the overdrive always being on. Just tinkering today with the functions of the amp, I plugged in a footswitch into the channel select jack. Even though the OD light remained on, the footswitch was able to turn on the clean channel, which the push button switch could not do before. Depending on how I wiggle the footswitch jack, the channel select function worked intermittently. I sprayed some anti-corrosive stuff and worked the jack with a cord. It worked briefly. Lifting the amp case or poking around the jack area made the channel select work then not work. Must be some cracked solder at the jacks? The previous owner actually reported having to prop up his output cord with something to get a steady output sound. He also said something about dropping the amp.

I checked the transformer's wiring and it is set for 120V.

I did the unplugging-the-transformer test and here's what I got:

High voltage disconnected and low voltage connected = no light
High voltage disconnected and low voltage disconnected = no light
High voltage connected and low voltage disconnected = light then dim
High voltage connected and low voltage connected = light

With both high and low voltage connected from the transformer to the board, I disconnected the preamp power supply and the light remained on. so that makes the short not in the preamp.
I also wanted to see if maybe it was the fan, so with both the high and low voltage still connected, I unplugged the fan and the light remained on.

During that brief time that I was able to get the channel select working, I connected the preamp out to a mixer and jammed a little...playing around with the channels. While playing I noticed that if the high voltage is disconnected from the transformer to the board, I get proper sounds coming out of the speakers: clean is clean, od is od...as soon I have both low and high voltage plugged in, the clean channel has a little bit of distortion. In addition, with the mute button pressed and a note is played on the bass, a tiny bit of the distorted note can be heard through the speakers.

I've been looking at the schematic and not knowing enough about circuits, I was trying to see where the low and high voltages might overlap in the circuit. My thinking is: if the short only appears on the light bulb limiter when both high and low voltage plugs are connected, then the short must be somewhere where both those voltages meet. Am I in the right direction? I'm guessing for now that the short is in the power amp section...??

Planning on completely removing the board out of the case tomorrow to take a look at the solder joints at all of thejacks...

[decades]

Problem with the overdrive is fixed!

I ended up re-soldering a dozen or so broken or cracked connections at all of the jacks and another dozen or so in places where the solder looked thin.

Now trying to fix the short. I pulled the outputs and the row of IRFP9240 on the lo side were shorted. I checked the gate resistors and source resistors and they're ok. I ended up replacing a couple of MPSA06, which I think are the drivers...Q106 and Q107 on the schematic...but I'm not sure...just replaced them anyways. The resistors around those are ok also. The zener diodes (D114, 115, 116, 117) are ok.

Anything else I should check and what caused that row of mosfets to die?

[g1]

Could you post the power amp schematic?
If by "low side" you mean the ones going to the neg. supply, they are IRFPS240 (edit: typo, should read IRFP9240), while the IRFP240 are on the positive side, yes?
Not sure about these particular ones, but Fet's sometimes measure low resistance even when good (unlike bi-polar transistors). Are you sure they are bad?

[Enzo]

IRFP240 and IRFP9240, positive and negative respectively.

[decades]

Could you post the power amp schematic?
If by "low side" you mean the ones going to the neg. supply, they are IRFPS240, while the IRFP240 are on the positive side, yes?
Not sure about these particular ones, but Fet's sometimes measure low resistance even when good (unlike bi-polar transistors). Are you sure they are bad?

IRFP240 and IRFP9240, positive and negative respectively.

What Enzo said...

I always have doubts when it comes to testing. I did the tests again just now and might have been previously interpreting the values incorrectly. What I did was, with the multimeter on diode check, put the red on source and black on gate. Next, I put the black on source and the red on drain. On the IRFP9240s, I got values from 340 to 390. On the IRFP240s, I got 620. I then moved the red to gate and then back to drain. On the IRFP9240s, I got OL. On the IRFP240s, I got 4.

It looks like the IRFP240s are the ones that are shorted (instead of what I've concluded on my previous post)...??? is this correct?

[g1]

When you are testing them with your meter, you can "charge up" the gate, turning it on so you get a low resistance reading from gate to source.
However you measured the 9240's to get OL (from source to drain), do the exact same thing in the exact same order to the 240's BUT reverse the probes.
Just like you need to reverse the probes depending on whether measuring pnp or npn bi-polars.

[decades]

I did it wrong. I was assuming the pins were the same and they were both P-channel. On top of that, I confused the source/drain/gate pin locations on both. Thanks for questioning.

ok, did the test again, this time while looking at the datasheet and making sure I can keep track of the g/s/d locations on both:

On the 9240s...red on source and black on gate, then black on source and red on drain = average of 90. Next, I moved the red to gate and then back to drain and got average of 640.

On the 240s...black on source and red on gate, then red on source and black on drain = average of 4. Next, I moved the black to gate and then back to drain and got an average of 616.

The first value should be OL, right?

[g1]

If you have a source to drain short it will measure low no matter how the probes are connected. Yours are only giving the low reading when the probes are connected a certain way, which is turning the Fet on, so the low reading in this case seems normal. Also, you mentioned all the source resistors are ok, and that your bulb limiter dims if the low voltage is disconnected but the high voltage (to the Fet's) is still connected.
So it doesn't seem to be a dead short causing your bulb to light up. Rather, something is not running correctly when both the low voltage and high voltage supplies are connected. My guess is the Fet's may be getting turned on hard by some other fault in the amp.
Here's another check for the Fet's, just to be sure. Short the gate to source with a jumper. Now measure drain to source, then reverse your meter leads measuring drain to source again. There is an internal diode from drain to source so it should measure like a diode, OL one way, normal diode drop the other way.
You should not get any low readings from drain to source (like the 4 or 90 you had before) with the jumper connected. And you should not get any low readings from gate to anything when the jumper is disconnected.