Ampeg B5R: Overdrive channel always on

[decades]

Thanks, g-one,

I jumped ahead this morning and did one more test to the fets, same to what you suggested. I probed gate to source and then gate to drain and got OL on both. I also jumped gate to source and probed drain to source both ways and got OL one way and some value in another. I was convinced the outputs were good so I soldered them back on the board.

I also pulled Q105 (but not Q102) to test and it was ok. Reason for not pulling Q102 was when both Q105 and Q102 were in the circuit, I probed them and they were giving the same values. I figured I only need to test one out of the circuit and it should also be true for the other.

Did the same for Q108 and Q109...

Q103 and Q104 seem to test ok. I didn't pull these out. Do I need to pull these out?

I've replaced Q106 and Q107.

I think I got all the resistors and diodes around these group of Q10X transistors tested and were ok.

After everything was back together, I fired it up (the light bulb limiter still lit) and measured voltage on the speaker output and got something that jumped around but was peaking at 4mV. I also measured across all the big .47 resistors connected to the source and the values ranged from 19.80 mV to 28 mV. According to the schematic, should be an average of 25 mV +- 5mV. The 19.80 was only on one resistor and the majority of them were in the 24 mV to 27 mV range.

I'm not sure what to check next. I'm just going down the line of things to check. Should I pull the voltage regulators and check those? By the way, when The high voltage supply is disconnected and the low voltage supply is connected, I can hear a "click" sound that seems to be coming from either of the voltage regulators, when turning on the power. I'm sure it wasn't coming from the fan. It doesn't happen when both high and low voltage supplies are connected or when only the high voltage supply is connected.

Also, with these units, is the fan suppose to turn on when switching the power on? or does it not kick in until later? When I power up the amp, the fan moves slightly and that's it: it doesn't spin.

[decades]

ok, I've verified that the clicking sound I mentioned above is coming from the relay. To repeat, the clicking on the relay only happens when only the low voltage supply is connected.

I found this quote...

When you turn on the Ampeg SVT 5 Pro amp, if it is cold the fan turns a few revolutions and then stops. When heat is sensed on the heatsink it will spin, progressively faster with increased heat.
The clicking is the protection circuit kicking off the output relays.
It is either faulty or doing its function: sensing Vdc on the output.
Which can mean a bad soldering job from the factory, a component(s) in the output section has (is) failed, the power supply is not outputing the correct voltages, a connector is on wrong.............

I know it's talking about an SVT-5 Pro but I believe it has the same circuits as a B5R with the exception of the preamp and 2 power amps.

I feel like I'm getting warmer (or not). I've been reading previous threads about the fan circuit causing all kinds of problems...

[Jazz P Bass]

Quote:"Also, with these units, is the fan suppose to turn on when switching the power on? or does it not kick in until later? When I power up the amp, the fan moves slightly and that's it: it doesn't spin."
That is normal behaviour of the fan circuit at power on.

[g1]

You are hearing the relay click, but it's hard to know if that means it is closing, or opening because the amp is going into protect.
So measuring for DC at the speaker jack won't show anything if the relay is open. You should check ahead of the relay, maybe at the output coil.
Also, back in post #3 you listed voltages for the power supplies. They were all low which is normal due to the limiter bulb. But none showed any negative voltages. Was that a typo or are there no negative voltages present?

[decades]

Thanks for the replies...

g-one...There might have been negative voltages, I'm still new to probing that I, most likely, wasn't paying attention to the negative symbol. I will do the TPs again.

I did some more testing and might have found a culprit. I've checked for DC several times before the relay: at the relay's NO and also after R173...The values jump around but the most I saw was 30 mV.

Just going down the line, checked the relay with the amp off, there's continuity between NO and NC, NO and COM, NO and C2, NO and C1...is the relay a molten mess?

It seems the output signal is being grounded directly because of the continuity between NO and NC. is that correct? is this what is increasing the current, causing the light bulb to light. ???

Anyone know of a replacement for the relay? The case says "Potter & Brumfield T-90 Series".

Will this work?
3 Potter Brumfield Series T90 General Purpose Relays New Old Stock | eBay

I can re-use the case from the existing relay. The pin outs look correct but I'm not sure about the rest of the specs.

[Enzo]

Just going down the line, checked the relay with the amp off, there's continuity between NO and NC, NO and COM, NO and C2, NO and C1...is the relay a molten mess?

ANything is possible, but this just seems really unlikely. You sure you were set on ohms, and a low scale as possible? And that your meter doesn't have a blown fuse or something?

What matters is what happens when power is on. You hear the relay click after a few seconds on? Read resistance across teh speaker te3rminals then, still shorted? or not?

If the old part was a T-90 I don't know why you couldn't replace it with another T-90. As long as the coil is the same voltage. Looks like your ebay parts are 12v, and teh circuit looks like it wants 12v.

[g1]

Try firing up the amp with the relay removed. If it was causing the problem, the limiter bulb should now be dim.

[decades]

ANything is possible, but this just seems really unlikely. You sure you were set on ohms, and a low scale as possible? And that your meter doesn't have a blown fuse or something?

What matters is what happens when power is on. You hear the relay click after a few seconds on? Read resistance across teh speaker te3rminals then, still shorted? or not?

If the old part was a T-90 I don't know why you couldn't replace it with another T-90. As long as the coil is the same voltage. Looks like your ebay parts are 12v, and teh circuit looks like it wants 12v.

ok, user error. I didn't have the meter set at the lowest scale possible. I checked the meter's fuse and it's good.

Pardon my erroneous measurements. Still learning (electronics 101)

The relay works intermittently. It clicks when the low voltage supply plug is connected while the high voltage is disconnected. Doesn't work when it is only the high voltage supply is connected. I can't remember checking if the speaker terminals are shorted after hearing the relay click.

Unfortunately, I can't check anymore since I've already pulled out the relay and jumped the output to the jack.

For sure though, is that the relay is failing. While it was out, I've applied 12v to it and it wouldn't click. The guy at the electronic shop also tried testing it and it would not click with voltage applied.

I've ordered a new replacement...but, unfortunately, I don't think the relay is what is causing the light bulb to light like I assume in the previous post.

Try firing up the amp with the relay removed. If it was causing the problem, the limiter bulb should now be dim.

After removing the relay and jumping the ouput to the jack, the light bulb limiter is still shining bright....so that wasn't the problem...

Anyway, I checked in to other things some more, especially the test points. I thought IC101, IC1, and Q101 were faulty so I replaced them. Good news is, after replacing those, that the light dims but not completely out. Should I call the amp good?

On the other hand, as of now, TP7 is the correct -5V but TP6 is showing -0.1 mV. I've tried tracing this negative voltage upstream and can see it up to TP3. Correct me if I'm mis-understanding the circuit, but TP6 to TP5 supplies the voltage to the gate of Q101? so what happens if TP5 ends up being low voltage like the .01mV that I measured?

The weird thing is that...if the light bulb limiter is brightly lit, I can make it dim by shorting the source and gate on the Q101 JFET.

Is the problem not enough voltage at the gate?

I think the root of the problem is TP6 showing a negative voltage but I don't know what is causing that.

[g1]

You may get all kinds of wrong voltage readings if your power supply voltages are not balanced. That is the first thing you need to sort out. They may be low due to the limiter lamp, but if it is dimmer now they should be closer. But the plus and minus sides need to be equal. Power supply drawing TP21&22, TP20&23, TP26&27.

[decades]

Thanks, g

With the limiter lamp, I tested the voltages at those test points and the -+ sides are balanced.

Took a deep breath and plugged the amp without the light bulb limiter and none of the fuses blew. I can hear/see the fan working also! No DC volts at the speaker outputs.

I checked the voltages again at those test points and +- sides are equal and the numbers pretty much jive with those on the chart (1v or so difference)

TP6 is still -3 mV

[g1]

Glad your leap of faith worked out, no fuses blew. Not sure if the fan should be running or not at idle. What is TP16 in Fan circuit on power supply drawing?
Are any of the output Fet's getting warm? (if so turn off)
How are TP9&10 voltages? TP7&8?
If TP6 is still around 0V, what is TP4 ? Does IC101 have it's proper supply voltages (+/- 15V) at pin 4 & 11 ?

[decades]

The fan is not always on. After switching the amp on, it takes, maybe, 5 minutes or so before the fan kicks in. Runs for about 5 minutes and then turns off. The fan cycles on and off. It doesn't seem to be turning too fast. I don't think the outputs are overheating but I can't verify now since I bolted and plugged everything down in they're place.

TP9 and 10 are balanced (close enough) with 2.98v and -2.87v. Schematic says +-3v

TP7 and 8 are also balanced with -4.85 and 4.89. Although, I think the schematic has an error since the chart says TP8 should 0V. ??

Yup, TP6 is still 0V ( or .8mv) and I'm getting -20mv at TP4.

The power supply voltages for IC101 and IC1 are the same with -14.9v and 14.8v.

[g1]

TP7 and 8 are also balanced with -4.85 and 4.89. Although, I think the schematic has an error since the chart says TP8 should 0V. ??

Good catch! I'm guessing the voltages they have listed for TP8 and TP6 are reversed, how does that work out?
How are your readings with regard to the bias info.?

[decades]

If the draftsman really switched those, the number I have for TP6 is ok then. I can relate, I'm a draftsman too (architectural)...staring and drawing lines all day, you're bound to mix things up.

The two ICs seemed to be the culprit for the extra current draw. Replacing them dimmed the bulb limiter. As far as why?, I need to read and study...

The output relay is working fine now.

I measured the bias voltages and they were cold as they were. What I measured (without the bulb limiter) were approx. 10mv lower than what is spec'ed. Dang trim pot was touchy but I was finally able to get an average of 22mv (with the fan running). The bias cools off a little with the fan not running. It's set a little bit colder than the specified 25mv+-average, on the other hand, I also didn't want to exceed the maximum bias voltage on any of the resistors, which is 30mv (25 +- 5mv according to the schematic)

It also looks like one mosfet is not matched closely to the rest: I measured a range of 20-28mv, while one was at 15mv. All the cement resistors were new.

[decades]

I just want to thank everyone for the help and having the patience to walk me through some of the stuff. I think I can file this project as fixed.

I finally plugged the amp into a cabinet, nothing is blowing, and it sounds great! All the functions seem to be working except for a scratchy tone selector.

This project was a big leap for me and I learned some basic things. (start with the simple visual inspection and poking around with a stick, isolate power supply/preamp/power amp sections, trace the power supply voltages throughout the whole amp, I need to improve on properly meter checking components, etc.)

Hoping to be less scatter-brained in the next project I attempt. Thanks again!