Ampeg B2R

[lowell]

Amp powers on. NO sound. Not even hum or hiss. Fuse hasn't blown and power transistor voltages seem fine. I'm suspecting the speaker jack relay. Maybe the relay is not the problem, but is doing its job and shutting off the output. Here's what I've got so far. I'm thinking if all these voltages seem good then maybe the relay IS the issue.

These are DC readings.

TP11: 5mv
TP18: 24v
TP17:6.6v
Q23 Base:.7v
+16v supply on top of R64:OK
D19 across relay coil:OK
+60v on cathode of D19:OK
Relay coil reads to spec: 1.44k

Ampeg B2-R.pdf

[Enzo]

OK, have you checked the speaker and speaker wiring?


Relay doing its job? No maybes allowed, find out. Either the relay clicks on or it doesn;t, do you hear the click? Or if you can;'t tell by listening, then use a volt meter. That +60v present at the top of D19 by the relay coil. Measure across D19, if the relay is energized, then there will be whatever working voltage the relay wants. If it is not energized there will be zero across the diode - in other words 60v at both ends.

And you can check the amp. Apply a test signal, and scope TP11. Got signal?

And you can use your ohm meter even. Unplug the speaker load. With the amp running, check the output jacks for shorts to ground. That would mean the relay was either not energized or its contacts had welded shut. And then measure resiatance between TP11 and TP12. If the relay is on and working then there should be continuity between TP11,12. NO continuity? Then the relay is bad or not energiazed.

[olddawg]

If you have a scope just put a signal on the amp input and look at the output signal on the input of the relay. If you don't have a scope you can put some leads on a speaker an check it that way. Relays and relay driver circuits fail all of the time.

[lowell]

OK, have you checked the speaker and speaker wiring?

No, however I've had NO load on the amp while testing it.

Relay doing its job? No maybes allowed, find out. Either the relay clicks on or it doesn;t, do you hear the click? Or if you can;'t tell by listening, then use a volt meter. That +60v present at the top of D19 by the relay coil. Measure across D19, if the relay is energized, then there will be whatever working voltage the relay wants. If it is not energized there will be zero across the diode - in other words 60v at both ends.

I hear a click, however the speaker jack HOT is grounded with amp off and with amp on. There is 37v across the relay coil so I guess it's energizing.

And you can check the amp. Apply a test signal, and scope TP11. Got signal?

Yes there is good signal.

And you can use your ohm meter even. Unplug the speaker load. With the amp running, check the output jacks for shorts to ground. That would mean the relay was either not energized or its contacts had welded shut. And then measure resiatance between TP11 and TP12. If the relay is on and working then there should be continuity between TP11,12. NO continuity? Then the relay is bad or not energiazed.

No continuity b/t TP11/TP12.

So it seems the relay is energizing and that the output section is good. I should note that the fan has been unplugged during all tests. The fan was plugged in and working however when I first started on the amp. Are you sure that the relay is energized when the speaker jacks are grounded and disconnected from the output? I'm not clear on how the relay transistor circuitry works.

[MarkusBass]

No continuity b/t TP11/TP12.
So it seems the relay is energizing and that the output section is good.

I think that it means exactly the oposite. The relay shorts TP11 and TP12 when the amp is turned on (asuming the relay is OK).

I should note that the fan has been unplugged during all tests. The fan was plugged in and working however when I first started on the amp.

The fan is not important in this case.

Are you sure that the relay is energized when the speaker jacks are grounded and disconnected from the output? I'm not clear on how the relay transistor circuitry works.

I don't think that Enzo said this.
The key is to understand how the relay works. The drawing is not the best but if you increase the drawing size, you will see that when the amp is OFF, the output jack is shorted throgh the relay (HOT to GROUND) and the output of the amp is not connected anywhere. And when the amp is ON, the relay connects the output of the amp (TP11) with the output jack (TP12).
If you don't have continuity between TP11 and TP12 when the amp is on, I'd say that there is something wrong with the relay. Either it should be replaced, or there may be another problem. Please check what is the relay voltage (it should be written on the relay). I don't remember but it can be 48V and you say that you have only 37V across the relay. It may be not enough to activate the relay. So please verify first the relay's voltage.
EDIT: I would also check the voltage on base of Q23. The relay may not work correctly if there is a problem either with Q23 or Q22. You can also plug in the fan just to be sure that disconnecting it does not cause any problems.

Mark

[lowell]

Ok Mark,
I stated in my initial post that Q23's base is .7v and that the surrounding voltages look good. I get what the relay does when ON vs OFF, just don't get the transistor circuitry at the bottom of the page. The relay is RTB14024 and has a 24v coil voltage so that seems fine... must be the relay eh?

[Enzo]

You said the output remains grounded whether the relay is on or off. That sounds to me like the contacts have welded together. You say there is 37v across the relay, so it would seem to be energized. You hear a click, put a finger on top of the relay to feel if that is what is clicking. CLicking means it is mechanically functioning, but that is no guarantee the contacts inside are working. Remove the relay, and clip a jumper betweeen TP11 and TP12. Got sound now?

When it is on, the relay normally turns on and shorts TP11 to TP12 so the ssignal can escape to the speakers. No continuity when the amp is on means the amp cannot get to the speakers.

[MarkusBass]

So it looks like a problem with the relay but 37V on the 24V relay is also strange. Is there any current flowing through it? Can you measure voltage on R53 (1k/5W in series with the relay)?
But on other hand, if you don't have continuity between TP11 and TP12 when the amp is on, removing it and testing outside the amp also makes sense. I hope you have standalone 24V supply.

Mark

[Enzo]

Obviously testing would tell, but considering that the supply is 60v, if the relay were open, there would be 60v across it. No current would mean no drop in the resistor or coil. So the 37v doesn't bother me.

[MarkusBass]

Obviously testing would tell, but considering that the supply is 60v, if the relay were open, there would be 60v across it. No current would mean no drop in the resistor or coil. So the 37v doesn't bother me.

I agree with this logic but I'm just curious what is the problem with the voltage on the relay. When I was fixing this amp, I measured the relay voltage and, as far as I remember, it was close to the nominal voltage of the relay (whether it was 24V or 48V). It could be 22V so 37V seems to be a bit strange.

Mark

[Enzo]

There is always the possibility that the 37v was a reading at the low end of the relay rather than truly "across" it. 37, plus the 24 for the relay, adds up to about 60v.

[MarkusBass]

Yes, this is exactly what I thought too. But Lowell wrote this:

There is 37v across the relay coil so I guess it's energizing.

Lowell, you measued the voltage not across the relay coil but between one pin of the relay and the ground. Across the relay you have 60V - 37V = 23V and this is exactly what is expected for a 24V relay. Since the voltage is OK, you hear a click and there is is no continuity between TP11 and TP12, the diagnose is simple: welded contacts in the relay. This could be diagnosed and fixed in 10 minutes if you had provided correct voltage readings .
Another question is why the contacts are welded? Were there shorted output devices is the amp? Do you see any signs of the output devices being replaced by someone else?

Mark

[Enzo]

Before we accuse, it was just a surmise on my part. As I reported elsewhere, correlations are not always causal. However, I am ready to wager my lunch money.

When relays weld themselves shut like that, I often wonder. But it is welded in the OFF position. All I can figure is the relay was opened at some output peak, and the inductive kick from the speakers collapsing inductive field caused a spark.


When I use the term across, I am thinking end to end of some part. But then I see people all the time checking PTs and reporting there is XXX voltage "on the red wire." With respect to WHAT?

[MarkusBass]

You are right - it is welded in the OFF position. And on the schematic it is also drawn in the OFF position, which may be very confusing for people who want to understand how it works.

Mark

[Enzo]

Generally relays are drawn in the relaxed position.