Velodyne CT100

[Jazz P Bass]

A word of caution.

Do not run the power amp without a heatsink attached to the output transistors & uPC1298.

[Jazz P Bass]

upc1298 Circuit.pdf

[tdlunsfo]

Juan thanks for the detailed reply. I have done some of the items you have requested already. Audio injected into the upc1298 pin 4 or 5 did not produce sound. The IC is the 4558 mentioned above and has the proper rail voltages. When injecting signal into pin 3 of the preamp board connector, I was losing signal on the input side of the op amp when I discovered a transistor pair setup between inverting and noninverting inputs. I was able to desolder one of the bases and get signal to pass (still not sure the purpose of these trans. are). I ended up with signal on pin 4 of upc1298 but distorted signal on the output. I am starting to wonder if the upc1298 is on the way out. If the outputs where shorted initially... seems likely.

[tdlunsfo]

This is the CT120. It's similar till you leave pin 1 of the 4558. Q501 and Q502 is what I was referencing with the transistor pair. Jazz P bass, caution noted. As of now it is doing no work.

[Jazz P Bass]

Jazz P bass, caution noted. As of now it is doing no work.

Even if the amp is 'only' at idle, without a load, there is a real chance of damaging the transistors when there is voltage applied to the transistors.
Power on, they must have a heat sink.

[J M Fahey]

[ATTACH=CONFIG]47944[/ATTACH]
This is the CT120. It's similar till you leave pin 1 of the 4558. Q501 and Q502 is what I was referencing with the transistor pair. Jazz P bass, caution noted. As of now it is doing no work.

Ugh!!! Q501/502 make a very crude compressor,think what was found in ALC (Automatic Level Control) in the 60´s.
Q502 does the actual attenuation, Q501 turns it into a Darlington to increase its sensitivity.
That system can only handle very low level signals, so it´s first attenuated 22k/1k , transistor is in parallel with the 1k leg, and then IC501A recovers original level because it has gain 22k/1k .
Clumsy and crude and out of vogue for at least 40 years, go figure.
Not too likely but if Q502 shorts it will kill all signal.

[tdlunsfo]

I desoldered the base of Q502 and was able to get signal down to the 1298. Prior to that I would lose it on the scope coming out of R502. The output out of the 1298 was distorted, so still something wrong or something wrong with my test setup. I was using around a 1k signal. Both Q501 and 502 test good with diode on the DMM. Today I am going to remove them both and test them out of circuit. A few days ago I desoldered each connector pin on the preamp board side to pin point what was firing the dim bulb. Pin 7 did the trick but as you can see, this schematic has it not used which obviously isn't the case on the CT100.

[J M Fahey]

Cool.
1) Q501 my be lossy, so always triggering Q502 turning it into a permanent mute, rather than a signal triggered attenuator

2) or maybe "something is triggering it all the time, signal or not.

*Measure DC voltage (no signal) at Q501 base: you should have way less than 1V , probably zero or a few mV.
About +1V there will mute the amp.

* measure DC at pin 7 IC501b , we want 0V there

* now I see how the "preamp" might influence the power amp, even with no signal.
Input connector not only connects Audio but also brings +15V to Q501, through R503 , allowing transistors to mute amp.

[tdlunsfo]

I went ahead and removed them both. They test fine on the bench. With them removed I am able to get a signal all the way to the output but when I connect the speaker it kills it and I get noise on the scope and no sound. I tested the speaker directly connected to my audio generator and it produces sound. Also ohms out around 9.5ohms. Makes me wonder if the power supply is healthy when under a load. As you can see from the pictures it has obviously gotten hot. I may replace the TIP29 and 30 just to see. I just don’t have any in stock.

I plan to put Q501 and 502 back in circuit and perform your tests when I get another free second from the kiddos. Thank you all for your help. My time in this is far beyond it’s worth and I am tempted to give it back to the store and tell them no thanks but it is a good example for me to learn more from.

Cool.
1) Q501 my be lossy, so always triggering Q502 turning it into a permanent mute, rather than a signal triggered attenuator

2) or maybe "something is triggering it all the time, signal or not.

*Measure DC voltage (no signal) at Q501 base: you should have way less than 1V , probably zero or a few mV.
About +1V there will mute the amp.

* measure DC at pin 7 IC501b , we want 0V there

* now I see how the "preamp" might influence the power amp, even with no signal.
Input connector not only connects Audio but also brings +15V to Q501, through R503 , allowing transistors to mute amp.

[tdlunsfo]

Well I discovered something interesting. When checking power supply voltages I touched my DMM probes between ground and pin 7 and low and behold I have audio. When I tie this point to ground through a 1 meg resistor I get the same effect. Anything less than a 500k lights the dim bulb. When i trace this pin7 it goes to the mute circuit of the 1298 pin 3 and also to the...you guest it..preamp board. I plan to trace out this complete circuit and see if you guys can help me understand this thing. It gets a little complicated on the preamp board side. More to come.

[Jazz P Bass]

Sounds like pin 7 may be part of a 'feedback' circuit.

[Jazz P Bass]

Have you tried to measure the actual dc voltage on pin 3 of the uPC1298?
(disconnected from the preamp)

From the block diagram it appears that it should be a rather high positive voltage.
Internally, it is the base of a PNP transistor that looks like the current source for the long tailed differential pair.

SEMI-UPC1298V.pdf

[tdlunsfo]

It's 58v. Based on what I have traced, this is all tied to the auto/ always on switch and auto mute. Looks like a 4558 looks for signal and turns on a transistor that grounds pin 7 and unmutes the circuit. Sounds simple but this thing has about 5 transistors and associated parts. They all test good but did just find15vdc (even when pin is unsoldered) on pin one of this 4558...that don't look right.

[J M Fahey]

Just thinking aloud:

Home Theater stuff, Subwoofers and similar, **LOVE** to have an Audio Triggered Mute.

Amp/cabinet seems to be OFF or at least asleep, and magically "wakes up" on receiving an Audio Signal.

There may be different depths of "sleepyness".
The signal detecting circuit may very well be in the preamp.
So on lack of input signal, your preamp is muted by Q502 ... AND for good measure power amp is also muted by its own Mute pin.

So when you troublesoot one section without the rest you get crazy and run in circles.

[tdlunsfo]

Ok here is how I have traced the schematic so far. I still can't understand why pin1of the 4558 is latched at 15v..even unsoldered. Sorry for my chicken scratch. Also, communications with Velodyne have been fruitless. Apparently they don't have much of a support program anymore.