Trying to repair an old Crown Power Tech 1

[SiliconKnight]

Hi all.

I'm trying to repair an old Crown Power Tech 1 (not the newer 1.1 series) linear amplifier.  I have found some schematics for the PT3 and service manual for the Macro Tech series (which is similar, but I suspect different enough to be risky to rely on heavily).

Does anyone have access to the Service Manual for the Crown Power Tech 1?

Thanks!

[andy3055]

https://elektrotanya.com/crown_power-tech_1.1_2.2_3.1_operation_manual_and_schematics.pdf/download.html

[andy3055]

Version 1 and 1.1 should not be too different from each other.

[SiliconKnight]

Version 1 and 1.1 should not be too different from each other.

Thanks, Andy3055.

I looked closely at the schematics for the PT 1.1 that you graciously provided.  Unfortunately, they seem to differ from the PT 1 amp that I have in several significant ways, including different connector sizes between the boards and different Test Point quantities.  This means that many of the "navigation points" I would normally use during troubleshooting are not aligning, making it very difficult to find much correlation between the schematics and the PCBs.

The symptoms are:  The amp powers on and displays a Red Fault LED and Yellow Distortion LED for Ch2.  So, I suspect that something is wrong with the Ch 2 side, but without a schematic that I can use to start intelligently tracing the fault, I'm afraid that I'll have to resort to Easter Egg troubleshooting (which is very time consuming).

I hope that someone else might have more documentation that help point me in the right direction.

[oPossum]

Check for shorted output transistors (the big ones). That is a very common failure on audio power amplifiers.

[andy3055]

Version 1 and 1.1 should not be too different from each other.

Thanks, Andy3055.

I looked closely at the schematics for the PT 1.1 that you graciously provided.  Unfortunately, they seem to differ from the PT 1 amp that I have in several significant ways, including different connector sizes between the boards and different Test Point quantities.  This means that many of the "navigation points" I would normally use during troubleshooting are not aligning, making it very difficult to find much correlation between the schematics and the PCBs.

The symptoms are:  The amp powers on and displays a Red Fault LED and Yellow Distortion LED for Ch2.  So, I suspect that something is wrong with the Ch 2 side, but without a schematic that I can use to start intelligently tracing the fault, I'm afraid that I'll have to resort to Easter Egg troubleshooting (which is very time consuming).

I hope that someone else might have more documentation that help point me in the right direction.

You might have to do this the hard way. One method is to compare the faulty channel to the good one with a meter. With no power, compare the resistance readings on identical points. Then with power on, compare voltages at identical points.

Checking the output transistors would be the first thing to do as mentioned above by oPossum. Also check if there is any DC coming out of the speaker terminals. Keep the speakers disconnected. If there are no capacitors on the outputs, you should be able to do this. If there are caps, you will need to check just before the caps. Those old amps very often had caps on the outputs. If your amp has them, check those as well.

[SiliconKnight]

Check for shorted output transistors (the big ones). That is a very common failure on audio power amplifiers.

Thanks for the suggestion.  That was my first guess, but quick in-circuit testing didn't reveal anything abnormal about the output transistors (Ch2 is faulted according to the LEDs and has no output, but Ch1 still works, the output transistors in both sides have the same resistance measurements forward and backward for both channels).

The Owner's Manual (I'm still trying get the Service Manual) says the Red Fault LED is "due to failure in the Low Voltage Circuit."  Looking at the Schematic for the PT 1.1 (and hoping that it's similar to the PT 1 that I have), the Red Fault LED seems to take an input from the Low Voltage circuits, as well as FB from the output of the Channel. 

If I'm interpreting the Fault Circuit Schematic correctly, I believe it's actually checking for 3 things:
1) Is the Output from the Channel above/below the voltage from the low voltage inputs to the output transistors (e.g. are the output transistors biased correctly and actually amplifying) - I've checked this and there a difference for Ch1, but not for Ch2 (and the signal at the Base of the transistors is good for Ch1, but just noise for Ch2)
2) The polarity of the low voltage input to the output transistors is the correct polarity (NPN above ground, PNP below ground) - I've checked this and it is.
3) Does the Output of the Channel exhibit a DC Offset > +/- 10V. - I've checked this and it is.

I've verified that the initial op-amps for both Ch1 and Ch2 (as well as the volume/gain pots) are producing the same output signal.  So, I think the issue is likely somewhere in the pre-amp, error amp or voltage translator stages (b/w initial op-amps and final output transistors), but without a more precise schematic, it's pretty challenging to follow the signal flow.

However, without a schematic that better matches my PCB, poking around in these circuits is very challenging.

[SiliconKnight]

You might have to do this the hard way. One method is to compare the faulty channel to the good one with a meter. With no power, compare the resistance readings on identical points. Then with power on, compare voltages at identical points.

Checking the output transistors would be the first thing to do as mentioned above by oPossum. Also check if there is any DC coming out of the speaker terminals. Keep the speakers disconnected. If there are no capacitors on the outputs, you should be able to do this. If there are caps, you will need to check just before the caps. Those old amps very often had caps on the outputs. If your amp has them, check those as well.

Yup - I just started doing that - put the O-scope is difference mode and started comparing various connectors/points b/w the channels.  Definitely the most time-consuming method.