CB amplifier troubleshooting

[tomcat671]

I'm trying to figure out why my positive peaks are higher than my negative peaks (see photo) and how to fix this. It's a Black Cat HD300 with 2X SD1446. Appears to be the same schematic as a Palomar Blue Lightning. Thanks in advance for the help.

 

[Handy Andy]

Is your amplifier biased with 15 ohm resistors?

I've seen this before, but the one I fixed was ... well, not of a reason for what many people would think would cause it.

The above uses this layout


Note where L1 is, and Note where T2's "B" wire is.

Deals with the "trombone" and the winds of T1 and T2 along with R5 and R4 - which is why I asked about those 15 ohm resistors.

The Trombone is the "twin tube" - need to verify that it's soldered and CENTERED in their "spots". Winds also, they need to go to specific spots - not just taken for granted.

So you may need to remove the tubing assembly from both sides to verify that the PCB board the tubes are supported by is soldered fully to their "tube" sections for Power to get to where it needs.

The other issue is L1 - is it even there?

This is a board to sink clearance problem inclusive - so if L1 exists - make sure it's not "spot welded" before it is supposed to be grounded - past the ferrite.

This Amp doesn't use Negative Feedback like T/Star does, It's non-existent - which tends to leave these at the mercy of ground loops and the inductive issues of high-gain Open loop - that Push Pulls' have as one of many problems fighting you.

The Techs I've dealt with me handling this - simply said to change the 15 ohm back to ten and leave it. But to me that didn't answer or fix the problem.

You running this thru a NPC radio over there? This never should come up because what comes out of the back of the Radio should be symmetrical - so wondering if you are seeing something of a BIAS problem in the scope than a NPC gone wrong display.

So I've got questions because how I fixed the previous one - required a blowtorch and a lot of patience to get the ferrite/trombones right as well as their coils - which required others to help from their memory because of the rework needed to be done to the amp, had from a previous owner, took much more than just a simple repair - it was a rebuild.

 

[ExitThirteen]

That scope reading does not look right at all, the amp is biased class C, but that's a non-issue at this point, I am curious as to what the radio is, what kind of wattage is being driven into the amp, and if there are any modulation modifications present in the radio. That's the kind of info needed to help this along. I'd also like to see a scope picture of JUST the radio to see how it looks before it goes into the amp.


~Cheers~

 

[tomcat671]

View attachment 42378

Is your amplifier biased with 15 ohm resistors?

Tomcat: It is.

I've seen this before, but the one I fixed was ... well, not of a reason for what many people would think would cause it.

The above uses this layout
View attachment 42379

Note where L1 is, and Note where T2's "B" wire is.

Deals with the "trombone" and the winds of T1 and T2 along with R5 and R4 - which is why I asked about those 15 ohm resistors.

The Trombone is the "twin tube" - need to verify that it's soldered and CENTERED in their "spots". Winds also, they need to go to specific spots - not just taken for granted.

So you may need to remove the tubing assembly from both sides to verify that the PCB board the tubes are supported by is soldered fully to their "tube" sections for Power to get to where it needs.

The other issue is L1 - is it even there?

Tomcat: It is.

This is a board to sink clearance problem inclusive - so if L1 exists - make sure it's not "spot welded" before it is supposed to be grounded - past the ferrite.

This Amp doesn't use Negative Feedback like T/Star does, It's non-existent - which tends to leave these at the mercy of ground loops and the inductive issues of high-gain Open loop - that Push Pulls' have as one of many problems fighting you.

The Techs I've dealt with me handling this - simply said to change the 15 ohm back to ten and leave it. But to me that didn't answer or fix the problem.

Tomcat: Setup worked just fine with the 15 Ohm resistors. Also, the amp is used in a base. Not getting bumped around. No reason why the tubes would have shifted.

You running this thru a NPC radio over there? This never should come up because what comes out of the back of the Radio should be symmetrical - so wondering if you are seeing something of a BIAS problem in the scope than a NPC gone wrong display.

Tomcat: not sure what NPC stands for.

So I've got questions because how I fixed the previous one - required a blowtorch and a lot of patience to get the ferrite/trombones right as well as their coils - which required others to help from their memory because of the rework needed to be done to the amp, had from a previous owner, took much more than just a simple repair - it was a rebuild.

 

[tomcat671]

That scope reading does not look right at all, the amp is biased class C, but that's a non-issue at this point, I am curious as to what the radio is, what kind of wattage is being driven into the amp, and if there are any modulation modifications present in the radio. That's the kind of info needed to help this along. I'd also like to see a scope picture of JUST the radio to see how it looks before it goes into the amp.


~Cheers~

The radio is a General AP Hill keying 3 Watts. I removed a mod to the limiter that was causing it to overmodulate. Attached is the barefoot signal. If I modulate it to 100% then the output of the amp will be overmodulated.

 

[tomcat671]

One other tidbit: the 27 Ohm resistor before T1 has been non-existent. I tried adding it back in but the amp would not work at all so I took it back out.

 

[ExitThirteen]

Sent you a PM!


~Cheers~