QSC HPR 122i QUESTIONS

[Bigdrums]

Working on the amp module for a QSC HPR 122i. I noticed a "^" symbol in front of many but not all values on the schematic. Diodes, resistors, transistors and caps.
I can't find any notation explaining this.
Does anyone know what this means?
The part I'm looking at is a surface mount transistor called Q44. ^A42 is written just below it. Anybody know what A42 means?
How can I attach the schematic for others to see?
Thanks

[Jon Snell]

That symbol means 'abreviation'. The transistor in question is either a PZTA42 or MMBTA42 depending upon the case required. The MMBT style case is slightly smaller but the lands are the same size.

[Jazz P Bass]

From the Parts List:

[Bigdrums]

Thank you both. One more question: what does CASCODE mean?

[Bigdrums]

Never mind, found a a good explanation of it on Wikipedia: "a two-stage amplifier that consists of a common emitter stage feeding into a common-base stage. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, higher output impedance, higher bandwidth".

[Jazz P Bass]

I tend to think of 'Chunkier'.

[Bigdrums]

I finally got the Q44 transistor and got it installed. When I got this powered speaker there was considerable damage to the circuit board and several components. Several years ago I repaired most of the damage and don't remember all the details, but some issue remained. Now it's all back together ready for testing, and I have it's functional pair sitting beside it. With a variac and light bulb limiter and a 90 watt bulb the working unit won't turn on with the bulb glowing fairly bright. Quite sure it will without the limiter though. I'm concerned about turning on the repaired unit without the limiter in place. Any suggestions?

[Kevina]

Glowing lamp limiter means something is drawing lots of power.

DO NOT POWER IT WITHOUT THE LIMITER; THERE WILL BE A BIG BANG.

Find out what is causing the current flow, bridge rectifier, output transistors etc.

When the unit is repaired, it will work with the lamp limiter quite happily.

[g1]

Do you have a speaker or load connected? If so, disconnect it and see if the bulb still comes on.
If not, check for DC at the output. If so, continue repairing without load.
If no DC on output, reconnect load without turning unit off.

[Bigdrums]

G1 Curious as to why "reconnect load without turning the unit off"

[g1]

It won't power up properly (power supply rails ramping up evenly) while on the bulb limiter if the load is connected. Once it is powered up and you see the unit is fine with no DC on output, you can attach the load.
If you power down when attaching the load, the cycle just starts again and the bulb lights.

It is a known phenomenon related to the bulb limiter. Not every model has the issue, but many do. As you have seen, it does it even with your good working unit.

[Bigdrums]

Found 1 shorted output transistor (2sc5200) and replaced it. Was then able to bring up the power on the variac (with bulb limiter) and no excess current draw, in fact next to nothing just like the working unit. However the power light isn't coming on. Checked the DC on the outputs (with no load) where the low speaker is 22v and 0v on the hi speaker. I presume this could be a imbalance in rail voltage or maybe something else? Getting close with this and happy about that.... LMK what you guys think I should check to resolve the DC off set. Thanks

[Bigdrums]

I thought I was making progress but alas. The reason the amp meter was showing such little draw was I didn't have the 15v supply plugged in. When I did the draw was low .3 amps at 50% on the variac, but each time I have blown out the previous mentioned 2sc5200. Never had such little current take out a large transistor that easily.
HPR122i_Schematic.pdf

The other thing I notice is the blue "on" light does not even start to come on like the working unit does even with the 15v supply plugged in.
There also seems to be a significant DC offset, some 22v on the woofer side, not sure about the tweeter side yet. But the tweeter side is where the 2sc5200 that keeps blowing is located, Q52
Without the 15v plugged in there is no blowing out the transistors and I can bring up the variac to 100% no problem.
Any help at this point would be greatly appreciated, thank you.

[Bigdrums]

OK well, please tell me if you can if it would be "safer" to use a lower wattage bulb say 25 watts or higher like 250 watts, instead of the 90 watt I'm using currently in the light bulb limiter?

[g1]

The higher power bulb you use, the more current is allowed at the device under test. So lower wattage bulb is safer.

Do you think all the parts in the 15V supplies are good and the supply is working properly? If not, you do not want to connect it.