PV XR400b mixer/amp with 200 H power module low output

[cablercdd1]

I have a PV XR400b mixer/amp with 200 H power module in my shop wit very low output. The problem appears to be in the 200h a series power amp. I am including the schematic. The only transistor that I have any gain from is Q3 and that's the level that goes to the speaker. I've tested the transistors and to the best of my knowledge are all good. All the voltages seem within reason. One thing I was wondering is if the power amp needs a minimum amount of drive to function. The level coming from the mixer board seems a bit low. I think I'll look at that next.
I haven't worked with too many solid state power amps so any help would be apreciated.

[Enzo]

Connect a spare cord from MAIN out to GRAPHIC in jacks. ANy help?

Your power amp is probably OK. Transistors don;t get "weak."

[cablercdd1]

I ran the main and monitor sends to another amp and it drives that amp fine. I plugged my sig gen into the aux input and still got low output. I'm going to check the coupling capacitors and resistors after q3 and see if I have an open part

[Enzo]

That is fine, but did you try what I said? Main out to Graphic in? Did you ever put a signal into the grqaphic in?

I am trying here to bypass the cutout contact of the graphic jack, which can cause this exact symptom.


There are no coupling caps in the power amp, other than C1,C2 right at the input. The power amp is direct coupled. If you had an open resistor, you woould hasve major trouble, like DC on your speakers.

[cablercdd1]

Thanks, I did as you asked. No diference in signal level.

[Enzo]

Look at the ppower amp board. On the end farthest from the power transformer wires, there is a four-wire connector for the cable to the front panel. Pull it off. Connect amp to speaker and power up. The four exposed male pins will be in that corner. The pin nearest the corner should be the amp input. Touch it with your finger. Hear hum? You can apply a test signal there with clip wires.

If that is strong, then the power amp is probably OK, if it iis weak, then we fix the power amp next. If the PA is OK, then we look to the whole graphic section up front. ANything between the GRAPHIN in jack and that four-wire cable.

[cablercdd1]

I unplugged the connector and touched the input pin with my finger and couln't hear any hum, just a low click. I plugged my signal generator at the pin and the PA puts out a low level, just like the when the mixer is plugged in, about .8 volts P-P at the speaker

[chipprogr]

Do you have a test meter that can give you a hfe, beta or gain test of a transistor? If not, then do you have at least 2 volt meters that can measure milliamps of current? I have had older amps like this before. On a diode test the transistors would show perfectly fine, but under a "real" test of its ability to amplify, they would fail. I would suspect the Q4,Q5 transistors , they may be limiting the output if not working properly. looks like your just right at the turn on point of them and then cant get any louder. ALso double check to make sure R19,20 are good and the 2 outputs connected to them are good as well.

Keep us informed- Eric

[Enzo]

OK, so it IS the power amp as you suspected, sorry, but I had to be sure.

With respect, I don;t read a low gain transistor so much.

Take off the speaker load, apply your test signal and measure the AC voltage at the output terminals without the speaker. Is it still .8v or whatever? Or did it comes back up to 20v or something?

Just my guts screaming at me, but pull C7 - next to Q2 on the schematic - and check it. In fact, just replace it. The little e-caps PV used when the amp was made 30 YEARS AGO are often a point of failure. Maces and similar were full of those 2uf/35v things and they are usually bad by now.

But back to reality. The schematic is strewn with DC voltage readings. Check them all.

COmpare signal levels through the first few parts. Q3 is the voltage gain stage, everything to the right of it is a current amplifier for the speakers. They just follow what Q3 tells them. Q3 is the biggest suspect of the transistors.

A bad Q4, Q5 is certainly a possibility as suggested. You can eliminate or incriminate them by disconnecting one end of CR4 and CR8.

[cablercdd1]

I haven't been able to get back to that amp. Too many other projects needing attention.

[cablercdd1]

Enzo and Eric. I finally got back to this amp. Something was bothering me about the amp kinda working so I attached another mixer to the input pins on the amp section and lo and behold I have plenty of nice clean volume. The transistors warm up nicely. I was sending the same level signal with my signal generator as was coming out of the amps mixer section which I see now was way too low to drive the amp to full capacity. With my mixer into the amp and the output just before clipping I measured a .3 v p-p at the input and had 6 v p-p at the output. I'm going to go back to the mixer section to figure out what's happening in there. I'll let you know how I make out.
Paul

[Down with SMT!]

Peavey XR400b- different problem

Wondering if any of you techs have had this problem:

There is a very, very faint output from the "Monitor Out" jack w/ pot set at 10. a clear strong source is being used & the subsequent output line level signal going into a known good mixer/amp & speaker.

[The "Monitor Out" reportedly had been putting out a useable output level]

Haven't gotten too deep into it yet, Factory says it could be the TLC74CN opto, but there is one per channel.
I initially deoxit'd the 1/4" jack, but the interior contacts (switchcraft) look a little discolored.

Appreciate your time & ideas.

[Enzo]

Hi Down, wlcome to the forum. it really would be best to start a new thread for your amp instead of tacking on the end of another. And we don;t want to get his problem and your problem confused.

The Monitor out jack sends the contents of the monitor bus to some other unit. The Monitor control in the master section controls its level. HOWEVER, you also have to send something onto the monitor bus for anything to happen. There is a monitor control for each channel for this purpose.


Feed a signal into a channel input - channel 1 for example. Turn up the channel 1 monitor control AND the master monitor control. DO you now have signal coming out the monitor jack? If you do, then you now know how it works.

If no monitor out happens on any channel even with their monitor controls up, then it is not likely a channel fault, it is more likely in the monitor master circuit, and that is one little op amp stage.

To be technical, your ICs are "op amps", not optos. We generally refer to things as optos when they are using some sort of photosensor. The part is a TL074, and is a very common IC. That is 0 as in zero, not the letter O, by the way.

There is one TL074 per input channel, but there are also a couple of them in the master section, and one quarter of one of them is the monitor out driver.

[Down with SMT!]

Peavey XR400b- very low monitor output

Hi Enzo, What an amazingly quick reply! Thank You!
Seems like my Mac doesn't like this toolbar...

it really would be best to start a new thread for your amp

(sorry, not sure how to correct that)

The Monitor out jack sends the contents of the monitor bus to some other unit. The Monitor control in the master section controls its level. HOWEVER, you also have to send something onto the monitor bus for anything to happen. There is a monitor control for each channel for this purpose.
Feed a signal into a channel input - channel 1 for example. Turn up the channel 1 monitor control AND the master monitor control. DO you now have signal coming out the monitor jack? If you do, then you now know how it works.

(Thanks for taking your time to explain- i was aware of the monitor functions, there is definitely no output)

If no monitor out happens on any channel even with their monitor controls up, then it is not likely a channel fault, it is more likely in the monitor master circuit,

(I assumed that- IF it wasn't a bad monitor output jack contact, which is possible, no?)

To be technical, your ICs are "op amps", not optos. We generally refer to things as optos when they are using some sort of photosensor.

(I must watch my sloppy slang, i'm more used to lighting gear)

The part is a TL074, and is a very common IC. That is 0 as in zero, not the letter O, by the way.

(Thanks for the typo correction)

There is one TL074 per input channel, but there are also a couple of them in the master section, and one quarter of one of them is the monitor out driver.

(There are only (4) TL074 on the whole board- that leaves 2 other possible IC's choices: the 14pin 4741CP's , or the 8pin RC4558P's)
Unless you know which one that is I will try to look that up.

thanks, john

[Enzo]

4741 - TL074 - interchangeable.

Starting threads is easy. On the index page for any forum section, right above the first thread on the menu, upper left, is a NEW THREAD button. CLick on it, and start typing. MAke up a thread title for the title line.

The 4741/TL074 just to the left of the five EQ sliders is U10. Try swapping it with one of the other ones. NOTE: some of the 074s point up and some down, so pay attention which way they are in the sockets. Then you will know if the IC is bad.

Make sure the monitor control itself is OK. And check to see if there is 15vDC sitting on the monitor out jack. That would be due to a shorted diode nearby if so.