I hope your in town Enzo. I asked customerservice@peavey.com for a schematic & layout Wednesday AM, and by Thursday PM when the computer I do email on died, no response. Bing can only find operating manuals, and those I would have to pay for. You have one of these schematics or a similar transformer output architectural product schematic? From 1995 according to the dates on the transistors.
It has no sound. It is a 75 w mono mixer amp I intend to use to mix the output of my Ensoniq keyboard into the speaker of my Shober Recital organ, along with the organ 1.5 vac signal. It has 4 ohm direct emitter output and transformer coupled 8 ohm 25 v & 70 v outputs. It has 4 XLR input boards with 5532 inside, and one dual RCA jack input board with the jacks 1000 ohms center pins from each other.
The OT rail is okay +-35, the op amp 4 & 8 pins are 0 and 31 v. ! measured to CT of the rail winding, the ground pins of the two rail capacitors. The CT of the main rail winding is disconnected from the "ground" output by the silence relay, which connects after 2 seconds. That is fine. But I can't find any sort of connection from the CT of the op amp winding (yes it has one, not a PV- or CS- regulation down from the main rails) to the driver transistors. The CT of the +-16 runs over to two 1 uf capacitors near the op amps, only.
I've found apparently a VI limiter TO92 transistor hooked to the base of a driver transistor, but not which op amp ? is driving the driver bases (TO126, maybe MJE340-350). Drivers pass the double diode test. It hasn't even popped into a speaker when I poked around with the meter with the power on, so maybe that output transformer has a shorted turn, but that is not tragic. My speakers are 8 ohms, I can direct connect one to 4 ohm screw and cut the wire to the output transformer if necessary. I think the off center op amp voltage is key, but haven't found a solution yet. On the theory that the output transistor bases center the op amp drive, I started checking them.
the output transistors were reading 74 ohms b-e both directions with no output or power transformer connected, and I coudn't find any 75 ohm resistors, so I pulled the OT's. They are fine, b-e is 420 to 440 ohms. So I'm wasting time here.
The op amps all read >1000 ohms from inputs & output to - & + pins, even the ca3094. It has "SPS" and "activity" leds on the front panel, I'm guessing that is what the Ca3094 is for? But no J174 to act as DDT. The two TO92 transistors are MPS6531/34. There are LM7815 and 7915 regulators, not shorted, and a LM340-24 regulator probably for the relay winding. Output capacitor on the LM7915 regulator is not shorted. ? .3 ohms ESR, 994 uF.
Thanks in advance.
It has no sound. It is a 75 w mono mixer amp I intend to use to mix the output of my Ensoniq keyboard into the speaker of my Shober Recital organ, along with the organ 1.5 vac signal. It has 4 ohm direct emitter output and transformer coupled 8 ohm 25 v & 70 v outputs. It has 4 XLR input boards with 5532 inside, and one dual RCA jack input board with the jacks 1000 ohms center pins from each other.
The OT rail is okay +-35, the op amp 4 & 8 pins are 0 and 31 v. ! measured to CT of the rail winding, the ground pins of the two rail capacitors. The CT of the main rail winding is disconnected from the "ground" output by the silence relay, which connects after 2 seconds. That is fine. But I can't find any sort of connection from the CT of the op amp winding (yes it has one, not a PV- or CS- regulation down from the main rails) to the driver transistors. The CT of the +-16 runs over to two 1 uf capacitors near the op amps, only.
I've found apparently a VI limiter TO92 transistor hooked to the base of a driver transistor, but not which op amp ? is driving the driver bases (TO126, maybe MJE340-350). Drivers pass the double diode test. It hasn't even popped into a speaker when I poked around with the meter with the power on, so maybe that output transformer has a shorted turn, but that is not tragic. My speakers are 8 ohms, I can direct connect one to 4 ohm screw and cut the wire to the output transformer if necessary. I think the off center op amp voltage is key, but haven't found a solution yet. On the theory that the output transistor bases center the op amp drive, I started checking them.
the output transistors were reading 74 ohms b-e both directions with no output or power transformer connected, and I coudn't find any 75 ohm resistors, so I pulled the OT's. They are fine, b-e is 420 to 440 ohms. So I'm wasting time here.
The op amps all read >1000 ohms from inputs & output to - & + pins, even the ca3094. It has "SPS" and "activity" leds on the front panel, I'm guessing that is what the Ca3094 is for? But no J174 to act as DDT. The two TO92 transistors are MPS6531/34. There are LM7815 and 7915 regulators, not shorted, and a LM340-24 regulator probably for the relay winding. Output capacitor on the LM7915 regulator is not shorted. ? .3 ohms ESR, 994 uF.
Thanks in advance.
I don't have the schematic of the MMA-875T but I think the power amp is the same one used in the Peavey UMA75T. Both are 75W.
If so, the center of the psu caps is not 0V but the amp output. The output is similar to the QSC collector ground but in this case, it's emitter ground.
Take the 0V at the center of the two 0.33 ohm emitter resistors or simply the chassis. The opamp rails should then read about +- 18V.
Regards
Mike
Attachments
Thanks, that does look similar. So the final op amp drives the TO-126 drivers through 100 ohms. the OT emitters drive the case ground and one side of the output transformer. The back side of the output transformer is connected to power transformer CT through a 1.8 mH coil. And in my unit through the contacts of the power up silence relay.
The two TO92 transistors are VI limiter.
I'll have to do some reassembly to check the op amp power voltages again. After I find where the 75 ohms b-e of the output transistors is. That strikes me as dingy, but both npn and pnp output transistors have it. My unit has heat sink for 4 output transistors, but only one pair installed: the other two 10 w emitter resistors are missing too. The OT are 704x0140 as shown which the interchange chart says are MJ15003/15053.
All those diodes in the driver stack!
The two TO92 transistors are VI limiter.
I'll have to do some reassembly to check the op amp power voltages again. After I find where the 75 ohms b-e of the output transistors is. That strikes me as dingy, but both npn and pnp output transistors have it. My unit has heat sink for 4 output transistors, but only one pair installed: the other two 10 w emitter resistors are missing too. The OT are 704x0140 as shown which the interchange chart says are MJ15003/15053.
All those diodes in the driver stack!
If I am not mistaken, there should be two transformers inside. The Mains transformer is wired to the bridge rectifier, followed by the filter caps with the center between the two caps for the amp output.
This output goes through a 1.8uH//22 ohm 1W followed by a zobel network (0.1uF+5.6 ohms 5W).
After that, it goes into the output transformer, which is an autoformer.
For direct out (ie bypassing the autoformer), connect speaker red wire to PIN 3 and black wire to PIN 1 (MAINS GROUND).
To use the output transformer, jump wire from PIN 3 to PIN 2. This will activate the output transformer.
Once this is done, you can connect the speaker red wire to any one of the terminals of your choice (4 ohms, 8 ohms, 16 ohms, 25V or 75V line distributed system).
Regards
Mike
Mine has a "gnd" connected permantly to the case, and to the OT emitters through a connector & the thermal switch. On the same side is a "4 ohm" screw which goes to the CT of the power transformer and the center of the rail caps when the silence relay closes. I think you can run speakers from 4 ohm screw to GND screw.
the other terminal strip is "COM" , 8 ohm, 25 v , 70 v. COM screw doesn't connect to "GND" screw. From COM screw to 8 ohm screw is 1 ohm on my DVM, which I find suspicious. I'm going to put a 5 A fuse between 4 ohm screw and the output transformer. Pulling the fuse will allow me to see if music comes out that way, without the transformer shorting the output. The output transistors don't seem damaged, though.
I found the 74 ohms between OT base & emitter. R67 100 ohms ties the two OT bases together, and merges current into a network of 100 ohm resistors on the VI limiter circuit. Why would they do that? Bleed off half their base drive current? They only have 1/2 watt driver transistors; the amps I've worked on before have trouble getting enough base drive into the OT's to make them put out serious wattage.
This one has two tech initials on the mainboard, but the OT's are dated 95. Wonder what they replaced? Rail caps are IC (Illinois Cap) and 3 ohms ESR, so they are probably original (and doomed to the trash can this time). The unit is strangely clean, I would suspect it to be a first year warrenty failure except for the tech initials. Cleanliness is one reason I'm suspicious of the output transformer, steam cleaning might have killed it.
the other terminal strip is "COM" , 8 ohm, 25 v , 70 v. COM screw doesn't connect to "GND" screw. From COM screw to 8 ohm screw is 1 ohm on my DVM, which I find suspicious. I'm going to put a 5 A fuse between 4 ohm screw and the output transformer. Pulling the fuse will allow me to see if music comes out that way, without the transformer shorting the output. The output transistors don't seem damaged, though.
I found the 74 ohms between OT base & emitter. R67 100 ohms ties the two OT bases together, and merges current into a network of 100 ohm resistors on the VI limiter circuit. Why would they do that? Bleed off half their base drive current? They only have 1/2 watt driver transistors; the amps I've worked on before have trouble getting enough base drive into the OT's to make them put out serious wattage.
This one has two tech initials on the mainboard, but the OT's are dated 95. Wonder what they replaced? Rail caps are IC (Illinois Cap) and 3 ohms ESR, so they are probably original (and doomed to the trash can this time). The unit is strangely clean, I would suspect it to be a first year warrenty failure except for the tech initials. Cleanliness is one reason I'm suspicious of the output transformer, steam cleaning might have killed it.
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Sorry for the confusion. Apparently the output transformer in the MMA 875T is different from the UMA75.
With the MMA 875T, the amp output goes through a relay and to a 4 ohm direct out. For this, the connection is 4 ohms and GND.
The amp output is also connected to the primary side of a line matching transformer.
On the secondary side, you have COM, 8 OHMS, 25V and 70V. This side is FLOATING, that's why COM is not connected to GND.
A word of caution. If you use direct 4 ohm OUT, do not connect any speakers to the transformer out. It will blow the outputs. It's either direct 4 ohms out or transformer out but not both at once.
"I'm going to put a 5 A fuse between 4 ohm screw and the output transformer"
I don't recommend doing that.
R67 100 ohms ties the two OT bases together, and merges current into a network of 100 ohm resistors on the VI limiter circuit. Why would they do that? Bleed off half their base drive current?
The conventional driver connection is the emitter resistors R69, R75 (100 ohms) both tied to the output.
R67 (180 ohms) ties the two driver emitters together. So, there are actually two biasing schemes at work.
These two biasing networks do not affect the VI limiting circuit. Over current is sense by 180 ohms (R65, R79) taken from the emitters of the output transistors.
Regards
Mike
With the MMA 875T, the amp output goes through a relay and to a 4 ohm direct out. For this, the connection is 4 ohms and GND.
The amp output is also connected to the primary side of a line matching transformer.
On the secondary side, you have COM, 8 OHMS, 25V and 70V. This side is FLOATING, that's why COM is not connected to GND.
A word of caution. If you use direct 4 ohm OUT, do not connect any speakers to the transformer out. It will blow the outputs. It's either direct 4 ohms out or transformer out but not both at once.
"I'm going to put a 5 A fuse between 4 ohm screw and the output transformer"
I don't recommend doing that.
R67 100 ohms ties the two OT bases together, and merges current into a network of 100 ohm resistors on the VI limiter circuit. Why would they do that? Bleed off half their base drive current?
The conventional driver connection is the emitter resistors R69, R75 (100 ohms) both tied to the output.
R67 (180 ohms) ties the two driver emitters together. So, there are actually two biasing schemes at work.
These two biasing networks do not affect the VI limiting circuit. Over current is sense by 180 ohms (R65, R79) taken from the emitters of the output transistors.
Regards
Mike
Attachments
Okay, thanks. Diagram post # 6 describes the situation I see on the table. I'll use one speaker, (3 element with crossover network) impedance 8 ohm, either on the 8 ohm tap to common, else on the 4 ohm to gnd screws. 17 vac will be loud enough in my music room. (half 35 v rails).
You're right, 100 ohm resistors from driver emitter to speaker ground is conventional. Something about leaving a path for the base charge of the output transistor to bleed off as it shuts off at crossover.
You're right, 100 ohm resistors from driver emitter to speaker ground is conventional. Something about leaving a path for the base charge of the output transistor to bleed off as it shuts off at crossover.
If you are using the amp "locally", not feeding faraway speakers, I suggest you disconnect output transformer, and use the direct output instead.
Not strictly necessary of course, but you remove one variable out of the equation and simplify your life.
That´s why manufacturers almost always provide for a jumper which can be installed or removed.
+/- 35V rails are more than enough for quite loud Music anyway 🙂
Not strictly necessary of course, but you remove one variable out of the equation and simplify your life.
That´s why manufacturers almost always provide for a jumper which can be installed or removed.
+/- 35V rails are more than enough for quite loud Music anyway 🙂
That is why I was thinking of cutting the hard splice from 4 ohm to output transformer and putting a ATO fuseholder in there. Pull out the fuse, no connection. This device apparently has swinging rail power supplies, so if something hangs up the CT feeds 40 v to the transformer as DC and magnetizes the core.
Right now output transistors (OKay, checked) are centered 4 ohm out at zero, but 5 op amps on power board have minus rail at 0 and plus rail at 31 v. ??? The op amp rail for output is apparently derived with 2.7k to 16 v zener/ 22 uf capacitors just like most Peavey PV amps. I've found the diode stack to driver bases and maybe the 100 ohm resistor from op amp, but haven't found which op amp drives the driver transistors yet. Keep running into diodes. There are four BA4560 and 1 ca3094, one of those drives the drivers, which one? Wish I had a part layout, tracing PC lands is getting old. One end of the 100 ohm resistor goes to 1n4148parallel33 ohms which is per schematic, the other end doesn't go to pin 1 or 7 of any 4560 on the board or pins 6 or 8 of the 3094, the ohmmeter says.
There are 7815 and 7915 regulators on the same board but apparently are for all the input boards & tone controls.Regulator Out doesn't connect to the power pins of the op amps on this board. Neither does low voltage CT (separate winding)
Something apparently was major wrong because the unit was so clean. Never been out of the box a month, looks likely. built 96? Bad solder joint somewhere? Based on the one I found in my PV-1.3k after 4 sets of output transistor replacements on that channel.
Right now output transistors (OKay, checked) are centered 4 ohm out at zero, but 5 op amps on power board have minus rail at 0 and plus rail at 31 v. ??? The op amp rail for output is apparently derived with 2.7k to 16 v zener/ 22 uf capacitors just like most Peavey PV amps. I've found the diode stack to driver bases and maybe the 100 ohm resistor from op amp, but haven't found which op amp drives the driver transistors yet. Keep running into diodes. There are four BA4560 and 1 ca3094, one of those drives the drivers, which one? Wish I had a part layout, tracing PC lands is getting old. One end of the 100 ohm resistor goes to 1n4148parallel33 ohms which is per schematic, the other end doesn't go to pin 1 or 7 of any 4560 on the board or pins 6 or 8 of the 3094, the ohmmeter says.
There are 7815 and 7915 regulators on the same board but apparently are for all the input boards & tone controls.Regulator Out doesn't connect to the power pins of the op amps on this board. Neither does low voltage CT (separate winding)
Something apparently was major wrong because the unit was so clean. Never been out of the box a month, looks likely. built 96? Bad solder joint somewhere? Based on the one I found in my PV-1.3k after 4 sets of output transistor replacements on that channel.
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The ca3094 is a transconductance amp (OTA) used for the DDT. So, it can be discounted. It is one of the 4560.
BTW, have you checked whether the problem is the amp or on the mixer side?
BTW, have you checked whether the problem is the amp or on the mixer side?
Injected fm radio earphone in the"pwr amp in" RCA jack on the back, no sound. All knobs at mid-range. Put FM radio in a XLR connector on the back, vol screw mid range, no sound. So working on the power amp part first. +-15 from 7815-7915 that don't go to the BA4560's on power board is fine. Traced -40 rail through 2700 ohm resistor to 15 v zener, to pin 4 of one BA4560. Hope that's the one that drives driver bases but pin 7 & 1 of it does not go to 100 ohm resistor connected to 1n4158//33 ohms. Next quiet evening will trace the lands from 1 & 7 that IC, could be a bad solder joint on bottom of op amp socket.
Thanks for interest.
Look for the other 2700 ohm at the +40V rail. You should be getting +15V at the zener. The same +15V should appear at pin 8 of the same BA4560.
If there's no +15V at the zener, the 2700 ohm is damaged (opened).
I'm not in town right now. It'll be easier for me to help you troubleshoot this amp after I fly back on Apr 13.
Meanwhile, I've attached a schematic of the PV-1200 that suffered a catastrophic failure. It took out all the parts circled in red.
It will give you an indication of the parts to look at.
Attachments
I've checked the output transistors and emitter resistors, they are fine. I've checked the drivers with the OT out, they are fine. There are no TO3 drivers since this is one pair OT, but there is a VI limiter, which is fine. So there was no meltdown.
I've traced both flying rail 15 v supplies to 4 & 8 of one BA4560. No other IC on the power board connects to this. Pin 7 drives a 10 k resistor and a cylindrical capacitor brown red black black black. Pin 1 drives a 22 k resistor. This doesn't look like the 4558 driving the 100 ohm resistor in the UMA75 schematic. The other 3 op amps on the power board run off the main 15 v regulator IC's, the ground of which is not tied to the earth of the flying rails power stage.
The base of the PNP driver goes to the negative input of a 4560 on the regulated 15 v . ??
I'm putting the board back in and will try to follow the radio music in from the "pwr amp in" jack with my analog VOM.
I've traced both flying rail 15 v supplies to 4 & 8 of one BA4560. No other IC on the power board connects to this. Pin 7 drives a 10 k resistor and a cylindrical capacitor brown red black black black. Pin 1 drives a 22 k resistor. This doesn't look like the 4558 driving the 100 ohm resistor in the UMA75 schematic. The other 3 op amps on the power board run off the main 15 v regulator IC's, the ground of which is not tied to the earth of the flying rails power stage.
The base of the PNP driver goes to the negative input of a 4560 on the regulated 15 v . ??
I'm putting the board back in and will try to follow the radio music in from the "pwr amp in" jack with my analog VOM.
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Okay, found the problem. Bad solder joint, which looked good. Tracing signal from the flying rails op amp to the driver bases, one jumper didn't have 0 ohms to the adjacent resistor pad unless I pushed real hard.
This unit probably has under 1 hour in 24 years. A little solder, new rail caps ( 4160 uf, ESR 0.27) and this unit will be better than new.
Don't tell me electrolytic caps don't deteriorate sitting on the shelf unpowered.
This unit probably has under 1 hour in 24 years. A little solder, new rail caps ( 4160 uf, ESR 0.27) and this unit will be better than new.
Don't tell me electrolytic caps don't deteriorate sitting on the shelf unpowered.
It's great that you got the amp working.
A common problem I encountered with such amps is blown rail caps. I had some with their tops ruptured, spilling electrolyte all over the pcb.
A common problem I encountered with such amps is blown rail caps. I had some with their tops ruptured, spilling electrolyte all over the pcb.
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