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indianajo 5th February 2012 06:55 PM

Peavey 1.3k versus PV-2000
My Peavey 1.3k that I have been monkeying with for several months, has a shorted B side power transformer. Reads continuous on ohms scale, but collapses to 3 volts on one side of center tap if both B top and bottom are loaded with 700 ohms to center tap. I had first replaced the output caps, thinking maybe one was leaking (they are 18 years old)
I think I'm going to have to buy a new transformer. The original peaks at 95 VDC per rail, which I calculate at about 66 Vaverage (as transformers are sold). I'm actually reading above 90 VDC on the A side. My first thought was connect both output stages to the A side of the transformer and make a Peavey 650, but I'm afraid the A side would go assymetric with any load with a shorted turn in there and blow output transistors with the DC offset. There is only a 4k resistor pair establishing speaker return voltage. Any thoughts on that?
So the rated output is 51 V 8 ohm. I was wondering if modern transistors can run closer to the rail voltage without distorting much. I've actually installed MJ21195G and MJ21196G transistors, they are cheaper. Original OTs are MJ15024 and MJ15025 by the cross ref list. So maybe I could buy a lower voltage transformer or SMPS and waste less heat. 58 V rating instead of 68V?
I looked at the Peavey PV-2000 schematic for guidance, and it only has 3 O.T. pairs instead of 5, same part number!!! ???? The rail voltage is not listed on the PV-2000 schematic, which would be interesting to know.
I'm looking at Antekinc 1500 VA toroids and connexelectronic A1000SMPS with the stranded wire (1200 W) option. Various voltages are available. Any thoughts about the switching noise of the connexelectronic unit? Do I have to build a steel box around it? I've got plenty of junk finned heat sinks from blown motor drives.
(Oh BTW, I'm putting speaker disconnect relays in there based on the 8v dc offset diac triggering them. If I can fit it in I may put in relays between the rail capacitors and the O.T.'s to save money on OTs. My speakers cost 3X the cost of this beast with the parts, I wish to save them, not the amp.)

Enzo 6th February 2012 06:02 PM

Did you ask Peavey if they still have the transformer? Big honking thing like that has to fit and mount in the chassis. The factory part is going to bolt right in.

You cannot run both channels off one secondary winding. Remember this amp has "flying rails." The CT of the power transformer does not go to ground. The power supply is not just sitting there.

As to throwing a lot of transistors at it to get closer to the rails, I don;t think that is a good move. The amp ratings are not pushing things, any limitations on how close to rails you get are not due to a fraction of a volt junction potential in the transistors. It puts out its rated power.

I don't see 4k resistors in the speaker circuit. WHat are the resistor numbers you refer to and where are they on the print?

indianajo 6th February 2012 06:33 PM

Thanks for listening. The center tap of the 95V on the PV1.3k is established by R265 and R267 on the diode bridge-rail capacitor PWB. They are 2 4k 5W resistors.Each winding CT connects directly to the emitters of the OT's on the two sides. Bottom of the eserviceinfo print.
Point taken about the flying rail whatever that means. I removed the A side capacitors and B side transformer leads, jumpered A side of PS to B side on the PS PWB, and tried to play it last night. The speaker on the B side was playing as well as the A side. The rail fuses (I installed the holders after getting tired of burned triac traces) were removed on the B side. ???? One datapoint is that the PNP collectors on the A side minus transistors are 650 ohms to the heat sink. Will be chasing that. Spent a couple of days removing rosin bridges to get the transistor quints to read <2ma current through a 47k resistor and a 19 V power supply from C to E on each quint with B to E shorted.
I don't hate my Peavey dealer but I've given up driving and he is 3 miles from the nearest bus stop down a lane with deep ditches, no sidewalks, and lots of impolite gravel & lumber trucks. I've put off walking up there to buy the double diode for the heat sink, and I really don't want to carry home a 40 lb transformer whatever it costs.
Besides, my speakers are either SP2-XT rated 49 Vrms, or T300 Hi-frequency-projectors rated not very much with 2 10" woofers. This 1.3k is really too hot to keep as is. If I sold it I'd only get $2-300. If I have to buy a transformer, I'll probably keep it for the T300's to play PBS great Performances on. So I want something a little less voltage spike inducing so I don't blow the cones out the grills. I really wanted a PV4 or something, but this came up first on craigslist. Drilling holes to mount weird transformers is not a problem. Actually I've already got 4 each @$10 132 VAC CT 2.5 A transformers that will fit in there. Some sort of 1 ohm balance resistors on each leg will be required to parallel 2 transformers, I suppose. Cut the turn on surge to 10000 uf, too.
SO why does the PV1.3k have 5 OT pairs per channel and the PV 2000 have 3? Noisier faster fan? Better heat sink design? Higher voltage rails?

Enzo 7th February 2012 12:22 AM

No no no, pick up the phone and CALL Peavey in Mississippi. Then you only have to carry the part from your door, where UPS drops it off. Your dealer won;t have one, he'll have to call and order it anyway. PV sells direct to anyone. DOn;t walk anywhere with it other than in the house.

I think you will find those 4k resistors are just there to even things out, the transformer center tap is connected to the speaker lead at J103. They are not an artificial center tap - they are not like the pair of 100 ohm resistors across a Fender 6v winding.

FLying rails? Grounded emitter to some? Look at the schematic. Your common everyday amp has output transistors from the V+ and V- rails through emitter ballast resistors and meeting at the speaker hot lead. Now look at this one. That point is grounded. The "output" goes to ground. The speaker is wired from ground to the center tap of the power transformer. SO when signal is amplifying, the +/-95v rails don;t just sit there, they are moving around with the music. Having two channels trying to run off it might be done if they are getting the exact same signal, but if the signal differes between channels, then one channel tries to move it one way while the other tries to move the same supplies the other way. PFFFT. It might be a little more clear on the PV2000 drawing.

I don;t know which drawing eservice offers, I always use what the factory provides, in thos case drawing 98872002 for the PV1.3K.

Before you go trying to rewire this, you really ought to study how this system works. It is not just a power supply sitting there. You don;t want to burn up a new transformer.

indianajo 7th February 2012 01:09 AM

Okay, thanks. I didn't understand the flying rails. The schematic I've got doesn't show where the tabs between PCB's connect, so I wrote the colors down on the PCBs with a sharpie, but that doesn't organize the concept in my mind all that well. Actually, the schematic doesn't have the transformer PN either. Calling Peavey- that is a thought. I thought Peavey would be stuffy about selling to dealers only.
Abandoning the jumpered PS rails off one winding, and beginning install of two independent 132 VCT (open circuit) transformers. About 96 VCT at 2.5 A (tested). The HDTV output is real stereo, so independent windings will be required. Don't need 1.3 kw, 200 w/channel 8 ohms will be fine. Thanks again.

Enzo 7th February 2012 06:50 AM

PV is absolutely THE most customer friendly comany in this business. They will sell you parts, they will send you schematics, you can call them on the phone for technical assistance, both for how to use their products and even in repair efforts.

I hope the experiment works.

If you can supply 2.5A into the load, why not calculate how much power that represents for say 8 ohms. Then calculate the current you need for the 200 watts. Remember, this is a solid state amp, it is going to produce an output signal voltage, then the load impedance will draw the current that demands... or at least try to. If these transformers can produce the full voltage the much larger stock transformer produces, then at full volume the load will try to draw the same power as well. The load has no idea the new transformer has a lot less current capability, it will still try to draw full current.

You still have the original transformer? The part number should be printed on it. PV transformer numbers start with 705.

indianajo 7th February 2012 06:19 PM

I've blown up some stuff on the B driver board with my short A center to B center, drive A with radio & not B, no power to B OT's experiment. Never ask a question on Super Bowl Sunday and hope for an answer before Monday. I'm probably the the only man in the USA that doesn't watch pro football.
1.3k OT's and driver transistors are passing the 19V no leak current test(b-e shorted) but at least the I blew C213, and the OT Base driveline 5A fuse I installed instead of jumpers, and the protection 3.3 V 1.3 W zener diodes I put between the Emmitter drive traces and ground (to keep from melting ground traces on the driver PWB when an OT goes). Will be lifting a lot of diodes & stuff near the emmiter line on the B driver PCB looking for blown stuff for a few hours.
I don't even need 200 watts for my HDTV, just that 2.5A @ 95VAC these US made transformers put out is 250 watts and some of that goes in the heat sink. I'm quite happy with the volume I get out of my dynakit ST120 for home use, 60 W/ch. I'm running 1.5 Vpp at normal listening volume. into the SP2-XT's at 8 ohms. The T300 Projector speakers near the HDTV are 8 ohms too. In the PV 1.3k I had installed 4 ea 4700 uf 100v 2011 manuf caps on the B side PS, and 6 ea 3300 uf 100V 2008 manufactured caps on the A side power supply. I'll probably delete half the 4700's and two of the 3300s to unstiffen the power supply for home use. I might even delete 2 pairs of O.T. per side, I don't need that much current on the speakers. Actually I'm still a little worried about lightning pops causing the speakers to blow even with 2x 6600 uf rail caps. I get them through the ST120 even though I have a 600V MOS supressor installed on the ST120 transformer input in addition to the dynakit designed .01 uf 1000V disk cap. I've blown two unmatchable tweeters on 2 sets of speakers with a tube ST70 amp, so I might be hypersensitive to blowing stuff. At least the SP2-XT's have available tweeter drivers, which is part of why I bought them.
Even if I just use one 2.5A@95VCT transformer per side (Open circuit 132 VAC) I'm going to put 1 ohm 25 watt resistors on the t-former hot leads to cut the turn on surge and unstiffen the power supply. I'm hoping the DDT circuit will cut the impact of lighting pops on the speakers, but who knows how to test that it works. Everything around the 3180 IC's was blown on A channel including the J174 JFET, but the 3180's weren't shorted between pins like the 4558 op amp was.
I'll figure out all what is blown before I call Peavey for the double diode heat sink sensors. I don't have long distance telephone service; I make such calls from the payphone at gas station a mile away where it is hard to hear, so thanks for your help over the internet.

cwahls 7th February 2012 07:57 PM

I bet I have a transformer for that. If you decide you need it you can pm me. I'll check tonight.

indianajo 1st March 2012 01:37 AM

Okay, thanks cwahls for the new transformer. Works. the next challenge was the ground rivit was loose on B side O.T. assy so I had the rails going +176V -2 v instead of +-84 for a while, with the speaker return at -76V. (I do have the 120 vAC on a 1500 watt heater element which damps down the violence a little if an O.T. explodes).I drilled the rivit out and put a 4-40 stainless screw in there with star washers on each side. Rails are now +-84Vdc and speaker return is back to 0. I have 10 ohm 450 watt resistors on A and B speaker outputs, with a car radio speaker across 5 ohms of each. The car radio speakers are protected by 50 uf 600 VDC motor caps in series.
So I can't get B side to make music at all. A just plays right along with the radio input. On B driver PCB, I found the Jfet shorted across, and the 3080 op amp shorted from an input to one power rail, and the 4558 op amp measured a little funny between pin 6 and the negative power supply, so I replaced them all. The 33k and other resistors around there measure okay. All the electrolytic caps in the entire amp are new. I've got the O.T's idling on B side at O v DC out, and if I touch pin 5 of the 4558 with a charged up .1 cap it pops in the speaker. By playing an FM radio into it at 4 VAC, turning the input pot all the way up on B (pos 1 on A), I can get music to go through the pin 2 of the 6 pin connector from the input board, through pin 1 out of the 4558, and then it stops dead right after C206. I took the input board out today to look for shorted or blown diodes, resistors, or traces, and nothing is wrong there. the 2.2 NP cap is new too. (2.2 uf 50 V ceramic). there is no diagram of that on eserviceinfo so I had to draw it out, and make sure corrosion wasn't stopping my signal.
So I checked with my 200000 ohm/v simpson 266 on the 2.5 VAC scale, ,and I can see 3 VAC music come out of the input op amp (4558) on B side pin 1, go through c206 the 20 uf cap into the 1.2 k input to pin 6 of the 4558, and it stops dead at the input of the resistor. The resistor reads 1.2 k, the 61 k resistors reads right, the other 1.2k and 61k resistors from O.T. feedback pin 9 read right, the op amp just sits there with 0vDC on 5 and 6 and 0.8V dc coming out of 7. Op amp power supplies are fine. The 4558 is new, the 3080 is new, the J174 is new. pins 9 of the output transistor board is reading 0 (the feedback) and pin 10 is reading 0. The power to the op amp is reading +-15 per spec. ????? Am going to have to button it up and clear off the table to start checking the power supply of my scope why the sweep quit working, but probably I don't think I can chase this current mode problem with that either if it does ever work again . ($40 B&k 2120 scope, worked fine till I left in on for 3 hours once. will replace E-caps and maybe blown diodes etc. beam sits still).

cwahls 1st March 2012 02:45 AM

Great news! I love recycling things to those that can use it.

I still have one (presumably) good driver board here......

Have not tied into the 8.5C yet but keep posting on your progress, you may just get something in the mail.

Don't over think this, it's a simple amp, if you don't have schematics contact Peavey, they are free. When it's up and running, it a damn stable amp for what you are using it for.

When they blow up, they only blow up so far back, not often do they take opamps.

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