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Peavey Renown

Started by Hawk, June 26, 2015, 09:12:28 AM

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Hawk

So, thought about it some more and fired up the amp with the limiter and slowly brought up the voltage to about 94 volts using a 100 W bulb. Bulb is very bright, current draw reads approx. .6 volts, 1mv DC on output which seems reasonable. But the rails are reading -/+ 1.47 volts. Seems way too low. Any thoughts? Ground to  Collector Voltage on Transistors 6,7,12 shows 1.47V, and the emitter voltage on 10,11,and 13 show -1.47.  Rail voltage on Schematic, when operating normally, is  +42 volts.  How much voltage is being dropped across the lightbulb? I checked the line volts on the amp and it reads 11 volts. hmmm,not a lot of voltage, the amp has far to go, so we should see an increase in rail voltage, but is this voltage too low at this stage and should I be concerned? ???

g1

  The quote about the bulb wattage said to use the wattage of the output power, not the input from the AC line (120V).  The 450W is input wattage, the actual amp power output is much less, I think around 150W ?
If the bulb lights up (more than dim), then there is a problem.  That is why the voltages are so low.
  Check for low resistances across the supply rails, which are usually caused by shorted power transistors or rectifier diodes.

Hawk

.7V AC across 42 volt supply, line side. 3.5v AC across 15v supply. As an aside I orginally checked all the Output Transistors and all the others on the power amp side and they checked out okay but one which was shorted and I replaced it and the other in last stage with a matched set from Peavey.
42 v supply diodes check out okay. What kind of resistance should I find across the 42 volt transformer winding?   Info so far...

Enzo

I have no idea what the transformer winding measures, I have never had to measure one.  The transformer is likely just fine, it is the last thing on the list of parts failing.  If you suspect the transformer, then pull the wires off the board, power up the transformer and see what AC you get on its wires.

450 watts is the maximum the amp can ever pull from the mains, not a steady need.  A 100 watt bulb is just fine.  When the bulb glows brightly, that means the amp is acting like a big short.  And when the bulb is glowing bright, that means very little voltage is getting into the amp itself, so of course any voltages in the amp will be very low.  That is the point of the bulb.  You are testing the amp at idle anyway, and it doesn't remotely draw 100 watts, let alone 450 of them.  At least when it is working.

The bulb is there to save the amp from blowing fuses.  it saves fuses, but also prevents full current from flowing in a damaged amp. It is not ever meant to be used while operating an amp.   Once the amp is stable and not making the bulb bright, we get rid of the bulb.

Hawk

Thanks gents!  So the AC voltage off the transformer is good. Checked the diodes using  a resistance check and they don't look so good. D36--8.45Mohm, 5.9Mohm  D34 .1ohm, 0 ohm,
D37 3.67 Mohm, 7.95 Mohm D35 0ohm, 0ohm. The voltage that is fed from the diodes is only 1.4 volts which is what I'm getting on the rails. The diodes look good visually. My readings were with the diodes in circuit.

I checked all the resistors in the power amp circuit and they checked out good except for R97 which should be 4.7K but reads as .8k, again, in circuit. How important is this to getting the amp up and running?

I'm including a photo of something wierd that I found. C53 and 54 don't exist in the amp. C53 only has the leads left, and C54 looks like a resistor and measures like one, 107kohm?. My friend didn't give this amp to anyone else to fix and it's been sitting in his basement for 20 years. I suppose C53 could  have broken off after I opened up the amp but I wasn't tinkering with that side of the amp. Could C53 have anything to do with the amp not working? :-\

Enzo

Visual inspection is important, because it something looks burnt or cracked, it needs replacing.  However, MOST defective parts do not look any different.  Does your car look different if a motor bearing is worn?

Those rectifiers are shorted, I'd just replace all four.  The shorted ones may have stressed the other two.  They are just 6 amp rectifiers, 100v or higher.  Whatever voltages are in the circuit now are irrelevant, because the diodes are shorted and because the bulb affects all voltages.

R97.  Look at your schematic.  One end is grounded, the other connects to a cap and R99.  The cap offers no DC path, but follow the line from R99 down and over and you will see it has a parallel pair of 0.1 ohm resistors to ground, R119,R120.  That makes 0.05 ohms, which is essentially zero, so the lower end of R99 is really grounded..  That means 1k R99 is now in parallel with 4.7k R97.  What do they total then?   I get 824 ohms, or 0.8k

Don't worry about resistor readings until the big stuff is fixed, unless the readings show open or shorted.

C53,C54.  C54 is there, it is the green one that looks like a resistor.  C53 has been snipped out.  The reason C54 reads like a resistor is that there are resistive paths to ground from either end, and your meter is measuring them.  Don't worry about it.

Hawk

Thanks Enzo, greatly appreciated. So I replaced the rectifiers and am now getting a mere .31 vDC on the rectifiers and on the rails. The 100 W bulb  is shining very brightly, at 90 V on the variac :'(

I was getting 1.4 volts with the shorted rectifiers...

Hawk

Okay, back to getting 1.4VDc on supply rail and other side of rectifiers....light bright as ever.

DrGonz78

#53
Try disconnecting the secondary leads of the transformer at this point. With all the secondary taps disconnected does the light bulb shine bright? I think at this point I would want to know the health of that transformer.

Edit: Perhaps I missed something here though...
Quote from: Hawk on July 30, 2015, 10:46:55 PM
Thanks gents!  So the AC voltage off the transformer is good.

So you have checked the AC voltage coming off the transformer that leads to the rectifier and it is good? Then are there burnt or open traces any where on that part of the board around the rectifiers etc. Should be even looking for shorts caused by solder bridges even.
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk

Yes, all the secondary taps  disconnected and the lightbulb shines very dimly at 120V so all appears well. Will check for solder bridges, open traces...

Hawk

Flipped board over--no solder bridges or burnt marks on rectifiers. I put in new rectifiers and the problem still exists.

DrGonz78

So what resistance readings are you getting across the +/- ends of C66 and C67? Are either of those caps shorted?
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk

QuoteSo what resistance readings are you getting across the +/- ends of C66 and C67? Are either of those caps shorted?
Thanks for this Dr. Gonz. I used my ESR meter and both caps checked out good.

So is  this a case of a shorted supply that could have something to do with the Output Transistors shorted out and pulling tons of current? I tested them with my Peak Atlas DC55, out of circuit, and they all showed good but one. I ordered a matched pair from Peavey and installed them. I haven't fired up the amp without a bulb limiter.

I installed them the same direction as the original transistors--serial number/ brand--unless for some wierd reason these new ouput transistors have different pinouts than the originals.

With the variac at 100V, .69Amps and the bulb limiter very bright, I have 4.9 VAC coming in for the rails and 1.4V DC leaving to feed the rails. So with the voltage drop of the rectifiers this DC voltage doesn't seem too far off.

Again, I've measured the resistors in the power amps  and they appear good. Is there anything else in the power amp circuit that could be hauling current through the amp? "Shorts between transistor case and heatsink, due to a punctured mica washer"? This is my toughest amp yet :grr

DrGonz78

Quote from: Hawk on July 31, 2015, 06:30:22 PMWith the variac at 100V, .69Amps and the bulb limiter very bright, I have 4.9 VAC coming in for the rails and 1.4V DC leaving to feed the rails. So with the voltage drop of the rectifiers this DC voltage doesn't seem too far off.

I am having a difficult time understanding exactly what this means. You say you only have 4.9VAC going in and 1.4VDC going out, but that seems very strange indeed. So we should be seeing a way higher AC voltage (of course) and I would expect the rectified DC voltage to be higher than the AC coming in. Then again these voltages are way off (of course).

One more thought is that there is a C69 0.1uf 1kv cap right between the two AC voltage inputs on the bridged rectifier circuit. How is that part doing? A quick test of shorted will be more than enough, but you'll have to remove at least one leg from circuit to get an accurate test.
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk

Sorry about that will try to be  clearer. 4.9VAC, 2.45VAC per side, on the secondary side of the transformer, measured from one side of the red wire connector to the other with the connector plugged in. 1.4VDC  rectified. And this same 1.4VDC shows up on the rails.

What's also strange is how, when I remove the red transformer wires from the board and test them, it has 53.6 AC volts from red to red, the light barely lit, but, again, when I connect it and measure, on the pins, I get 4.9VAC and the light goes very bright!

Silly question but if the cap is short should I get an infinite reading? The .1uf is too low a value for the ESR meter to read it.  I have a multi-meter. Also if this cap is shorted then will it be pulling the voltage to ground?Thanks.