Peavey KB300 combo keyboard amp repair - diyAudio
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Old 11th December 2012, 04:07 AM   #1
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Default Peavey KB300 combo keyboard amp repair

Hi everyone ! This is my first posting on that forum.

I've got the schematic matching exactly my KB300 (300CH board from 1986).
Iíve got this amp for years. I changed a few noisy pots in it years ago and it worked like a charm with my bass guitar, voice, guitar, anything I throw at it.

Yesterday, It worked on all channels except for an unusual background noise, then it started to make random loud pops and cracking even with nothing connected and all volumes at zero. I suspected a bad contact or loose wire, but inspection of wiring, main speaker and horns shows no problem and 4 ohm impedance. All fuses are intact, so I reassembled it. This combo amp has only a polarized speaker connector and an another one connecting the spring reverb unit... hard to make any mistake !

Now I no longer get the red LED light on when I turn it on. It now sounds and looks dead. Power levels on board are identical to schematics specs. First impression is IC or output transistor failure. Since the problem was independent from the preamp, I think something is wrong with the power amp.

Iím not a qualified technician but an experienced soundman and musician (bass and guitar). I donít mind changing a few pots or defective connectors, but I have no idea how to troubleshoot my precious amp.

From the symptoms, do you think changing the output transistors and all IC on sockets (pretty inexpensive and simple task) has chances to fix the problem ?

I'm used to order all original parts from Peavey's or reputable vendors.

Any ideas ???

Thanks
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Old 11th December 2012, 12:19 PM   #2
JMFahey is offline JMFahey  Argentina
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Quote:
From the symptoms, do you think changing the output transistors and all IC on sockets (pretty inexpensive and simple task) has chances to fix the problem ?
No. Please do not replace parts at random.
You probably had a poor contact, such as cracked solder or track, and while messing with the PCB by pushing and pulling those connectors, it became worse.
Imagine what will happen after needlessly replacing a ton of parts!
You need to get the schematic and start working backwards from the speaker.
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Old 11th December 2012, 04:41 PM   #3
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Yeah, the only thing I replace shotgun fashion is the electrolytic caps, and that is based on the calender. 1986 is 26 years, my limit is 20 years on e-caps once something goes wrong. If it works, I don't monkey with it. Electrolytic caps have a plus on one end, or a minus in balls pointed at one lead. Some near the speaker have "voltage NP" printed on them for non-polarized electrolytic cap. They are full of water, sealed with rubber, and oxygen attacks the rubber whether powered on (fast) or off (slowly). Buy caps service life rated >3000 hours if you do replace them, 500 hour caps are quite popular, and really stupid if you want to use the equipment more than a month. Farnell.com has the hours service life in the selector table. 105 deg C rated hours are better than 85 deg C rated hours.
However, you say your power supply rails are reading correctly (collector voltages on the output transistors). If that is correct, Second step is to measure for any DC voltage on the speaker hot terminal. If you have it, you have output transistor problems.
Next I would start testing the emitter resistors power off with the ohms scale. These tend to blow up if the Output Transistors go. You have to pull the base lead of the output transistors loose to test them. The double diode test is the diode check or ohms 2000 scale of your DVM, base to emitter, base to collector, 450-610 ohms one way (.6 v drop) infinity the other way, they are not bad. I won't say they are good, I found some in my PV1.3k amp that passed the double diode test and still exploded. Once I put 17 v on the OT's through 47kohms from a car battery charger+capacitor, and measured leakage current with the 200 ma scale of the DVM, I could see some difference in leakage current between the transistors that tended to blow up, and new ones or old ones that were going to survive rail voltage. Short the base to emitter when you do this, that junction is not rated at 17 V. The new/okay used transistors leak less. This simulates the Vceo test done at the factory without the 200 VDC which hurts and could kill you. Datasheets on transistors are on datasheetcatalog.com.
Probable third step is to get a bad switch room heater, or a light bulb box, and put in series with the AC power to limit the size of transistor explosions if there is a problem. 120 VAC will blow the die right through the metal case into your eye. My room heater element (inside past the switch) cut the rail voltages of my PV1.3k from 85 to 67, and stopped the failed transistors from exploding.
Next is to buy 225 Watt resistors the same value as your KB300 speaker and replace the speaker temporarily, so you can test without blowing up the speaker with DC voltage. You may as well get OT's and emitter resistors on the same order to save freight.
In Canada, some people like digikey in milwaukee, but get shipment royal mail, not Fed Ex to save customs loan and paperwork fees.
If you're amp has Peavey # 70483180 or 70484200 OT those are On Semiconductor MJ15024 and 70473180 or 70474200 are MJ15025. I buy my On Semi stuff from farnell or mouser, no e-bay silk screen re-marked transistors for me.
Peavey probably has the schematic under support on their website. If not, maybe eserviceinfo.com has it.
Oh, check the fan while you are at it, that is another component that 10000 hours of operation is not kind to, and can cause blowups if it sticks.
Oh, and I just fixed my third amplifier without an oscilloscope. A non-autoranging DVM is useful for DC problems, for AC you need a fast VOM like a simpson 266 with a 2 VAC scale. I exercise the amp with a radio on the input, and look for the beats of rock music with the VOM if I have to go into AC diagnosis.
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Last edited by indianajo; 11th December 2012 at 05:07 PM.
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Old 11th December 2012, 09:49 PM   #4
Enzo is online now Enzo  United States
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COnsider the symptom, the amp is dead. The amp is not distorted, or acting funny, it is just dead. No power light. All fuses OK. You say voltages are all OK per schematic, but those voltages are all on the power amp.

He says he already has the schematic.

The power amp is also the power supply for the whole system.

The lack of a power light tells me you at least have no +16v on the preamp. SO either the power amp is not making +16v, or it is just not getting to the preamp board.

If your output transistors were shorted or open, the light would still come on, or the fuses would have blown. Not them. Your filter caps might have broken loose or dried out, but in those cases, you get loud hum or other noise. So not them either.

This thing worked before, regardless of how noisy, and after you went inside it, it came out dead. That pretty much tells us your work was involved. First suspect is a missed connection. Is the connector on the end of the preamp board fully pushed down? And is it properly placed, not off one pin? On the power amp, there are the red wires and the orange wires from the transformer. Are both those connectors fully on? And at the other end of the power amp, the connector to the preamp. Is that conector on correctly and fully?

You checked the fuses. Did you pull them from their clips and check them with a meter? And remember, two fuses on the board and another on the amp chassis.
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Old 12th December 2012, 02:55 AM   #5
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Default Peavey KB300 diagnostic and repair

Thanks for your advice... remember, I'm rather green in electronic repair procedures. I have only studied basic electronic and soundman's oriented stuff.

Anyway, I've started to work out by trying to isolate potential problems.

First thing I did was to check voltages on both amp and preamp leads and they all match schematic specs.

I saw the past owners replaced a few diodes, so I checked them first. So far, I found one diode on the preamp and 8 out of 13 diodes on the amp board (all IN4148) that appear to be fried (way out of specs, no loger polarized). I have no idea what could possibly have caused such high failure rate.

I found diodes that failed in different sections of the main amp:
A pair between the 7815 and 7915 voltage regulator, another between the amp and the preamp 15V line, another is next to the 5331 voltage regulator (45V line), another pair between the power supply and the +15V and -15V and the amp board. There is also an isolated one next to the 6531 transistor.

I've got about 40 catalytic capacitors on that board, from 5000 MFD to 2,2 mcFD. Your comments on lifespan of such parts really concerns me. I have not found a single one that is not polarized. This board was manufactured in mid 80's. I rarely use this amp, but still... it is 30 years old.

I am now checking for out of specs resistors (I found a few that are completely out of their specs range).

All I've got for testing is a digital ohm meter and basic understanding of component specs sheets. I have no oscilloscope I have no practical experience on testing transistors and capacitors. I can only understand 75% of Indianajo's comments on testing procedures.

The extend of damages I found so far makes me feel I have reached the limit of what I'm able to fix with a reasonnable chance of fixing it successfully.

So, it looks like my best bet is to get a replacement amp board altogether (if available and affordable). I'm waiting from Peavey's parts department detailed response. They reply within 24h. I will keep you posted.

As a last resource I am quite likely to get an estimate from a qualified service center. The KB300 is an exceptionnally rugged and adaptable combo amp. ideally suited for my needs.
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Old 12th December 2012, 04:17 PM   #6
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I can guess you haven't checked for DC on your speaker output yet. That could prove the output transistor failure theory if you have it.
Some devices like semiconductor regulators can load a diode (1n4148) with another diode drop, where you can't tell if the 4148 is blown in circuit. You have to unsolder one leg. However, not unusual to blow a lot of diode, I replaced more than a dozen in my PV-1.3k project. Also a lot of resistors, and a few non-electrolytic capacitors. My output transistors were pretty obviously blown, one had a hole in the top. Rail voltage rampaging through the system can blow a lot of parts. I had over 100 bad, diodes, resistors, 50v ceramic capacitors, transistors, IC's. Most parts were salvageble from old power supplies or CRT terminals, but I had to buy new transistors, IC's, and one value capacitor. If you are not employed like me, it is the kind of puzzle I like, but that amp took me over a year to puzzle out, mostly just with the DVM. If you are employed or do volunteer work, new boards or service or a new amp might be more the thing to do.
Since you know how a diode test works with a DVM, you can prove transistors bad by doing two of those tests on them. Disconnect the base from anything. Put a clip lead on base to plus of the meter for NPN transistors. Check resistance on 2000 ohms scale or voltage drop on diode (depending how your meter works) from collector and emitter to base, should be 450 to 620 ohms or .45v to .62 v. Then put the negative on the base. Should be infinity ohms or volts to collector or emitter. If anything reads near zero, the transistor is blown. However, that doesn't prove them good. Like I said, you need to check with base to emmiter shorted how much collector to emitter leakage current there is at a voltage higher than 2 v that meters produce on ohms or diode scale. I used 17 v out of a battery charger/capacitor arrangement, limited in current by a 47k resistor in case I get forwards on the diode by mistake. No particular value is correct, I just checked forwards and backwards, they had to be way different, an similar leakage current to new parts I had in the junk box.
The one thing I needed AC analysis on the PV-1.3k on, it would work sometimes and not others, so I put 2 VAC on the input from a portable radio, and used the 2 VAC scale of a Simpson 266 VOM (100kohm/volt), and looked for the beats of rock music coming from the input in until I found the problem. I had made a bad solder joint at one electrolytic capacitor, that would pass music sometimes, and not others. You have to put a .47 uf 600v capacitor in series with the AC scale of a VOM or they read DC voltages on AC scale. If you buy a VTVM they come with a radio probe that has a capacitor already built in it. The VOM or VTVM is fast enough to go up and down on rock music beats, whereas DVM's average the reading over 4 seconds, But the VOM was a **** of a lot cheaper than a reliable scope with new e-caps and 2 new $50 probes. I bought a $40 B&K2120 scope last year, it worked a couple of times then I forgot and left it on and the horizontal sweep quit. Needs new e-caps, I'm pretty sure, plus perhaps a diode or resistor, but the controls are glued together with epoxy and I can't get them off to take the PWB's out to replace the e-caps. !@#$% If your VOM or VTVM has a steady voltage on it not correlated with music, you might have an oscillation. You can prove that by decreasing the cap size, music will disappear on a series with probe 150 pf cap, but RF oscillations will pass right through.
Anyway, good luck. If you are near Ann Arbor Enzo is a very nice and knowledgeable owner of a Peavey service center. He can't advertise, but as a grateful recipient of a lot of good info from him, I can.
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Last edited by indianajo; 12th December 2012 at 04:24 PM.
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Old 12th December 2012, 11:13 PM   #7
Enzo is online now Enzo  United States
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Thanks, actually I am in Lansing, but our friend seems to be on the Canadian side of the river.

Look on the preamp board, pick one of the 8-leg op amp ICs, the 4558s. Verify you have +15 on pin 8 and -15 on pin 4 right at the IC. It is not enough those voltages get to the preamp board, they have to get all the way to the ICs.

PV is not going to have 30 year old complete boards. Or I will be VERY surprised.

I don't know what your local techs charge, but in my shop, a repair like this is probably not going to be complex, and would likely cost under $100.

Your amp is dead, no sign of life, but not blowing fuses. You are not going to find a resistor off value causing this. A bad cap or off value resistor won;t stop the lights from coming on, they woulod just make it sound funny.

By the way, when you switch power on the first time each day, does the speaker make a little thump or pop?

I don;t think we did this either: plug your signal into the power amp in jack. Any sound that way?
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Old 22nd December 2012, 09:03 PM   #8
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Quote:
Originally Posted by Enzo View Post
(...)
PV is not going to have 30 year old complete boards. Or I will be VERY surprised.
(...)_By the way, when you switch power on the first time each day, does the speaker make a little thump or pop? (...)
Actually, Peavey had the power amp board and the output board in stock. They no longer had the preamp board but I did not need it either because I was able to get all components and every single pot I needed as a spare if one gets too noisy.

The power switch never did produce a pop or any noise at all.

I've repaired this preamp board a few years back (preamp IC and changed one pot). This time I am going trough the schematics and will replace every single defective component. So far I found 15 resistors and one 4148 diode that was defective. I'm on standby until I get proper meter to perform an ESR test on capacitors. I only replace what is clearly out of specs.

I should be able to put back the whole thing together in about a month.

Thanks and happy New Year !
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