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K200A has hum/low output [message #9820]

Thu, 30 October 2008 21:00

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

Just got a K200A-2. It worked alright for a few days until this afternoon. I flicked it on and was greeted by a mild hum and greatly reduced output. The low output is on both channels. I was thinking something in the power amp or power supply portion of the amp. The amp never had the "thump" that my B models do when I turned it on. It does have a thump when turning it off. Also, the volume controls on both channels acted different than my B models even before these problems. My B models would be very loud right off the start and continue from there. This amp wouldn't start to get LOUD until just before the very end of the pot's sweep. Of course, I'm new to this A model game as well.

Any suggestions on what to check out? I'm supposed to have a new Fluke 179 DMM show up any day and will be able check on things. My electronics knowledge is so-so. I built a 5F2 Fender princeton, so I know well enough not catch the business end of a power supply cap, but will still pretty much have to be told in detail where to measure or check at. Thanks guys!

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Re: K200A has hum/low output [message #9821 is a reply to message #9820]

Fri, 31 October 2008 08:00

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

heres my first guess by what I found in pre A model frankenstein head.
On the driver board which is mounted on the rear wall of the amp or on the rear floor of the amp you will find a 25 mfd cap that is covered in black or red type card board. One end will be marked + and the other -, replace that and any others you find that look like that with a new one of the same or close value(20 to 35 mfd) but atleast the same or greater voltage. I think they where rated at 50 volts.
Also the two big can caps clamped down to the chassie floor could be going bad, measure the dc voltage on the case of one of any of the 4 output transistors mounted in a row on the bottom of the amp, and then set the volt meter for low low ac volts and measure again, and report back with a post, dc wise you should see between 39 and 40 volts if I recall.

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Re: K200A has hum/low output [message #9822 is a reply to message #9821]

Fri, 31 October 2008 11:33

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

So on the transistors, will I set my probes on the base and emmiter pins then to take the measurements?

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Re: K200A has hum/low output [message #9823 is a reply to message #9820]

Fri, 31 October 2008 12:11

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

No, you go ground to the chassie and the hot lead to the metal case of the a output transistor on the underside of the amp, or the red wire off of one output transistor inside the amp.

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Re: K200A has hum/low output [message #9824 is a reply to message #9822]

Fri, 31 October 2008 12:12

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I'd agree with Steve and suggest you start by checking the power supplies.

Your 200-A head will have the two large filter caps that supply the power output stage and if you look at the back panel of the amp you'll find two small pc boards that contain the low voltage regulators for the pre-amp sections.

Start by measuring the voltages at the filter caps. Ground the black lead to the common connection between the two filter caps and then read the voltages on each of the other cap terminals. There will probably be somewhere around 40vdc, plus and minus. So that I'm clear here, what I mean is that one filter cap will have positive 40vdc on it, and the other filter cap will have negative 40vdc on it. I think that the power supply wires are color coded the same on the A heads as on the B's, so the positive voltages will be carried by red wires and the negative voltages will be carried by green wires.

The two supplies should be close in value, so if one side is significantly lower than the other that would be something to investigate. While you are here, read the ac voltage level on each of the supplies. This will sort of tell you how well your filter caps are working.

If both supplies check out, then check the output of the two regulator boards. I'm gonna guess (because I can't remember), that there should be 8-9 volts output from each of the regulators. I know that in my 200A head, I found two open caps on the regulator board. I think they were 30mF caps.

This weekend I'll pull my 200A chassis out and review my posting here, so that I'll know if anything I said was wrong. In the mean time, check the supplies and let us know what you find.

IMPORTANT! I've detailed how to check the supply voltages in your amp, which will require you to work on it with the amp plugged in and turned on. There are dangers involved in working on a live chassis, and if you are not comfortable or qualified to do this please refer servicing to a qualified tech.

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Re: K200A has hum/low output [message #9825 is a reply to message #9824]

Fri, 31 October 2008 19:53

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

First, I am greatful for all the assistance. Okay, I did the test I knew how to do correctly. Let's say you're looking down on the amp so the faceplate is facing toward you. We'll start with the transistors from left to right. I measured -25,40,40,40,-3 gradually climbing to around +.5, same. The caps came out +40 and -40. I also believe those two little preamp driver boards are supposed to give out -8 and +8. I came up with -25 and +25.

Also, when checking AC on the caps, do I use my probes in the same fashion and just switch the meter to AC? Another observation is the that the far left transistor looks a little funny. It's stil an RCA TO-3, but is rather oxidized and nasty looking compared to the rest of the transistors. Please don't let the fact that I used the term "TO-3" fool you, I'm still learning this game. Laughing

[Updated on: Fri, 31 October 2008 20:18]

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Re: K200A has hum/low output [message #9826 is a reply to message #9820]

Fri, 31 October 2008 22:14

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

So, I decided to randomly probe around and MAY have found my problem. I set my meter for diode test and began inspecting the small black transistors on the little preamp driver board on the far left. Apparently this meter will give a "beep" tone if a diode is shorted out. I have one 2N3638 transistor that I can get to beep when I test it. I tried getting the other ones next to it to do it, but could get. I measure a few k resistance on all my other tests, but the bad combination gives me only 3 ohm of resistance. Think this shorted out and is causing a 25 volt reading instead of an 8 volt?

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Re: K200A has hum/low output [message #9830 is a reply to message #9826]

Sat, 01 November 2008 12:23

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

First off let me correct the earlier post by noting that the 200A series heads use plus and minus 25Vdc, not the 8Vdc that the 200B series uses. So the values you got are correct.

Yes, measure the ac ripple the same way that you measured the dc voltage at the filter caps, just switch the meter to read ac volts. On my 200A head, I read + and - 41Vdc with 83mV ac on the positive cap and 68mV ac on the negative cap. These values are just typical ones, and will give you a reference for your readings.

You are calling two small boards "pre-amps", are you talking about the two leftmost boards on the rear panel (with the front of the amp facing you)? If you are, then you are refering to the two voltage regulator boards. These boards take the 40 volts from the power supply and reduce and filter the voltage down to 25V, for use in all of the pre-amp circuits.

As for your meter, I'm not sure what the beeping shows in the diode test mode. My Fluke will beep when the diode or transistor junction under test conducts. So it will beep if the transistor is good or is shorted. Only when you reverse the lead connections will you be able to tell the difference between a short and a forward conducting transistor. A typical transistor junction should conduct in one direction only. So until you reverse the meter connections you can't tell if the transistor is just conducting as it should or is shorted.

To test a transistor, make sure the amp is off and unplugged. Set you meter to the diode test position. Place one probe, let's say the red one on the base lead. Now take the other meter probe (black) and touch it to the emitter lead. Depending upon the polarity of the transistor, your meter will either beep or not. If you look at the meter it will either read low, around .6 or will read open or infinity. Now reverse the probe connections. If the meter had originally beeped, with the probes reversed it should now not beep. Perform the same tests between the base and collector and between the collector and emitter. In all of the transistors in your amp you should find one way conduction between all bases and emitters and bases and collectors. There should be no conduction at all between emitters and collectors.

Now as a caution, there will be some transistors that will test bad or questionable due to other elements in the circuit, like a resistor between the base and collector. So sometimes to get a true reading the transistor will need to be removed from the circuit. What you really need to look for are gross anomalies, dead shorts or complete opens between transistor leads.

As for your voltage readings on the power transistors, there should be 6 TO-3 power transistors. The two leftmost are part of the voltage regulators and the other four are the power amp outputs. They are all basically 2N3055 transistors. The voltages that you got are probably in the ballpark.

As for your original problem, you are right in your approach. It is happening to both channels, so look at the power supply or the power amp. Hum makes me think power supply first. Check the ac ripple at the main filter caps and at the regulator outputs. If your meter has a capacitor setting, check the values of the caps on the regulator boards.

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Re: K200A has hum/low output [message #9832 is a reply to message #9830]

Sat, 01 November 2008 17:58

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

Me again

. Yes, I was referring to the two small boards in the back of the amp that supply voltage for the preamps. I measured around 98mv AC on each of the caps and 5,500uf on the capacitor test (caps were disconnected and discharged). I measured 61Vac going to the rectifier from the transformer and a recitifier output of +/-40Vdc. As for the transistor. My meter gives a very short beep when your probes make contact during a diode test. According to the manual, it will make a steady beep (like it does) if it fails a diode test. I do have excellent pictures of the front and back of this little board, but it probably is something that I'm getting a false reading on.

However, I do have some interesting new devolpments. I decided to hook my cabinet back up to see if anything was new. For a brief moment and I had much more output coupled by horrible static. The output fell back again to very faint (again, each channel was the same). So, naturally I turned a volume control up full and strummed my Telecaster, only to have something make connection again with a 4x10 cabinet right next to me. You would have laughed at how hard I jumped. Anyways though, I have sporratic, loud crackiling noises at all times. If I take a pen and tap ANYTHING on the inside or outside of the amp, I get more crackling. I mean even the faintest of taps will set if off. Am I right to assume that I have bad solder somewhere in the power amp then?

[Updated on: Sat, 01 November 2008 19:41]

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Re: K200A has hum/low output [message #9833 is a reply to message #9820]

Sat, 01 November 2008 18:25

ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA

Member

Fixed it! All it took was a $.02 screw. I started inspecting the poweramp board and noticed I was missing one screw on the back panel. Those screws do more than hold the board down, they provide a ground! I put in a new screw, flicked the power switch, and things are perfect. Well, it nice to know all my voltages are right on spec and there are very helpful people on here. I have a copy of all the K200B schematics comming in a few days. I'll snap some pics and make them available. Get a little, give a little I guess.

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Re: K200A has hum/low output [message #9840 is a reply to message #9820]

Mon, 03 November 2008 06:26

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Nice fix! sometimes the cure can be so darn easy yet time consuming.

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