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K250 Overhauling [message #15247] Thu, 10 February 2011 19:08 Go to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
From Costa Rica HEllo Everyone,

I just got an old K250, it looks very very wear and a lot of parts in the inside were altered by someone (probably without idea of what was doing).

Then I decide to take the time to overhauling it. Most of the resistors and caps were changed so I took the task to putting it back to specifications of the original schematichs. A couple of transistors were missing, so I replace them. New bridge rectifier in place, new fuse (it used to have a 5amp fuse Shocked ).

Keep the filter caps and the transformer, the thing is when I turn it on inmediately a BIG hum (similar to when a guitar is short) comes out of it whithout any input, I tried all the pot and nothing change. Tested the volts from caps, transistors and ICs, and they seem into tolerance.

Does anybody have any advise I could take?
Thank You in advance.

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Re: K250 Overhauling [message #15248 is a reply to message #15247] Fri, 11 February 2011 13:10 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
If it seems that your power supply is all working ok, then the problem may be in the power amp.

You may have a power amp problem that is putting dc voltage on the output of the amp. I'd start by taking a dc voltage reading across the output jack with no speaker connected. If you do have dc voltage there, look for a bad power transistor or driver transistor. And if there is dc on the output jack, do not plug in a speaker until you have this fixed, as it will damage the speaker.

If you replaced all of the things that you said you did, you may have installed something in the wrong place or in the wrong direction. Double check your earlier work.

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Re: K250 Overhauling [message #15249 is a reply to message #15248] Fri, 11 February 2011 15:44 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
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Thank you for the replay Chicagobill, I did the test and the output jack is reading -39.5V DC Sad . The strange thing is that if I desconnect the Poweramp PC5065 completely and read the Imput from PC5066 or PC5067 it reads 0.04V DC. Could this be correct?

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Re: K250 Overhauling [message #15250 is a reply to message #15247] Fri, 11 February 2011 16:07 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Yes, the signal input voltage has nothing to do with the amount of dc voltage on the speaker jack. There is a problem with the power amp that is causing the full negative power supply to appear on the output.

As I noted before, check for shorted output transistors or shorted driver transistors, or for that matter wrong transistors in the power amp circuit. The output should be balanced at zero volts between the two sides of the power supply. Something is causing the circuit to become un-centered and the negative supply voltage is at the output.

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Re: K250 Overhauling [message #15251 is a reply to message #15250] Fri, 11 February 2011 16:21 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Thank You Again, I just found something really interesting that I totally oversaw Embarassed . Q12 and Q13 transistors were replaced with 2N3773 but Q10 and Q11 still RCA36892, and as per your replacement list previously posted, the equivalent part is 2N3055. The 2N3773 have completely different specs so I asume the system is unbalanced because 2 transistors are higher than the others.

Think I got it, I'll buy the 4 new 2N3055 tomorrow and see if it works better.

[Updated on: Sun, 13 February 2011 09:24]

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Re: K250 Overhauling [message #15253 is a reply to message #15251] Sun, 13 February 2011 13:29 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Well new 2N3055s on place and the same readings are present on the output jack.

Stranger than before after I placed the new transistors a couple of resistors burn out. Double checked all transistors pinout and seems to be fine after 10 times. Confused

Any other idea will be apreciated? Does anybody think that a defective capacitor can be producing this voltages?

Thank You all.

[Updated on: Sun, 13 February 2011 13:30]

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Re: K250 Overhauling [message #15254 is a reply to message #15247] Sun, 13 February 2011 15:10 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Hello.
Here are 3 checks you need to make.

1) Pull out and test that all 4 of your new 2N3055 transistors to confirm that they are still ok.

I think you will now find the at least one of them that run off of the negative side of the 39.5 volt DC power supply rails is bad.
also you might find that one or both of the driver transistors on that side is shorted.

You wll need to replace the burned up 5 watt resistor(s).

2) Check that your + side of the 39.5 volt power supply rail is working by hooking up your meter to the red wire on one of the main power supply filters and the chassie of the amp.
Confirm that this +39.5 volts is getting to both 2N3055 outputs that run off it.

3) Confrim/check that you have NPN type driver transistors on the + side of the circuit driving the two outputs, and that you have PNP type transistors on the - side of the driver/output stage.
Unplug/unhook the + AND - wire that brings in these voltages two the two preamp boards. The reason for this is that if you only have one side of the power supply rails working right the 14 pin IC chips will be toast!

For trouble shotting reasons replace the fuse with a 1 or 1.5 amp fuse. Doing this may help stop you from burning out more new parts if you still have a problem going on.

I hope these tips help you, so let us know how things go.

[Updated on: Sun, 13 February 2011 15:11]

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Re: K250 Overhauling [message #15255 is a reply to message #15254] Sun, 13 February 2011 20:42 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
WOW Shocked
Nice crystal ball you should have. Q5 and Q12 are shorted on the negrative side. I also find out that C7 (27uf caps) is bad Evil or Very Mad.

Tomorrow I'll exchange all the parts and see what happen, I hope the tatalum caps were the original problem disbalancing the poweramp.

I really apreciate the inputs and advises from everyone. This forum has been a great help. In Costa Rica there's no real culture on reparing vintage amps, if it's not working to the garbage can will go, so there's very few people I can go.

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Re: K250 Overhauling [message #15256 is a reply to message #15255] Mon, 14 February 2011 12:18 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Sad Seems that changing the parts didn't work. Now the output jack is now +39.5 VDC. I'll check the routings of the PCB if there's any bad connections or bridges on the soldering.

Thank You

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Re: K250 Overhauling [message #15257 is a reply to message #15247] Mon, 14 February 2011 14:38 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Well you are making progress and still finding bad parts, so that is good!

Having a + voltage now on the output jack would most likly mean that you have shorted or wrong type transistors on that side of the output stage.

Unless caps C7 and/or C8 test out shorted I do not think they are part of the problem, these caps( any Tantalum caps in these amps)tend to drift in value alot as they age, but if they are not shorted you can just change them out latter on.

Make sure that doiode CR1 thru CR5 on the board and diode CR3 mounted in the metal clip inbetween the inner pairs of output transistors all test good also.

Other than these tips that Bill and myself have layed out it should just be a matter of find bad parts and replacing them.

NOTE, also make sure that the ground connections to the chassie from the two main power supply filters is making a good solid connection, and that the Black center tap wire on the power transformer is making a good connection to ground also.

Keep us infomed on your progress!

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Re: K250 Overhauling [message #15267 is a reply to message #15257] Thu, 17 February 2011 08:42 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Well, no bridge connections so far on the soldering. Yesterday I replaced all the transistors on the board except for Q1 - Q3. Q4 & Q8 are NTE128, Q6 2N4401, Q7 Q5 Q9 are 2N4033, and Q10 - Q13 are 2N3055. Also replaced CR3 and CR4 with 1N5402, CR1 / CR2 / CR5 are the originals but test good.

The amp turned on with a 1amp fuse on, but no negative readings in all the board, only in the collector from Q1 and Q2. Output jack was 37V DC. Then I turned off, move a little the connections to the 2N3055, turn it on and fuse blows in a snap.

I spent 5 hours yesterday making sure of the board tracings and the directions of the transistors, no good luck.

I'll try to contemplate the board a couple of times today.

[Updated on: Thu, 17 February 2011 09:52]

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Re: K250 Overhauling [message #15279 is a reply to message #15247] Sun, 20 February 2011 16:14 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
You need to confirm that you have the slip on connectors for the output transistors on right.

Since the left and right pairs are mounted facing oposite each other you must confirm that the yellow and blue wires are going to the right pins.
Also make sure that all of your mica insulators for the 4 output transistors are not cracked or split and shorting that transistor to ground.

Other than these tips we have given so far you should be able to get this driver and output stage up and running as long as all of your transistors are #1 good,#2 any subsitutes are the right ones to work in that spot and#3 the transistors are in the circuit the right way in regards to E,B,C placments.
Keep at it and let us know how things are going.

[Updated on: Sun, 20 February 2011 16:14]

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Re: K250 Overhauling [message #15306 is a reply to message #15279] Fri, 25 February 2011 18:15 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Hello everyone.

Finally the power amp is up and running, the problem were diodes CR1 and CR2, first time I tested they were ok but today it showed leaking Mad , so new pair of diodes and no DC on the output jack and all voltages are on the spot.

It seems that the work doesn't stop there for me; since I connected the two boards PC5066 and PC5067 and both have issues.

PC5066 has no sound on neither jack only a "hiss", but it's modulated accordingly the Bass, Treble and Bright pots.

PC5067 has sound on both jacks both no modulation from neither pot, not pulling the bright or the Selectone. All voltages are on the spot. I hope it's not an IC problem.

Tomorrow I try again, and let you know.

[Updated on: Fri, 25 February 2011 18:16]

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Re: K250 Overhauling [message #15316 is a reply to message #15306] Sat, 26 February 2011 14:27 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Hello again.
Well its good to hear that kept at it and got your driver and power amp section of the 250 up and running!

Unfortunatly in looking at my schematics for both of these preamp sections I think that the 14 pin IC chip (NTE725, or ECG725 for replacment types)on both boards are toast, and heres why.
The output of the IC on the 5067 board gets feed into A side/second gain section side of the IC chip on the 5066 board and from the problem you are discribing both of the chips have a blown out gain section.
The reason for this more than likly is that these chips need both a +12 and -12 volt supply when powered up to live and when your output stage was shorted and blowing fuses you lost eitheir one of these + and - power legs.
To repair this and prove out my informed guess on the cause I would first get two 14 pin IC sockets ( your going to need them anyway) and install them when you have carefully unsoldered the two ICs.
After installing the sockets you can plug the ICs back in but swap them and I will bet that you have the same problems going on with each board.
The only other thing that could be happening would be that your problem with the 5067 board is due to a pot that has had its carbon face section broken due to someone trying to work on and remove the nut that holds the pot to the front of the amp.

Sometimes these steel nuts corrode really bad to the threaded shaft of the pot and breaking the nut free free in order to remove these boards from the face of the amp breaks the pot when enough force is applyed to get the nut free.

Since you have these boards removed I would take the time to check that all of the pots work ok.
The best way to do this is to unsolder them all to do the ohm check.
Once again, hang in there and you will get it all fixed up!

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Re: K250 Overhauling [message #15325 is a reply to message #15316] Sun, 27 February 2011 12:44 Go to previous messageGo to next message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
Let me know if you cant find a source for those 739 chips. I know where I can get them for around 3 to 5 US dollars. The sockets may be around 1 or 2 dollars. Keep up the good work. Steve C

Steve C

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Re: K250 Overhauling [message #15329 is a reply to message #15247] Mon, 28 February 2011 18:32 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
You have not posted this info, but if your 250 is a -1 model ( just a 2 channel non-effect model) make sure their is a jumper connector on the 5066 board.
This is a terminal plug that goes into that female on the back of the board.
If you do not have this connector bridging the #1 and #2 terminals than the 5066 preamp will have no output.
The #2 terminal brings the signal out of the Q2 transistor and goes back in on the #1 terminal to feed Q3.

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Re: K250 Overhauling [message #15337 is a reply to message #15329] Tue, 01 March 2011 10:06 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
After teaking a little and cleaning the input jacks, I have PC5066 working fine, all modulation and switching works for both inputs. The only problem is that PC5067 has no sound at all, and no modulation seems to affect in that channel. Tested the mollex connections and everything has continuity.

It give me a little hope thinking that the ICs are fine, tested all the resistors and caps and seems to be fine, diodes are new since I didn't wanted to risk me with the previous power amp experience; making sure place them on the right direction.

Now I have no idea if the PC5067 is not sending signal or maybe the PC5066 is not receiving it.


PS: Yes Steve it is the 250-1 with no effect, and the jumper is in place and making good connection.

[Updated on: Tue, 01 March 2011 10:07]

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Re: K250 Overhauling [message #15338 is a reply to message #15247] Tue, 01 March 2011 11:50 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
I'd start by reading the voltages on the input and output pins of the chip, before anything else.

Make sure that there is +&-12v on pins 7&14. There should be no or almost no voltage at pins 4, 5, 6, and 8, 9, 13. If you get any voltage readings on the inputs or outputs, the chip is probably bad.

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Re: K250 Overhauling [message #15342 is a reply to message #15338] Tue, 01 March 2011 14:09 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Here are the readings I'm having on this IC.


1:0----------- 14:13.54
2:0----------- 13:-0.044
3:0----------- 12:0
4:0----------- 11:0
5:0----------- 10:0
6:0----------- 9:0
7:-13.16----- 8:-0.05

I don't know if those variations on pins 8 and 13 could be significative.

[Updated on: Tue, 01 March 2011 14:10]

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Re: K250 Overhauling [message #15343 is a reply to message #15247] Tue, 01 March 2011 14:58 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
No, -0.05 volts is what I call almost nothing. The readings are good, but that doesn't mean that the chip is ok.

Try this, apply an signal to the input jack and monitor the signal at pin 1 of the chip. Is it getting there? You can use a guitar or an ipod or if you have one a signal generator for the input signal. To monitor the output at pin 1, you can read the ac voltage level there, or you can jumper the signal line directly to the input of the power amp at pin # 1 of the Molex connector.

If the signal is getting to pin 1 of the chip, continue to trace the signal through the rest of the preamp. If the signal does not get to the output at pin 1, check to see that the signal is getting to the front of the chip at pins 5&6.

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Re: K250 Overhauling [message #15344 is a reply to message #15343] Tue, 01 March 2011 19:37 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
Thank you for the advise Chicagobill.

I loaded a 400Hz signal on the input; pin 5 get signal with no problem but pin 6 don't. Pin 1 has just a minimal ammount of AC, so I think the problem is that the chip is unbalanced.

I traced back the signal lost and it happens on R2 (49.9K Ohms), after it to C3 there's 0 ACV.

Changed it twice and no difference. Confused

[Updated on: Tue, 01 March 2011 19:40]

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Re: K250 Overhauling [message #15346 is a reply to message #15247] Wed, 02 March 2011 11:39 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Don't worry about the signal at pin 6, that's normal.

It seems that the IC is bad. Replacements may not be easy to get where you are, but the generic number is 739 or 4739. It is a dual opamp in a 14 pin dip case.

You can also rig up a common 1458 or TL072 to work in the board with jumpers. There used to be an adapter board that did this, available from PAIA. You just need to figure out the input pins and the output pins of each chip and get the new chip's pins to the correct circuit points.

There may or not be compensation components that were necessary on the original chip that can be removed with the modern ICs.

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Re: K250 Overhauling [message #15350 is a reply to message #15346] Thu, 03 March 2011 20:09 Go to previous messageGo to next message
jmaroto is currently offline  jmaroto
Messages: 14
Registered: February 2011
Junior Member
YEEEYYY!!!!!! Very Happy


Finally up and running, TL072 worked like a charm. Spent a couple of hours on the mod wiring but it worth it. Now PC5067 is a lot brighter and clear than PC5066, but I guest is a good thing to have the vintage original sound in one and the upgraded IC on the other.

Thank you very much to all of you whom share your knowledge.

Now just time to rock this amp and set the world afire.

[Updated on: Thu, 03 March 2011 20:09]

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Re: K250 Overhauling [message #15351 is a reply to message #15247] Fri, 04 March 2011 12:41 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Congratulations! Glad to hear that it's up and running.

As for the difference in tone, the new chip does have better specs than the original ones, but the design of the preamps are different as well. So normally there is a difference in tone.

Now get out there and start those fires. Just stay away from Chicago, we've already had our big one.

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Re: K250 Overhauling [message #15362 is a reply to message #15247] Sat, 05 March 2011 13:37 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Since you are good with elelctronics her is a tip to keep your now working Baby alive.
The hardest time for a SS amp or circuit to over come is at start up, the instant in rush of current is a big load on the semiconductors.
If you do a search on the Web you will find ways of building cheap slow start up circuits that you can add to them amp.
In short, its better to keep anything SS on all time even if its at idle then to turn it on and off all the time.
Over 40 years ago now I stopped turning my amp off durning brakes when I was playing a gig.

[Updated on: Sat, 05 March 2011 13:37]

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Re: K250 Overhauling [message #15373 is a reply to message #15362] Sun, 06 March 2011 09:44 Go to previous message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
Man, Thats some more good info! I usually leave them on myself only cause of the cool ambiance all those purple (violet) power lights give to the stage. Besides theres been too many times we got up to start another set and someone didn't get turned on and its like "Hey Get Me Some Monitor" or "wheres that damn steel guitar" or "Help me out here Bass". You never know who got rushed to the stage waiting on a cold one for the next set and just didn't think to turn it back on. All for the love of music, you gotta love it!!! Steve C

PS: I'll see if I can get a couple of pictures on the Web board I brought a camera to the last gig where I used that little Challenger. Good little amp, but it was a good thing I brought the K100C-8 and my Morley direct-combine box. We were kind of loud and it took both the get on top the mix during leads. It sure was awesome to here them scream though!!!

Steve C

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