| Kustom 200 B1 blowing fuses [message #21094] |
Sun, 10 August 2014 09:22  |
Tone Meister
Messages: 42
Registered: August 2014
Location: NC
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Hi, using my first post seeking information on a 200 B1 that is/was blowing fuses. Amp serial number is 59412, no tremolo or reverb. Power amp board is PC703 but I haven't pulled either preamp board so I haven't yet determined exactly which boards they are.
The amp had been sitting for years and was blowing fuses upon power up. Found a shorted driver transistor (38736 = RCA 40409) and was able to source some NOS replacements for all the driver transistors, RCA 40409 and 40410.
There is at least one output that is shorted, so I want to just refurbish the power board and both preamp boards while I have it open. I plan to use 2N3055 replacements unless there is a better choice these days. Please don't suggest the NTE130MP because they cost 20X what I'd have to pay for the 2N3055s.
The main filter caps are good, but I do want to swap out all the tantalum caps on all the boards and all the driver and output transistors, plus the 1 ohm 5 watt resistors. Is there a definitive parts list for these amps, or does someone have parts suggestions for a solid, basic circuit overhaul without going too far overboard? I just want to make the amp functional and reliable.
Any parts suggestions or technical recommendations are welcome.
Thanks in advance.
-Stan
[Updated on: Sun, 10 August 2014 10:35] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21097 is a reply to message #21094] |
Sun, 10 August 2014 12:51  |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Stan:
If you look down near the bottom of the topic Steve suggested ("more power from a Frank head") you will see my detail of the refurbishment of my K200 last week. This includes a full parts listing of the transistors used, and some other parts. The last two posts in that thread give all the details.
I used MJ15015 output transistors, which are heavy duty, industrial versions of the 2N3055. (see the data sheet from ON Semi for the details... http://www.onsemi.com/pub_link/Collateral/2N3055A-D.PDF
I happened to have a bunch of them in stock. I'm a broadcast engineer, and those transistors are used as modulators in AM transmitters. The main advantage to using such a heavy duty transistor in a K200 is that they are also very closely matched. Besides, I now know I'll never need to replace them ever again. They are rated at 180 watts each.
NTE has a version of these. Newark has them in stock for $3.65 each. You will only need 4. Be very careful about buying them from other cheap sources like fleabay. This transistor is frequently counterfeited! A relabeled 2N3055 might work for a while in a transmitter, but will fail eventually...and we've seen a bunch of them.
Clearly your problem is on the power amp board. Have you changed all the electrolytic caps yet? Kustom used very high quality parts in these amps, but carbon comp resistors can go way out of tolerance after 45 years. I found some on the output board that were as much as 250% of the value they should have been. I replaced about 80% of the resistors on that board...and a lot of them on the pre-amp boards in mine.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Kustom 200 B1 blowing fuses [message #21100 is a reply to message #21097] |
Sun, 10 August 2014 15:03 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Hey Stan, welcome to the place. These circuits are very basic, and nearly any power transistor with sufficient ratings will work. I've used 2N3055 and the MJ15000 series transistors in the past with no problems. The only problem could come from a transistor with too low of a gain rating. In a few cases some tweaking is needed to stop some unwanted oscillations, but not too often.
Dave points out a very real problem with counterfeit transistors. Try and buy whatever you use from reputable sources.
Any 40 year old amp will have problems, but these amps are easily brought back to 100%.
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| Re: Kustom 200 B1 blowing fuses [message #21106 is a reply to message #21094] |
Sun, 10 August 2014 19:10 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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A none effects preamp channel on a k200 B head has 4 knobs, the top right knob being a brite switch .
The pot and brite / boost switch layout at the bottom center of the pc203 schematic has these parts in the wrong location even when viewed from the rear of the board.
[Updated on: Sun, 10 August 2014 19:17] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21114 is a reply to message #21094] |
Mon, 11 August 2014 08:20 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Those are the T03 types for the positive and negative regulator sections and are not under a heck of a lot of load by any means, although I have seen them go, but most times when that takes place at least one other transistor in that section is gone and that voltage spike due to that takes out the electrolytic caps in that section.
If yours are good that do what you want with the extra two you have on order, place then in the regulator circuit, or play around some and sub them in the output stage to try and get a better match on those 4 transistors for less DC voltage off set on the amps output.
[Updated on: Mon, 11 August 2014 08:23] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21502 is a reply to message #21094] |
Tue, 23 September 2014 16:12 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Please explain dead short. Do not connect a load to the amp until you get it stable. If you have a light bulb limiter, use it.
Have you checked the mounting for Q1 and Q2? Check to be sure that there is no connection from the chassis to the collectors.
You said that you replaced the two bias diodes, what about the chassis mounted one, is it okay?
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| Re: Kustom 200 B1 blowing fuses [message #21503 is a reply to message #21502] |
Tue, 23 September 2014 16:43 |
Tone Meister
Messages: 42
Registered: August 2014
Location: NC
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Dead short = dead short. I know well enough not to load the amp and I am using a lightbulb limiter connected to a variac until the amp is stable. When Q1 or Q2 red wires are connected my 150 watt light bulb lights fully. I can connect Q3 and Q4 red wires and there is no short as long as Q1 and Q2 are disconnected. I've tested all remaining diodes including the clip-mounted one, caps, and transistors and thus far cannot pinpoint the short. I've checked all my work sites for solder bridges and for other conditions that could cause a physical short.
It will be tomorrow morning before I can get back to the amp, but I will definitely check the mounting for Q1 and Q2. I made sure that each mounting screw has the plastic insulating grommet and I used Q-Pads and didn't over-tighten the mounting screws. Perhaps I should remove Q1 and Q2 and fire up the amp to see if I can isolate the problem to the power board or those transistors?
What do you think?
-Stan
[Updated on: Tue, 23 September 2014 16:46] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21504 is a reply to message #21094] |
Tue, 23 September 2014 17:39 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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So I take it that what you mean is that the amp is drawing a lot of current when the two top output transistors are connected to the circuit.
To me a dead short would be like there was a metal screwdriver directly across the power supply or something. Like a component has fused internally and is like a direct wire from supply to ground.
I will assume that when the amp is powered up without the transistors connected, there are normal power supply voltages present. With the limiter installed, they will be lower than normal, but they should still be there. Is there any voltage on the output terminal?
So check your output transistors. Have they shorted? Has a pad or screw insulator failed allowing the case/collector to touch the chassis? Do you have the plug on B-E connectors plugged on correctly? Did you install the bias diodes with the correct polarity? Have you installed the new drivers in the correct positions?
Please don't take offense to the questions, I have no idea as to your skill set. And even old pros can still get something wrong.
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| Re: Kustom 200 B1 blowing fuses [message #21505 is a reply to message #21504] |
Wed, 24 September 2014 11:34 |
Tone Meister
Messages: 42
Registered: August 2014
Location: NC
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No offense taken Bill. The amp is (was) drawing more than 1A of current with no load, so I consider that a short.
I'm still a novice when it comes to solid state circuits, even the basic ones such as this, but I can follow directions. I have the skills and all the tools and test equipment to do the job with a little guidance here and there. I appreciate forums like this one where folks can offer specialized knowledge on a single line of amplifiers.
Diodes are oriented correctly, B-E plugs were never connected. Drivers are in the correct position (Q704, Q708 = 40409) (Q705, Q709 = 40410)
Q2 is shorted to chassis when mounted, so my mounting failed in some way. The pad looks good and the transistor is still ok, so I will have to mount it more carefully this time.
I'm gonna try re-mounting Q2 and power it up again, and if it powers up ok, then I'll post back here shortly. I hope that is the only short.
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| Re: Kustom 200 B1 blowing fuses [message #21508 is a reply to message #21094] |
Wed, 24 September 2014 12:30 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I've never used Q-Pads, but I have used other silicon thermal insulators. Even with the mess of the silicon grease, I still find it works best for me.
If the pads are okay, look at the plastic shoulder washers that keep the mounting screws centered in the chassis holes. Sometimes they can crack or thin out and allow the screw to contact the chassis.
As long as you are using the light bulb limiter, there is no real reason to lower the fuse rating. The light bulb will work faster than the fuse and will save your new parts.
The reason for the other questions was to make sure that the two halves of the amp were working in balance. The top and bottom half of the power amp are basically at odds with each other. When the signal comes in, the top half pulls and the bottom half pushes. When there is no signal the amp should center itself waiting for the input signal to tell it what to do.
When one half is not working correctly, then the two halves can both push at the same time. This is when the amp draws too much current and starts the avalanche of shorting transistor junctions. This will continue until the fuse blows.
The bias string of diodes is what keeps the two halves in sync. It tells the top half when to push and when to pull. If the string opens then the communication is lost and both halves turn on at the same time and the amp shorts out.
So in your case I didn't know if there was the physical short causing the current draw or if it was an imbalance of the power amp doing it.
Keep us informed as to your progress.
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| Re: Kustom 200 B1 blowing fuses [message #21509 is a reply to message #21508] |
Wed, 24 September 2014 13:21 |
Tone Meister
Messages: 42
Registered: August 2014
Location: NC
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Member |
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OK, re-installing Q2 has not solved the problem yet, but it did eliminate an E-B short that was there before. All eight of the mounting screws on all four output Qs are shorted to the chassis somehow some way. I'm gonna pull them all and take a real close look to find out why. I think I may have some of those washers on hand and will replace them all if I do
Solid voltages are present from all the driver transistors.
+40.5V DC on the Red wires at Q1 and Q2
-40.7V DC on the Yellow wires of Q3 and Q4
[Updated on: Wed, 24 September 2014 13:22] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21511 is a reply to message #21094] |
Wed, 24 September 2014 18:24 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Fix the positive regulator and the negative should fix itself. The negative side tracks the positive side so if the pos is low so will be the neg.
Was Q714 the one that was dead? How do the schematic voltages on the rest of the transistors compare to what you have?
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| Re: Kustom 200 B1 blowing fuses [message #21514 is a reply to message #21094] |
Thu, 25 September 2014 13:13 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It doesn't matter, I was just wondering. Depending upon how is went bad, it could have taken out some other transistors as well.
I don't understand your numbers are these voltages at the collectors? As I mentioned before the positive regulator is what you should focus on, so Q710-Q714.
Q714 is the main output pass transistor, so there should be +40 volts on the collector, about +9 volts on the base and +8 volts on the emitter. The +9 volts on the base controls and turns on the transistor enough to make it pass +8 volts to the emitter.
The control voltage at the base of Q714 comes from the emitter of Q712 which in turn is controlled by the voltage at its' base that is set by Q710 and Q711.
I have to assume that you have meter tested all of the transistors in the regulators, but if you haven't done so please do. Did you change the tantalum caps in the regulators when you did the audio circuits?
Edit: I got so caught up in the regulator theory, I forgot to ask if you have tried unloading the circuit by disconnecting the preamp circuits from the output of the regulators? Maybe there's a short in one of the preamp boards.
[Updated on: Thu, 25 September 2014 13:25] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21519 is a reply to message #21094] |
Thu, 25 September 2014 16:45 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If you haven't put the limiter back on yet, please do so until you get this sorted out.
Did you test the new 2n3055 replacements as well? You've had a catastrophic failure of the low voltage power supply regulators. There are three basic transistors that you need to replace, 2N3567 NPN, 2N3638 PNP, and SE4002 NPN.
For this usage, I would recommend you use 2N4401 NPN and 2N4403 PNP. I'd also suggest that you disconnect the red and green 8 volt supply wires from the preamp boards until the regulators are fixed. I don't know if there was any damage done to the preamps, but better to be safe than sorry.
Q711 was originally a reverse biased 2N3638 transistor. Later it was a 6.2 volt Zener diode. I don't know what your board had, but either will work.
What was the problem with the output transistor mounting?
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| Re: Kustom 200 B1 blowing fuses [message #21521 is a reply to message #21094] |
Thu, 25 September 2014 21:25 |
Tone Meister
Messages: 42
Registered: August 2014
Location: NC
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Member |
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Bill,
This amp had been sitting for many years and was brought to me for resurrection. It had a blown fuse when I got it, so it had never been off the limiter until yesterday. Any damage done would have happened eons ago because I've been careful not to exacerbate the existing problems. Basically, the entire power supply and output section was toasted. The power supply caps are still in great shape, ESR less than 1/4 ohm and capacitance of ~5600uF each, so I kept those.
I had already disconnected the Red and Green wires to the preamp before pulling all the small signal transistors in the regulator section.
I have a 1N4735 Zener (6.2V 1W) on hand that I'll use at Q711 but I'll have to get the 2N4401 and 2N4403 tomorrow at the local Rat Shack - they have 15-packs of both NPN and PNP that include 5 each of the ones I need, so that will save me from having to order. I have gazillions of semiconductors in stock but nothing that will replace the toasted ones in this amp.
The 2N3055s at Q714 and Q718 are still fine. I never ran the amp without the limiter until I was certain there were no more shorts.
The problem with the outputs shorting was that I had used just the Q-Pads and no other insulator. I had never used the pads before and somehow had it in my mind that was all I needed. Wrong! I added new insulators and new shoulder washers and problem solved.
[Updated on: Thu, 25 September 2014 21:26] Report message to a moderator |
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| Re: Kustom 200 B1 blowing fuses [message #21540 is a reply to message #21094] |
Fri, 26 September 2014 12:53 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Interesting about the Q-Pads. I've never seen them so I read the literature and find that they are, as you well know, non-insulating.
I've used the gray Silpads before and they do not require insulators, but they do not like being squeezed too much. I've seen a few of them fail, causing the transistor to short to the heatsink. I guess that's why I stick with the old mica insulators and silicon compound.
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| Re: Kustom 200 B1 blowing fuses [message #21612 is a reply to message #21094] |
Thu, 02 October 2014 18:05 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The diode looks like it is installed correctly. The transistor in the photo looks like it is in wrong though. Most of those transistors are EBC looking at the flat face, is that one marked?
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