K200A-1 Power Amp Rescue Needed [message #27251] |
Fri, 19 April 2019 13:28 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
thetragichero
Messages: 33 Registered: April 2019
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hello i picked up this early 1968 k200a-1 bass head (by ross inc on the front, no 200) for a song because the owner said all it was doing was crackling
measuring the voltage on the speaker at -12V, i can see why!
i checked the power transistors using the resistance setting on the multimeter and the only one that tested odd was the one closest to the power transformer (resistance between emitter and collector around 1M, shorting base to collector and measuring again gave around 77k. all the other power transistors measured >4M and 700k respectively). replaced that transistor with a 2n3055 i pulled from an organ and that gave me -20V across the speaker terminals
i'm having a hard time finding a schematic for this unit as the ones on the site do not match up. boards along the back of the chassis are PC702 and PC602. there's no internal reverb or anything. there does not appear to be a transformer on the output as the K200-Ax schematic would lead me to believe
another thing of concern is that it is fitted with 5 power transistors, even though there is a chassis hole for a sixth. obviously an accurate schematic would work. any help would be much appreciated
i took voltage readings on the power transistors (from left to right)
C -18V -18V 38V 38V 38V
B -33V -33V -12V -13V 26V
E -33V -33V -12V -14V 26V
filter caps look relatively new, and besides being attached to the chassis by a zip tie and hot glue (some tagboard would make me much more confident in its durability), i measure 38V and -32V respectively
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Re: K200A-1 Power Amp Rescue Needed [message #27255 is a reply to message #27251] |
Sat, 20 April 2019 12:59 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Yes, my guess is that is a very early K200A amp with a single sided preamp power supply. I will assume that there is no negative power supply going to the preamp boards?
Normally, when an output goes bad in these amps, they will blow fuses and take out other components as well. If the outputs don't test as shorted, I would look at the rest of the power amp circuit.
Check for burned or open resistors, shorted caps, shorted transistors, etc.
If your meter has a diode test setting, use that to test the transistors, if not use a low ohm setting. 4 meg and 700K readings don't mean much here as the meter may not be applying enough voltage to forward bias the junctions.
Be sure not to connect the speaker to the amp until you have the voltage on the output fixed. You don't need to ruin the speaker while you are working on the head.
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Re: K200A-1 Power Amp Rescue Needed [message #27257 is a reply to message #27255] |
Sat, 20 April 2019 15:10 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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Bill he lists a PC602 board, wouldn't that mean he should also have or need a PC502 board and the "missing" power transistor? Schematic I have shows the 602 as the minus supply and 502 as the positive supply. Just to double check went down to my bench. My A without effects just has a PC502 board in it with space (holes) in the chassis for an additional board. It is the positive regulator. The other unit with effects has both the boards a PC502 positive regulator and a PC602 negative regulator. While this should have nothing to do with his voltage on the speaker jack it will affect the units ability to function if he truly only has a 602 board along with the driver / output board on the back wall. Unless someone cobbled a +24volt supply out of zeners.
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
[Updated on: Sat, 20 April 2019 16:02] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27259 is a reply to message #27251] |
Sat, 20 April 2019 22:09 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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That's a good point, I didn't look up the pc numbers to see if that was the positive or negative regulator board. I assumed that it was a positive one.
We should get some more info about what this one has.
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Re: K200A-1 Power Amp Rescue Needed [message #27260 is a reply to message #27251] |
Sun, 21 April 2019 06:06 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4768 Registered: June 2004 Location: NY
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Yes, the sperate pc502 and pc602 are only used in the A serise amp, in the B series amps they where put on the pc703 board which is a combo regulator and driver board.
The A model preamps run on a + and - 23 volts and as in the later B series amps the negitive regulators output tracks the positive regulator.
This all means that the - 23 volts will never be correct if the positive regulators output is not right.
Both of these boards have 15 volt electrolytic caps that will short out if Transistors on the board are bad and thus smack the caps with way more voltage then they are rated at..
Note that if you go into our tech section and choose the pc board listing you will atleast find schematics for all the versions of your 702 board and the schematic for a 502 board which it would seem your missing!
I guess you can bread board yourself one, but first let's get the amps driver and output stage up and running.
Just for now I would unhook the 34 volt power supply wire from the regulator board that you do have .
PS, by the way there's no such thing as a vintage era Kustom bass head, they where just non effects heads and had a normal and brite channel.
[Updated on: Sun, 21 April 2019 06:25] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27261 is a reply to message #27251] |
Mon, 22 April 2019 02:41 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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Just a heads up. Been working with thetragichero on this. He redid the power supply and added a ic regulator to supply the 24 volts for the preamps. This amp has been "worked" on by someone else before and they mangled it. In addition.to out of tolerance resistors, incorrect parts put in in the wrong places, the 1N3754 was substituted and just hanging in mid air. He will be checking the rest of the diodes and transistors tomorrow along with the rest of the resistors. Steve he has a couple of messages in queue that is why I put this up to let you guys know where we were at.
It was in fact a 602 board in the amp. Someone tried to "rewire" it to make it work for the 24 volt supply.
Time to put these east coast eyes to bed.
John
Oh yeah, it actually is a K200A-1 amp with no effects just like the one I have, so it has no need for -24.
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
[Updated on: Mon, 22 April 2019 02:43] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27263 is a reply to message #27262] |
Mon, 22 April 2019 10:27 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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I had questioned thetragichero on the 602 board in his initial post because I knew it was the negative regulator board.
After several pm's he sent me photos - front and back of the board and it was in fact a 602! Someone in the past "worked"on the amp and literally mangled it. The issue of the 602 board is now moot. He rebuilt the power supply section on a board and added an IC regulator to provide the +23 volt supply for the 102 boards. Now have the +23 and 40 volt +/- supplies solid. Tackling issues with the 702 board now. Discovered several "replacement" components out of tolerance, connected to the wrong points, etc. Also as I stated in post above, the 1N3754 had been replaced with a substitute diode that was covered in thermal grease and hanging in mid air. After replacing all the caps on the 702 board he is in the process of checking the rest of the components.
RE messages in queue - I had only passed that on as he had stated it tome at the time he was constantly booted off after logging in. Did not mean that you had missed anything Steve.
John
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
[Updated on: Mon, 22 April 2019 10:32] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27265 is a reply to message #27251] |
Mon, 22 April 2019 12:19 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4768 Registered: June 2004 Location: NY
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No problem or apology's need gang!
Every morning I release/ approve the post that folks make until they have 5 of them at which time the site should allow him to post without going thru the approval process.
So let's all get on the same page here, he now has newly made regulator board supplying both a + and minus 23 volts for the preamps?
[Updated on: Mon, 22 April 2019 12:24] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27282 is a reply to message #27251] |
Thu, 25 April 2019 13:18 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Glad to hear that you got it back to life. Now just a little tweaking to get it 100%.
I'm not a big fan of replacing parts for the sake of replacing things, so I wouldn't replace the outputs if they are all working as they should. The only one I would question is the used one from the organ, if it's still in there.
Again, congrats on the repair.
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Re: K200A-1 Power Amp Rescue Needed [message #27284 is a reply to message #27282] |
Thu, 25 April 2019 14:10 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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thetragichero
Junction of Q701 / Q702 is supposed to be about 0.60 volts NOT 60.
Also base of Q706 is supposed to be 1.5 volts NOT 15
Anyone that has an A series amp with a PC702 board take note. These measurement are from a working 100% good K200A-1 amp I have on the bench right now.
Not recommended to chance running the amp with that diode out of the clip. Chance of both sides firing at the same time too great for my comfort zone.
Congratulations on the progress.
John
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
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Re: K200A-1 Power Amp Rescue Needed [message #27285 is a reply to message #27282] |
Thu, 25 April 2019 14:17 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
thetragichero
Messages: 33 Registered: April 2019
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oh i agree in theory that shotgunning parts is a poor way of trying to diagnose a problem, since you may end up creating more problems
only one of the original rca power transistors was in the unit when i received it (because it had their part number instead of a 2n series designation, and because it appeared like it had been around the block a few more times than the others), and that was hooked up to the 602 board that wasn't even needed
i'm researching how to match output transistors because my assumption is that with all of the other bobo "repairs" done on it, i'd be willing to wager that the last guy didn't bother matching the 2n3232 output transistors
at the very least i'm hoping to learn something, which is really where the fun is in this
edit: YAY i can post without moderator approval. guess i've proven i'm not a spam bot, or at least a really sneaky spam bot
[Updated on: Thu, 25 April 2019 14:17] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27286 is a reply to message #27285] |
Thu, 25 April 2019 14:48 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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I use 2N3055's for output
I use my handy dandy multi-tester to match them based on the hfe and uf readings. It is one of those 9.95 multi-testers found on flea bay but it works.
If you want to spend the $$$ NTE130MP will fill the bill.
John
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
[Updated on: Thu, 25 April 2019 14:52] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27287 is a reply to message #27251] |
Thu, 25 April 2019 15:21 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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I use 2N3055's as well. I usually buy them 10 at a time and gain match them when I get them.
When working on any of these amps, I leave the compensation diode loose from the clip until I'm done removing and replacing the boards, that way the leads will not be flexed any more than necessary. I worry more that the diode lead will break and causing the amp to go into meltdown mode.
I should have guessed that the outputs were mixed and matched from the description of the 24 volt regulator board!
I have nothing against NTE parts, but I hate to pay all that money for them when it isn't needed.
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Re: K200A-1 Power Amp Rescue Needed [message #27291 is a reply to message #27251] |
Sun, 28 April 2019 06:04 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4768 Registered: June 2004 Location: NY
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Please explain!
I do not understand how you are cascading 1 into 2?
If you mean you are pulling your gtrs signal out of input 2 of channel one and the pumping that into channel two then you are putting the channels in parallel.
To me the term cascade imply's a series connection!
Please also note that the first gain stage Transistors in these amps can be blown out if you apply more then 1 volt of signal to them.
[Updated on: Sun, 28 April 2019 06:06] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27299 is a reply to message #27298] |
Wed, 08 May 2019 18:39 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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All ... replying to thetragichero at this time going to try to talk him through a step by step to resolve this.
Update May 9 - had phone phone call with thetragichero and determined the accidental short wiped three of the four outputs.
He is going through the board rechecking diodes / transistors and resistors then ordering what he needs. This beast will roar again.
I highly recommended as others have that he implement a light bulb limiter into the process.
John
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
[Updated on: Thu, 09 May 2019 16:45] Report message to a moderator
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Re: K200A-1 Power Amp Rescue Needed [message #27307 is a reply to message #27251] |
Fri, 10 May 2019 12:07 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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The driver transistors are welded to the heat sinks. If you place them leg side up on your bench, you can hit the bottom of the transistor case with a large screwdriver and the transistor will break free from the heatsink.
I then use a tapered reamer to slightly open up the hole in the heatsink so that a new transistor can be pushed in from the bottom. I will tack solder the new transistor's case to the heatsink.
The simplest light bulb limiter can be made from a wired lamp socket, you know the kind of socket with the wires already attached and two insulated alligator clips. Put an alligator clip on each of the two socket wires. Then use the clips to attach the bulb to the two terminals of the fuse holder, so that the light bulb replaces the blown fuse.
100 watt incandescent lamps are difficult to find, I'd just use a 60 watt one that are still being sold everywhere.
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Re: K200A-1 Power Amp Rescue Needed [message #27326 is a reply to message #27251] |
Sun, 19 May 2019 08:37 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
JDinPA17603
Messages: 69 Registered: May 2018 Location: Lancaster, PA
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Glad to hear it is up and running!
Counterfeit 2N3055's unfortunately abound.I lucked out with my last purchase - got 10 matched pair for $30.00. Guy had bought a storage locker and it was half full of new parts still in bags. I just checked with him and in six months he managed to sell it all.
Enjoy the amp.
John
Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.
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