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KUSTOM K200B-6 [message #29443]

Sun, 23 February 2025 10:03

Voxman2027
Messages: 11
Registered: November 2024

Junior Member

Well the continuing story of Bungalow Bill here on this Kustom K200B-6.
Came in with distortion caused by (something ) I thought wouldn't be too hard to find. It was missing the negative half of the sine wave.

Initially all the components tested in and out of the circuit tested normal. The 2 separate input channels outputs were fine with good sine wave.

So I concentrated on the only other PCB in amp.
I felt that there was a transistor that wasn't conducting properly so I spent plenty of hours going over all the voltages.
Ended up going through amp and replacing ALL the old tantalum capacitors with Nichion from Digi-Key. Replaced every old carbon resistor that was not dead on value with 1% metal film. The diodes all checked good so I left them alone.

Then I replaced ALL the transistors except the 4 final output ones (tested good). I had trouble finding some of the original transistors so I was forced to go with equivalent substitution recommended by various posts I found for substitution them.
After all those components changed I powered it up on variac and was very disappointed to see the current surpass 3 amps at which time the 3 amp fuse went bye bye. Yup I was erroneously mistaken to think everything would be fine and I have no excuse other than being stupid.

So then after this unfortunate event I set about pulling the PCB out of the amp to really troubleshoot the issue.
Although the amp drew over 3 amps, I didn't see anything smoke which puzzles me. I would have thought if something was drawing that much current that something physical would have been happening.

Subsequent component testing everything was still good except 2 of the positive rail transistors. These were a substitute of a substitute and I suspect not suitable for this amp circuit. I doublechecked them for proper orientation (BEC) before soldering them in. They are shorted but only show one diode working in each.

At this point I'm stuck... I doublechecked every transistor, resistor and capacitor I replaced to make sure they were the proper ones and installed correctly. If I put 2 new transistors in, then I would expect the same thing to happen immediately. I'm afraid that I won't be able to get this puppy up and running again without major analysis of circuits (above my pay grade). It's a goofy design IMHO.

Not sure if anyone can really help me with this situation but just thought I'd throw it out there in case someone has been in this situation before.

Thanks for reading my story. I think the butler did it but I can't be sure yet.

I WOULD HAVE ADDED A PHOTO OF CIRCUIT BUT THAT ISNT ALLOWED HERE.
This has been fun so far....

Gary
PS The "transistor location diagram" that is supposed to be accurate for this amp.... well it isn't. Luckily I replaced each transistor and was absolutely certain to replace it with same.

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Re: KUSTOM K200B-6 [message #29444 is a reply to message #29443]

Mon, 24 February 2025 10:02

stevem
Messages: 4799
Registered: June 2004
Location: NY

Senior Member

you should not have problems finding the 2N3055 / NTE130 output transistors , there’s tons of them on EBay.

What circuit board are you taking about the transistor layout being wrong, the 703 board?

[Updated on: Mon, 24 February 2025 10:02]

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Re: KUSTOM K200B-6 [message #29457 is a reply to message #29444]

Fri, 28 February 2025 08:46

Voxman2027
Messages: 11
Registered: November 2024

Junior Member

Stevem,
Thank you for your interest in my post, I was beginning to wonder if the post was viewable to the forum folks.

I have found transistors ( some very expensive) however the 2N3055 (sub for RCA 36892) you mention isn't any of the transistors I needed.

Regarding the LAYOUT DIAGRAM.... I have NO IDEA how or where I found it as I have not been able to find it again anywhere on the website. It is
labeled PC703 which should be accurate for a PC703 schematic. However it isn't ... it's different, Q704 is actually Q708 and therefore Q708 is Q704.
Whoever made up the layout must have been cross-eyed or dyslexic 😎.

This repair has been a nightmare compared to any other amp I've worked on throughout the years.

I received the final transistors (40410 sub for 38736) I needed and installed. Checked and doublechecked all again before applying power.
Brought up AC slowing while watching current draw and was happy to have nothing unusual happening. Looking at the input signal, it was miraculously normal looking. But just when I thought all was ok, I saw a little wisp of smoke so immediately shut down.
Long story short... the -/+ 8vdc supplies coming from that conglomeration of transistors (Q710 thru Q718) were too high at -/+ 12vdc and had affected the input circuit of PC5027... specifically R9 (220 Ω) and C3 (33uf) on the +8vdc supply. It probably damaged Q2 (2N3638) too so they were all replaced.
I couldn't find anything else amiss on PC105 which is the other input pre-amp although they share the same supply voltages.

So now I disconnected the supply voltages going to these 2 input channel PCBs from the main PC703 PCB. Using a dual +/- 8vdc from a DC supply, I powered up the 2 input channel PCBs. Everything worked fine now with a good sine wave.

So this is where I'm currently at in troubleshooting this amp. I know there's a problem with the -/+ 8vdc supplies coming from PC703 but I'm not looking forward to tracking that down. At this point I'm very discouraged in my attempts to get this amp fixed. The issues with not finding exact replacements has me wondering if I'm ever getting this amp fixed. I've already given up charging customer for all the numerous components replaced as they haven't fixed the problem and in reality caused more issues than the original problem.

I honestly don't expect anyone to be able to really help but maybe I've missed something or who knows what. I probably will let this amp sit for a while before coming back to it. Maybe with a fresh approach I can uncover what's going on.

Thanks anyways for your comments and suggestions.
Much appreciated
Gary

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Re: KUSTOM K200B-6 [message #29458 is a reply to message #29443]

Sat, 01 March 2025 07:23

stevem
Messages: 4799
Registered: June 2004
Location: NY

Senior Member

Hi .
There are many revisions of that PC703 board.

This site shows 3 revisions, 1,2 and 4, it’s missing number 3 which is a revision I have never seen to this day all these years later!

You can find them in the technical section here under the heading of boards by PC number.
One thing I notice is that Q711 on one revision is a NPN type and on another it’s a PNP type.

This I would have to think is your issue with that regulation section.

Also you posted that Q704 and 708 are swapped.
That would not be an issue since they are both the same part number heat sinked NPN output driver transistor.

Please make 3 more separate post that only say TEST.
Once I approve these then delete them all of your future post will just go right up to the board.

[Updated on: Sat, 01 March 2025 07:27]

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