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Old 15th January 2007, 08:14 PM   #1
Monamis is offline Monamis  United Kingdom
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Default Resurection of dead KSA80

Many years ago I bought a KAS80. It have performed well over the years until last year when I noticed that the right channel (as you look at it) was not warming up as it usually does. I decided to investigate to see if there were any dry joints. I took the driver board out checked underneath etc moved some of the components and reassembled. Explosion. The stabiliser transistor and zener diode on +ve rail exploded and some of the smoothing caps drooled horrible goo everywhere.
I would like to repair it but I would appreciate any advice on how to check every transistor and bearing in mind the original fault, restore it to its former glory. First thought is to replace every transistor but since most are matched pairs and some of them are obsolete and to cap it all they are darned difficult to get off the board.

So where is the best place to start? Source of suitable components at least the electrolytics need changing. I have found some at RS components but they are a little too long. What about the semis and the passives.

Or to put it another way....HELP
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Old 15th January 2007, 10:01 PM   #2
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Default Depends...

...on your personal priorities and perhaps capabilities...

The chassis, heatsinks, transformers, etc are the most expensive parts of the amp, and AFAIK yours are working perfectly. Seems to me you have 3 options:

1) Identical rebuild: You can repopulate the boards you have, using components as close to original as possible. There will be some that are no longer available, but as far as I can tell the modern equivalents are as good or better, but you will need to do some research. Stripping the boards of the original components without damaging the tracks and pads may be a challenge, but is definitely doable. You can find out about some of the modern component equivalents in the threads mentioned below.

2) Not original but same family: rebuild the amp using new "Krell Clone" boards, you can check the ksa50 or ksa100mkII threads for info on the boards and which components to use with them. Performance should be as good or better depending on your choice of boards and ancillary components.

3) Totally new: rebuild the amp with a completely new topology, perhaps an aleph or some other high quality class A amp (A40, A75, JLH, Madrigal etc) for which schematics, boards etc are available. Depending on your speakers this could give you the biggest bang for the buck, the right amp and speaker combination sounding much better than the sum of the parts.

HTH

Stuart
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Old 15th January 2007, 10:05 PM   #3
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I think you can find the basic schematics under the thread

"Reverse engineering Krell KMA 160"

You can also call or email Krell in the USA to purchase any special parts like the high current relays.

You can look at the date codes on the large power supply capacitors to determine if you want to sway them out.

Naturally, with 2 channels signal comparison helps.
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Old 15th January 2007, 10:23 PM   #4
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These amps are getting to the end of their useful life as far as electrolytic capicators go. To preserve an amp like this or orther make re-capping after 10 to 12 years is almost a mandatory happenning. Tweeks of installing more modern semiconductors to improve performance is also not a bad idea but not mandatory... although if it passed across my bench all the plastic tab TO-220 devices would get replaced. They get tired after many thermal cycles and can randomly fail. Also tweeking the bias and DC balance if applicable at three year intervals is also a good idea. I bet very few folks that own these amps have followed this guideline.

Stuarts idea of a new channel is not a bad idea... although you will need to investuigate Linesources thread to do it properly. Keep in mind that it will not be chaeap to do... and you still need to replace all the electrolytics! The KSA-100 board will work in there biased to give 80 watts of class A power. Which sounds better I can't say. Been a long time since I listened to a KSA-80.

Also... If you don't want to repair it I buy blown up Krells! At any rate it is worth repairing!! E-mail me if you decide you want to let go of it.

Mark
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Old 15th January 2007, 11:12 PM   #5
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yeah i would like to buy some blown up krells too! But shipping from the UK would be HUGE!!


How skilled at amplifier repair are you? Do you have the proper equipment to carefully match transistors? check components, set BIAS desoldering tools etc If not, personally i would seek help from someone that can help with the restoration!

If you are a skilled tech, then all the electrolytic caps need to go! but i wouldnt just replace them with any old cap the locale spares house stocks!

This may just be my opinion, but working on a Krell is akin to working on a Ferrari, or Porsche, Aston or whatever your favorite mega buck car is.

I have to believe that when these amps were built, extreme care was taken in component selection. So replacing caps should be done cautiously. using the best caps you can get your hands on.

testing items like Diodes and transistors can be done easily with almost any VOM that has a diode check setting. this at least will spot the shorted or open devices and you can work your way from there.

The first thing i do with any amp that comes in for repair is check every transistor and diode with a meter to find the devices that are shorted or open and take inventory of how bad off the amp is. output transistors usually have to be pulled and tested one on one as if one is shorted, it will show as the whole bank being shorted.

and extreme care should be taken in Matching transistors if any need to be replaced.

If there are circuit traces that need to be repaired. there are PCB trace repair kits available but they are not cheap. But PCB's should be repaired and not just patched.

Vintage Krell amps are worth every penny when in original condition. and almost as much in restored condition.

Please dont gut it and roll your own. Please restore it and if you want to build your own, sell it and use the money to build a clone.

Zc
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Old 17th January 2007, 09:52 PM   #6
Monamis is offline Monamis  United Kingdom
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Thank you all for your informative replies.


Stuart

I think I would like to repair the amp as in option 1. It would maintain the value of it. I think these amps in working order are still worth around 1000UKP so I could keep my options open once it is repaired.
Yes the power supply works fine as the other channel is working and heating up properly.

The power caps appear to be good; they still had some charge in them even 3 months after turning the beast off!!

I think my plan is to replace all the electrolytics on the board and identify which transistors are caput. One of them is easy, its in two pieces..... I think there are quite a few matched transistors on the board so I would either have to buy matched pairs OR buy a bundle and match them myself. Would the stabilzer transistors need to be matched? Wouldn't have thought so but to use the car analogy maybe for best performance they should be matched.

Mark

Cost? Well I have a yardstick for that is Krell importer in the UK quoted me 800UKP to refurbish the amp so if it even approaches half that to do the job I think having an official refurb job would make it more saleable if and when the time comes. However I don't really want to spend that on an amp that will be worth at best 1200UKP

The question is where to get the best suited replacement parts. You mention Linesources thread. I will investigate.

How does the VOM show up dud FETS? I have never tried it on a FET, I have in the past tried it on Transistors but not in circuit.

Zero Cool

Yes I fear your right shipping would be crippling from UK to US with the wiegth of these thing probably need a ship al to itself....

Fortunately the board is in good nick but so much heat has to be applied to get the solder to melt all the way through and stay melted while you pull on the device. I have solder wick and a vacuum pump but I think to do a good job a proper heated vacuum desolderer is called for.
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Old 17th January 2007, 10:07 PM   #7
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Default transistor matches...

Hi,

While there are a few transistors that will make the amp work 'better' if they are matched, IIRC nothing in the schematic requires them. If you have the time and a simple test setup you can do as good or better job matching than the originals, so don't be intimidated by the idea that you need matched parts.

The small signal transistors are quite inexpensive, on the order of a few pence each, so you can buy a couple of dozen to find a few really close pairs. The output transistors are the most expensive semis, but parts bought from the same lot code will be fairly close by default, and the high value emitter resistors make exact matches much less critical.

I've rebuilt a couple of old amps, and it's mostly scutt work, removing, cleaning, testing, replacing and resolding. But be really careful about values of replacement components, checking and double checking and all that. I bought an amp an upgrader was selling because it simply 'didn't work'. He had misread 2.5k as 25k, resoldered a resistor with the incorrect value and the amp never worked for him again...

HTH

Stuart
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Old 18th January 2007, 12:06 AM   #8
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Quote:
Well I have a yardstick for that is Krell importer in the UK quoted me 800UKP to refurbish the amp so if it even approaches half that to do the job I think having an official refurb job would make it more saleable if and when the time comes. However I don't really want to spend that on an amp that will be worth at best 1200UKP
Well, I've seen them on Audiogon advertised that they were just gone through by Krell and the price they've sold for probably represents a loss to the seller if he had Krell do a full number on it. At best about $1500.00 U.S. for one gone through and the chassis in mint condition and $1500.00 is pushing it for a KSA-80B.

Line source's thread is the best place to start. He shows his circuit for matching devices someplace in the thread. In doing work on the KSA-100 MK-2 we have in fact found better MOSFETS to replace the obsolete devices in both the KSA-100 MK-2 and later balanced KSA series. There are actually three choices when it comes to those MOSFETS other than the ones Linesource used. I have personally tested two of them so far and there are no audible differences I could hear between them but two of the three are better apt to dissipate the heat alot better. UNfortunately neother of those will plug directly into the 80B circuit boards. One package is SMT and the other is a 4 dip package.

MOSFETS tested so far....

ZVN2110A / ZVP2110A

IRF110 / IRF9110

ZVN2110GTA / ZVN2110GTA....SMT versions that dissipate heat better than the plastic stuff provided there is enbough copper trace.

The KSA-100 MK-2 boards thata re soon to arrive are set up to accept all three of these MOSFET options.

Mark
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Old 19th January 2007, 03:19 AM   #9
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Quote:
Originally posted by Mark A. Gulbrandsen

Line source's thread is the best place to start. He shows his circuit for matching devices someplace in the thread.

Mark
What thread is that. is that the "Reverse engineering Krell KMA 160" Thread??


Thanks


Zc
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Old 19th January 2007, 04:25 AM   #10
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Hmm - you say it blew up after you disassembled it, checked for problems, and re-assembled it. Off the top of my head, and this is pure speculation since I'm not familiar with the KSA-80:

1) electrolytic caps typically don't blow up unless the polarity is reversed - did you perhaps get something backwards (like the supply connections) when you put it back together? That might also explain the diode going.

2) Another possibility is that in your disassembly/re-assembly the insulators between the output transistors and/or the hardware holding them to the heatsink was displaced. I'm thinking TO-3 packages, since the case is the collector, if the insulator got displaced then you'd have the supply rail shorted to the heatsink and perhaps to ground or even the other rail if you had bad enough luck to displace the insulators on both sides.

I'm not trying to say you're stupid (I know I have had my extraordinarily moronic moments), just trying to come up with a theory that explains the symptoms.

I just completed ressurecting an old NAD receiver that had a blown set of output transistors, driver transistors, and bias transistor. I acquired the schematic and did pretty much the same thing as your plan - started working back from the outputs, checking transistors, and pulling anything that was dead or suspect. When I got to the point where the power supply over-current protection stopped cutting in I checked to see that all the voltages in the remaining circuitry made sense. While I was waiting for parts to arrive, I plugged the circuit into LTspice and played with it in simulation until I understood it well. Now it is all back together and making music again.

In summary, I think that the passives should all be ok unless they were electrolytic caps subjected to reverse voltage polarity, or a diode subjected to a reverse voltage higher than its reverse breakdown voltage. The small signal transistors in the input circuit, voltage amplification stage, and any current sources are probably also ok unless there was a disaster like getting the rails hooked up backwards.

Good luck.
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