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 Post subject: Scott LK-60B Amplifier Problem
PostPosted: Mar Fri 02, 2018 1:34 am 
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Joined: Mar Thu 01, 2018 11:21 pm
Posts: 6
I am not versed in electronic circuitry except for being able to solder and use a digital multimeter in checking voltage and resistance, etc. I recently decided to check the operation of the LK-60B Scott Amplifier kit I built in the late 1960s. It has been in storage since I purchased a surround sound system some 20 years ago.

In using Section 4 of the manual to check the balance and bias without instruments, I found that the left channel bias read 10 on the meter but the balance was OK. I thought that perhaps the electrolytic capacitors were at fault since many years had elapsed since it was constructed. I started checking the capacitors on the left amplifier driver board by measuring the AC voltage across them. I found that the voltage was low across all of them except for C6 which measured 25 VAC.

Since the right channel bias and balance was OK I decided to check the voltage across the C6 cap on the right channel board for comparison but in the process the probe slipped and I believe I either grounded the collector of Q4 or I shorted the collector of Q4 to the emitter of Q3. At any rate the 1-1/4 amp slo-blo fuze blew. When I replaced the fuse and turned the unit on, R18 got hot and started to smoke.

I feel very foolish having screwed up, especially since I don't know where to start in the repair process. Hopefully I didn't do a lot of damage.

Can anyone help with this? I scanned a copy of the schematic and the driver board layout from the manual. I also took resistance measurements which are penciled in on the board layout if that helps.


Attachments:
schematic.pdf [173.73 KiB]
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rt chan driver bd.pdf [217.75 KiB]
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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Sun 04, 2018 3:15 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
Most likely you shorted a driver transistor which may have carried over and taken out an output. There are two ways to approach this problem. easiest is to reduce supply voltage and bring it up slowly, comparing the voltages between the left and right channels. Often you can see a trend where the voltage on one side or the other of the bootstrap is pulling toward one side or the other. this is indicative of a transistor conducting too much, often because of a short on that side. You then replace the transistor and repeat the process.

You say you have limited test equipment so I assume you may not have the variac or limiting supply needed to do this. An alternative requires pulling outputs and drivers on the bad channel and checking them out of circuit for shorts. This should be done on a high resistance scale of your meter. Also be on the lookout for any where leakage in the reverse direction is found. Those, as well as any that read open in both directions, should be changed out. While they are out of circuit, also check all diodes in the driver circuit for open / shorts. Don't forget to also check D1 -D2 which may be mounted on the heatsink.

Next reapply power (a variac would be helpful here, as you could do as stated in the first paragraph, otherwise you are taking a chance!). One trick I have used is to use a smaller fuse which will be less forgiving of problems. (.75 or 1 A for this one) Solid state amplifiers like this do not use full supply current until the volume is turned loud, so voltages should come up good as long as you keep the volume low. If the voltages do look good, then you can put the specified fuse in and treat it as normal.

To go into more will require I have some actual voltages and conditions to make a more indepth evaluation.


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Mon 05, 2018 12:00 am 
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Joined: Mar Thu 01, 2018 11:21 pm
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Thanks for the reply CaveRat. Yes, you are correct in assuming that I don't have a way to reduce the supply voltage. I'm guessing the best I can do is the alternative you mention in your second paragraph. I'm not sure what you mean by "pulling outputs and drivers" although I can isolate the right channel driver board by unsoldering 8 wires (not including ground) which connect it to the rest of the circuitry. What do I need to do to "pull outputs"? D101 is on the driver board but I'll have to look for D102. As you say, it must be on the chassis somewhere.


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Tue 06, 2018 3:45 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
By "pulling" I mean you will need to physically remove the transistors from the circuit to take the resistance measurements. If you don't you will get false readings through other components. Silicon transistors should read about half scale on a conventional VOM in one direction, and when the leads are reversed they will read open. Check from base to emitter and base to collector to obtain these readings. The emitter to collector reading should be open both ways in most cases. This will isolate a shorted transistor as well as one which is open. It does not address gain issues, but in your case it looks like a case of short / open. (Usually what happens when a "mistake" is made!)

Diodes should read half scale in one direction and open in the other.

Note too these readings must be made with a conventional analog Volt /Ohm meter, not a digital one since the voltage supplied by digital meters often do not allow the barrier voltage of the transistor junction to be overcome. (they read open in both directions even on a good transistor)


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Tue 06, 2018 8:01 pm 
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Joined: Mar Thu 01, 2018 11:21 pm
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I do have an old Simpson 260-2 VOM I can use instead of my digital meter as you suggest. I'm hoping that Q110 and Q111 are not damaged because they are a "matched pair" (whatever that means) and I don't know how I would be able to purchase another "matched pair" to replace them.
OK, I'll dig out the old Simpson and see what I can find. Fortunately Q1, Q3, and Q4 plug into sockets on the board and are easily removable for checking. They also have winged heatsinks on them which is what my probe touched to cause the problem in the first place. It looks like I will be able to pull Q110 and Q111 from the circuit by unsoldering wires from their leads to terminal strips. I'll let you know when I get the results. Thanks again for your help.


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Mon 12, 2018 9:34 pm 
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Hey CaveRat, I was incorrect in my last post. Q1 is not plugged into a socket, it is soldered into the board.
At any rate, I isolated what I could by unsoldering and cutting connecting wires and here is what I found:
Q3 and Q4 are definitely bad - shorted in all measurements
D101 and D102 are OK
Q110 and Q111 are OK
D201 and D202 are OK
As for Q1, Q2, D1, C6, and R18, I don't know since they are soldered into the board. The results I got for Q1, Q2, and D1 are not good but, as you say, I may be seeing false readings through other components. What do you suggest? Should I replace Q3 and Q4 and see what happens when I apply power?


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Tue 13, 2018 5:04 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
This type of failure is unusual; most times outputs take out the drivers but since the accident originated in the driver stage I would assume that high current may have extended further upstream. Thus I would have reservations regarding the condition of Q2 in particular.. If you simply replace Q3 and Q4 alone it is possible Q2 and Q1 are also bad. Powering up under those conditions could easily take out your replacement transistors again and also the outputs.

I would recommend at least pulling Q1 and Q2 and verify they are good before powering up. If they are good there is still a possibility of taking out your replacements if something else is bad, but that possibility would be reduced if you know for certain Q1 and Q2 are good. You have already established the outputs are good according to your previous post so that removes the most common cause of driver failures.

Makes it hard without a variac, but when I get these type of failures a trick I use is to remove the outputs and power up using the variac to reduce supply voltage. As I raise the voltage I monitor the bootstrap voltage. It should remain about half of supply voltage as you increase the line volts. If something is amiss, the voltage will swing either too high or low and you need to find out why. Once you reach full supply voltage and if everything else looks good, then measure the difference (emitter - base) voltage going to the output transistor sockets. It should be under 1 volt on each transistor. if so, power down and reinstall the outputs and test again. Check bias current and adjust according to manufacturer specs. That should have it working.


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Tue 13, 2018 6:16 pm 
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While I don't have a variac to use I have taken the liberty of ordering extra parts such as Q1 and Q2 replacements including the sockets which I can solder on the board to avoid having to solder the Q1 and Q2 leads - this always makes me nervous. I've also purchased an extra D1 and R18 just in case. Do you think I can get away with replacing Q!, Q2, Q3, and Q4 or should I also replace D1 and/or R18. Could these last two cause a major problem if they are bad and I don't replace them?


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Wed 14, 2018 1:41 pm 
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Location: Warner Robins, GA
scott60 wrote:
I do have an old Simpson 260-2 VOM I can use instead of my digital meter as you suggest. I'm hoping that Q110 and Q111 are not damaged because they are a "matched pair" (whatever that means) and I don't know how I would be able to purchase another "matched pair" to replace them.
OK, I'll dig out the old Simpson and see what I can find. Fortunately Q1, Q3, and Q4 plug into sockets on the board and are easily removable for checking. They also have winged heatsinks on them which is what my probe touched to cause the problem in the first place. It looks like I will be able to pull Q110 and Q111 from the circuit by unsoldering wires from their leads to terminal strips. I'll let you know when I get the results. Thanks again for your help.


If the digital meter has a diode test function you can use that for most transistors.


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Wed 14, 2018 10:37 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
If you have them available I would change Q1 and Q2 just as a precaution since it is unknown what stress they may have experienced. As for the diode and resistor if they do not appear overheated you may be OK. Just check all voltages around the drivers, watch for potential high current on the outputs, and be aware if something isn't quite right these are possible causes. (The resistor moreso than the diode)


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 Post subject: Re: Scott LK-60B Amplifier Problem
PostPosted: Mar Thu 15, 2018 2:42 pm 
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Joined: Mar Thu 01, 2018 11:21 pm
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Great, thanks for the advice. I'll replace Q1 and Q2 as you suggest using sockets and get back to you with the results after I fire it up - probably over the weekend.


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