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 Post subject: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 3:28 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I have a strange problem with a KLH 20 stereo. The set works properly until I put it into mono, then I get a low frequency thump in the speakers. I can disconnect the speakers and it's still there. And move the switch to stereo and it's clean. You can see the wave slowly build up when you switch it to mono. It works in both channels.
The signal is clean before C100, and then a big sine wave at C102. The -13 volts is clean, but reads low at -10.8. The resistor feeding the -13 volts is good, so maybe something is drawing a bit too much current, but it still works in stereo.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 12:39 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4610
Location: Sunbury, Ohio 43074
Try shorting the inputs to ground and see if the problem still occurs. If not, you're picking up some kind of crud on one of the input lines. You didn't mention if this occurs with or without anything connected to the INPUTS. It appears that the inputs sort of just "float" here unless something is connected to them.

I'd also scope the power supplies and see if there is excessive ripple present.

EDIT: you stated ... I get a low frequency thump in the speakers. I can disconnect the speakers and it's still there

You mean the speakers thump if they are disconnected??? 8) :wink: sorry, someone was bound to say it. I got there first.

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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 3:42 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 958
Location: South Jersey East of Philly
OK..so it's on the scope when the speakers are out of the circuit?
Some questions to try to nail this down...
Can you hear it thru good headphones?
Does it repeat in a cyclical manner?
Does it do it on all input functions?
Interesting that it only occurs when you bridge the channels with the mono/stereo switch...which really doesn't introduce anything new to the circuit.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 4:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
Barry H Bennett wrote:
Try shorting the inputs to ground and see if the problem still occurs. If not, you're picking up some kind of crud on one of the input lines. You didn't mention if this occurs with or without anything connected to the INPUTS. It appears that the inputs sort of just "float" here unless something is connected to them.


Yep shorted the inputs and it still happens.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 5:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
Tbirdkid wrote:
OK..so it's on the scope when the speakers are out of the circuit?
Some questions to try to nail this down...
Can you hear it thru good headphones?
Does it repeat in a cyclical manner?
Does it do it on all input functions?
Interesting that it only occurs when you bridge the channels with the mono/stereo switch...which really doesn't introduce anything new to the circuit.


Yep, the signal is there at the Record Out RCAs. It takes a few seconds to show up. If you switch it from mono to stereo then to mono, it takes about 1 1/2 seconds for the sine wave to fully develop.

There's alot more to the schematic than what I posted, this is just the preamplifier. KLH attenuated the AUX input signal and then amplified though these 2 transistors. Since there is no feedback from the output transistors to the preamp, I left that part out.

I haven't tried phono, don't remember hearing it on FM, it does it on AUX mono, and AUX stereo when switched from stereo to mono.

The schematic shows 8 electrolytic capacitors on the input board, but I only see 6. I've replaced them all, with the correct values, and rechecked the polarity.

Here's a picture of the board before I replaced the capacitors. It's a jungle in there.


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20181118_140511.jpg
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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 5:10 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
Let's try a different approach. Hook your speakers back up and leave them there ... running the amp without a load is probably not a great idea anyway.

First, let's see if both channels are working. Switch it into stereo and put a signal into L and R and verify that both channels have an output. If not, you have probably found the problem.

Next step, if it passes the L/R output test .... switch the amp into the MONO position and see if it thumps with signal in but turned down. btw, what frequency is the tumping at? 60Hz, 6KHz? 1HZ ??? rough idea.

Let's see what the results of the above are, cuz next step will probably be grounding, with a clip lead, the + sides of C100 and C150 to absolutely eliminate any chance that some input circuitry is causing the problem. You might also try grounding the junction of R39 and R54 just for kicks.

Let's see what these tests do........

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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Mon 17, 2018 5:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
Barry H Bennett wrote:
Let's try a different approach. Hook your speakers back up and leave them there ... running the amp without a load is probably not a great idea anyway.

First, let's see if both channels are working. Switch it into stereo and put a signal into L and R and verify that both channels have an output. If not, you have probably found the problem.


Both channels work and sound good. Volume, Bass and Treble work.

Barry H Bennett wrote:
Next step, if it passes the L/R output test .... switch the amp into the MONO position and see if it thumps with signal in but turned down. btw, what frequency is the tumping at? 60Hz, 6KHz? 1HZ ??? rough idea.

The volume control also controls the thump. The volume control is just at the right edge of the schematic, R200 at 50KOhms. It sounds like it's thumping at about 2 to 3 times a second.

Barry H Bennett wrote:
Let's see what the results of the above are, cuz next step will probably be grounding, with a clip lead, the + sides of C100 and C150 to absolutely eliminate any chance that some input circuitry is causing the problem. You might also try grounding the junction of R39 and R54 just for kicks.

Let's see what these tests do........

Grounding R39 and R54 is a good idea. I bet it would keep thumping, but I'll check it when I get home.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 2:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I grounded R39 and R54 and went through the inputs with the selector switch: AUX 1, AUX 2, and FM all had the thump sine wave in mono. Phono started with a giant thump sine wave and slowly went to zero. In stereo you can see the giant sine wave start in phono mode, then it's gone. You can just see a spike in the other modes.
There is no sign of the sine wave at the front of C100 and C150, but you can see a small sine wave after the capacitors, at the base of Q110 and Q112.
With no input, I have a voltage of 0.6 volts on one channel's input and 0.2 volts on the other. Q110 has a base voltage of 1.05 volts and Q112's base is at 0.995 volts.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 4:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3291
Location: Seattle WA US
Look to the -13volt power supply. Dried out electrolytic capacitors in the power supply could lead to "motorboat" oscillation, which is what you seem to have. The electrolytic closest to the junction of R100 and R103 should be first suspect.

C101, C103, C151, and C153 are also suspect.

-Chuck


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 4:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I've replaced all electrolytic capacitors. The -13 volts is low, reading -10.8. The -15 is good and there is a 1KOhm resistor between the -15 and -13, and it's good, reading 999Ohms. Bu I don't know it -13 is correct, the output capacitors C102 and 152 are not 16uf, they are 0.22 ceramic. Let me get the voltages of those four transistors.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 5:13 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
Here's the voltages on the four transistors.


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KLH 20 voltages.gif
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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 4:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I found that adjusting the volume does very little to the sine wave, but even with the volume all the way down, if I turn the balance to the left or right the sine wave vanishes, and the bass control will vary the sine wave, even though both controls are on another board and after the volume control. I do believe both boards share the -13 volts and a ground, so I need to check that.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 5:31 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4610
Location: Sunbury, Ohio 43074
It occurs to me that a sine wave does not thump. If it's really a sine wave, you should be hearing some low frequency hum or buzz, not a thump. Thumping noises only happen with spikes (impulse noise in other words).

What is the frequency, and p-p level of this sine wave, and is it symmetrical around 0v (ground) on the scope? Photo of the scope trace might be helpful

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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Tue 18, 2018 6:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13916
Location: S. Dartmouth MA 02748-1225 USA
Thumps may come from capacitors... Monitor the suspect power source to be sure that it is not the that, if so, replace with caps of a greater MTBF, use longer leads and heat sink when soldering. Only go to next "step" in voltage rating, for example: 16 volts to 25 volts, not only will there be a size, space issue but also there is the possibility fidelity. O.K. in the P.S. though. No need to do the entire amp, just in the suspect areas...

YMMV

Chas

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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 2:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I checked the grounds at the 2 boards, they are good. The sine wave does appear on the -10.7 volts when in mono.
Here's the scope view, it's set at 50msec per division, so it has about 165msec wavelength or about 6Hz at about 0.5 volts at the record out RCAs.


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20181218_182850.jpg
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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 3:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I pulled the power connector from the tone control board, and it did not affect the sine wave at the record out RCA. So the whole problem seems to be in the preamp board.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 5:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
Is the B- still low at -10.8 volts? Do any of the power supply voltages drop when the thump occurs? I recommend using an analog meter for testing, or a min/max digital meter.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 6:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
I put the old caps back in and the thumping went away. Totally clean at the record out RCAs. Tried it several times all good. Put in the only other 10uf (the schematic says 15uf but 10uf 15v were in there) caps I had, UKW Nichicon 10uf 63 volts, I did have 50 volts in, and the thumping started back up. Put the old ones back in and clean again. So I guess I'll look for some 10uf 15 volt caps.


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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 6:54 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4610
Location: Sunbury, Ohio 43074
The 63v cap is not the issue. It may be BAD ... but higher voltage rating is perfectly ok to use.

Is the cap perhaps a NON POLARIZED electrolyitc? That would definitely make a difference

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 Post subject: Re: KLH 20 thumping problem
PostPosted: Dec Wed 19, 2018 7:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3982
Location: Cortez, Colorado
It's not polarized. The caps I got to replace the axial 10uf 15 volts were Nichicon TVX 10uf 50 volts. They couldn't be bad, because I also tried Nichicon's UKW 10uf 63 volts. I just tried it again with the old caps and it's still good. Maybe I'll just keep those 2 old caps in it.


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