Dave wrote:
wrote in message
ups.com...
On 1 May, 18:49, "Dave" wrote:

The still bad switch probably needs disassembling & reassembling, or
else replacing the individual switch or switch block. Rows of
pushbutton things on PCBs were a standard design around for many
years in the 70s & 80s, so finding a replacement from some scrap
item is likely.


I re-cleaned the volume pot with some lubricating cleaner and dried
it out with a hair dryer (on low heat setting). The crackling in the
right channel has now all but disappeared.

The left channel, however, will now work for about 10-30 seconds when
the amp is first powered up, then distorts rapidly over about 1-3
seconds and cuts out completely after that.

The amp is set up as follows:

Input jacks -- input selector PCB -- volume knob/loudness PCB --
tone control PCB -- power amp PCB.

I'm going to take out the input caps off the tone control and route
the signal directly from the volume knob to the power amp. That will
at least tell me if the problem is with the preamp or power amp. I'm
pretty sure it's going to be in the power amp, as the second-to-last
component in the pre-amp (tone control board) is the stereo/reverse
stereo/mono switch. If the problem was in the preamp prior to this
point, changing this switch should move the problem from one speaker
to the other. Which it does not. The only downstream component from
the stereo/reverse stereo/mono switch is the balance pot.

When I replaced the caps on the power amp, I cleaned the board with
spray cleaner. It was filthy, like 30 years of dust and grime. In
retrospect, I probably shouldn't have bothered, maybe the cold
temperature of the spray cracked off an internal lead in one of the
transisotrs or something.
The last component in the amp is "speaker protector". This is a white
rectangular box about 1-1/4"L x 3/16"W x 1/2"H (approx, I'm not
looking at it right now.). I don't know how this works, but could it
possibly be my problem? Or should I be thinking transistors? I
guess if I've got the whole amp apart it wouldn't be such a big deal
to measure bias currents on both channels at idle... there are 14
transistors per channel including the outputs.


Re the off balance, there are reasons amps have balance controls. If
yours has none you can fit a potential divider in the channel with
the higher output, and set it up and replace the covers. If youre
lcky and the amp has external connections between pre and power
sections, you can put what you want between those 2, such as a new
volume & balance control.

My amp is integrated, but fortunately is very simple to split as the
connection between pre-amp and amp is a three-conductor wire. I've
been meaning to do this for awhile, I've just purchased a sub with
line inputs and outputs... will have to cannibalize some piece of
junk for a block of RCA jacks.

All of my signal path connections are wire-wound, not soldered. Is
there some tool to do this if I wanted to stay true to design and
wind the connections on my pre-out/main-in connections? Is there any
benefit to doing it this way? I'm guessing there probably is as it
looks like a fair bit more work than just plonking a blob of solder
on a ribbon cable lead.
Thanks for the help.

Dave



NT

The posts I've seen in this thread do seem to suggest a problem with
coupling caps. Also, I've seen bad (open circuit or intermittent)
polystyrene caps on HK's in the past.

If you're not already using them, a sine/square generator and an
oscilloscope would be most useful here. In addition to rapidly narrowing
down to the affected stage, a square wave might show that you have a
frequency response problem in the affected stage. If so, the source of the
problem may well be more obvious.

Remember that with coupling caps, you can often get a clue with DC voltages.
There may be DC, or a higher DC than the other channel, on the output side
of a bad cap, or the following transistor stage may show a low collector
voltage.

If indeed the static was not controlled by the volume or balance, then your
problem is narrowed down a bit, assuming there is only the one problem
involved here

I have the service manual pdf if you need it.


Mark Z.