Teac AG15D Tuner-Amplifier
I bought this rather nice
equipment ages ago to replace my old Tandberg tuner amplifier
and I've been very pleased with it since, until fairly recently
when the volume control knob stopped working so we had to use
the remote control, then very recently every so often a loud
crack from the speakers would deafen you. This loud crack would
daily set the amplifier back into standby, but it could then
be put back on again until the next occurrence of the loud crack...
then the other night the loud crack changed to a quiet crack
but every few minutes instead of once a night. I diagnosed dry
joints and decided it was time to disconnect everything and fix
the problem. Disconnecting stuff is a horrible job as the tangle
of connections have materialised over many years and then removing
the lid from the amplifier reveals it's not going to be an easy
Once you are familiar
with an equipment it's a lot easier to work on it and of this
Teac was a perfect example. Firstly it looked problematic to
remove the main circuit board, but after removing 46 screws I
had the thing out on the bench. At this point I can advise that
the rear panel needs not to be detached. I initially removed
countless screws and did this, but later when still looking for
the problem, discovered the rear panel can be left attached to
the main board.
Inspecting the solder
side of the main board revealed lots of cracked solder joints.
A clue to their locations were several areas of discolouration
due to local heating. The worst dry joints were the connections
to three voltage regulators fastened to a vertically mounted
heatsink. As there were three small electrolytic capacitors inaccessibly
close the the heatsink, I removed the heatsink complete with
the three regulators then fitted two new 1uF and a new 10uF which
are connected to the regulator outputs. I then checked the other
electrolytics in the area where heat had discoloured the board
and found that the dreaded securing compound that I used to encounter
when I repaired VCRs had been used to secure the larger capacitors.
This compond gradually turns into a brown brittle substance and
during this process liberates a highly corrosive by-product that
eats copper and tin. Fortunately only a single wire strap (which
was anyway duplicated) had been partly corroded. Before I'd realised
it was that nasty securing compound I'd detached a 4700uF capacitor
that I thought had leaked. In fact the capacitor seemed to have
leaked but possibly due to the effects of the securing compound.
As it looked pretty horrible I cleaned the circuit board and
fitted a new capacitor. I also removed the 5 volt smoothing capacitor
and fitted three 2200uF capacitors wired in parallel as I did
not have a small 6800uF handy. After carefully refitting the
heatsink assembly and resoldering around 30 cracked solder joints
I reassembled everything and refitted the multitude of cables.
Incidentally.. that nasty gluey
stuff that turned brittle and corrosive was once used in a complex
chip used in a Fujitsu hard drive. All examples eventually failed
in slightly different ways causing their owners lots of grief.
A large computer firm called "Tiny Computers" who had
used these hard drives went bust. I think eventually the chip
manufacturer bit the dust and most hard drives got a bad name
until memories faded.
Switching on I was alarmed
to hear a loud hum from the speakers followed by the amplifier
switching itself off. I must have shorted something in the resolder
exercise or perhaps even fitted a capacitor back to front. All
the cables had to be disconnected again and I settled down to
identify the problem. This time I'd realised that the rear panel
can be left attached so the zillions of screws remained in place.
First... had I fitted a capacitor wrongly? No all were fine.
Had I shorted something when resoldering? I didn't think so but
decided to buzz out the connections as that loud hum I'd heard
might have been a power rail shorting, but all were OK.
Below, immediately left of that
central heatsink are three defunct capacitors, looking just as
innocent as the scores of similar, but good ones.
Just in case an audio
power transistor was bad I removed the output assembly. This
is secured by three screws underneath and two screws either side
of the assembly. At each end is a small board that needs to be
loosened then the whole amplifier board can be lifted out. There's
a long row of power transistors which appear to be complementary
NPN/PNP pairs with a bias transistor between them. A check with
my multi-meter showed all six circuits were perfectly symmetrical
so no problems there. I also tested the standby circuit board
in case it's capacitors were bad and were somehow turning off
the equipment but no problems found here either.
Here are the suspect capacitors.
Left to right 1uF open circuit, 1uF open circuit, 10uF which
had an ESR greater than 20 ohms and measured about 2uF, 1000uF
which had a good ESR but was only 800uF so was on its way out.
The other two are 4700uF and 6800uF which I swapped but then
found the horrible gunge was remnanants of a securing and highly
corrosive glue. This would eventually eat the terminals.
As I progressed, occasionally powering
the main board whilst carefully balanced outside the chassis
and hearing the thing turning off after a few seconds, I came
across something rather odd. The 5 volt ground connection at
the regulator pin wasn't connected to the main copper ground
circuit. Both the adjacent 7812 12 volt regulators were grounded
but not the centre pin of the 7805. I traced the circuit and
found the 7805 connections were indeed marked GND but these were
megohms away from chassis. Neither was the main power supply
which is around +/- 40 volts connected to chassis. As this is
also marked GND I was puzzled. Maybe that last loud crack had
finally blown a hole in one of the printed circuit tracks? That
would explain why it had turned into a quiet crack?
I found a repair manual on the
Net and studied the track layout for the main board circuit.
This showed the 5 volt ground was indeed not connected to the
12 volt regulator ground connectons. But why were both circuits
In studying the circuit I noticed a
selection of small lower power circuits that seemed to be sensing
various voltages and connected to the circuit that turns off
the amplifier. Maybe one of the dozen capacitors in this area
had failed and was making the protect relay operate? This can
happen if a delay circuit of say 5 seconds using say a 100uF
capacitor whose value had dramatically dropped or whose ESR had
risen sky high was not allowing time for a voltage to establish?
However all the capacitors, although not perfect were certainly
good enough for purpose. I then decided to look with a magnifier
for a break in the ground circuit perhaps caused by a component
failure. I found two interesting things. These were metal tabs,
through which two, otherwise discrete and isolated, ground circuits
connect to the chassis. That was the answer to the problem. When
I'd detached the main circuit board for the second time I'd noticed
one loose securing screw. This (because of Sod's Law) connected
the main 40-0-40 volt supply ground to chassis (hence the loud
hum when first installing the repaired amplifier). The second
tab, which had been tight, connected the 5 volt ground to chassis.
This method of grounding is probably used to minimise circulating
ground currents which might result in hum or modulation of sensitive
amplifier circuitry. Below... the critical securing screws which
must be tight.
I refitted the main board
and plugged in all the cables, turned on the amp and waited for
the usual few seconds... but this time the amplifier stayed on.
I suspect the initial fault
was just a selection of dry joints at the 5 volt and 12 volt
regulators. Before I reassemble everything I'll look into that
duff volume control... possibly more dry joints? And of course
in these digital days the control is not a simple potentiometer
but a pulse generator whose pulses are counted by a microprocessor.
No wonder it's failed (mechanical plus elelectronic = eventual
failure) and of course switch cleaner would be ueless. I decided
to leave the thing in the end because it uses a special rotary