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
I opened up the old Teac
once again and looked at the main circuit board. For some reason
the amplifier had been cutting out and we relied on a good thump
on its case to resurrect the output. I found two probable reasons.
One was dry joints at relay coil connections and the second was
a slip-up I'd made when fitting a cooling fan. I'd used the one
of the three regulator outputs to provide me with the fan supply
voltage but inadvertently increased the regulator current making
it even hotter. I rewired the fan to the main supply voltage.
There were two.. plus 47 and the minus 47 volts. I decided to
check the fan to see if it would run more slowly than at its
rated 12 volts and decided on 4 volts, at which it drew 110mA.
I fitted a large resistor of 390 ohms (47V-4V/110mA= 391 ohms)
and this worked with exactly 4 volts across the fan terminals
with fairly unobtrusive noise. Having put back the AG-15D into
service I tackled the AG-D200. I'd been using this for a week
or so and found it had several faults. One was failure to drive
several of its speakers, and after some period it would make
sharp cracking noises. Also, I found the whole HDMI system was
inoperative (which is its main feature).
Below, the newer Teac doesn't
use too many straightforward plugs and sockets so must be dismantled
Below a view of the main
culprit for cracking noises...
What you can see above
is a circuit board (mounted in front of the power output transistors
on seven plugs). To remove this board needs some extensive levering
but it then pops off for scrutiny. There are seven identical-looking
preamp and driver transistor sets. You'll notice the pre-amp
transistors have heatsinks (those vertical silvery-coloured things).
Using a magnifier you can see that at least 50% of the solder
joints have circular marks which are cracks in the solder. I
started by resoldering all the transistor legs which were by
far the worst affected but to make a lasting repair I'll need
to resolder nearly all the solder joints.
Why is this board so bad? My
guess is the solder is almost certainly an early lead-free type
whose composition is not up to its job. I can also see that the
amount of solder on many joints is rather lacking in quantity.
Interestingly all seven pcb connector soldering is fine, but
where even the slightest heat is present the more heat the more
cracking so it's clear the fatigue strength of the solder is
very poor. I noticed lots of red markings on this and other boards
and I wonder if this represents evidence of QA examinations during
Here's a small area affected
by solder cracking. The shiny joints, I've already resoldered
as these were the worst, but you can see that nearly all are
affected to some extent. In this area the audio signals are extremely
small and readily susceptible to extraneous noise.
I counted 7 amplifier circuits
each carrying 100 joints. I resoldered 700 joints on this board
and around 100 on the main board plus a couple of others. I realised
I wasn't the first because of copious numbers of red ink marks
on many of the solder blobs.
I managed to get all the bits
put back together except a small metal bracket which could of
course belong to virtually anything I've had in for repair. I
plugged in and switched on and everything seemed OK but I'll
wait until after Christmas before refitting into my Hi-Fi system
as there are far too many leads to move around. In the meantime
my old AG15D is doing a sterling job.