Hi All,
I'm struggling to repair my beloved Technics A1, as a few might know these are somewhat rare and little known about. I had this working well for years with only a dry joint on a diode giving me issues a while back (the failed joint caused the class A side on one channel to run at 15amps opposed to 7!) thankfully it was a relatively easy find and fix, if only by chance.
I now have issues with both channels, strangely one side the class A part is not working properly and the other channel it's the high voltage class B, I got this channel working with some new transistors that had been taken out but it soon failed again and I'm not sure the issue isn't the genuiness of the NEC 2SA1006B part I bought along with others.
Anyway I have managed to obtain probably the only set of schematics in existence for it, these were thought not to exist outside the factory so a lucky find! There's a lot of detail with them so invaluable for the repair but I'm looking for some help repairing it, it's rediculously complex and I'm at the extent of my electronics knowledge to get to the bottom of the issues.
Any help appreciated, perhaps there is someone UK based who has greater abilities than mine to look at it?
I'm struggling to repair my beloved Technics A1, as a few might know these are somewhat rare and little known about. I had this working well for years with only a dry joint on a diode giving me issues a while back (the failed joint caused the class A side on one channel to run at 15amps opposed to 7!) thankfully it was a relatively easy find and fix, if only by chance.
I now have issues with both channels, strangely one side the class A part is not working properly and the other channel it's the high voltage class B, I got this channel working with some new transistors that had been taken out but it soon failed again and I'm not sure the issue isn't the genuiness of the NEC 2SA1006B part I bought along with others.
Anyway I have managed to obtain probably the only set of schematics in existence for it, these were thought not to exist outside the factory so a lucky find! There's a lot of detail with them so invaluable for the repair but I'm looking for some help repairing it, it's rediculously complex and I'm at the extent of my electronics knowledge to get to the bottom of the issues.
Any help appreciated, perhaps there is someone UK based who has greater abilities than mine to look at it?
as an easy suggestion, reflow the solder joints, (add a dab of flux, and use a wattage-
appropriate pencil - not a plumber's gun), inspect all traces looking for cracks.
your amp runs hot so there's a lot of heat cycling around those joints and can cause
joint and trace damage.
next - using your schematic, follow the voltages and measure them. Warning buy
a cheap VOM/DMM and use ONLY insulated probe with miniature clips. clip to measure
ment point, then turn on. do not attempt live probing - one slip and you will
destroy the amp.
appropriate pencil - not a plumber's gun), inspect all traces looking for cracks.
your amp runs hot so there's a lot of heat cycling around those joints and can cause
joint and trace damage.
next - using your schematic, follow the voltages and measure them. Warning buy
a cheap VOM/DMM and use ONLY insulated probe with miniature clips. clip to measure
ment point, then turn on. do not attempt live probing - one slip and you will
destroy the amp.
Thanks Bob....
I've reflowed all joints and checked for track breaks, connector issues, corroded VR's etc. The actually hot bit is limited to the 5v per rail class A side and I've been through that thoroughly. I've had it on for hours and the class A part is only warm not hot so something is amiss with the drive side.
I've been through the schematic and pretty much all voltage reference points check out and I'm able to adjust with the VR's if needed. I am live probing but being very careful as like you say one slip and it's a big pop! The class B side runs +/-105v so it's a bit lively in there.
Certainly with 6 capacitors the size of Coke cans! 100000 mf!
I've reflowed all joints and checked for track breaks, connector issues, corroded VR's etc. The actually hot bit is limited to the 5v per rail class A side and I've been through that thoroughly. I've had it on for hours and the class A part is only warm not hot so something is amiss with the drive side.
I've been through the schematic and pretty much all voltage reference points check out and I'm able to adjust with the VR's if needed. I am live probing but being very careful as like you say one slip and it's a big pop! The class B side runs +/-105v so it's a bit lively in there.
Certainly with 6 capacitors the size of Coke cans! 100000 mf!
if both channels seem to have problems (hopefully identical) then look
into the power supply - ensure all is well there, voltages. then if the
channels are not identically wrong, then it may be a second problem.
I'd hate to simply introduce this without cause/reason, but it could be
caps in the power supply and the rest of the amp.
if you have access to a scope you can introduce a sinewave at the input
then look along the signal path to see where any visual anomalies crop
up.
the issue would be to describe the "bad" sound for each channel. this
assumes, as you indicate, all points with electrical information is
verified/confirmed. and both channels are somewhat identical at
each corresponding point (within margins/reason).
into the power supply - ensure all is well there, voltages. then if the
channels are not identically wrong, then it may be a second problem.
I'd hate to simply introduce this without cause/reason, but it could be
caps in the power supply and the rest of the amp.
if you have access to a scope you can introduce a sinewave at the input
then look along the signal path to see where any visual anomalies crop
up.
the issue would be to describe the "bad" sound for each channel. this
assumes, as you indicate, all points with electrical information is
verified/confirmed. and both channels are somewhat identical at
each corresponding point (within margins/reason).
Unfortunately its the class A part on one channel of the amp and the class B on the other channel, all power supplies are good with the main caps having been replaced a few years ago and more recently I've done the smaller ones in the low current PSU.
The main amp PCB cards are fairly devoid of caps, electrolytics there are virtually none, there are some of the non polarised polyester ones, I've pulled a few and they seem to test fine.
The problem is the amp has a total of 8 power supplies (4 per channel) and a very dedicated protection circuit that looks for DC on the input and output plus the usual shorts/overloads at the speakers & input etc. The protection circuit not only switches the relays (4 of them for the 4 speaker outlets) but also a further two relays per channel that switch the high voltage 105v class B and low voltage 5v class A side out of the equation. All of this buried in the middle of the amp so it makes it near impossible to work on it without stripping it down to get to even simple things like fuses where there are 14 of them buried!
So I can't hear anything as the protection circuit won't switch on, one side always worked then has since stopped working, the other side keeps blowing the 2SA1006B transistor that drives the main power transistors on the negative rail, its counterpart does the other 4 on the positive rail. I've tried using a variac to bring it up to voltage slowly and this works and I get it to continue to work for hours but a cold switch on kills it, I wonder if I'm buying fake/non original parts that aren't up to spec? Otherwise I can set it up and it sounds great. There are countless pots to adjust the circuit and I've kind of got most of them figured out now to know what they do, thankfully I have found the schematics (believed not to exist outside the factory) so those have helped as they have voltage reference points to work to.
I need to post pics somehow?
There are a few on my Instagram account, along with all the other crap I'm into! I appreciate any help.
Toby Carter on Instagram: “My little work station......project Technics SE-A1....#clampmeter #multimeter #thermalprobe #schematics #1977 #classA+ #technics #SEA1…”
The main amp PCB cards are fairly devoid of caps, electrolytics there are virtually none, there are some of the non polarised polyester ones, I've pulled a few and they seem to test fine.
The problem is the amp has a total of 8 power supplies (4 per channel) and a very dedicated protection circuit that looks for DC on the input and output plus the usual shorts/overloads at the speakers & input etc. The protection circuit not only switches the relays (4 of them for the 4 speaker outlets) but also a further two relays per channel that switch the high voltage 105v class B and low voltage 5v class A side out of the equation. All of this buried in the middle of the amp so it makes it near impossible to work on it without stripping it down to get to even simple things like fuses where there are 14 of them buried!
So I can't hear anything as the protection circuit won't switch on, one side always worked then has since stopped working, the other side keeps blowing the 2SA1006B transistor that drives the main power transistors on the negative rail, its counterpart does the other 4 on the positive rail. I've tried using a variac to bring it up to voltage slowly and this works and I get it to continue to work for hours but a cold switch on kills it, I wonder if I'm buying fake/non original parts that aren't up to spec? Otherwise I can set it up and it sounds great. There are countless pots to adjust the circuit and I've kind of got most of them figured out now to know what they do, thankfully I have found the schematics (believed not to exist outside the factory) so those have helped as they have voltage reference points to work to.
I need to post pics somehow?
There are a few on my Instagram account, along with all the other crap I'm into! I appreciate any help.
Toby Carter on Instagram: “My little work station......project Technics SE-A1....#clampmeter #multimeter #thermalprobe #schematics #1977 #classA+ #technics #SEA1…”
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