Akai GX 653D blowing fuse capstan motor stops

Search and download GX-635 service manual , circuit diagram.
See "syscon" capstan motor PrintedCircuitBoard:
Capstan AC motor is driven by a "misused" bridge rectifier working as
DC-AC decoupler.
If a transistor "shorts partially" the DC connections of that rectifier ,
also motor AC will vary depending on regulator circuit.
Often that power transistor direct connected to the rectifier is blown,
because in old 635 versions this T is located on hot small tin cooling strip located on syscon pcb (upper pcb mounted "reverse"),
later versions place it on chassis near transformer and connect by wires for better cooling and lifetime.
Replacement transistor can be BUX84 or any other normal T having same or more max voltage and current like original.
BUX84 works in my 635 for years.
All works on syscon are difficult , be careful not to rip wire wrap connections if you loose screws and turn it out chassis .
Works on audio pcb are easy possible without demounting pcb from chassis frame.

Look for transistors type 2SC458 on audio board (lower pcb).
They tend to generate distortions due to aging.
Replace all of 2SC458 for 2SC2240 , 2SC945 or any small signal T fulfilling electrical data of 2SC458 (or being better/stronger).
2SC2240 is first choice and "everywhere" to get.

If possible desolder multi-stage reverse switch on audio pcb.
This switch changes magnetic heads depending on play direction.
If it is dirty (oxidated) it will cause tone interrupts at play and rec mode in both directions.
If desolder not possible apply spray "teslanol oszillin T6" INTO that long multiswitch.

Also apply T6 into play speed pitch potentiometer (located between reels).
If its contact fails play speed will not be constant.

Clean rubber tape press roll with gum-cleaner until it looks clean and deep dark.
Demagnetize tape heads.
If possible check better adjust play and rec head azimuth , reclevel and bias , so the 635 plays back audio signal near as possible as rec original.

A wire connected remote control can be built self quite easy , only some
pushbuttons/switches in case necessary , no electronics.

Changing electrolytic caps on audio pcb will improve sound quality.

DO NOT apply electrical power to the 635 while pcbs are NOT mounted with ALL their screws onto chassis ! GND power line is connected via chassis to pcbs and 635 will work crazy if GND on pcbs is missing.

bukongahelas

To check if capsmot driver T is blown , connect its collector and emitter direct. Then capsmot will run with max speed.
If not also the rectifier connected to that T can be blown.
On syscon pcb are some more of these weak rectifiers , all tend to blow due to aging.
Replace them all with standard types , but obey pin connection order !
Single-in-line rects can be pin-wired AC , + , AC , - or + , AC , AC , - .
Confirm whats type is original built in and replace by standard type with same pinout.
bukongahelas

Thanks for your help there, very detailed I'll get cracking and let you know the outcome
I'm in Perth Western Australia so parts could be an issue but well see how i go

cheers

Only Akai mechanical parts are rare , electronics is standard and such parts are available even down under except outback.
Akai reeltapes are dinosaurs , hardly to crash , they withstand even
funnelweb spiders inside , dust devils , crocodiles and poison snakes.
bukongahelas

Thanks Bukongahelas I've got my soldering iron and knife at the ready !!!

I have just started to have a look at this reel to reel and I can see two transistors on the metal strip. One is the c1061 and the other is D794
Which is the one you want me to replace with the BUX84 ?

Sorry to ask this as I can solder and take things apart but I am a bit uneasy at finding the equivalents

Search and download service manual of that 635 , look at circuit diagram.
CapsMot regulator transistor is TR13 (2SC2336B).
Maybe another but function ident T is mounted eg 2SC2362 .
For that TR13 you can take BUX84 or BUX85 as replace part , also any similar T.
Mount (and isolate electrical;"metalbackplane"transistors lead voltage on their metalcase ; use mica isolator pad and white thermo conduct grease ;
or gum pad) that T on metal chassis for better cooling.
Or use full isolated "plastic" type case transistor.
Also check or replace that rectifier connected to TR13.
Diagram says an SV810-200 .
If you connect base and emitter of TR13 , motor will run fullspeed if rectifier is OK. If not rectifier maybe blown.
If TR13 and "his" rectifier are ok but still no motor run , you must check
whole servo circuit and AC motor supply.
Confirm that switches and relay contacts give good contact.
Speed switch , pitch potentiometer , contacts of AC motor revolution relay.
If you ask any further question , please refer to circuit diagram and use the labels written there.
bukongahelas

Thanks for the reply this has made it a lot clearer what you are talking about I do have the service manual so any other questions I will refer to it

I really appreciate you taking the time to answer my questions I am away for 4 days and will order the parts when I get back
Hopefully this will work and will stop it blowing the F2 fuse

Cheers

Brian

Obey for pin-order of rectifier to get the one that fits exactly.
There are two kinds: (+ AC AC -) or (+ AC - AC) .
Stronger (voltage+current) types live longer.
I check suspicious semiconductors and order only if really defect.
Check other Diodes+Transistors , if defect , so you have to only once order parts.
If you want to improve audio quality , change all transistor 2SC458 on
(lower) audio pcb for 2SC2240 , which is 100% compatible.
Also change of all electrolyt caps is recommended on syscon + audio pcb.
The long shift-multiswitch for play-reverse toneheads needs to be cleaned.
It can be opened after desolder. If you want to do that i will tell how.
Lazy method is applying !acid-free! contact spray INTO the multiswitch.
And there is a small signal mini relay like finder series 30 type that tends to fail sometimes. Its function is enable pitch potentiometer in play mode and disable it in rec mode. Bad contact these relay results in not constant play speed , also as bad contact in pitch pot , also as in speed switch.If they give inconstant "values" to servo mot electronic , play/rec speed will be also inconstant .
The 635 is a litte bit weak because of that many contacts of switches,pots and relays.
If i use my 635 i first push all switches and turn all pots several times and change tape play direction some times to "clean" internal multswitch.

For optimum rec-play audio performance (no sound deviation) any tape recorder should be adjusted to magnetic parameters of tape type/manufacturer used.
This is done at trimpots+trimcaps on audio pcb by using a reference prerecorded test tape , which also allows check+adjust of tape head orientation (track , tilt , azimuth).
Do NOT turn any head align screws or trimpots/caps by try and error.
You need some measuring equipment to do that successfully.

bukongahelas

Thanks again, I will report when I have success. I have ordered the TR13 and D 9 parts now while I wait I will start investigating further

Cheers
Brian

Well the parts have arrived and I have replaced Tr13 and the rectifier D9 but when I apply power the capstan motor spins and
F4 - 1amp fuse blows straight away and the capstan motor stops

Not too sure what to do as it is so hard to get to the components
Might have to get aTech to look at it

Success I have just replaced D1 rectifier and it has solved the problem !!!
All seems to be working well and no fuses are blowing

Many thanks to Bukongahelas for all your advice and help

Cheers
Brian

Replace all rects on syscon pcb.
Replace types are more powerful.
Mount TR13 on chassis for better cooling , connect TR13 to original pcb solderpoints by 3 colored wires.

Now take a record and switch source/tape to check recording quality.
If nearly no difference is audible , you are lucky and tape type fits to
adjustment of your reeltape. Slight tape hiss and crosstalk from opposite direction tracks are normal with 4 track reeltape systems.
If audio level or high frequencies come more or less your RT needs adjustment or take other tape type that fits adjustment.
Record level and bias are the adjustments you can do self without further test equipment.
Do not move screws that adjust head positition.
For this (and playback level adjust) you need a prerecorded reference tape.

bukongahelas

Thanks again
Will do...... As we say here in Australia. " No Worries "

Cheers
Brian

Just a quick question

I also have a Gx 636 as well as the 635D both are in good condition
If I was to sell one which one would you sell and which one would you keep ?

Hi Guy , Im new here... so ,
i have the same problems exept ,i already replaced 21 transistors on lower sound board ( preamplifier)and now , when i turn it on , fuse 1A blew .
i didnt touched syscom board, my thinking that i scuwed up something on sound board , check everything , all T in right direction , no links when i souder , evething looks good .
i check power board and traced wire which supply 22v to sound board , disconnected it , now cap motor is running but no sound becouse no 22v , as soon a contact 22v to sound board , BUM , fuse blew .
i test for resistanse bitveen contacts 43 (22v ) and 42 (GR) on sound board , if it blowing fuse, should be shortage there ( 0 oahm) but i got about 500 oahm , Where to look next ?
Sorry for gramma mistakes , this is my second lunguage.

What is your first language ?

22V supplies many circuit parts on audio pcb , a parallel network with current paths.
Isolate all current paths . Connect them again part by part (path by path).
Fuse will stay ok until defect path is connected , then will blow.
The error/short must be in that current path (see diagram).
Isolation can be done by desoldering one leg of resistors or cut circuit traces (later connect them by solder point).
Work systematic by excluding working parts/ current paths until locating the bad short path/part/solder error.
bukongahelas

Isolation can be done by desoldering one leg of resistors or cut circuit traces (later connect them by solder point).
That is exactly what i need ..
I know that i have to isolate parallel network, but didnt know how phisicaly to do that( just cut tracers was too scary for me ) , now i feel confedence , it going to happend over weekend .
THANK U VERY MUCH .
Alex Grigoriev
Im russian.

Use a mini drill eraser or sharp knife to cut pcb traces.
A very small gap is enough. Check cut-isolation with Ohm-Meter.
Scrap solder stop paint (near trace cut) for later connection by solder point/bridge , better a short wire.
Be careful not to rip traces or solder "eyes" when desoldering parts/legs.
Best use a vacuum driven professional desolder station .
If not available warm up solder point/leg and pull out leg of part with tool (we call "pinzette" , a micro grip tool).
When leg is lifted it is easy to desolder the "eye".
To remove parts with more than 2 legs "jump" fast with solder iron from leg to leg while pulling with tool.
Being fast enough will make all solder points fluid for a short moment , then part will release.

If possible change all electrolyt capacitors for new ones , if value possible use film capacitors (WIMA).
! Change all transistors type 2SC458 to 2SC2240 (or similar) full electric and pin compatible ; PinOrder ECB. !!
2SC458 causes sporadic noise .

If possible desolder and open/clean that long play-rev switch on audio pcb.
I posted some threads where i cleaned such similar switches.

As russian i think you are flexible in solving problems with minimum tool and spare part.
Improvisation and vodka.

bukongahelas

HI
Some progres been made,
i fund a part of " network" which cost fuse to be flowen... i cut off 22v supply like u explaned to me , fuse is OK but now, how to find what cost a problem .?
this part, right on center of schematic with T13 .. i remove T13 and fuse blow , R64 blow , r68 blow , D2 blow... ( all transistors 458 not 2240 been replaced before this problem ,only Tr16 is original , i couldnt find it )
next step , will cut connection #39 which goes to OSC ( not sure what it does ) , i just run out of fuses ( 0.5$) each, willing to buy more tonight and keep looking .
In Your opinion ,what could be common in this area to fell to blow the fuse
Thank s Alex

Confirm using schematic what sub-modules (like Osc) are supplied by +22VDC .
Disconnect these modules until fuse stays OK.
Unfortunately OSC is located near edge of pcb , hidden below chassis bar.
Loosen pcb screws to get a little more space , bend pcb carefully , insert a small wooden/plastic piece to keep gap open.
That module that blows fuse will contain the error , which must be a hard short , for blowing the fuse.
Other way is to locate +22VDC modules by schematic and look there for short semiconductors.
Who needs +22VDC and who knocks them down ?
Start with osc submodule , often there shorted semiconductors.
This cost more time but less fuses.
bukongahelas

I removed the bar and supply good GRD wire , it is planty of room to work...
I keep looking ...
Thank you