Any electrical gurus here?

[ClarkNovak]

So, I was given a very nice Akai 202D-SS reel-to-reel deck. Mechanically it's awesome - even has the smoked plastic reel cover intact! Transport works flawlessly, heads in great condition.

Electronically, time has taken a toll. Left Front channel and both Rear channels output great audio. Right front channel does not. It exhibits the following symptoms:

• Gives a power-on "thump" like all the other channels, but then...
• Within about 10 seconds of power-on, static is heard in channel and...
• Audio fades out and disappears rapidly, and...
• Channel meter shows all of this activity.

Above is the Playback Amp block from the repair manual. I see three power transistors and several caps that could be culprits here. So my question is: does this sound like one or more of the caps has failed, or is it one of the transistors that can make enough current to power up initially, then craps out? The fact that the diagram shows the meter reading directly off of the playback amp channel and not off the final amp output channel leads me to believe that the problem is located on this board.

Anyone here familiar with this machine or with this symptom enough to advise me where to start? I know, I should "take it somewhere"... but times are tough and I'm a good disassembler/solderer, so if I can fix it myself, I'd prefer it.

Thanks!

 

[quadsearcher]

It looks like the drive to meter drive transistor TR3 actually comes from off the board through pin 5. If that's the case the problem could be in any circuitry between the head and and where meter drive is tapped from. The output of TR2 leaves the board on pin 4.
I don't know this model, but this is where signal tracing could save a lot of time. A regular scope or even an old-school audio signal tracer to check the output of each gain stage to find where it is being lost. It is likely a leaky small-signal transistor (breaking down under load), and while that can often be found by testing each junction in the reverse-bias direction with an ohmeter set to the highest (20 Meg) scale, a scope would save time and the removal/replacement of all those parts.

There's only 9 leads to this bd., you could jig it outside the unit with extensions to access each stage. Noise or hum shouldn't matter for just seeing where the signal is lost. The place to start would be input to this board on pin 2 and output on pins 3 or 4.
Or if there's enough space I like using something like Pomona mini-grabber clips to get where the scope probe won't fit.

Capacitors usually are worst at turn-on, and improve with heat.
Transistors when borderline break down quickly after startup.

I like your signature, I test myself on Ohm's Law every chance I get to stay in practice.

 

[ClarkNovak]

Thank you so much! That is very clear and easy to understand. I have a good idea where to start now Thanks again!

 

[donaldpatten]

I would replace the caps in this order:C2,C7,C9,C10,C5,C3,C4.
The transistors would cause noise, static, level changes etc. Try heating/cooling the suspect parts with air while playing a tone or music.
You could swap parts to sort out the bad ones.
Spray the switch and pot contacts, and work the contact cleaner in: do this first.

Don

PS
To Quickly find bad caps bridge (parallel) the suspect one with a known good one. With low voltage circuits 30 VDC and less it is fast to just touch the leads and not have to solder the bridged cap.

Don

 

[kap'n krunch]

Wish I understood this stuff...
this is the coolest and most helpful forum I've ever seen...I know about signal path charts , but that's child's play compared to this...

Good luck and may it be fixed , ClarkNovak!!!!

 

[ClarkNovak]

Well, I ordered a bunch of new transistors from Mouser a couple of days ago - enough to re-power the entire unit, if need be. Cross yer' fingers

 

[donaldpatten]

I hope you have caps too.
Don

 

[Doug G.]

The first thing I would do is swap a good channel lead from the head with the bad channel lead where they connect to the board. This will quickly determine whether the problem is with the head or electronicals.

Say, swap the front left with front right. If the problem is now on the other channel, the head is bad. if it stays the same, it's in the electronicals.

It does, admittedly, sound like a typical capacitor problem, however.



Doug