Need help with Pioneer cassette deck circuits

[martytoo]

I recently bought 2 very inexpensive dual well Pioneer decks. They are the CT-W54 and the CT-W79. They have some minor differences on the cosmetics of the faceplates but once the cover is off they appear identical. I think it might be a model year difference, although one has a Canadian sticker on the back so it could be a difference in where the decks were marketed. They are close to mint mechanically. The rollers and heads look great. The seller was upfront in that he explained that each had display issues and that one well would only record on each machine. Lucky for me that one records on the left only and the other on the right. He thought the troubles might be the common voltage regulator issue that many have written about on the Internet. I was hoping that the 7812 regulators were the problem but replacing them did not help either of my decks.

This was the reason I thought the fix might be simple - I found this without the seller's input:

http://www.tapeheads.net/showthread.php?p=629890

As I said above,replacing the 7812 regulators did not help. I then tried to check simple issues on the circuit boards such as failing diodes. I didn't find any differences in the two decks when testing diodes. (Although I can't be sure I checked them all.)

I then compared the DC voltage on the circuit boards to see if there were any differences between the voltages printed on the boards and the readings. I also checked for unlabeled spots on the boards. It seems that because both the left and right decks record that an extra circuit board is mounted in a plug in on the main board to control the bias and such needed for the left recorder. I found a spot on that board that the voltages are not the same. The deck that is sick on the left side has a lower voltage than the deck that records on the left.

At W148 the functioning deck measures 5.5 volts and the non-functioning deck measures only 3.9.

Does anyone reading this have any ideas as to what components on this board might be causing the drop in voltage. I can swap out the boards to see if I am correct. However, I would like to fix the deck that isn't working and I would not like to randomly replace components on the board until I find the failing piece.

Help please!

 

[martytoo]

i always think twin decks are not worth repairing

I know that the decks are worth almost nothing.

I am:

1) Trying to have some fun
2) Working on decks that I won't worry about if I wreck them- learning as I go. It's like learning to drive on an old piece of junk instead of a new dent free beauty.

So far I have soldered in some 7812's, resoldered the display panel connections, decided that silver solder isn't for me an ordered traditional solder and transplanted a working transport from one machine to the other to verify that the transport wasn't the issue. I would love to learn how to diagnose more than a bad connection or a bad switch and this is a way that I can practice without ruining anything of value.

 

[martytoo]

I fixed the voltage to the left recording board issue by doing some resoldering under the sockets the board drops into.

The left transport will not record even after this "fix". Should I look at the ribbon cable that controls the left transport next or is there some other area that I need should concentrate on?