Pioneer RT-707 Servo Board Troubles

[CaseyWKC]

Hey all, I'm trying to get an RT-707 back into shape.

It had been worked on before, caps that see the most usage have been replaced and the problematic small signal transistors that keep up the recording bias have been replaced with modern subs as well.

It doesn't response to any speed controls whatsoever.

I've seen the other threads - replaced the .1 uf sanyo caps that often short, I have cleaned the molex connections, deoxed and clean all switches and pots in the unit.

On the positive pin of the rectifier on the servo assembly board I get wild readings between 75-130 volts. Violently swinging up and down. I have not been able to trace it to anywhere specific. It seems to stay stable at around 142v dc for a couple of seconds before it starts swinging up and down.

I have replaced the rectifiers on both the power supply board and the servo assembly board and see no change.

Thanks anybody!

 

[CaseyWKC]

I have another unit I just went through that had shorted caps causing speed to run loose - on that unit the molex connection from the motor of the unit - I get around 180 ohms between pins 3 + 4.

On the good unit between pins 5 + 6 I get no reading of ohms but my MM reads a voltage drop of .300ish mv.

On the bad unit I get no reading between pins 5 + 6 coming from the main motor.

Thinking maybe the motor is the culprit?

 

[aldena007]

When you power up the unit without any tape mounted/threaded the deck should illuminate the fwd mode indicator and the capstan should be turning ccw. If this is not happening on initial power up the issue is most likely on the RWG-70[Control Assembly PCB]. Transistors Q403,404& 405 are involved in starting up the Capstan motor. If the capstan motor is not spinning ccw on power up then nothing is going to happen when the tape movement controls are activated. If you see no tape direction indicator lights turn on or the reverse light comes on with out the capstan motor running you have the same issue with RWG-70. The capstan motor will turn on without the tape direction indicators if the lamps are burnt out. Until you get the motor running on power up you are wasting time/effort. After you have the deck in the correct startup mode then load a tape and try to play it. The transport system will do nothing until a tape is loaded and threaded. If the unit fails at this point then you can start testing other parts of the transport system. The deck has several cascading steps with switching transistors and relays. The troubleshooting at this juncture is not for the easily frustrated. The capstan motor is a very rugged unit and will most likely be OK if it will spin freely by hand and slowly spin down. If is turns with difficulty it is bad. The currently available service manual for the RT-707 has a few errors and the KU [USA] and KC [Canada] models have some differences so also follow them with a "jaundiced eye". The RT-707 is a very nice example of Pioneer engineering and worth getting up and running.

 

[aldena007]

Just had a thought. Make sure the Pitch control is clean and functioning properly. If that pot has not moved for awhile it will give issues like flutter and wow, etc.

 

[AE Bob]

Also clean the record button switch. The pitch control signal goes through the record switch so that in record the pitch is obtained from the 50% tap on the pitch pot, but in play it is obtained from the wiper of the pot. Although I believe bad contacts in the record switch causes the capstan motor to run too slow (not sure). I had one RT707 in for repair that had intermittent speed run away, it had a hole in the TO3 transistor heatsink mica washer, that was arcing through to the heatsink causing the motor to intermittently go to full speed.

 

[CaseyWKC]

When you power up the unit without any tape mounted/threaded the deck should illuminate the fwd mode indicator and the capstan should be turning ccw. If this is not happening on initial power up the issue is most likely on the RWG-70[Control Assembly PCB]. Transistors Q403,404& 405 are involved in starting up the Capstan motor. If the capstan motor is not spinning ccw on power up then nothing is going to happen when the tape movement controls are activated. If you see no tape direction indicator lights turn on or the reverse light comes on with out the capstan motor running you have the same issue with RWG-70. The capstan motor will turn on without the tape direction indicators if the lamps are burnt out. Until you get the motor running on power up you are wasting time/effort. After you have the deck in the correct startup mode then load a tape and try to play it. The transport system will do nothing until a tape is loaded and threaded. If the unit fails at this point then you can start testing other parts of the transport system. The deck has several cascading steps with switching transistors and relays. The troubleshooting at this juncture is not for the easily frustrated. The capstan motor is a very rugged unit and will most likely be OK if it will spin freely by hand and slowly spin down. If is turns with difficulty it is bad. The currently available service manual for the RT-707 has a few errors and the KU [USA] and KC [Canada] models have some differences so also follow them with a "jaundiced eye". The RT-707 is a very nice example of Pioneer engineering and worth getting up and running.

The unit spins CCW with no tape installed on power up with the forward direction light illuminated.

Also clean the record button switch. The pitch control signal goes through the record switch so that in record the pitch is obtained from the 50% tap on the pitch pot, but in play it is obtained from the wiper of the pot. Although I believe bad contacts in the record switch causes the capstan motor to run too slow (not sure). I had one RT707 in for repair that had intermittent speed run away, it had a hole in the TO3 transistor heatsink mica washer, that was arcing through to the heatsink causing the motor to intermittently go to full speed.

This has been done and that mica insulator and transistor has been replaced.

Just had a thought. Make sure the Pitch control is clean and functioning properly. If that pot has not moved for awhile it will give issues like flutter and wow, etc.

This was cleaned multiple times.

 

[CaseyWKC]

I am missing my -24v I should be seeing at pin 11 on the servo assembly control board - RWG-076

I see 0v at pin 11 and at pin 9 I have +25v.

 

[CaseyWKC]

Still trying to figure this one out, going back to the power supply board on pin 5 I see voltage fluctuating from 10-20volts and pins 2+3 jumping anywhere from -10 to around a max of -18v.

Those pins all hang off the rectifier according to the schem. On the rest of the power supply board all voltages seem stable.

Then running to to the motor and motor run caps the voltage is swinging there quite wildly as well.

Still 0 volts where I should see -25v at pin 11 on the servo assembly board.

Wish it was a bit easier to trace the power supply board on these but the heatsinks and the way the two 2sd525s are mounted makes that a bit of a challenge.