M90 Successful Deck Repair HOW TO

[docs]

I'll spend a bit more time on trying to test with the scope to see what it looks like at pin 15 and Q711 base.

 

[docs]

With Q711 out of board and both wires to solenoid disconnected, I checked at pin 15 which is meant to be low by default, and corroborated against pins 1 (L) and pin 2 (H) to see what each one looked like, pin15 remains low as is base of Q711.

After adding Q711 back into circuit, my oscilloscope is providing different visuals and now pins 1 and 2 and 15 on IC701 look the same which they didn't before.

 

[caution]

Interesting about pin 1. Try pulling out one side of R728, and see if there's any improvement. That's connecting pin 15 to a circuit that kills the ff/rew motor when the solenoid engages for recording. You can even see on the schematic how pin 1 ties into it.

 

[docs]

Hi mate, I think r728 is built into the board, next step is c711, could try there.

 

[docs]

No dice, I'm thinking that the solenoid itself is shot, or the actual control IC.

 

[caution]

If pin 15 looked fine without Q711 and the solenoid, and you reinstalled them at the same time, maybe check Q711 without the solenoid. Pin 15 could go high if D752 on the solenoid board shorted out, and the solenoid's power is allowed to flow backwards through Q711 to pin 15 when the solenoid isn't supposed to be running. Power is always present on the input side of the solenoid and D752, waiting for Q711 to switch on. Power isn't supposed to flow to ground until Q711's base is energized, so if it was allowed to reach the collector without switching on, it might decide to go backwards through the base and over to pin 15 instead. Once the base goes high and connects the collector to the base, now the base sits at a higher voltage than the emitter, which is grounded and connected to the collector now, giving power a much easier path to ground.

 

[RaleighRon]

Great info in this thread. I need to order belts for my M90. Saw another post debating size also. Really like to get links for where to buy each belt if possible. There's a set on ebay but the guy had like 89 negatives the last few months lol.
If there's a good source in the USA you use links would be greatly appreciated to eliminate me buying the wrong belts. Thanks.

 

[Pavel61]

How to replace the diode D 702 10E 1? I can't find the original.

 

[caution]

A 1N4004 will work.

 

[D_Nice]

Attached is image from same location as resistor you replaced. Mine appears to be missing completely!

 

[caution]

FR704 can also be found soldered on the back, what do you see on the other side of that spot?

 

[Superduper]

FR704 can also be found soldered on the back, what do you see on the other side of that spot?

I could just see the 2 glass heatshield tubing horns peeking over the top of board, I would say that resistor is definitely back there.

 

[Transistorized]

I could just see the 2 glass heatshield tubing horns peeking over the top of board, I would say that resistor is definitely back there.

I think you are correct. Even though mine has been replaced, that is exactly where mine is. On the back side.