RT-707 Playback relay problem?

[Trevor White]

Hi all, it's been a while since I last visited.

My lovely old RT-707 is still performing well, until today. I started playing a tape and noticed that the left channel was silent. No action on the VU meter. But then after a couple of seconds the left channel suddenly came to life. The audio quality was excellent and the machine continued to play normally.

I found though that if I stopped the tape, even for just a moment, when starting to play again the left channel would be silent but would suddenly come to life after a moment or two. Very strange. I checked all the obvious things like the head wires etc. Everything looks normal.

I was wondering if the relay's on the main switch board might be playing up. In the description in the Service Manual they talk about the RY601 and RY602 relays being energized, but I can't be sure that either of these relays affect signal being passed from the heads to the amplifier section.

I have also found that if I leave the tape playing for some time and then press stop. When I press play after a short time the playback starts immediately on both channels.

Can any of you guys offer a suggestion of where to look? And could it be those relay's?

Cheers,
Trevor

 

[GPS16]

Hi Trevor,

the relays mentioned are for Motor Control and Fwd/Rev Play Torque shifts. The Audio does not go through them. The first hurdle is the Tape Monitor switch. Give that a tickle when it is running and see if you get a response. If so power the machine off and do the usual 20 changes of state and take things from there. If this is a red herring you need to check around the muting circuits to see if they are shutting off to let the audio through.

As usual, keep us posted.

Cheers

GPS16

 

[GPS16]

Hi Trevor,

check the voltage levels around Q105/6 as these transistors short the audio to ground when muting. The 1.4V is coming from Q118 collector which gets a delay from the two caps C125/6 when the Fwd/Rev play changes state. For just the left channel, something odd around Q105 looks favourite.

Cheers.

GPS16

 

[Trevor White]

Hello again GPS16, great to hear from you.

I was fairly sure that those relays on the main function switch board were not anything to do with audio, but I wanted to be sure. Also I did the cycling of the tape/source switch with no change in its behavior.

Your suggestion regarding the mute behavior sounds much more promising. Although if I use the reverse direction switches the sound comes back on both left and right channels together after the mute period. The left channel seems to be off for a short time only after engaging Play from the fully stopped state. It's all a bit odd.

I will endeavor to get the machine down and open it up for a look at those transistors you have suggested.

Thanks heaps mate!
Trevor

 

[xpert911]

Hello Trevor,

I just saw your slightly older post about the RT-707.
At the moment I'm also preparing my old RT-707 and I've actually finished replacing the electrolytic capacitors on the power supply and oscillator board. I've also already done the servo board.

Now I wanted to do a final test after reworking the power supply and... now the capstan starts up briefly... only very briefly and then stops. I then checked the voltage on the servo board and found that the 100v for the motor collapsed.

Do you have any idea where I could start troubleshooting?
Sorry if I "bother" you so directly with my question... I'm desperate and you obviously know your way around.

 

[GPS16]

Hi xpert911,

when you say the 100v has collapsed. Where did you measure it? At the motor or at the transformer?

Does REW/FFW work OK? They are ran from the same 100V supply. Check FU2 with a meter (if you have one). With empty reels fitted. Grab them with your hand and assess the torque when in REW/FFW.

Check the plug/socket near the power switch and also the plug from the motor itself to the Servo PCB. This is a well know port of call for Capstan Servo issues.

Keep us posted.

Cheers

GPS16

 

[Trevor White]

Hi there xpert911,

I'm afraid I have no idea what your problem might be. I was hoping that GPS16 might also see your post and respond, which he has. Thanks GPS16.

The only thing I would do if I encountered that behavior would be to very carefully review all the solder joints on everything I had replaced, just to be sure.

Good Luck,
Trevor

 

[xpert911]

Ok, thank you for the answer. I solved the problem myself...I found that this effect only occurs at high speed and then I followed the full line of speed and found that a wire (green) on the servo board had come loose. Unfortunately, you couldn't see it straight away. Everything is going fine now. Thanks.

 

[Trevor White]

Hi there everyone.

Well, I finally got my RT-707 down and disconnected from my stereo so that I could properly investigate where my left channel was going. I is a weird problem as I found that if I left the machine powered on for some time before pressing 'Play' then both channels would play immediately. If I pressed 'Play' as soon as the machine was powered up the left channel would be silent for a while.

Over the past week or so this problem has been getting worse so I decided to open the machine up and have a look. GPS16 made an excellent suggestion regarding the transistors for the mute circuit and I thought I would look into that possibility.

First of all though I took all the covers off and the bottom plate and with the machine on it's side I began by gently probing various components on the Playback Amp board with a plastic probe. I was amazed that if I gently pushed down on the circuit board the left channel would come back, once pressure was released the signal would stop. Dry joint, I said to myself and began carefully probing around on the foil side of the Playback Amp.

After not too long I found this. Not sure how this happened but it must have been hanging on by a thread of solder. The offending component was a small value capacitor.

What a relief! After a bit more solder it now looks like this.

The left channel is back now without any odd behavior.

Thanks guys!
Trevor

 

[GPS16]

Hi Trevor,

Nice one! Glad you got it sorted. I hope this was not one you replaced!!!! Could you give us a "C" number so that we can see where in the beast the problem was. We can then file this away with other such problems and their rectification issues.

Cheers

GPS16

 

[Trevor White]

Hi there GPS16,

Well to my considerable embarrassment that was one of the caps that I replaced Just how I missed that when checking my work I will never know. The joint must have had a very small quantity of solder making the connection which over time fractured and caused the problem.

As for which capacitor it was I reckon it is probably C131 or perhaps C132. Unfortunately my Service Manual has the Playback Amp as RWF-080 whereas my machine which is a 'D' (Australian) model has RWF-078, and they are different!

Anyway, all good now. The machine still sounds amazing and makes spectacularly good recordings. Not bad for a machine that is just over 40 years of age.

Cheers mate,
Trevor