Pioneer RT-707 service & rebuild

[retox]

I have read through this thread and feel encouraged to try and revive a RT-707 but need some help. I have a digital multi-meter, enough skill to follow directions and the ability to read a schematic. I do have soldering skills. I don't have the knowledge to trouble shoot from the schematic to the circuit without help. So, here is the problem as I understand it. The deck play recorded music without problem, but won't record on the right channel. I can hear a slight bit of distorted music, but it is at a very low volume. I have cleaned the heads several times, looked them over with a 10X eye loop and don't see any unusual wear, used deox-it on the pots and switches several times. Don't know what to do next. Any help would be greatly appreciated.

 

[GPS16]

Hi retox,

on first impressions, it sounds like you have lost the bias.

Start a recording and have the monitor switch set to Tape when you have previously set the levels at the setting Source. With the switch set to Tape, repeatedly set the Right Record Switch on and off. this must be done slowly as there is a delay involved to stop head and tape magnetisation. When the switch has changed state you should hear a relay click. It should go "On" pretty quickly. The "Off" should be the delayed bit.

We want to know if the relay clicks and also if there is a momentary "good" recording.

So:-
1. Does the relay click?
2. Does this get a good recording at all?

As an extra test, with the Left Switch set to off, does the Rec LED follow the Right Switch when you change it's state.

Are we on the "Control" side or the "Lack of Bias" side. The above tests should help us home in!!!

Cheers.

GPS16

 

[retox]

Thanks for the quick response.
1. The relay clicks when I switch it on and off.
2. No, the recording is very low in volume and distorted.

The Rec LED follows the Right Switch when engaged/disengaged.

Thanks

retox

 

[smurfer77]

Retox, 'low and distorted' recording is often associated with lack of bias. If you have 'exercised' the machine as GPS suggested try the extra test GPS mentioned. If not something there, you may have to trace the audio signal along its path, but I would triple check the bias switches etc before going nuts hunting.

 

[GPS16]

Hi Retox,

With the machine set to record, find VR304, which is the Right channel Bias pot and give it a small tap with the plastic handle of a small screw driver. If this makes a difference. Then the pot needs attention. Do not move the setting. When it has been removed, it can be measured and a replacement set to the same resistance before fitting. If you look at the diagram, the resistor is shorted at one end to the wiper. This means that it is a variable resistance and not a Potential Divider. As the Left channel works OK, then this is the only real component in the Bias which could cause trouble.

If VR304 is innocent then there is a wiring/continuity problem.

GPS16

 

[retox]

The results of tapping VR304 made no difference. I did it with headphones on and no input, I heard no response without input in record mode while tapping on VR304. While recording music I got the same low volume distorted playback results while tapping on VR304.

 

[GPS16]

Hi retox,

a scope would be handy to go any further now. Do you have access to one?

We now need to see what you have got on TP302 and the relay RY302. This is where the audio and bias are mixed.

Also if you have a DMM measure the resistance from the ends of VR304, it should be between 0 and 33K Ohms.

GPS16

 

[retox]

I have access to a scope but don't know how to use it. I could also purchase a SEEED 109990013 Handheld Oscilloscopes DSO Nano v3 if you thought it would work.
https://www.mouser.com/ProductDetail/Seeed-Studio/109990013/?qs=Hlcjo/O3pQ4lH8EgIM66uw==&gclid=EAIaIQobChMI-cmJz5O51wIVgx2BCh2SwQlrEAQYASABEgJ0ivD_BwE

Also looking at this scope...
https://www.ebay.com/itm/Mini-Pocke...%3Ab0f290de15f0ab605f97b0f5ffff390f%7Ciid%3A1

 

[retox]

If I am measuring correctly I read 27 ohms on both VR304 and VR302 across the resistor. When I am measuring across the wipe I get .9K on VR304 and 2.0K on VR303. Hope this makes sense.

 

[retox]

This is what I measured.

 

[GPS16]

Hi Retox,

The scopes listed may be on the edge when looking at the Bias as this is up at 125 kHz. You will get a false Sine Wave represented as any harmonics which would change the waveform will be outside of the bandwidth of the scope. The base Lab 20MHz scope would be much better in this case. But I do understand that it will be "Funds Permitting".

The readings you mention across the resistors do not make sense!!! The points in red which give a reading of 0.9 have PCB print between them so they should read a short circuit of less than one Ohm!!!. This is usually contributed by the test leads!!!

27 Ohms would seem a bit low too!!! If that was where the "Setpoint " should be why use a 33k pot???

At this point in time things are not making sense.

The first step is still to check that the Audio to be recorded is present and correct at TP302.

If you get hold of the scope I can guide you through it's operation!!! Then take things from there.

GPS16

 

[retox]

Thanks, not sure how soon I can get the scope, and will be out the week of Thanksgiving. But I will borrow a scope from work.
Again, thanks for the help.

-retox

 

[dave99]

I have read through this thread and feel encouraged to try and revive a RT-707 but need some help. I have a digital multi-meter, enough skill to follow directions and the ability to read a schematic. I do have soldering skills. I don't have the knowledge to trouble shoot from the schematic to the circuit without help. So, here is the problem as I understand it. The deck play recorded music without problem, but won't record on the right channel. I can hear a slight bit of distorted music, but it is at a very low volume. I have cleaned the heads several times, looked them over with a 10X eye loop and don't see any unusual wear, used deox-it on the pots and switches several times. Don't know what to do next. Any help would be greatly appreciated.

Hello, just a suggestion as I had the identical problem on my RT-707 after taking it out of decades of storage. I replaced Q302 and Q304 on the Power and Oscillation board. Fixed my recording issues.

Dave

 

[retox]

Good to know, I haven't had time to get back into this. Hope to work on it over the holidays.

Thanks

-retox

 

[Trevor White]

Hello, just a suggestion as I had the identical problem on my RT-707 after taking it out of decades of storage. I replaced Q302 and Q304 on the Power and Oscillation board. Fixed my recording issues.

Dave

Hi there Dave,

Just as a matter of interest, what equivalent transistors did you use to replace the 2SC1344 (Q302) and 2SC1213 (Q304)

Cheers,
Trevor

 

[smurfer77]

I replaced no

Hi there Dave,

Just as a matter of interest, what equivalent transistors did you use to replace the 2SC1344 (Q302) and 2SC1213 (Q304)

Cheers,
Trevor

I replaced no transistors in my RT-707. I am known for being rather aggressive in replacing old transistors in vintage japanese amplifiers, but the fact is that those used in the RT-707 are neither trouble makers (i.e. unreliable), nor noisy. As my RT-707 measured better than spec in the various departments I left semiconductor alone. If I used the mic input, I might however replace those to push noise lower but that is the only area i though about it (I briefly though about main amp, but really spec is good!). I see no need to replace other transistors unless they are broken... spec on the RT-707 is very good and I see no point. That said, if you really want replacement suggestions let me know and I will look into it.

 

[Trevor White]

I replaced no


I replaced no transistors in my RT-707. I am known for being rather aggressive in replacing old transistors in vintage japanese amplifiers, but the fact is that those used in the RT-707 are neither trouble makers (i.e. unreliable), nor noisy. As my RT-707 measured better than spec in the various departments I left semiconductor alone. If I used the mic input, I might however replace those to push noise lower but that is the only area i though about it (I briefly though about main amp, but really spec is good!). I see no need to replace other transistors unless they are broken... spec on the RT-707 is very good and I see no point. That said, if you really want replacement suggestions let me know and I will look into it.

Whoops, sorry Smurf, somehow I replied to a comment by dave99, Did not mean to bother you. Although I accept what you are saying. It would be of interest to know just what others have used for these replacements.

Cheers.

 

[Trevor White]

Hello, just a suggestion as I had the identical problem on my RT-707 after taking it out of decades of storage. I replaced Q302 and Q304 on the Power and Oscillation board. Fixed my recording issues.

Dave

Hi there Dave, (dave99)

Just as a matter of interest, what equivalent transistors did you use to replace the 2SC1344 (Q302) and 2SC1213 (Q304)

Cheers,
Trevor

 

[BobbyN]

I used KSA992 to replace the 1344's.

 

[buffdriver]

I used KSA992 to replace the 1344's.

I believe that you mean its complement, KSC1845 in place of the 1344. KSA992 is a PNP transistor.