Pioneer RT-707 Capacitor Upgrade and Service

[GPS16]

Hi Trevor,

this is a measure of Torque. This means a tension multiplied by the distance from the centre. An empty 7" reel is required on which you wind a length of string with a loop on the end. The loop you attach to a spring balance.

There are 4 Torque settings for each motor. These are Full Wind Speed, Take Up Torque, Back Tension Torque and Fast Wind Back Tension. The Full Wind speed is just full power. The other 3 are done by adjusting the movable taps on the big green resistive droppers. Check the measurements first and see if they are in spec. If they are LEAVE THE GREEN RESISTORS ALONE. They are very brittle and have been in service for a good few years now. They may crumble if moved so you don't want to be messing with them if you don't have to. Smurfer has the luxury of a donor machine for bits. So the pressure was off a bit when he did this.

To check the readings you will need to over ride the Autostop. With no tape loaded press the required function (Play,FF or Rew) and hold the button in. The loop will drop to activate the Auto Stop but this is just an impulse. After the clunk, the button can be released and will stay in that mode until STOP is pressed.

The same method is used to check the operation of the brakes. There is a reading for both directions as the brakes are differential. Hence the advice on the differing reel materials. They are important to as there is no help from the electronics in any way here. Not like on the bigger Otari, Ampex and Studer Gear!!!

Hope that makes things a bit clearer.

GPS16

 

[GPS16]

Hi Trevor,

As an aside, put an empty 7" Reel on the RHS Spindle. Leave the LHS one empty. Hold the reel tightly and select FF Wind. The torque will be the strongest. Then select Fwd Play. After the delay the machine will restart at a much lower torque. Press Rev Play and the torque will drop again. Then press Rewind and the torque will still be there but hardly any thing at all. This is to stop loops during fast wind if the reels or tape pack are eccentric and the tape tries to snatch.

This just illustrates the motor control on a bit more of a practical basis.

GPS16

 

[Trevor White]

That's great GPS.

So the objective of having the length of string wound on the reel is to apply enough tension to the reel/string to prevent it from moving, the reading on the tension scale then gives the measure in grams?

I hear you in regard to the value of those big adjustable resistors. My machine seems to operate well with no apparent winding or stopping issues. I thought that it might be prudent to check them though.

Cheers, and thanks for all your advice.

 

[dhnash]

The string is "attached" to the reel hub with the machine in stop mode. I suppose this could be accomplished by tying a knot in the end of the string and placing the knot in the opening in the hub that the tape leader would be placed. The string is then wrapped around the hub in the appropriate direction based on the type of measurement being made (forward or reverse tension). The tension (or spring) gauge is then connected to the free end of the string and the machine is placed into the appropriate fast wind or play mode based on the instructions shown in the manual.

Another method of making these measurements is to use a tension gauge called a tentelometer. These instruments measure tape tension directly without the need for using string. The following is an example of one that would be suitable for this application:

https://www.ebay.com/itm/Tentel-TEN...490958?hash=item1a3868058e:g:mCgAAOSwEEBZ7xeB

Some benefits of having the tape tension set correctly is it helps to reduce head wear caused by excessive tape tension and also helps to ensure that the tape maintains proper contact with the heads during recording and playback.

 

[Trevor White]

The string is "attached" to the reel hub with the machine in stop mode. I suppose this could be accomplished by tying a knot in the end of the string and placing the knot in the opening in the hub that the tape leader would be placed. The string is then wrapped around the hub in the appropriate direction based on the type of measurement being made (forward or reverse tension). The tension (or spring) gauge is then connected to the free end of the string and the machine is placed into the appropriate fast wind or play mode based on the instructions shown in the manual.

Another method of making these measurements is to use a tension gauge called a tentelometer. These instruments measure tape tension directly without the need for using string. The following is an example of one that would be suitable for this application:

https://www.ebay.com/itm/Tentel-TEN...490958?hash=item1a3868058e:g:mCgAAOSwEEBZ7xeB

Some benefits of having the tape tension set correctly is it helps to reduce head wear caused by excessive tape tension and also helps to ensure that the tape maintains proper contact with the heads during recording and playback.

Thanks for that dhnash.

I was aware of the Tentelometer and may have to consider something like that. I have been looking for a small hanging scale, spring or digital, that will measure in the small ranges we are dealing with here, but everything seems to be setup for loads up to 40 and 50kg. Not too accurate at 30 - 40grams I would have thought.

Cheers.

 

[dhnash]

If you look closely at the photographs in the listing I posted the link to you can see that this tentelometer has a maximum range of 200 gm and a 50 gm reading falls near the middle of the scale which is ideal for making adjustments on my Teac X-1000R. The resolution in the 30-40 gm range which you indicated would be appropriate in your case is also very good.

 

[Trevor White]

If you look closely at the photographs in the listing I posted the link to you can see that this tentelometer has a maximum range of 200 gm and a 50 gm reading falls near the middle of the scale which is ideal for making adjustments on my Teac X-1000R. The resolution in the 30-40 gm range which you indicated would be appropriate in your case is also very good.

Thanks mate. Looking closely at the pictures of that unit I can see that the manual is describing use on a VHS VCR. Presumably that does not matter, running the open reel tape over the rods would yield a similar result.

EDIT: Looking again at the pictures I can see diagrams describing use on an open reel deck. Although a picture showing the side of the box indicates the unit was factory calibrated for VHS. Does that matter?

Cheers.

 

[dhnash]

The instrument works fine with both VCRs and reel to reel machines. It is calibrated using a test weight attached to the end of a short length 1/2” magnetic tape. Ideally the calibration of reel to reel machines designed for 1/4” tape would be done with the test weight attached to 1/4” tape but more knowledgeable folks than I have told me that the error introduced by using the 1/2” tape is not significant. I actually replaced the 1/2” tape with 1/4” tape myself which is easy to do. There is a user manual for the tentelometer available for download if you do a Google search for the model number. The manual provides clear instructions for calibrating and using the instrument.

 

[Trevor White]

Excellent, thanks very much.

Actually that item on eBay you pointed out has all it's documentation as well.

Cheers.

 

[smurfer77]

i've been away for a few days but it looks like the AK community is taking good care of you! Great work on the recapping. Did you daughter boards come out smoothly? I remember fighting with those for the better part of an hour, until I learned how to use my solder sucker properly. Also great to see a lot of details here that i breezed over, such as the rough maps for the VRs and discussion of the torque adjustment. You can get relatively econmical force spring gauges, and if you work on r2r more you will end up needed a few (becomes some specs call for tens of grams, some a hundred to two hundred grams, and on one of my machines i needed 1kg or so (on the Revox PR99 pinch roller if I recall correctly)!). Recently i bought an Extech 475040 which has sub-gram precision, but goes up to 5kg, so I am looking forward to doing my annual service on my machines and use that next time to check tension and forces etc.

Keep up the good work!

 

[Trevor White]

i've been away for a few days but it looks like the AK community is taking good care of you! Great work on the recapping. Did you daughter boards come out smoothly? I remember fighting with those for the better part of an hour, until I learned how to use my solder sucker properly. Also great to see a lot of details here that i breezed over, such as the rough maps for the VRs and discussion of the torque adjustment. You can get relatively econmical force spring gauges, and if you work on r2r more you will end up needed a few (becomes some specs call for tens of grams, some a hundred to two hundred grams, and on one of my machines i needed 1kg or so (on the Revox PR99 pinch roller if I recall correctly)!). Recently i bought an Extech 475040 which has sub-gram precision, but goes up to 5kg, so I am looking forward to doing my annual service on my machines and use that next time to check tension and forces etc.

Keep up the good work!

Hi there mate,

No, I had no trouble with the daughter boards. I have almost exclusively used Goot Solder Wick during this project. On those larger daughter board pins I used a 3mm wick, and it slurped up the solder very well. Impressive stuff. I have always found a solder sucker to be an awkward thing to use and I never had a lot of success with it, whereas the Goot Solder wick cleans up the pins and pads like new!

I also had a look around at the Extech 475040. Wow! they are not cheap, but I can see why it would be really useful.

Cheers,
Trevor

 

[Trevor White]

Hi All,

So I thought I would just document the completion of the electrical work on my machine. In the last installment I had not replaced the two VR's that control the VU meter levels, VR109 and VR110.

Seeing as how I was doing all the trimmers on this board I thought I may as well replace these guys as well.

Here is the corner of the Playback Amp with the original VR's in place. You can see that the Control board is still mounted here which makes accessing these VR's from above a bit fiddly. So once again I carefully removed the Control board and it's alloy frame, this gave plenty of clearance.

These are the Bourns multiturn 10K VR's

The corner of the RWF-078 board from the foil side. Plenty of that sealing goo on here.

And now with the VR's carefully removed and the pads cleaned up.

Here are the new VR's in place.

Finally the machine is complete (I hope). Ready for calibration and testing.

Here are a couple of mandatory tools that I picked up for adjusting these new VR's.

Once again I made a careful diagram of the physical connections of these VR's. Because my board the RWF-078 now has the VU meter driver circuitry included the layout, the connection of VR109 and VR110 does not match the foil side circuit diagram at all. I found that one of the VR's had the Wiper pin shorted to one end whilst the other did not. So once again I took careful measurements of the exiting resistances both in situ and when unmounted.

The two original VR's were 6.8K parts, although just why Pioneer used such large values I'm not sure. Both of the resistances on my original VR's were only around 400 ohms ?? Anyway, I dialed up the same values on the new Bourns VR's and everything worked well afterwards. Needless to say the machine now requires a full calibration. But I am waiting on some additional tools before proceeding.

That's all for now.

Cheers.

 

[GPS16]

Hi Trevor,

For a full alignment you will need the proper adjusting tools before going near to the variable inductors/transformers. If you have studied Smurfer's thread then you will have already witnessed the possible pit falls.

Ferrite adjusting slugs are fragile and brittle to the extreme. Great care must be taken with these.

Apologies for not mentioning this before you ordered the parts you have used already. My set lives in the accessories pouch on the scope, so it did not spring to mind!!!

Cheers.

GPS16

 

[Trevor White]

Hi Trevor,

For a full alignment you will need the proper adjusting tools before going near to the variable inductors/transformers. If you have studied Smurfer's thread then you will have already witnessed the possible pit falls.

Ferrite adjusting slugs are fragile and brittle to the extreme. Great care must be taken with these.

Apologies for not mentioning this before you ordered the parts you have used already. My set lives in the accessories pouch on the scope, so it did not spring to mind!!!

Cheers.

GPS16

Yes indeed, I have sourced a nice set of plastic? tools with various shaped ends, square, hex slot and cross etc. All in various sizes. Hopefully these should be OK.

Having seen the disaster that befell Smurfer however, I will be ULTRA careful if I venture into the recording EQ and calibration.

Cheers,
Trevor

 

[smurfer77]

Yup. I demolished that little slug. These particular ones are so soft I might even worry about using my ceramic tools on them. I bought some soft plastic FM alignment tools; not the right shape or fit, but they work and are too flexible, which has the benefit that there is no way to apply too much force because the plastic deforms before any real forces can be applied.

Looking forward to your alignment posts.

 

[Trevor White]

Hmmm, I think I may have run into a problem.

Up to now all the testing I had done was in playback by mounting a tape I had previously recorded and having a listen. All good so far.

However today I decided I had better plug some input into the machine and check out the line in function. All I did was run an RCA cable from my Amp to the line in on the 707. I made sure the 707 chassis was grounded to the earth post on the Amp. I plugged in my headphones to the 707 and played some music via the Amp.

I dialed up the Line in, and fabulous sounding music, for a while. Suddenly, without me touching anything, there was this loud electronic noise, a bit like an earth loop hum on steroids, both VU meters pegged, but after only perhaps 2 seconds the noise disappeared and the music was back clear as a bell. The VU meters behaved normally, no sign of trouble.

I then started using a plastic probe to carefully push and tap all the components I had changed, all the ones I could easily reach that is, but no recurrence of the noise. Very odd!

Then, later on, I hooked it up again, no tape mounted, just listening to the input with the headphones. Everything seemed normal, so I started pushing buttons. All the small buttons for Bias, EQ Record On/Off etc did nothing. So I put the unit in 'Play' and then the noise recurred, and stopped just as quickly. I tried FF and RW, and the noise appeared again, just for a very short time. After occurring two or three times nothing else I could do would provoke it to happen again. The Play, RW and FF controls caused no problem.

I have been carefully using a plastic probe to gently move all the wiring connections, but I'll be damned if I can figure out what it might be. Obviously something in the input chain. I was wondering about those new transistors I put on the Mic Amplifier board. Looking at the foil diagram it looks like at least a couple of the 2SC1344's may be on the Line In circuit after the Line In volume pots. I could be completely wrong about that though.

Can you guys offer any suggestion of just what may be wrong, or at least the type of thing I should look for?

Cheers,
Trevor

 

[GPS16]

Hi Trevor,

isolate the input circuits by putting the pots to Zero. If the noise re occurs the problem is afterwards. By the sounds of it it is Oscillation!!!

In case it is a strange sort of feedback, make sure the output pots at the back are at the indent position and not full up. With my preamp, the tape loops can give odd reactions with too much gain.

With the amp disconnected, use the phones output on the 707 and see if the same problems occur. The annoying part is that you don't know what sets it off as yet.

If you have instability do not have the speaker in circuit until you have resolved the problem. Oscillations are pure Sine Wave as a rule and you will be hurling RMS power right up the spout to your speakers. If the frequency is high you may fraz your tweeters.

Beware.

GPS16

 

[Trevor White]

Hi Trevor,

isolate the input circuits by putting the pots to Zero. If the noise re occurs the problem is afterwards. By the sounds of it it is Oscillation!!!

In case it is a strange sort of feedback, make sure the output pots at the back are at the indent position and not full up. With my preamp, the tape loops can give odd reactions with too much gain.

With the amp disconnected, use the phones output on the 707 and see if the same problems occur. The annoying part is that you don't know what sets it off as yet.

If you have instability do not have the speaker in circuit until you have resolved the problem. Oscillations are pure Sine Wave as a rule and you will be hurling RMS power right up the spout to your speakers. If the frequency is high you may fraz your tweeters.

Beware.

GPS16

Hello mate,

Yes I think you are right, it does have an 'oscillation' sound to it. It only occurs on input. In this situation I am only listening via headphones on the 707 itself. There is no RCA cable output to the Amp, so no risk there. If I have the Line In level pots at zero then I have not heard the noise occur, it only seems to happen when there is some signal coming in.

The fact that it might be oscillation was what made me think of the transistors. I replaced the 2SC1344's on the Mic & Line input board for KSC1845F's. Can this sort of noise be caused by a bad capacitor? Or is it indeed more likely to be a bad transistor?

Thanks mate,
Trevor

 

[smurfer77]

Trev, at work now and no time to check the schematic but a couple of points; (1) when you hear the noise it is on both channels? this may narrow things down; (2) are the voltages at the transistor pins reasonable; (3) the KSC1845 is not up to spec with 0.05A max collector current with the original being 0.1A. Probably not your issue, but as I mentioned earlier (possibly over in my thread) I would have chosen KSC1815. Also, probably not your issue, but which gain version of KSC1845 did you choose?

 

[Trevor White]

Trev, at work now and no time to check the schematic but a couple of points; (1) when you hear the noise it is on both channels? this may narrow things down; (2) are the voltages at the transistor pins reasonable; (3) the KSC1845 is not up to spec with 0.05A max collector current with the original being 0.1A. Probably not your issue, but as I mentioned earlier (possibly over in my thread) I would have chosen KSC1815. Also, probably not your issue, but which gain version of KSC1845 did you choose?

Hi mate,

The noise occurs on both channels evenly, and at the same time, but only for quite short periods. Very frustrating.

Yes I remember well your warnings about those KSC1845's. The versions I ended up with were the "F" spec. All of my examples have hFe values from 360 up to around 380. These values are not too far different from the original 2SC1344's which were all in the low 300's. The real issue with hFe comes with the 2SA672 replacements the KSA992F. All my examples had hFe values between 400 and 430, whereas the original 2SA672's had hFe values of only 104 and 116.

I have a good supply of the KSC1815Y and KSA1015Y. The KSC1815Y's have hFe's ranging from 185 to 195, so a bit lower than the 2SC1344's. On the other hand the KSA1015Y's have hFe vales ranging from 190 to 210, so much closer to the original 2SA672's. Also there is the current and frequency specs which you have rightly pointed out as well that sees the KSC1815 and KSA1015 much closer to the originals.

It has been pointed out to me on other forums that the specs of the transistors does not mean that they will necessarily be driven to those levels in circuit. It is all up to the circuit design. I ended up using the KSC1845F and KSA992F pairing because others had done so, in RT-707's, with apparent success. However in my machine something is rotten in Denmark, as they say.

Of course it may have nothing to do with these transistors, but I can't think of what else I have replaced on the input side of things that might cause this behaviour.

Unfortunately I don't really have the electronics skills to necessarily diagnose and trace the fault, especially with it being so intermittent. I figure I have nothing to loose by replacing the KSC1845F's and KSA992F's with the KSC1815Y's and KSA1015Y's and see what happens. I mean, it has to be to do with something I have changed wouldn't you think?

I also have a supply of original old stock Hitachi 2SC1344-E transistors and also some 2SA672-B's. The 2SC1344's came from an American supplier and I'm pretty confident they are real. The 2SA672's though came from Asia and who really knows what their provenance might be. But their hFe ratings match the originals pretty closely.

Thanks,
Trevor