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RT-707 rising from the grave

QSilver

QSilver

Super Member
I posted years back about an RT-707 that had been badly damaged during shipping.

The crippling blow was that I would need a new front chassis plate to get the tape to run properly again and this looked unlikely. However, I've recently been able to buy one and having now received it, I pulled the big box with the rest of the RT-707 in from off the shelf and onto the bench. Done about 3-5 hours on it so far just rebuilding it to make sure I remember how it all goes together...

So far, I've got the motors, run caps and everything else around the motors re mounted, solenoids, brakes, tape guides and pinch roller arms all back on and the tape sensor and the capstan motor.

Next step, which I think will be the harder one, remounting the head assembly (its all in pieces) and head alignment....
 

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I've now rebuilt the whole machine mechanically and refitted the heads to the head block, wired them up and tried the deck out minus the VUs just to see if my original tape skew problem was cured. After a slight adjustment to pinch pressure... so that it pinches, All seems to be well mechanically and the tape is fed smoothly through the whole mechanism.

I have given the heads the initial line up procedure in the service manual and the FWD play head sounds very good after adjusting azimuth etc. However, the REV play head isn't quite right and I can't get the HF anywhere close the the FWD play head. If I lift the tape detection lever out of the way, it improves quite a lot and I wondered if it might be an issue of the tape detection lever pressing down on the tape too hard... any ideas?
 
It turned out to be the back tension not being high enough and now that the deck has been put through all the mechanical adjustments its working very well - I'm very pleased with it.

I've now turned my attention to the electronics and have a lot of hiss on the source input. I replaced all the transistors on the Mic Amplifier assembly with KSC1845 and KSA992 which has reduced the hiss but if I turn the Mic volume pot up I get more hiss. I would like to lower this further and after now finishing a recap of the mic amp board, the hiss is still noticeably higher than the hiss from the playback amp boards. Where would I look next? Power supply?
 
I was told by a reliable tech that that will happen just don't use the Mic input. I have not the time to read the schematics. He told me about that in another forum. It was not on this one. It was explained in detail how it all work, so feel free to Goggle, Im sure it will come up.
 
QSilver said:
It turned out to be the back tension not being high enough and now that the deck has been put through all the mechanical adjustments its working very well - I'm very pleased with it.

I've now turned my attention to the electronics and have a lot of hiss on the source input. I replaced all the transistors on the Mic Amplifier assembly with KSC1845 and KSA992 which has reduced the hiss but if I turn the Mic volume pot up I get more hiss. I would like to lower this further and after now finishing a recap of the mic amp board, the hiss is still noticeably higher than the hiss from the playback amp boards. Where would I look next? Power supply?
Click to expand...

Could it be noise inherent to the mic amp circuit design, rather than noisy parts? Also, congrats on your getting this machine back together. I have an RT-707 thread going on my impending recap over on the AK tape forum right now - love this machine.
 
Eminence1963 said:
I was told by a reliable tech that that will happen just don't use the Mic input. I have not the time to read the schematics. He told me about that in another forum. It was not on this one. It was explained in detail how it all work, so feel free to Goggle, Im sure it will come up.
Click to expand...

Yes, I did expect to get some noise on the mic circuit but I didn't expect the hissing to be there when the mic volume is turned to the bottom. There is always a hiss much higher than the tape head amp. After about 12 o'clock on the mic pot it gets worse so that extra hiss must be the mic preamp. But I don't know where the rest would be coming from. The problem is, that the line input of the deck uses the second half of the mic preamp to amplify the Line In signal. So I can't get a clean line in yet. :no:

smurfer77 said:
Could it be noise inherent to the mic amp circuit design, rather than noisy parts? Also, congrats on your getting this machine back together. I have an RT-707 thread going on my impending recap over on the AK tape forum right now - love this machine.
Click to expand...

I guess it could be a design issue but I find it strange that it would hiss this much as the head amps hiss far less. What would be the point of having a noisier recording feed than a playback feed? Thanks, I have two of these now that this is working again, the second one will be finding its way onto the bench as soon as this one is done... it's suffering from gummed up lube so the tape cancel and pause don't work...

I'm going to try and disconnect the initial mic amp stage from the section that feeds into the line input by removing the the two resistors that feed into the next stage and see if that lowers the noise floor. Maybe it won't but this circuit is pretty noisy and turning the line level pot to the top doesn't give me any extra hiss at all...
 
I tried a few things and even removed the dual mic volume pot but I couldn't get the hiss to get much lower. I recapped the power supply and oscillator assembly for longevity and the hiss is lower than the hiss you get from recording. I also compared it to my other 707 and my RT-909 and they both have some hiss on the input source monitor switch. My Revox PR-99 doesn't however that is a bit more high end that these other decks so I' not too surprised. I tried recording with the RT-707 and its sounding pretty good. Live version of Hotel California sounds great.

I'll move onto re capping the servo and control PCBs next and then the playback assembly. Funnily enough the wow and flutter is horrible by comparison on my other RT-707 so my thoughts on re capping the servo board look to see how low I can get the fluttering before looking into the other one.

Picture of the deck and the rebuilt preamp board with huuuge 4.7uF film caps.... maybe overkill but they fit. :D

I'm half tempted to look into redesigning the input stage to be much quieter but that's not a project for now...
 

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Wouldn't surprise me if the mic circuit made a bit more hiss than line since its starting from lower level and employs more gain.

I set my 707 to source and turned up the line and mic until i could hear hiss. Let's say i usually set my normal listening volume to about 2/10 on my sanui au-888, then I had to set the volume to about 4.5/10 before line/mic at max setting made audible hiss from about 2ft away from my speakers. Mic noise was a slightly louder and higher pitch hiss, but on similar order of magnitude. with mic and line turned to zero i get audible hiss at about 6/10 (which is a volume level much higher than I ever use with the RT-707 and this amp/speakers).

Is that something like what you have or the mic is much noisier?

Nice film caps. I'm debating film vs electrolytic in that position. Were the through holes conveniently wide, or you had to use pretty long leads from the caps to narrowly separated holes?
 
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Thanks for checking. I was using headphones from the jack at the front of the deck for reference. The hiss is less noticeable from the amplifier connected to the amplifier but its still there. If I disconnect the mic circuit entirely, the hiss doesn't really drop at all. If the mic volume is over 5/10 it builds up and gets worse... Thing that gets me is that the line level shouldn't be so low so it shouldn't need a lot of gain to lift it... so why does the circuit hiss so much... looking at the schematic.... I think I see some 100k resistors in the path of the line in. Unless I'm getting mixed up here... its like its being attenuated so that the mic circuit isn't overdriven by the larger line signal and then to be reamplified by the circuit...?

For the film caps, I had to bend the wires to get them to go through two narrowly separated holes. The good bit was that the caps had long legs so I just lined up one leg, then bent the other one neatly underneath the capacitor then straight down to line up with the two holes I then covered the rest of the bare leg the ran along the capacitor with some heat-shrink just in case it got close to something it shouldn't. The was plenty of room for them... almost as if they had meant to be... :thumbsup:

I could try to take a picture of how I did it if you would find it helpful?
 
I repeated using headphones and like in your case the hiss becomes quite audible when the mic goes above 5/10. However, I hear almost no noise when turning up the line input pot, even at max it's barely audible - my line input is quiet (not sure why i was hearing noise on that before - will look into that). Level diagram shows mic starts at -66dBV, compared to line level which is -10dBV. About the 100kOhm resistors (R143/4), i see what you mean. I'm new to the 707 and don't know much about mic amp circuits so maybe someone else will chime in :). I don't see me using that mic input much. I have nice external mic amps and would just hook that into the line input. Would be great if the mic amp could be made quiet with some minor changes though... (i'm hesitant to replace the circuit outright, but each to their own :))

Thanks for the offer of pics of the film caps but don't worry, I will make it work.
 
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The only reason I want to keep the mic circuit is for the DIN socket at the back which I believe uses the mic preamp. I wonder if connecting something into the DIN connector would help drop the hiss... though I doubt it... At the moment it doesn't matter so much anymore as the hiss on the tape is louder than the source but if re-bias for tape and get it lower the hiss might become a problem again. Maybe a simple opamp would do better? I'm not sure as I haven't experimented with them much. All I know is that the Revox PR-99 I have uses opamps all the way through and it has almost no hiss at all.

When I am recording off a CD now on the RT-707 you can't hear the difference between the CD and the tape maybe the film caps helped with that. :D
 
Maybe you will be okay if you use the DIN anyway, since the signal is 36dBV higher (-30 dbV compared to -66 dbV), and you will have the mic pot at a lower position?

I think the PR-99 has opamps used throughout but also uses a lot of discrete transistors for amplification, no? (At least, that is the situation with the Revox B77, which I think is very very similar to the PR-99)
 
From what I can see on the schematics, the PR-99 looks to just use op amps all the way through the audio sections but I could be wrong... Yes, the B77 is really similar - nearly identicle. I think the PR-99 is said to the "Professional" version of the B77. I was looking for a B77 but a PR99 came up sooner for a good price and I really do like the tape counter and control.

I will test using the DIN input tomorrow as I think I've tried it in the past and the gain was very high - almost too much... I'll test it and see how it goes.

Got the face plate back on the RT-707 now and its looking good! Hardest thing next will be to try and get the color back into the right VU dial. I think its sun damage... They're printed directly onto metal plates so I'm not sure what the best thing to do will be... Need one new bulb for it too... any ideas where you can get them as they're smaller than the fuse bulbs I have.

Second picture is of an almost fully recapped power and oscillation board. A bit more room around the capacitors ought to help keep it running a bit cooler!
 

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QSilver said:
Need one new bulb for it too... any ideas where you can get them as they're smaller than the fuse bulbs I have.
Click to expand...

I might convert to LED if my bulbs blow. But definitely will use warm colour - can't stand those bright blueish white LEDs that show up on ebay 707s
 
smurfer77 said:
I might convert to LED if my bulbs blow. But definitely will use warm colour - can't stand those bright blueish white LEDs that show up on ebay 707s
Click to expand...

Me too. I like to keep the gear looking as original as possible. I used warm white ultra bright LEDs with a 15K resistor in line to reduce the brightness - You can't tell they're LEDs now but I'll never need to change them again!

I filled the edge off the bottom of the LEDs and they fit perfectly into the existing bulb holder - made up a new wiring loom for the pair of them and soldered it in.
 
I've been continuing my restoration - I have now recapped all of the boards and regreased the transistors on the control board and the voltage regulator on the power and oscillator board which has significantly dropped the temperature from hot to warm. I finished by recapping the servo board yesterday replacing what I could with film caps. My only problem is.... I don't know the test point for checking capstan speed. The manual indicates that for 19cm/s I should get a reading of 3000Hz and for 9.5cm/s I should get 1500Hz. The question is, where is this? It's not on the TP point on the board.... is it simply found by connecting my counter between the two terminals of the FG coming to the board from the motor?
 
Any ideas on where to connect a frequency counter? The last thing I want to do is get a replacement level meter for the right channel - as the one it currently has is faded and the red has gone pink. The bulb is blown too but I think I know where to get the bulbs. Does anyone have a spares machine?
 
No, the 3000Hz is the output of the Frequency generator on the back of the capstan motor. The service manual specifies that the output should be 3000Hz at 19cm/s
 
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