8mm Forum: ST600M stereo playback conversion

This is topic ST600M stereo playback conversion in forum 8mm Forum at 8mm Forum.

To visit this topic, use this URL:
https://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=003960

Posted by Martin Jones (Member # 1163) on August 28, 2008, 02:33 PM:

This week's diary...
Monday: Settled down to convert my ST600M for Stereo Playback. Had planned this 5 years ago and after spending the last 3 years extending and renovating our retirement home I had at long last been allowed to spend time on a hobby. Stripped the machine down to the bare amplifier and examined it along with the notes I made 5 years ago. Something was wrong; I discovered that I had done the conversion 5 years back .... and forgotten about it!. Put it all back together.
Wednesday: (Tuesday is a day off and the wife insists we go out for the day). Dug out my copy of "June is bustin' out all over" from Carousel (Derrann print, one of my favourites), set everything up through an external power amp and switched on. Didn't sound very Stereo to me! Even through headphones. Switched off, stripped it back down to the amplifier. A few checks; found dead short between the output channels, so I was getting Mono! Remembered that I had discovered that 5 years ago and had put it to one side for investigation.... and forgotten about it!
Thursday: Spent the morning trying to remember what I had done 5 years ago and reconciling it with the notes I made at the time. It turns out that I am not suffering from senile dementia (despite what the wife says), and the fault was a stupid one; one channel's leadout wire was soldered to the wrong PCB land.
Connected it all up again, switched on and... crisp, sparkling Stereo! Useful three days, despite me being so thick.
Now I have a Stereo Elmo and a Stereo (S926) Eumig. Now all I need is the time to compare them to decide which is best. I bet somebody out there can save me the trouble?

Posted by Steven J Kirk (Member # 1135) on August 28, 2008, 05:49 PM:

How difficult is this to do? I have 2 Elmo 600s although I do have a Eumig S938 anyway.

Posted by Kevin Faulkner (Member # 6) on August 28, 2008, 05:59 PM:

I take it Martin that you had to make up a pre-amp especially for this to work as the ST600 is single Chanel right the way through? How about some schematics and pics to show exactly what you did.

I'm in the lucky position to have an ST600D M-O (silver) which has a circuit similar to the ST1200HD and has 2 separate pre-amps and monitor outputs so can be fed out to an external amp without any mods. The results I get from this machine are awesome compared to the ST1200HD's I have tried this with. There is virtually no hum etc and the frequency response is unbelievable.

Well done to you sir [Cool]

Kev.

Posted by Maurice Leakey (Member # 916) on August 29, 2008, 03:58 AM:

I assume from what's been said that the ST600M and the ST600D differ with their amplifier outputs.

I have an ST600D M (Twin Track) which has stereo outputs so I didn't quite understand Martin's story, but Kev now seems to have explained the situation.

Posted by Martin Jones (Member # 1163) on August 29, 2008, 07:50 AM:

Kevin and Steven,
I'll prepare a post with schematics to try and clarify this.I had intended to provide a seperate external preamp for one channel, but actually ended up using only what was already in the machine, with some component value changes. And I agree, the resulting sound is superb; I've not played anything yet on my recently restored Eumig S926, so have no yardstick to compare it to. Watch this space...

Martin

Posted by Steven J Kirk (Member # 1135) on August 29, 2008, 06:08 PM:

Maurice, so the D is for 'dual' outputs? Mine are ST600M and have a mono 3.5mm jack marked for aux/headphones. Goes up and down with the volume control. Bit hummy. These are twin-track but it's either track or summed together. No stereo. I'd be interested in creating a higher quality output, if only mono.

Kevin ( Mr Elmo ) is it worth getting a hum-absorbing device as they have at Maplins to reduce hum?

Posted by Martin Jones (Member # 1163) on August 30, 2008, 04:00 AM:

Steven,
There's a simple mod that will enable you to feed out your existing mono signals BEFORE they go through the internal Power Amplifier which will not affect any of the existing functions. That would enable you to use an external HiFi amplifier.
I'm preparing a post on my stereo playback conversion; I'll put it in the same post as a seperate item.

Martin

Posted by Maurice Leakey (Member # 916) on August 30, 2008, 04:15 AM:

Steven

On my ST-600 D M there is a three-position sliding switch above the concentric Vol/Balance control.

On either side of the sliding switch are sockets marked MONI-1 and MONI-2.

On page 17 of the instruction book under REPRODUCING SOUND THROUGH A STEREO AMPLIFIER :- Connect the green monitor jack 1 with left channel, and the green monitor jack 2 with right channel.

Posted by Martin Jones (Member # 1163) on August 30, 2008, 05:41 AM:

Maurice,
On the ST-600 M (or MO) two track mono models the problem is that the AUX output is taken AFTER the internal power amplifier, so contains any noise (including hum) added in that amplifier, and is of EXACTLY the same quality as that delivered to both the internal and any external speaker. Picking the sound off BEFORE that amplifier ensures that the extra noise (and hum) is not present, allowing a HiFi low noise amplifier to be used.
I don't know the circuit of your DM machine; do you know if those outputs are taken BEFORE or AFTER the power amplifiers? If they vary in level with settings of the concentric volume controls, they are most likely taken AFTER the amplifier.

Martin

Posted by Maurice Leakey (Member # 916) on August 30, 2008, 06:01 AM:

Martin

The two concentric controls are:- small outer one for volume, and the larger inner one is for balance.

I don't use the stereo outputs. If I need to run a stereo film I use the GS-1200, or Sankyo Stereo 800. Both of these have two in-built speakers and separate Din outputs. These I feed into twin Elmo speakers which I purchased from a fellow Forum member.

Posted by Martin Jones (Member # 1163) on August 30, 2008, 09:52 AM:

Kevin and Steven,

htt://8mmforum.film-tech.com/ubbpics/ST600M0007.pdf
(as far as I can tell, you will have to copy this address to your web browser to get to the .pdf, and then print it for reference)

Here's the description promised..
The idea arose back in the 90's when I was the proud owner of only two projectors, the ST-600M and a Eumig 824 Dual Gauge. I would have liked a Stereo machine but at that time both new and used machines were out of my budget. When my Elmo motor stopped working, I got in touch with Hanimex at Swindon, who sent me a replacement Motor Control board, which didn't cure the problem. They were very helpful and sent me a complete set of schematics and layouts for the machine, from which I was able to solve the problem...Their new board had been incorrectly assembled! The connection socket had been soldered the wrong way round (pin 1 in the pin 8 position)!
The amplifier schematic (which I hope reaches this post as I have had to upload it as a PDF because of its size) proved interesting as the microphone recording preamp uses half of an IC with two identical seperate amplifiers; only the external components are different. Also, the amplifier of this model has all the switching and connections for the version with Optical sound, only with the exciter lamp and photocell not present ( the boxes on the shcematic identify those). The Magnetic-Silent-Optical switch is there; the Optical position is inhibited by the size of the aperture in the Control panel. A little ingenuity (please refer to the PDF as needed, the mods are shown in red) makes a conversion to Stereo Playback possible.It should be noted that the point of the exercise was to enable Stereo playback; the question of whether recording would still be possible was of secondary importance since recordings of Main or Balance tracks could be made on my other machine if needed.

1. The M-S-O switch aperture was enlarged to allow the switch to access the O position. This makes available some unused switch contacts.
2. The PCB tracks were cut at the points shown with "X". These......
a. makes sure that SW103-1 does not ground the Main Track head in the Play position.
b. isolates IC 101-2 from the main output amplifier, and
c. isolates SW102-3 from the 11V supply.
3. Additional connections were added as per the dotted lines; The first provides a Ground at the two centre contacts of SW102-3, the next one
grounds the common connection of the two heads via SW102-3 in the Optical position, the third replaces the deleted ground connection at
SW 103-1 via SW102-3 in the Magnetic position and allows head switching to revert to normal in the Magnetic position.
The last connects the "hot" end of the main track head to IC 101-2 input, via the Optical position of switch SW102-1.
4. Various component values were changed to make the circuit of IC 101-2 the same as IC 101-1, identified by the large dots...
a. R120 was removed completely.
b. C120 was changed to 680pf.
c. R116 was changed to 56 ohms.
d. R115 was replaced with 150kohms in
parallel with 0.033 uF, both in series wth 3kohms.
5. A 3.5mm Stereo ouput socket was fitted in the rear panel
alongside the AUX OUT socket, and connected to R108 and R127
where shown
by the arrows.

In use the Main-Mix-Sub switch is left in the Mix position and the Magnetic-Silent-Optical switch in the Optical position. Stereo at 30mV level is taken from the Stereo output jack. The internal monitor does not work, neither is there an output to an external speaker.
In the Magnetic position, Mono play back is available in all three modes (Main, Sub and Mix) because of the new grounding arrangements for the heads and SW103-1 via SW102-3. The internal monitor, amplifier and mono speaker output works normally in the Magnetic position.
Obviously, the Microphone input no longer works. The effect on recording (now via AUX IN only) in the various mono modes has not yet been tested but, as said above, it's the playback facilities that are important. There is a full set of contacts available on SW102 in case they are needed (nothing envisaged at present!).
The extra connections appear to be very long in some cases, but the amplifier board is very compact so the actual links are actually very short. The biggest problem is identifying components, switch contacts and PCB tracks, so the job is not recommended for the inexperienced in electronic repair and assembly.

A previous post refers to a simple mod on a basic ST-600 M (or MO) for bringing out a mono signal to a HiFi power Amplifier. To achieve this it is only necessary to cut the connection between J102 and the "R" contact on SW103-2 and reconnect J102 to the two centre contacts of SW102-2. This will enable J102 to be used as an AUX Input on both Record and Playback, and as a low (30mV) level AUX Output during both Magnetic and (in the case of an MO machine) Optical Playback.

Martin

[ August 30, 2008, 11:30 AM: Message edited by: Martin Jones ]

Posted by Steven J Kirk (Member # 1135) on August 30, 2008, 12:16 PM:

Very interesting! Thanks so much Martin. I've saved the pdf and your post and will look those over closely. I think the stereo mod would be beyond me but the mono I would try. Luckily I have a 'tame' local electronics man who I can also call on! Thanks again.

Posted by Martin Jones (Member # 1163) on August 31, 2008, 03:56 AM:

Steven,
Thank you for your comments; if you decide to try the mono conversion contact me first at blagdonian(at)btinternet(dot)com so I can pass on a couple of tips regarding dis-assembly and send you an annotated PC board layout.

General,
My remarks regarding the effect on recording in the last post were precautionary (to avoid egg on my face!), but having fully analyzed what I have done (and no doubt others are checking) I am happy that when the Mode switch is in the Magnetic position all circuitry other than the microphone preamp is effectively back to where it was before the conversion, and will work for both recording on seperate tracks and for playback of tracks 1 and 2 individually or combined.
The only real anomaly with regard to normal Audio Practice is the signal level at the outputs (30mV instead of the more usual 150mV of a "Line" input or output) but this can be allowed for at the external amplifier input. It is strange that Elmo has adopted this level, just as it is strange that they designate the "Moni" socket, which is intended for an earpiece and is part of the final speaker circuit, as (Aux Out) also.
I don't know if this is the case on the ST-600D M/MO, as it has a different amplifier. I do have a copy of the Operating Manual for that machine; there is no circuit diagram and Elmo do not list sensitivities (levels) in their Specifications, another departure from normal audio practice.
Martin

[ March 04, 2014, 08:03 AM: Message edited by: Martin Jones ]

Posted by Kevin Faulkner (Member # 6) on August 31, 2008, 06:02 PM:

Martin, Congratulations go to you on a superb mod. I have now studied your notes and circuit diagram and I for one think you have done a superb job.
Using the other half of the pre-amp ic is one superb idea. I think in my 600D M-O they have added a second IC because it still needs the optical pre amp. I suspect they have used a similar layout to you and used the one dual channel ic for the mag heads and added a second for the mic and optical cell. The 600D also has the same output IC as used in the 16CL as it gives about 10W output. Yes the moni outputs on this model are also at a low level just like the ST1200HD and I too have never worked out why Elmo chose such a non std output voltage except that at this point it's before the vol and tone controls etc. You could of course put a buffer amp at this point (dual opamp) to bring that level back up to a more normal level for feeding into an external amp and that would mean that you would have very little loading effect then on the internal amp and its circuitry. This is in effect what they have done on the GS1200 where the output is a bit higher and allows you to feed a tracks output back into the aux input of the other track for track to track transfers. Also on the GS this circuit drives the meters.
On the GS800 I think the circuit is again similar to yours but with 2 output ICs but I have not been able to track down circuit diagrams for the GS800.

Whatever, at the end of the day your mods make complete sense and I think you will find may others having a go at this conversion.

Well done, great ingenuity [Smile]

Kev.

Posted by Martin Jones (Member # 1163) on September 01, 2008, 04:22 AM:

Thank you Kevin, praise indeed!
Ruminating over the weekend about the loss of Mic recording facility; I'm going to try an idea to restore this....
A patch lead with 12dB attentuation (the amount the Mic. amp. gain has been increased) built in and connected from the new channel's output jack (at R127) back to the AUX.IN port (J102).
This will restore the facility but the changes in frequency response could have dire effects on my voice! (But probably no more than what the wife has been threatening to do to me for years).

Posted by Kevin Faulkner (Member # 6) on September 02, 2008, 11:06 AM:

What about doing this as an article for FFTC? See Tom's topic in this section: http://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=003943

Kev.

Posted by Martin Jones (Member # 1163) on September 02, 2008, 12:47 PM:

Steven,
Earlier you asked about a hum absorbing device from Maplin..The only one I can find at Maplin is for dealing with ground loop problems,; it would not help in this case as the hum in question comes directly from pickup in the heads. Or did you find something else?
A proper tuned in line hum filter will help between an AUX output and an external amplifier. Devices for the speaker line, such as the old CRAVEN Hum Filter/ Tone control are totally useless, no more than a cynical con. Pity, because a lot of their products were excellent.

Martin

Posted by Kevin Faulkner (Member # 6) on September 02, 2008, 04:07 PM:

Maurice/Martin, The moni outs are before the vol controls etc. If you think about they have to be much like Martins conversion as you only have the one single power amp.

Sound quality is extremely good at this point although as Martin found the level is a bit lower than one would hope for so you need an amp with a fairly good sensitivity at line in.

Posted by Patrick McGrath (Member # 1210) on March 03, 2014, 04:08 PM:

Sorry for bringing up an old topic.

I've been experimenting with this idea of a stereo output conversion for my ST180MO. So far I have built an external preamp and bypassed the internal electronics with mixed results.

I wonder if anyone can tell me, is the ST180MO the same circuit design as the ST600 Martin describes here?

Posted by Martin Jones (Member # 1163) on March 03, 2014, 04:14 PM:

Yes.

Posted by David Ollerearnshaw (Member # 3296) on March 03, 2014, 04:53 PM:

The PDF link.

Posted by Patrick McGrath (Member # 1210) on March 03, 2014, 05:18 PM:

Excellent! Thank you.

My St180 is my guinea pig, so to speak. I've already done a manual thread conversion and replaced the tensioner with a nylon roller.

Would be fun to see if I could do this mod as well. I have a Sankyo 800 already but I can't use my long throw 1.2 lens in anything but the elmo. Might be nice for scope/stereo in the living room.

Posted by Martin Jones (Member # 1163) on March 04, 2014, 03:19 AM:

Notes on the .PDF

Red Boxes: Optical parts that are not present in Mag Only machines. In Mag/Opt machines should be disconnected from circuit

Red dotted lines: New connections.

Red Arrows: Output points.. connect to new RCA jacks, or a 3.5mm Stereo socket.

Red Xs: Cut PCB track.

Red Dots: Replace components with same values as those in other channel (remove if no equivalent in other channel)

The extra switch positions can only be accessed if the switch panel hole is lengthened in Mag only machines.

The PCB is double sided; this is NOT a conversion for anyone WITHOUT the relevant Electronic knowledge and PC Board "reading"
skills.

See notes on Recording earlier in the thread. There are also quite detailed notes on the theory and practice behind the conversion for those technically minded.

BTW: ST180 (180 meters) = ST600 (600 feet)

[ March 04, 2014, 08:13 AM: Message edited by: Martin Jones ]

UBB.classic^TM 6.3.1.2