Fostex G24s Reel to Reel player help - Everything works except play & record!

SilverBall

New member
I recently picked up a Fostex G24s. My collection is mostly 60s, 70s, and 80s TEAC Reel to Reel players but I couldn't resist a studio Reel to Reel player/recorder with 24 tracks. I also have quite a few AMPEX Grand Master 1" tapes which should have some original music recordings I'd like to check out. They all had sticky shed which is temporarily gone after using a dehydrator on them. The Fostex G24s looks like new. All voltages are spot on, just one large cap slightly out of spec that I replaced (along with the original rechargeable battery). All controls (FF, RW, Play, etc.) work flawlessly. This thing is amazing!

The problem is recording and playback just won't do anything. Tapes that should have content don't move the sound meters at all on the 24 cards. Setting it to input monitor and sending it music to record also does nothing on the 24 meters (and doesn't appear to record anything). Here's the really strange thing - If I run the RCA cables into my receiver from ANY of the 24 inputs or outputs, all I get is a buzzing sound. It also happens when the G24s is off but still plugged in! It makes no sense to me. All 24 cards are present, and yes - the SMD capacitors that notoriously leak will be replaced eventually (around 700 total), but I find it impossible that all 24 cards are completely bad and won't allow anything to play or record. I'm guessing that there's a bad amplifier chip or something somewhere upstream that's shared by all the channels and causing this strange behavior, but I'm just not sure where to even start. Visually everything looks perfect, and all connectors are on the way they should be. Maybe a short to ground somewhere?

Any help at all would be greatly appreciated! I just need something to point me in the right direction of where the problem might be. It's much more difficult to trace down an issue on SMD boards (and other tiny boards crammed full of components) than on 60s to 80s solid state equipment. I really think this is a great piece of history and I'd love to get it back to the operating state it was in when it came from the factory back in the 90s! Of course, without the soon to be discovered defective capacitors.
 
Last edited:
I recently picked up a Fostex G24s. My collection is mostly 60s, 70s, and 80s TEAC Reel to Reel players but I couldn't resist a studio Reel to Reel player/recorder with 24 tracks. I also have quite a few AMPEX Grand Master 1" tapes which should have some original music recordings I'd like to check out. They all had sticky shed which is temporarily gone after using a dehydrator on them. The Fostex G24s looks like new. All voltages are spot on, just one large cap slightly out of spec that I replaced (along with the original rechargeable battery). All controls (FF, RW, Play, etc.) work flawlessly. This thing is amazing!

The problem is recording and playback just won't do anything. Tapes that should have content don't move the sound meters at all on the 24 cards. Setting it to input monitor and sending it music to record also does nothing on the 24 meters (and doesn't appear to record anything). Here's the really strange thing - If I run the RCA cables into my receiver from ANY of the 24 inputs or outputs, all I get is a buzzing sound. It also happens when the G24s is off but still plugged in! It makes no sense to me. All 24 cards are present, and yes - the SMD capacitors that notoriously leak will be replaced eventually (around 700 total), but I find it impossible that all 24 cards are completely bad and won't allow anything to play or record. I'm guessing that there's a bad amplifier chip or something somewhere upstream that's shared by all the channels and causing this strange behavior, but I'm just not sure where to even start. Visually everything looks perfect, and all connectors are on the way they should be. Maybe a short to ground somewhere?

Any help at all would be greatly appreciated! I just need something to point me in the right direction of where the problem might be. It's much more difficult to trace down an issue on SMD boards (and other tiny boards crammed full of components) than on 60s to 80s solid state equipment. I really think this is a great piece of history and I'd love to get it back to the operating state it was in when it came from the factory back in the 90s! Of course, without the soon to be discovered defective capacitors.

Have you followed the steps for playback and recording as per the manual? Don't mean to sound condescending but a global issue like that sounds fairly uncommon and would point more towards a likely user error or potentially the cabling/input source you are using to test.

Try switching the noise reduction off and test again as well (although I don't think this will explain why you get no signal when just monitoring input).
 
Thanks for the input! I actually hope it is user error, that would save me more troubleshooting. I believe I did test with noise reduction off, but I'll double check that. I also did follow the steps for playing and recording in the manual thinking I was doing something wrong, but I'll verify that too.

Thanks!
 
I've spent a fair amount of time carefully following the steps in the manual, and here's the key thing I've found that likely will help with a solution.

For recording, I hit the save/rdy buttons on channels 1-24 I want to record on. Next, I press the record button to enter "input monitoring". The LED "Input Mon" blinks (as it indicates in the manual"). Now, I can check the quality of the source before actually making a recording (which is ultimately done by holding the play and record buttons down). This all makes sense and I've done it, but this is where the problem is. When I feed any of the "input" channels with the RCA plugs coming from my source music, I get no activity on any of the 24 meters. If I feed it into any of the "output" channels (I know, that makes no sense) the meters respond exactly as they should! Of course, this is completely backwards. This also likely explains why I get absolutely no output on the meters when I play a tape that should have recordings on it. It's almost like a locked on relay or bad transistor that forces input monitoring to go to the input channels. I believe this also explains why I couldn't record anything (on previous attempts feeding music into the input channels).

So, it appears I really do have a problem as I originally indicated (and an odd one I hadn't expected). Any ideas on what the culprit might be?
 
Last edited:
I'm pretty sure this has to be a bad component on the system control board, it's the only thing that makes sense to me. Now, I just need to determine which component might be responsible for the behavior I'm experiencing.
 
Everything I've tested on the system control board so far has been fine, with the only issue being the original rechargeable battery. I've replaced that, and now all settings are being saved correctly on power off. Unfortunately, the exact same issues remain with the inputs and outputs, and play and record. Any help on what could be causing this would be greatly appreciated!
 
Hi SilverBall

I realise this is a number of months after the last post but if the machine is back in a box or store-room awaiting help then I offer the below:

A couple of possible causes I've found from experience. The first, and easiest, is that there may be a number of broken solder joints on the motherboard that connects to all the audio cards. Re-solder them and it might be all that's needed. Also check the condition of the solder joints of the RCA connectors. The physical stress of repeated connections and disconnections may fracture them.

Secondly, (from experience with my G24S and G16S) the problem is quite possibly as bad as you may have already feared - all surface-mount capacitors on all channel cards may need to be replaced.

You'll have to wash each circuit board to clean the leaked electrolyte, and repair any damage done to copper traces. The electrolyte also weakens the trace-PCB adhesive, so sometimes the copper lifts from the board. In my 2 machines' worth of experience that damage isn't too bad - just a here-and-there job, with the worst offenders being the 47uF caps. The electrolyte leakage also forms a low-enough impedance path to interfere with the gate activity of the FET transistors that perform switching between Input and Repro, meaning one or the other (or both) refuse to pass audio. Cleanliness, and good solder joints are critical here.

Your observation of meter activity when you feed a signal to the output jack is expected - the reason being the op-amp that drives the output socket also feeds the input stage of the op-amp that provides the meter signal. So, yes, you are back-feeding the output op-amp, but you're also directly feeding the meter amp. (Imagine you've driven onto the freeway via the OFF-ramp, and then done a U-turn to join the normal flow of traffic).

The channel amp cards are single-supply, so there are a number of voltage dividers present to form virtual grounds at each stage of amplification. Each divider uses a capacitor to stabilise its output, and since this voltage directly feeds op-amp input stages for biasing, it needs to be in optimum condition (stable, with as little noise as possible). Inter-stage connection of the audio signal is also done via capacitors, to eliminate DC current. So again, these caps needs to be in optimum condition to allow the amplifier to function properly.

Having listed the above work, I'd reassure you it certainly can be done, and the machines sound pretty good for what they are... bearing in mind some of them are nearly 30 years old now!

Regards
Bryan
 
Last edited:
I deal with bad SMD parts all the time on Tascam servo cards- they get replaced with through hole types as SMD parts were never meant for 30-60 year old equipment. They are cheap and are used on cards that are throw away. This is why I can still fix some Tascam cards except the 238S and getting away from SMD is a very good idea. If there are a lot of them then you need to keep this in mind when you buy something. Fostex was not all that great of a product- they often took shortcuts.
 
Everything I've tested on the system control board so far has been fine, with the only issue being the original rechargeable battery. I've replaced that, and now all settings are being saved correctly on power off. Unfortunately, the exact same issues remain with the inputs and outputs, and play and record. Any help on what could be causing this would be greatly appreciated!
Hi SilverBall
I have G24S and I need to replace the rechargeable battery. Would you be able to guide me through what’s involved?
 
Back
Top