I killed my 388!

  • Thread starter Thread starter Gavin137
  • Start date Start date
Just to add:

You stated the fuse blew even with the compromised servo board out of the circuit. I'm assuming from the previous posts that F1 is on the 35v power system. Have you checked the PSU yet in complete isolation?

E.g. I had a fuse blowing constantly on an Otari. I don't know what part failed first, but I replaced a bridge rectifier and a filter capacitor on that rail. (My assumption is the cap failed and brought down the rectifier, but I have no way to prove that hypothesis.)
 
Don, that's where the extender cables come in. He has yet to isolate the supply but the only way to do on the 388 is make some extender cables because when you plug in the PSU board there are a couple systems that load the supply so you have to build the extender cables to eliminate the loads but enable the inputs to the supply. I'm hoping there is something bad on the supply...it would be an easier fix.
 
Ah, now I get it. I've not owned a 388 so not familiar with the internal arrangement. (there's one for sale now on PDX CL....) :-)
 
I've not tested the psu in isolation yet, mostly due to not having had time to build the cables. The 388 project is at the back of a very long bus itself right now. I did take a look at it for things hooked up wrong, given that I had the psu installed wrong I thought it couldn't hurt to take a look. I did find a screw that had fallen down onto one of the heat sink circuit boards and could be shorting something out.

Sweetbeats, I am interested in those connectors you spoke of so I could test the power supply. What would you want for them? I was also thinking that if I pulled the bias cards out and disconnected a few of the plastic connectors I would be able to test the psu in relative isolation, with it only connected to the mother 2 board. Thoughts?
 
I'll PM you on the parts. I need to review what I used before and confirm I have the stuff. Give me a couple days.

As far as pulling what you can and testing the psu in "relative isolation", well, you can DO that but you are asking for some tail chasing. In order to confirm the psu is operating properly you need to isolate it, period.

I somehow missed that you'd installed the psu incorrectly...can you tell me more about that? That could be very important. And yes, a metal part on a live circuit board is not a good thing...any way you can power it down, disconnect the mains, pull the psu board and then put the screw back where you found it and take a pic and post it up here?

EVERY bad mistake I have made working on audio gear has been the result of being impatient, hasty, careless, etc. Take note all ye who read this...a moment of haste can literally turn your gear into waste.
 
:facepalm:
sweetbeats said:
EVERY bad mistake I have made working on audio gear has been the result of being impatient, hasty, careless, etc. Take note all ye who read this...a moment of haste can literally turn your gear into waste.
Click to expand...

Absolutely! The PSU installed incorrectly was just the latest in a series of blunders caused by not paying attention. Anyway what I did was put the PSU card where the reel servo card goes :facepalm:. The connections are the same, and I wasn't paying attention. When I powered it up nothing happened, presumably because there was no power supply where it should be. So I put the PSU where it belongs, powered on the machine, and that's when F1 blew.

Then, since I had already made a couple of dumb mistakes, I decided to check out the whole machine for anything that looked wrong (such as a screw on a live circuit), and that is exactly what I found: a screw from the VU meter bracket had fallen down onto one of the small circuit boards that connects to the heat sink in the back of the unit. I'll post a picture of that here in a few.

Anyway, point taken: it's bad enough trying to figure out what is currently wrong with this machine without making more things wrong as I go.
 
Yeah...and I'm not trying to jab. I'm really bad about getting impatient and going all "hammer mechanic" on whatever is frustrating me and that's the time to stop and step back...do something else for a bit so you can reapproach the task with a clear and patient mind.

I don't think and damage can be done by plugging the psu card into the reel servo slot and powering the device but I'll double-check and get back to you.
 
I'm not 100% that it was *exactly* right here, but definitely close.
 
OK let's try this
 

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Can't tell for sure, but you may have shorted that ?regulator? there. Remove the screw for sure. Are those transistors or voltage regulators?
 
And BTW, the screw is resting on the +5V regulator IC. I don't think that would have killed it but you would be double-checking that with the PSU out of the chassis and isolated; making sure the +5V rail is outputting close to +5V dc and also passing only scant amounts of AC current (like in the low single digit mVac). So I really think your next step is to build up some extender cables and get your PSU checked out and then go from there.
 
Gavin137, I just sent you a PM regarding PSU extender cable materials.
 
update

So, I had to walk away from the 388 for a while - lots of other things going on. But I did remove the screw that had fallen down into the works. I just got around to picking up some new fuses, and this time when I powered it up, PSU in place, no reel servo, the unit power on, the capstan spins, the tape counter lights go on, and no fuses blew! Now I did locate and purchase a replacement reel servo board from someone who was parting out a 388 on ebay, so when it arrives I may be well on the way to fixing this thing. I am worried that if something is still wrong I could blow up the new reel servo board, but I'm not really sure how to find that out without installing the board and seeing if it works. I am considering trying to repair my old one, just to test things out, but as sweetbeats pointed out there are some components of that that could be bad as well. Not sure how to proceed at this point.
 
I soldered in a new transistor to replace the blown one in the reel servo board, and replaced it into the machine. Upon being powered up it immediately went into "play" mode, with the pinch roller snapping into place and the take up reel table spinning. Then I put a tape on the machine and powered it up again. And again it went right into play mode and played the tape as if I had pressed play. However, none of the transport controls work at all, and the tape counter just shows zeros.

It seems to me that the remaining problem is likely the reel servo board, given some of the issues it has, and I am thinking that with a new servo board the machine should function (hopefully) normally. Any thoughts / cautions?
 
Odd stuff, I've been struggling with an issue with the capstan not spinning on my 388 for a few months now. Today I tracked it down to a resistor that had for some reason taken the place of a jumper connecting the 35v rail to the capstan servo board. Upon replacing the resistor with a jumper and powering on, the capstan spun up and mine too went into play mode. Now when I power it on, I get a flash of numbers on the clock before it quickly dims off entirely. The solenoids stay engaged in play mode. All of the voltages going to the control board are where they should be, and the only thing I currently have connected to the Mother 2 board are the PSU and control board. I suspect a problem with control board, or maybe a transistor on the TR board. Also, the 6v reset power supply for the control board is suspect. Ive noticed that it hovers at about 9.5v no load and about 8.2v loaded. On the reset rail, I measure 2v AC, not sure if these numbers are what they should be or not. The 2w resistor on the control board that traces back to the reset line gets too hot to touch, and produces a hot smell. This business with the reset rail and 2W resistor had been going on before I got the capstan spinning, when the transport controls all worked as they should, albiet without capstan movement. Sound like we might be able to help each other here. I have a full service manual with schematics and part numbers, if they would be of help.
 
I have pretty limited experience trying to track these kinds of thing down. The guy I got the reel servo board from also had many other 388 boards and parts on ebay, including a cap servo board. I don't see them for sale any more, but nobody but me was bidding on this stuff, so my guess is he still has much of it and might relist. The guy's username is stukalo on ebay.
 
I got the replacement reel servo pcb today, put it in place, powered up the 388 and it goes directly to play mode: pinch roller snaps into place, capstan spins, but the take up reel doesn't move. Then the tension arm lets off pretty quickly because the take up reel is not moving, and so the capstan stops. I took the pcb out and powered it on again with the exact same result. So with the new servo board in it behaves exactly as it does with no servo board in place at all.

ThenI put the board I had tried to repair back in and powered it up again, with the same result except now the take up reel turns and takes up the tape. Through all this the counter readout is not functioning / flickering a bit, and none of the transport controls have any effect. Also a couple of the vu's started moving, so I believe it was playing the tape. Perplexing and discouraging for the moment.

Shepdog - when you say control board are you talking about the board for the transport controls? Because I'm suspecting this could be the problem with mine as well.
 
Thats the one. Of curiosity, could you disconnect everything from the control board except for the yellow plug(412) and tell me if the 2W resistor(biggest resistor on the board) gets hot about 30 seconds after powering up. How do your transistors on the TR board look? Anything smell fried? I can confirm that the reel servo has nothing to do with this, well not anymore if it caused this problem for you. The reel servo was out when this started with mine. Im hoping its one or more of the transistors on the TR board. I'm planning to cross reference and order some suitable replacements for all of them once I get some time.
 
I disconnected everything except the yellow connector and powered on. Capstan spins but no counter lights or reel movement. Powered down after about a minute. the large resistor (405) did not feel hot to the touch, but there are signs that it has been hot in the past. In the picture you can clearly see heat damage around the upper leg. Other resistors (notably 424) did feel a little warm, but not hot as in melting. I also found a ceramic capacitor (c412) that looks like it has become fused with one of the electrolytic ones. I pulled it back, and you can see it in the picture.
 

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