MCI JH-16 - Fuse keeps blowing

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Mailliw

Well-known member
Joined
Jan 13, 2005
Messages
196
Location
Halifax, Canada
Hi,

I am trying to figure out why the supply reel on my brother's 8-track jh100 machine won't spin. The manual has an analogue torque adjustment procedure, where you adjust a pot to get a proper looking sine wave. So I try this, just to see if it tells me anything...

The supply and takeup reel motors both have test jacks inside the chassis, and I hooked my scope probe upto one incorrectly.
The test jacks take banana plugs, but I didn't have any, so I hooked my scope up from behind, and didn't see two resistors:

test_diagram.jpg


When I put a piece of paper in the optical sensor to start the reel motors, I heard a fuse blow...

Now the 5A fuse labeled "main deck" blows everytime I turn the machine on..

I don't understand how this is happening!
Any ideas?

Thanks
 
One side of your 'scope is grounded. The 22K resistors protect the "whatever" from the short to ground. You went past the 22K and shorted the whatever to ground. If it is sick even with the 'scope not connected, then the whatever is blown-up in a way that pops the fuse.

Without knowing a lot more about the whatever, that's the best I can guess.
 
you probably shorted one of the motor driver transistors.

there are 3 small power supplies on the back of the machine. 2 are identical-one for the supply reel and one for the take up. (the other for everything else)

you can unplug the black cinch-jones connectors from both of these and see if your fuse still blows. then if not plug in one side only. then you should know which side is shorted.

there are also 2 or 4 transistors mounted one the back side of the deck plate that are the capstan drivers. maybe you zapped them, so if no luck with the "chimneys"(that's what they call the 2 power supplies in the back), try the capstan transistors. stranger things have happened.
 
I thought oscilloscopes were like multimeters in the fact you couldn't cause damage by being an "outside observer"

Here is a snippet from the wiring harness schematic; is anything here susceptibe to damage from what I did? Hopefully not the motor :(

takeup_motor.jpg


The capacitors are huge, they are in metal cans with rubber seals on the connectors.

Thanks,
William
 
[quote author="amorris"]you probably shorted one of the motor driver transistors.

there are 3 small power supplies on the back of the machine. 2 are identical-one for the supply reel and one for the take up. (the other for everything else)

you can unplug the black cinch-jones connectors from both of these and see if your fuse still blows. then if not plug in one side only. then you should know which side is shorted.

there are also 2 or 4 transistors mounted one the back side of the deck plate that are the capstan drivers. maybe you zapped them, so if no luck with the "chimneys"(that's what they call the 2 power supplies in the back), try the capstan transistors. stranger things have happened.[/quote]

This machine only has 2 power supplies in the back, 1 for the transport and 1 for the electronics.

There are many transistors in the power supply, I am not sure which ones are "motor driver" ones..

Thanks,
William
 
Here is a schematic of the power supply, with the motor driver board inside...

also a schematic showing how the power supply hooks upto the motors

jh-100_1000px.jpg

JH-100 POWER SUPPLY
bigger version here

wiring_1000px.jpg

WIRING
bigger version here
 
Q7 Q8 are fried. One of them may have been froed from before, which is why you were trying to debug it; now it't really fried.

I must say, this scheme begs to be fried. Not a trace of short protection, or even hot-leakage bias.

How painful is it to replace the board that Q7 Q8 are on with a known-good board? If you think it is cheaper to repair it yourself, be SURE to replace ALL of Q3 Q4 Q7 Q8 and CR6 CR7 CR8 CR9. I'd also be suspicious of CR1. All these parts have to be rated bare minimum 200V, and 400V parts would be wiser.
 
Hello,

Unfortunately I don't have any spare parts for the machine, so I will have to repair it myself.

I took apart the power supply and Q7 through Q10 are not individual DTS-411 transistors like on the schematic, they are pairs of transistors tied together, like this:

pair_sketch.jpg


pictures:

2trans_640.jpg

transistor pair (top)
bigger

2trans_under_640.jpg

transistor pair (bottom)
bigger

Can the pair be substituted with one bulkier transistor, or do they have a special function?

Q1, Q2, Q3, Q4 look like they've been changed before and as a result, some of the traces are lifting.

top_640.jpg

driver board (top)
bigger

board_bottom_640.jpg

driver board (bottom)
bigger

side_640.jpg

driver board enclosure
bigger

supply_640.jpg

entire JH-100 power supply
bigger

Thanks
 
> Can the pair be substituted

I don't know.

I think you need a JH100 guru. I could guess, but on a 600 watt amplifier guessing can get real dangerous real fast.
 
Not a Guru, but a dude from Ft Lauderdale who has seen those wretched things for 25 years...I can see using two beefier replacements just to keep the heat dissapation even across the heatsink.

But Brian Roth is a man who could give you a replacement part number in the modern age. Yo, Bri..
 
Gee, thanks Larry..... <g>

The older designs, such as Ampex MM-1100/1200, and early MCI designs, used a goofy circuit with essentially a bridge rectifier and a "pass" transistor that all ran from AC mains.

I dunno of a suggestion. The power transistor has to have a VERY high VCE breakdown to handle the "mains side" of the problem, plus plenty of power dissipation capabilities.

Once upon a decade, these parts were semi-commonplace. Nowadays, everything runs from lower rails.

Bri
 
Hello,

I am going to replace the DTS-423s and the ECG94s with some 2N5240s I got off ebay: http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=7571486567&sspagename=ADME:L:RTQ:CA:1

I read a page on replacing transistors and the author suggests you try to match the current gain as closely as possible. The min/max H(FE) for these replacements is 20/80 and the original parts have 30/90... close enough?

It seems digikey has suitable replacements for the TO-39 package transistors, mm3003 and mm3004, check it out: 2N5416

I hope they work out,
William
 
mike spitz would add another transistor in parrallel to up the current rating. he does it on the mm1200 but maybe he/you could apply here. atrservice.com
 
Hello,

Thanks for all the replies!

I took out all the transistors and tested them with the diode function on my DMM. Only two are bad.

I figured out why the transistor pairs have two different flavours of transistors; only one in each pair gets blown in a short, and they have been replaced before. The delcos are the originals.

None of the to-39 transistors seem bad. The diodes seem fine too.
I did order replacements, should I change them anyway?

Thanks,
William
 
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