Motor transistors keep dying on me

[EmilFrid]

If it's a grounding issue, maybe I should just tie the reel motor control card's ground to chassis? Motor control's ground measures 0,8 ohms to chassis

 

[map2112]

Do you have a copy of all of the schematics for the unit, including the control unit and the motor control boards we could look at?

I looked in the forum for the schematics but the link was missing.

Are the 3 pairs of 100VAC transformer windings separate? If you disconnect the transformer is there any connection between the 3 sets of windings? I assume these should all be isolated from each others, both when disconnected and also when the motors are connected.

Mark

 

[EmilFrid]

@map2112 yes, they are indeed separate and don't show any continuity between each other.

I will post pics from my service manual tomorrow. Im thinking PSU, reel motor control and some more stuff. Thanks for chiming in, I appreciate that

 

[map2112]

Maybe this has been asked, but are the two transistors for the supply and take up motors mounted to a common heat sink/plate? With the transistors mounted, is there any continuity between the two transistor cases (collector) or to the heatsink?

 

[EmilFrid]

@map2112 no continuity to chassis or between the transistors. That was among the first things I checked

 

[EmilFrid]

Will work on it tonight and hopefully move forward with this

 

[Evertide]

no continuity to chassis or between the transistors. That was among the first things I checked

Electrically isolated , But terminal well connected ?

 

[EmilFrid]

@Evertide yes it's well connected, screwed onto the casing like it was originally

 

[EmilFrid]

@map2112 here come the relevant schemos.

 

[map2112]

Why were the feedback resistor values (R14 and R18) changed from 10K to 47K on the supply and take up motor controller op amps?

I see their values marked up on the latest schematic you just posted. This increases the gain of those op amps by 5x.

What voltage do you measure at the output of these two op amps in the various tape modes?

Does your manual describe how to set the values of R101 and R102 for example and what the output of the op amp should be in each tape modes (which ultimately control the motors)?

 

[EmilFrid]

@map2112

They were changed in an experiment where I used another inverter that didn't put out as high voltage as the original. But it has now been changed back as I have the original 7406 again. The values i measured before the inverter swap was 5X higher. But this is no longer an issue. This was just used once during my hunt for this overarching problem, so the faults have nothing to do with that. All measurements posted in this thread are taken from the original sets of chips (opamp changed though as it for some reason died already long time ago)

What I find troubling, though, is that when I measure the motor voltage as per the picture I add to this message, it shows 220 volts in rewind (supply motor measured) and around 180 during fast forward. That seems way too much as in the manual it says 35 V +/- 5 volts in fast forward

 

[map2112]

So I assume the primary winding of your power transformer is wired for 240V (not shown in the schematic of the PSU)?

What do the 100V windings measure?

Mark

 

[EmilFrid]

Yes mine is for 220 (Sweden). The windings all measure 108 V each which is close enough to the stated 100 volts.

 

[map2112]

How about the motor voltages on the take up motor in those modes?

Do the motors have connectors that they can be temporarily swapped for testing (plug the supply motor into the take up controller, obviously without tape loaded)

If so do these strange behaviors follow one motor in particular?

 

[EmilFrid]

Take up measures 220V in fast forward and 180V in rewind. The fault is jumping randomly between the two motors so I don't think swapping the leads will yield much knowledge. It works perfectly as long as I only have one transistor mounted, and once I add the other one, one and sometimes both transistors fail.

I've never encountered something this strange

 

[map2112]

How about disconnecting one motor? Does that change things?

 

[EmilFrid]

i will give it a try right away

 

[EmilFrid]

Unplugged take up motor and supply motor transistor broke immediately at rewind.

 

[map2112]

Sorry to hear that! This is quite perplexing. Did you mean the supply motor transistor blew out with the supply motor unplugged?

How about if you temporarily replace the two motors with two 220V incandescent bulbs as loads. Maybe it will let you troubleshoot this while taking the motors out of the equation?

Also Ian’s earlier idea about installing a bulb in series with the motor could be another approach to try after that?

 

[EmilFrid]

the bulb approach is good. I'll see if I can find a couple of bulbs. I only have two good transistors left... Feels kindof hopeless.

The supply motor transistor blew with the take up disconnected.