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Old 17th Sep 2021, 10:07 am   #1
D4rkS4v4nt
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Default Otari MTR-10 Power Supply Issue

Hi Folks,

I'm new to the forum but it has come up many times when googling for different topics!

I have acquired an Otari MTR-10 with a power supply issue, the status LEDs show the following -
  • +42V = ON
  • +30V = ON
  • +24V = ON
  • +26V = OFF
  • -26V = OFF
  • +10V = ON
  • +5V = OFF
  • READY = OFF

So far I've performed the following troubleshooting steps -
  1. Checked visually and for continuity all power supply fuses
  2. Checked and re-seated all power supply connectors
  3. Checked for raw DC voltage from the rectifier responsible for the +26 / -26V rails, this was OK (+32V and -32V)
  4. Examined the circuit board and found a few solder fractures around the pins of the molex type connectors, these were reflowed with fresh solder

Any pointers or assistance greatfully received

I've attached the schematic for the PSU to this post.

Thanks!
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Old 17th Sep 2021, 10:20 am   #2
Cobaltblue
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Default Re: Otari MTR-10 Power Supply Issue

Hi and welcome to the forum

Just a brief look but your 26V supplies won't come on if the 5V is missing (disconnected by RL1) so I would start with the 5V supply.

Now I have had a closer look that's not true but whatever the 5V needs to be heath for the unit to work so I would still start with the 5V supply

Cheers

Mike T
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Last edited by Cobaltblue; 17th Sep 2021 at 10:29 am. Reason: Closer Look at what RL1 does
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Old 17th Sep 2021, 12:09 pm   #3
D4rkS4v4nt
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Default Re: Otari MTR-10 Power Supply Issue

Hi Mike,

I've been told that the Fairchild regulator in the 5V supply can be a usual suspect - I've checked the voltage at the IN pin via resistor R5 and I'm getting 0V. This is why I was concentrating on the 26V circuit as the 5V supply is connected via the regulator.

On the 10V / 5V supply I can see the raw DC voltage from the rectifier D15 (13V).

I noted that one of the smaller capacitors in the READY circuit (rhd side of schematic) may have a bulging top, I can see this is connected by a jump cable (a) and (b) so wondering if a short in this area could cause a problem...

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Old 17th Sep 2021, 11:21 pm   #4
jamesperrett
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Default Re: Otari MTR-10 Power Supply Issue

I'd start by checking the voltages on all the pins of Q10.
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Old 18th Sep 2021, 3:59 am   #5
Refugee
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Default Re: Otari MTR-10 Power Supply Issue

I have had a look at the circuit.
The chip in the 5V regulator gets power from the +26 volt rail so no 26 volts means no 5 volts due to 0V on the base of the 5 volt power transistor.
As both 26 volt rails are down first measure the voltages across the two 0.33 ohm resistors connected to the collectors of the 26 volt regulator power transistors. If there voltages are close to 0.6 volts the regulator is shut down due to overload.
Change that dodgy capacitor in the 5V regulator as that may have caused the relay to chatter and cause the main fault.
We need to find out if there is a short circuit on the output of the 26 volt regulator before going much further.
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Old 23rd Sep 2021, 2:57 pm   #6
D4rkS4v4nt
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Default Re: Otari MTR-10 Power Supply Issue

Hi Folks,

Thanks for the advice received so far

I'm also a member of tapeheads forum and had some useful advice from one of the regulars. It took me awhile to get my head around how the power supply circuit (for +26V and -26V) works but what really helped me was simulating the circuit via the Paul Falstad Circuit Simulator application - https://www.falstad.com/circuit/.

OK so so far the following troubleshooting was performed -
  1. Checked visually and for continuity all power supply fuses
  2. Checked and re-seated all power supply connectors
  3. Checked for raw DC voltage from the rectifier (at resistors R2 and R3) responsible for the +26 / -26V rails, this was OK (+32V and -32V)
  4. Examined the circuit board and found a few solder fractures around the pins of the molex type connectors, these were reflowed with fresh solder
  5. Disabled "control/tracking" loop by lifting R101 and R112 - this resulted in the +26V rail coming live, but no -26V rail, so appeared that the fault was within the -26V rail.
  6. Found that Q106 transistor was bad (part of the control loop) and replaced.
  7. Found an error in the schematic (there is a connection from the base of the pass transistor to the base of Q102/Q106 that doesn't exist on the actual board).
  8. Tested C2, C3 and C10 capacitors - C2=5300 (should be 6800), C3=4000 (should be 4700), C10=4200 (should be 4700).

After experimenting with failure modes in the Circuit Simulator I came to the conclusion that the fault must be a short in one or more of the following components on the -26V rail, capacitor or transistor. My first suspect was the pass transistor Q12. I found a dead short between the base and collector and an open circuit between the base and emitter.

It appears that the transistors have either been replaced or never matched the schematic as I have a matched pair of Sanyo transistors for Q11 and Q12 -

2SD1061(S) - Q11, VCBO 60V, VCEO 50V, VEBO 6V, IC 7A, PC 40W
2SB825(S) - Q12, VCBO -60V, VCEO -50V, VEBO -6V, IC -7A, PC 40W

The schematic I have specifies higher spec Toshiba parts -

2SD1148(O) - Q11, VCBO 140V, VCEO 140V, VEBO 5V, IC 10A, PC 100W
2SB863(O) - Q12, VCBO -140V, VCEO -140V, VEBO -5V, IC -10A, PC 100W

Unfortunately these are no longer available so I've looked in the Toshiba catalog which recommended 2SA1941 as a replacement for 2SB863 but that is also in short supply and looks like the replacement for that is the 2SA1943.

So I've ordered the matched pair of these from Fairchild/onsemi, hopefully as they are better than the original specification they should be OK?

2SC5200O - VCBO 250V, VCEO 250V, VEBO 5V, IC 17A, PC 150W
2SA1943O - VCBO -250V, VCEO -250V, VEBO -5V, IC -17A, PC 150W

Would the 2SC5200O also be a good replacement for Q10?

Q10 - 2SC2565 - VCBO 160V, VCEO 160V, VEBO 5V, IC 15A, PC 150W

David
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Old 29th Sep 2021, 10:59 pm   #7
D4rkS4v4nt
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Default Re: Otari MTR-10 Power Supply Issue

So whilst I was waiting for my new pass transistors to arrive I did some more diagnostics as I wanted to be sure that I didn't nuke the new transistors in the same way that the existing ones.

The pass transistors Q11 and Q12 are externally mounted on the rear heatsink and connected via a cable and plug. I'm also replacing Q10 as it is a non-original transistor made by iscsemi who have a mixed reputation online. Q10 is also connected via a cable and plug. I disconnected Q10, Q11 and Q12 from the main board.

I then took voltage readings at various places around the -26V circuit and compared them to what was shown in the simulation. I found that the zener diode was reporting an incorrect voltage (-0.6V) and from the readings I found I concluded there was a short somewhere within regulation circuit preventing the reference voltage from operating. So I decided to replace the remaining transistors, Q105, Q107 and Q108 that I'd only tested so far. I tested each one after removal before replacing with a brand new replacement. Upon reaching Q105 I found that it had failed and appeared to have failed in an open state (when tested with a diode tester). I'm pretty sure I tested Q105 earlier, maybe it went pop when I soldered it back to the board?!?!

To recreate the exact issue in the simulator I had to short the collector to emitter so wonder if it was behaving differently when in-circuit.

After replacing all three I powered up the PSU and took the same voltage readings again and this time they were as expected! Now both +26V and -26V operation matches.

Problem solved
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Old 2nd Oct 2021, 11:44 am   #8
DMcMahon
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Default Re: Otari MTR-10 Power Supply Issue

Well done, good diagnostics.

David
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