48k Issue 4b - TR4 shorting, 4116 all removed

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lrochfort
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Joined: Fri Jun 21, 2024 5:31 pm

48k Issue 4b - TR4 shorting, 4116 all removed

Post by lrochfort »

Hello all,

I had an edge connector mishap that shorted ZTX650 transistor TR4. Immediately afterwards all voltages at the 4116 RAM were incorrect.

Subsequently I have:
  • Replaced the 7805 regulator
  • Replaced scorched D15 and D17
  • Replaced TR4 and TR5
  • Removed all 4116 RAM
Powering up with the 4116 RAM removed immediately shorts TR4.

Some voltage measurements with all 4116 and TR4 removed:
  • C34: 4.88v
  • C50: 11.3v
  • C52 LT: 1.7v
  • IC6 P1: -200mV
  • IC6 P8: 1.7v
  • IC6 P9: 4.86v
My multimeter doesn't do manual resistance range, but auto mode shows 0.3 ohms across each side of the coil.

Any suggestions as to what to check next would be much appreciated.
Rabs
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by Rabs »

Does anything from my previous repair help?

[https://www.sinclairzxworld.com/viewtop ... 805#p54805]
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1024MAK
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by 1024MAK »

For others following this, the OP has also posted on Spectrum Computing forums here.

With the power disconnected and TR4 removed, test the resistances as in the list below:
  • R60, R61, R62,
  • Between the pad for TR4 collector and 0V/GND
  • Across C44
  • Across C45
  • Between 0V/GND and edge-connector contact 20 upper
  • Between edge-connector contact 23 upper and 0V/GND
A photo of your board showing an in focus close up of the circuitry around TR4, TR5 and D15, D17 etc. would be useful.

Other points

It’s possible that the coil has shorted turns. However, a multimeter will not show this.
Double check the connections and polarity of TR5, D15 and D17.

What type of transistors are you using for TR4 and TR5? What type of diodes are you using for D15 and D17?

Do you have a DC Power supply that has variable/adjustable output voltage and an variable/adjustable current limit?

Mark
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lrochfort
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by lrochfort »

Many thanks for the help. Please find replies inline.
1024MAK wrote: Fri Jun 28, 2024 11:21 pm
With the power disconnected and TR4 removed, test the resistances as in the list below:
  • R60, R61, R62,
  • Between the pad for TR4 collector and 0V/GND
  • Across C44
  • Across C45
  • Between 0V/GND and edge-connector contact 20 upper
  • Between edge-connector contact 23 upper and 0V/GND
  • R60: 217 ohms
  • R61: 15 ohms
  • R62: 15 ohms
  • Between the pad for TR4 collector and 0V/GND: 11M Ohms
  • Across C44: 2.8k Ohms
  • Across C45: 2.8k Ohms
  • Between 0V/GND and edge-connector contact 20 upper: Inconsistent. Sometimes 1.2M Ohms, other 5M Ohms
  • Between edge-connector contact 23 upper and 0V/GND: Inconsistent. Sometimes 30M Ohms, other overload
A photo of your board showing an in focus close up of the circuitry around TR4, TR5 and D15, D17 etc. would be useful.

Other points

It’s possible that the coil has shorted turns. However, a multimeter will not show this.
Double check the connections and polarity of TR5, D15 and D17.
I'm getting unreliable readings in-circuit for these. I'll be able to de-solder them and test on Tuesday.
What type of transistors are you using for TR4 and TR5? What type of diodes are you using for D15 and D17?
  • TR4: ZTX650
  • TR5: ZTX213 (That's what was in the board. The schematic says ZTX315
  • D15 and D17: B157
Do you have a DC Power supply that has variable/adjustable output voltage and an variable/adjustable current limit?
Yes
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1024MAK
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by 1024MAK »

lrochfort wrote: Sat Jun 29, 2024 11:13 am
  • R60: 217 ohms
  • R61: 15 ohms
  • R62: 15 ohms
  • Between the pad for TR4 collector and 0V/GND: 11M Ohms
  • Across C44: 2.8k Ohms
  • Across C45: 2.8k Ohms
  • Between 0V/GND and edge-connector contact 20 upper: Inconsistent. Sometimes 1.2M Ohms, other 5M Ohms
  • Between edge-connector contact 23 upper and 0V/GND: Inconsistent. Sometimes 30M Ohms, other overload
These results don't show up any items of concern.
lrochfort wrote: Sat Jun 29, 2024 11:13 am
1024MAK wrote: Fri Jun 28, 2024 11:21 pm Double check the connections and polarity of TR5, D15 and D17.
I'm getting unreliable readings in-circuit for these. I'll be able to de-solder them and test on Tuesday.
No, this is a visual check, not me asking you to test (although some multimeters should be able to test in circuit diodes).
lrochfort wrote: Sat Jun 29, 2024 11:13 am
  • TR4: ZTX650
  • TR5: ZTX213 (That's what was in the board. The schematic says ZTX315
  • D15 and D17: B157
A ZTX650 for TR4 is fine (or a ZTX651 or a ZTX653 are also okay).
Yes, ignore the ZTX315 on the schematic, that's an error. A ZTX213 is fine. Various other small signal PNP transistors can be used, but their orientation may differ when fitting to the board.
The diodes should be BA157. Again, the schematic has errors.

Mark
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lrochfort
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by lrochfort »

Thanks very much, Mark.

So you think at this point it sounds like I should investigate the coil?
Last edited by 1024MAK on Sat Jun 29, 2024 7:41 pm, edited 1 time in total.
Reason: Quote removed
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1024MAK
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by 1024MAK »

Next, my suggestion would be to use your DC PSU set to +12V with say, a current limit of 100mA.
Connect the negative/0V output to the ZX Spectrum 0V/GND.
Connect the +V output via your multimeter (set to a suitable DC current range, say 1A or higher to allow for any in rush current) to the positive (+) lead of capacitor C44.

The ULA, the 4116 (lower) DRAM and TR4 should NOT be fitted. And there is no need to power up the ZX Spectrum via its normal power supply.

Please report back on what current the board actually draws.

Mark
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lrochfort
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by lrochfort »

1024MAK wrote: Sat Jun 29, 2024 7:40 pm Next, my suggestion would be to use your DC PSU set to +12V with say, a current limit of 100mA.
Connect the negative/0V output to the ZX Spectrum 0V/GND.
Connect the +V output via your multimeter (set to a suitable DC current range, say 1A or higher to allow for any in rush current) to the positive (+) lead of capacitor C44.

The ULA, the 4116 (lower) DRAM and TR4 should NOT be fitted. And there is no need to power up the ZX Spectrum via its normal power supply.

Please report back on what current the board actually draws.

Mark
It draws 0.03A
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1024MAK
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by 1024MAK »

lrochfort wrote: Tue Jul 02, 2024 3:32 pm It draws 0.03A
So, a current of 30mA is fine. There is no short circuit in the DC side of the +12V rail. The fault is within the DC/DC converter circuitry.

Mark
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Re: 48k Issue 4b - TR4 shorting, 4116 all removed

Post by 1024MAK »

So next, check and test resistors R58 (1k) and R59(1.8k). Then transistor TR5. If D11, D15, D17 and D18 test okay, then it is indeed looking like either a short that's bypassing the coil, or the coil itself.

To fully test the coil, an ac signal is needed as well as a 'scope. But I've never needed to test one, so can't tell you much more. With a multimeter, you can test between the two windings to check that they are not electrically connected.

Original coils are no longer mass produced. But a while ago I noticed some rewound ones on eBay. It is also possible to rewind it yourself. You will have to measure the wire diameter, note the direction of each winding and the number of turns as you disassemble it. Buy the same enamelled copper wire (also known as transformer wire) and wind on the same number of turns in the same direction. The correct direction is very important.

Mark
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