Issue 2 TR4 keeps blowing and DRAM is tested OK

[zx64]

Yes very interesting info about ULA, I agree.

My ULA doesn't get hot very much, only slightly warm.

[1024MAK]

ULA info, points to keep in mind...

The ULA uses bipolar transistor technology, whereas modern CPUs use modern ‘CMOS’ technology. Bipolar transistor technology is much more power hungry compared to CMOS technology.
The ULA technology of this period, requires higher internal switching currents for faster speed operation. Lower power ULA types were available for low speed applications.

The ULA is working well within the temperature and electrical power ratings of the DIL package used. Semiconductor junctions can work without problems at between 100°C and 125°C. So what may burn your finger may be normal for the chip. The biggest problem is that the 7805 voltage regulator heats up the inside of case, a case with not much airflow.

The ULA type used in later board versions were a different series, they do run considerably cooler compared to the first type that was used.

The ULA does not run at just 4MHz. The primary function of the ULA is the video system. That’s a high speed speed system. Hence the main oscillator is 14MHz... (although note that this is divided down internally to lower speeds).

Mark

[1024MAK]

Back to my topic, I think I have found the problem with the constantly blowing TR4 transistor. I was using a ZTX450 which is only rated at 1A and of course it should have been a ZTX650 or ZTX651 rated at 2A I feel rather embarrased actually as I should know better. I still don't understand why it would blow when the vanilla spectrum only uses around 750mA, it is still within the operating range for a ZTX450.

I read somewhere that the ZTX450 was a suitable replacement for transisor TR4 as the wattage is the same. I guess the HFE and bias voltages make a difference also in the boosting circuit as I compared both on my transistor tester and they are not the same.

When comparing transistors in this kind of application, there are many parts of the specification and characteristics that have to be looked at. One important point is that with DC-DC converters, the switching currents are very different to the input supply, or output currents. In fact, the switching currents can be considerably higher. This is also part of the reason that the electrolytic capacitors in this part of the ZX Spectrum are critical to normal operation.

Also when switching an inductor (coil, transformer), when the magnetic field collapses, high reverse voltages will be generated.

You also want a transistor that has a high gain at a high current, and which has very fast switch on and importantly, a very fast switch off speed. If the transistor is too slow, it will overheat. Hence selection of the correct transistor is critical. ZTX651 and ZTX653 transistors are still in production, and are the only recommended replacements for TR4 (Sinclair used the ZTX650 which is a lower voltage version of the ZTX651, and which is obsolete).

Mark

[rimmeruk]

Also when switching an inductor (coil, transformer), when the magnetic field collapses, high reverse voltages will be generated.

You also want a transistor that has a high gain at a high current, and which has very fast switch on and importantly, a very fast switch off speed. If the transistor is too slow, it will overheat. Hence selection of the correct transistor is critical. ZTX651 and ZTX653 transistors are still in production, and are the only recommended replacements for TR4 (Sinclair used the ZTX650 which is a lower voltage version of the ZTX651, and which is obsolete).

Mark

Yes, seems it is important that the transistor can handle the higher reverse current as > 1A is being generated when the boost circuit is starting because that was the only time the transistor was blowing.