2230 PSU Replacement Component Choices


alastair.knights@...
 

I’d be grateful for comments on my component choices below, which are for an upgrade of my 2230 PSU as per Tek’s Product Modification 060-2239-03 (‘Q935…Q9070 Replacement’). Although my scope works, I’m wary given some of the cautionary tales told in this group and the early SMPS design concepts used and improved on by Tek over the years.

The Tek modification involves replacing most of the critical semiconductors in the pre-regulator, crowbar and inverter, so correct component choice is vital. Your comments and the reasons behind the advice would be welcome. NB In the UK the mains can be up to 245Vac, which affects component voltage ratings in the pre-regulator.

Circuit Ref, ‘Tek description’, originally specified (as fitted in italics) part, my suggested replacement, followed by my reasons for selection:

Q935, ‘Thyristor, SCR 8A, 200V sens gate, TO-220’, C10682X283 (GE C10682), ST Micro TS820-600T – An old GE datasheet for the C106 series lists the ‘On’ condition with 1mA of gate current and the TS820-600T has Ig and Vg as 0.2mA and 0.8V respectively, together with higher voltage and current ratings. NB R935 will shunt the first 6mA of any leakage in VR935 and CR948 before Vg reaches 0.8V.
Ref. - https://www.st.com/resource/en/datasheet/ts820.pdf

Q946 and Q947, ‘Transistor NPN, 50V, 150mA, 200mW, inverter’, SJE389, On Semi MJE15032G – The manual shows 80Vp-p swings at Q947c and the later TIP31C fitted was rated at 100V and 3A. The MJE1503G is rated at 250V for extra margin and has a greater SOR (safe operating area) than the TIP31C (some obvious replacements don’t). The hFE is a bit higher at 100 rather than 40 for the TIP31C but as close as I could find while having the higher voltage and SOR rating. It’s not obvious how or if a higher hFE would affect operation of the inverter and I can’t model it, hence my caution. Ft is higher at 30MHz but hopefully that won’t be a problem.
Ref. - https://www.onsemi.com/pdf/datasheet/mje15032-d.pdf

Q9070, ‘MOSFET, N-channel, TO-220’, STP3000 (Motorola 8630), Toshiba TK10E60W – The Toshiba TK10E60W has higher voltage and current ratings, while keeping the Rds On, gate capacitance and gate charge below that of the IRF730 Tek suggest as a replacement. The gate threshold voltage is within range and modelling in LTSpice suggests it should work well.
Ref. - https://toshiba.semicon-storage.com/info/docget.jsp?did=13500&prodName=TK10E60W

VR935, ‘Zener, 51V, 5%’, SZG35009K7 (1N978B), Nexperia BZX79-C51, 143 - It is not obvious why the original is being replaced, but the mod sheet says all listed components should be replaced. A low power 500mW Zener is indicated to ensure that the full 51V develops when a few mA passes through it. My PSU already has a 1N978B fitted, so I’m strongly tempted to leave it in, given that it shouldn’t be under stress and the available replacements don’t appear to offer an advantage in performance or reliability.
Ref. - https://assets.nexperia.com/documents/data-sheet/BZX79.pdf

CR907 (and CR906), ‘Rectifier diode, silicon, 400V, 1.5A, 50ns’, DSR3400X (BDY73), Vishay MUR460 – The Tek modification sheet recommends 2 x BYD73G in parallel, which makes sense given their rating. LTSpice modelling shows currents of around 1.7A ave and 1.8A rms in CR907. I have a single BDY73G fitted which sounds unhealthy, so the MUR460 looks better with its 4A and 600V ratings, together with a short 50ns recovery time.
Ref. - https://www.vishay.com/docs/88686/mur440-e3.pdf

Finally, I’m reluctantly replacing all the electrolytics in the PSU for safety. While the existing capacitors all read okay in terms of value, leakage and ESR, I’d rather not take the risk given the early nature of the design and known component stress in places. I’m also replacing C907 (1uF non-polarised) as modelling shows that this takes a lot of punishment – 1.2A during the ‘on’ period with 245Vac mains – and failure would break a lot of expensive parts!

Everything else is being kept original. I also have a parts list for Mouser is this is of use to anyone.

Sorry for the long post and thanks in advance for your comments.

Alastair Knights


Bert Haskins
 

On 3/24/2021 12:18 PM, alastair.knights@gmail.com wrote:
I’d be grateful for comments on my component choices below, which are for an upgrade of my 2230 PSU as per Tek’s Product Modification 060-2239-03 (‘Q935…Q9070 Replacement’). Although my scope works, I’m wary given some of the cautionary tales told in this group and the early SMPS design concepts used and improved on by Tek over the years.

The Tek modification involves replacing most of the critical semiconductors in the pre-regulator, crowbar and inverter, so correct component choice is vital. Your comments and the reasons behind the advice would be welcome. NB In the UK the mains can be up to 245Vac, which affects component voltage ratings in the pre-regulator.

Circuit Ref, ‘Tek description’, originally specified (as fitted in italics) part, my suggested replacement, followed by my reasons for selection:

Q935, ‘Thyristor, SCR 8A, 200V sens gate, TO-220’, C10682X283 (GE C10682), ST Micro TS820-600T – An old GE datasheet for the C106 series lists the ‘On’ condition with 1mA of gate current and the TS820-600T has Ig and Vg as 0.2mA and 0.8V respectively, together with higher voltage and current ratings. NB R935 will shunt the first 6mA of any leakage in VR935 and CR948 before Vg reaches 0.8V.
Ref. - https://www.st.com/resource/en/datasheet/ts820.pdf

Q946 and Q947, ‘Transistor NPN, 50V, 150mA, 200mW, inverter’, SJE389, On Semi MJE15032G – The manual shows 80Vp-p swings at Q947c and the later TIP31C fitted was rated at 100V and 3A. The MJE1503G is rated at 250V for extra margin and has a greater SOR (safe operating area) than the TIP31C (some obvious replacements don’t). The hFE is a bit higher at 100 rather than 40 for the TIP31C but as close as I could find while having the higher voltage and SOR rating. It’s not obvious how or if a higher hFE would affect operation of the inverter and I can’t model it, hence my caution. Ft is higher at 30MHz but hopefully that won’t be a problem.
Ref. - https://www.onsemi.com/pdf/datasheet/mje15032-d.pdf

Q9070, ‘MOSFET, N-channel, TO-220’, STP3000 (Motorola 8630), Toshiba TK10E60W – The Toshiba TK10E60W has higher voltage and current ratings, while keeping the Rds On, gate capacitance and gate charge below that of the IRF730 Tek suggest as a replacement. The gate threshold voltage is within range and modelling in LTSpice suggests it should work well.
Ref. - https://toshiba.semicon-storage.com/info/docget.jsp?did=13500&prodName=TK10E60W

VR935, ‘Zener, 51V, 5%’, SZG35009K7 (1N978B), Nexperia BZX79-C51, 143 - It is not obvious why the original is being replaced, but the mod sheet says all listed components should be replaced. A low power 500mW Zener is indicated to ensure that the full 51V develops when a few mA passes through it. My PSU already has a 1N978B fitted, so I’m strongly tempted to leave it in, given that it shouldn’t be under stress and the available replacements don’t appear to offer an advantage in performance or reliability.
Ref. - https://assets.nexperia.com/documents/data-sheet/BZX79.pdf

CR907 (and CR906), ‘Rectifier diode, silicon, 400V, 1.5A, 50ns’, DSR3400X (BDY73), Vishay MUR460 – The Tek modification sheet recommends 2 x BYD73G in parallel, which makes sense given their rating. LTSpice modelling shows currents of around 1.7A ave and 1.8A rms in CR907. I have a single BDY73G fitted which sounds unhealthy, so the MUR460 looks better with its 4A and 600V ratings, together with a short 50ns recovery time.
Ref. - https://www.vishay.com/docs/88686/mur440-e3.pdf

Finally, I’m reluctantly replacing all the electrolytics in the PSU for safety. While the existing capacitors all read okay in terms of value, leakage and ESR, I’d rather not take the risk given the early nature of the design and known component stress in places. I’m also replacing C907 (1uF non-polarised) as modelling shows that this takes a lot of punishment – 1.2A during the ‘on’ period with 245Vac mains – and failure would break a lot of expensive parts!

Everything else is being kept original. I also have a parts list for Mouser is this is of use to anyone.

Sorry for the long post and thanks in advance for your comments.

Alastair Knights
Hi Alastair
FWIW
I have 2221A 2230, 2232 2235 versions 0f the 22 series Teks.
The only PS failure in any of these has been the line input filter, three so far and counting.
These scopes have only been used on US power.

The only other chronic problem that I've had with these is the focus resistors.


In the past I have had many! PS problems in the 2213, 2215, and the 'A' versions of these models.

YMMV.

Bert


alastair.knights@...
 

Hi Bert,

Thanks for supplying your experience. Yes, I may be going overboard, but the long repair stories on the EEV Blog bring out the preventative engineer in me!

The RIFAs capacitors in my input filter already have cracks (are will be replaced), and an open-circuit focus chain resistor was the original fault when I got the scope.

Thanks again,

Alastair


Bert Haskins
 

On 3/24/2021 5:25 PM, alastair.knights@gmail.com wrote:
Hi Bert,

Thanks for supplying your experience. Yes, I may be going overboard, but the long repair stories on the EEV Blog bring out the preventative engineer in me!

The RIFAs capacitors in my input filter already have cracks (are will be replaced), and an open-circuit focus chain resistor was the original fault when I got the scope.
When I replace these resistors I replace the high value ones with two resistors of half the value in series to split the voltage drop.

They seem to last longer that way.


Thanks again,

Alastair




alastair.knights@...
 

I thought I ought to answer my original question in case it helps others. Back in March 2021, I asked for advice about suitable replacement parts for a 2230 PSU and based on tests, the answers seem to be:

Q935, ‘Thyristor, SCR 8A, 200V sens gate - I tried using a ST Micro TS820-600T as a replacement, but it triggered unexpectedly and burn out, so this device may be too sensitive. So, I kept the old GE C10682 part in service.

Q946 and Q947, ‘Transistor NPN, 50V, 150mA, 200mW, inverter’ - I tried using On Semi MJE15032G as replacements, but the inverter would not oscillate reliably or continuously. It seems likely that hFE and possibly capacitance are major factors in whether the inverter oscillates. My MJE15032Gs have an hFE=96 while the original transistors are only hFE=48. Once again the original transistors were retained.

Q9070, ‘MOSFET, N-channel, TO-220’ - I used a Toshiba TK10E60W as a replacement and this worked fine. I also bolted a small finned heatsink onto the back of the main mounting plate using the Q9070 fixing screw and this lowered the temperature of the mounting plate by around 10 degrees C (max temp was 46C after 30mins).

(VR935, ‘Zener, 51V, 5%’, SZG35009K7 (1N978B), There was no need to replace this as it only needs replacing to make room for the revised SCR location described in Tek Product Modification 060-2239-03.)

CR907 (and CR906) - I used a MUR460 as a replacement but the leads are too large to fit in the holes on the PCB. However, binding the thick MUR460 leads onto the stumps of the old diode is a simple fix (please don't drill out the THP hole!)

Apart from the RIFA caps in the Line Input Filter, the only other component that I felt needed replacing was C907, which has to pass very high ripple currents of many amps. Despite the original being a high-quality TRW film capacitor, given its 35 years of service and critical location, I replaced it with a 630V Panasonic ECQE6105JF cap.

Hope this helps someone as previous postings have helped me.

Alastair Knights