Tektronix 2221 Oscilloscope Repair

[Allen49]

I have a Tektronix 2221 that I am trying to repair. The previous owner stated he was using the scope when he heard a pop and something started smelling burnt. I found that R909 was burnt and also one of the legs on Q9070, IRF710, was burnt off. The IRF710 was placed on a heat sink with a piece of heat sink tape but I don't know if it was ground through the center pin to the heat sink. Could anyone help me out with this.

[Alex_Baker]

I don't have a ton of experience with this but I think grounding the center pin to the heatsink is generally avoided outside some edge cases. If there are other transistors on the same heatsink I think it is safe to assume that it should not be.

[BlownUpCapacitor]

I'm not sure if this applies to the 2221 specifically, but I know some 2200 series scopes had to be modified because of parts in the PSU prematurely failing, so tek sent out mod kits to replace some transistor and rectifiers because they couldn't handle the stress.

Also general rule of thumb if something smokes in a 2200 series scope, check the RIFAs.

[TERRA Operative]

Check the 2235 tekwiki page, there are pdf files with details on PSU mods (I cross referenced the parts to modern part numbers).
It may be close enough to apply.

https://w140.com/tekwiki/wiki/2235

[p.larner]

Is that a pre regulator like in the tek2215? scope?.

[BlownUpCapacitor]

Is that a pre regulator like in the tek2215? scope?.

The Tek 2221 was introduced 6 years later than the 2215, so I would assume that the 2221 has a PSU that already fixes what the pre-regulator board in the 2215 was improving. I think this is further supported by the fact neither the 2235 or the 2221 has a pre-regulator board option available.

[tppc]

Hi there,

I found that R909 was burnt and also one of the legs on Q9070, IRF710, was burnt off. The IRF710 was placed on a heat sink with a piece of heat sink tape but I don't know if it was ground through the center pin to the heat sink. Could anyone help me out with this.

Assuming that the PSU on the 2221 is the same as the 2221A, I would say that the center pin of Q9070 must be isolated from the heatsink.

If Q9070 and R909, blew up, you can for sure replace U930 and test all transistors around it.

Best Regards.

[tppc]

BTW,
IRF710 (rated 400V) is not a suitable replacement for Q9070.

Edit :
(or maybe 400V is good enough if your mains voltage is 110V which seems to be the case)

The SM indicates Q9070 = MTP6N60 (rated 600V)

[fmashockie]

The 2221 is very similar to the 2230 which I own.  In the 2230, Q9070 is on its own daughter board and the TO-220 tab is attached to chassis frame; it is then connected to the mainboard via connector W9070.  It looks like there's a sheet of insulating tape underneath the tab. The schematic that tppc shared of the 2221 is exactly the same for the 2230.  You can see that drain for Q9070 is not chassis ground otherwise it would be indicated in the schematic as such.  Therefore, the TO-220 tab is not grounded. 

Here's a link to some troubleshooting tips for PSU repair for the 2221 series https://www.hakanh.com/dl/docs/troubleshooting_tips_on_2200_ps.pdf

[BlownUpCapacitor]

The 2221 and 2230 are sister models. I think the only difference between the 2230 and 2221 is that the 2230 has a higher bandwidth and a dual time-base.

[tppc]

Service manual with schematics for the 2221 can be found here : https://w140.com/tekwiki/wiki/2221

The snapshot in my previous post was from the 2221A which I own.

[bdunham7]

The 2221 and 2230 are sister models. I think the only difference between the 2230 and 2221 is that the 2230 has a higher bandwidth and a dual time-base.

Sort of.  The 2221 actually came in both 60MHz and 100MHz versions.

[BlownUpCapacitor]

Sort of.  The 2221 actually came in both 60MHz and 100MHz versions.

True, but the 100MHz version was the A model, which is comparable with the 2232, with 100MHz and 100Ms/s, only without the dual time-base and waveform reference button.

I'm not too sure how much later the A model was released, as w140 doesn't specify.

[bdunham7]

True, but the 100MHz version was the A model,

No, there is a 2221A which is 100MHz (I have one) but there was also a 2221 100MHz.  Apparently the last year of 2221 production was 100MHz and it was not simply an unlabelled 'A' version because the 2221A has significant differences in the digitizing system.  This can lead to some issues when trying to repair these as there are enough differences between the models to matter. The 2221 100MHz is almost the same as the 2221 60MHz (and I haven't seen a separate schematic or manual for it) but not the same at all as the 2221A.

[BlownUpCapacitor]

This is interesting. I didn't know this.

Thanks for the correction.