Wobbulator Woes

[Refugee]

They can be replaced with electrolytic types in most places. With RF gear there might be some value in adding a ceramic SMD part between the legs on the print side of the board.

[Craig]

Yes there are 27 tantalum caps according to the parts list - that's the bad news.   However, slightly better news is 24 of them are the same value so a wholesale replacement will be that bit easier.  Caps on order.

Cheers,   John

I agree with Refugee's comment above (see below)

Tant beads tend to fail (usually shorted) only in two circumstances

1. When used as a local decoupling capacitor on a power supply rail.
2. If the voltage rating is close to the rail; so 16V on a 15V rail or 6V on a 5V rail.

When used as signal level capacitors, they really don't tend to fail at all.

Unfortunately 2. above was very common because the apps notes for those capacitors suggested those voltage ratings, I recall. I too designed using those voltages back in the day.

I tend to now replace the ones on power rails with aluminium electrolytic. Back in the 70's and 80's tantalum had a lower ESR that aluminium electrolytic. But that is no longer the case, because very high performance parts are available driven by requirements of switched mode supplies.

But since you have ordered replacement Ta ones, I hope you have chosen a higher voltage headroom - so for example 25V on 15V rail and 10V on 5V rail.

[Radio Fixer]

Seems far too complicated for just old radio work to me. I might not know whether it was the test gear or the radio that wasn't giving me the result I wanted.

But GL with it John.

Gary

[Geaney]

Wholesale replacement is probably over the top since the main supplies are +/- 15v and most of the caps are all rated 35v.  There is one however that is rated 6v on a +5v rail so I'll certainly upgrade that one - and there are two rated at 16v which also get the upgrade treatment.   I'm assuming the 24 others are de-coupling so have plenty of headroom.  Thanks for the info on the failure modes of these caps.

Cheers,  John

[ppppenguin]

I still wouldn't trust old tants as decouplers, even if well rated. It's just easier to replace the lot with ordinary electrolytics while you've got the thing apart.

[Craig]

I'm 100% with you on that Jeffrey. Had too many bead tants fail in Tektronix gear often with smoke and chaos.....

[Geaney]

Short Update.   Started at the power supply - there are four rail pairs +/-5v, +/-15v, and +/-25v.  Both the 5v and 15v rails were low - about 4.7v and 14v respectively.  According to the manual the tolerances are very tight so 15v has to be within +/- 0.04v.   I managed to adjust it to the right level.  There is no adjustment on the +5v - it come straight out of a 7805.  There is however a tantalum between input and ground so I replaced it and to my surprise 5v is now spot on.   The 15v had then risen to 16v so I adjusted is back to the correct level.  I tested the old cap in my Chinese tester and the value showed fine but ESR was high at just under 2 Ohms.

There is no + or - 25v.   Transformer output looks good, diodes look good, clean 35v on -25v side electrolytic but lots of ripple on the cap for the +25v side.   Will need to order up some electrolytics.   On closer examination this 25v circuit has signs of extensive work done in the past so I guess I'm walking in someone else's footsteps at this stage.   Hopefully I'll get further than he/she did!

Cheers,

John


        

[Geaney]

Some progress:  +25v had a dead short on it.  Turned out to be the third tantalum I replaced.  Having had another tantalum dragging down the +5v I figured I'd bite the bullet and gunshot the lot of them.   The forest of red caps is now replaced by a forest of mustard coloured caps!

Time for a cup of tea and a biscuit 


Cheers,   John

[Geaney]

Looks like those two tantalums were the only problem.  The device works fine now - at least for the basic functions I will need.  It has lots of pulse modes that doubt I'll ever need but I can read up on them and check them later on.   One wafer switch is still a bit dodgy but hopefully a few more rounds of contact cleaner and it will sort itself out.

I'll leave it running for a few hours before I put it back in its case then hopefully that's a nice addition to the test gear for the workshop.

Cheers,   John

[Radio Fixer]

Nice job John: are we going to see some traces from a radio soon?

IF bandwidth, FM discriminator etc

Gary