SHP3 & new meter blues

Well.. Got to the point of setting bias, and instead of measuring voltage I set the meter for a capitance test. I let the smoke out of a few transistors feeling kinda silly.

Doug,
OK--50 lashes with a wet noodle !! Don't feel too badly--you AINNA, the first one to do something like this!!
Ron Rich

..Are you sure it wasn't the dreaded dead spots on bias pots?

Rob

Thanks guys -

Im sure its not the bias pots, I replaced those with 15 turn cermets. I'm afraid it's caused from pure boneheadedness

Also, it was on a dim bulb tester, but not even powered on. Basically, when I accidentally applied voltage to the bases of Q15 & Q16, it zapped Q12 Q13 at the least. It also opened up Q15 & Q16 - the test provides far in excess of breakdown voltage. To be safe I will remove them all from circuit and test them for leakage an replace the damaged ones.

Think I also found an error in Tony's book that I haven't seen listed in errata -

Page 54 Q12, Q13, Q22 & Q23 are listed as part# 309687. On page 55 they are crossed but incorrectly identified as part# 309867.

Hi Doug,
What is a "15 turn cermet", and where can they be obtained ??
I added the correction to the above error info, plus one more--If you find any more, just add them above, where they hopefully will be found ! Knowing Tony, he would want it done !! Ron Rich

Ron, it a material used in this case to construct precision potentiometers. The number 15 just states the range of mechanical adjustment. These are typically used in small-signal gain trimming where a wide variation must be accommodated.

It is -way- overkill here...but if it makes the owner happy.........

https://en.wikipedia.org/wiki/Cermet

Rob

Rob & Ron, in Tony's book, page 24, figure 17

"Replace the wire wound pot with a cermet unit of comparable resistance"

One paragraph up he states "it is therefore highly recommended" to do this. I agree it's overkill, but I figured he had a good reason to put it in there.

Hi Doug,
Yep--attza eggzactly whut he rote !!
Where did you purchase it/them ??
Ron Rich

Got mine from eBay, Amazon has them as well "bourne 15 turn cermet" . I got them from eBay because I didn't have a big enough order for Mouser or Digikey, and MCM electronics doesn't carry them. I used Bourne 3006, which is a 100ohm 15 turn variant. Think it was $7 for 5 of them or close to it.

Doug,
Thanks--I am familiar with Bourne brand, "pots"--just did not realize they were "cermet" pots--never saw / heard that word before--
Ron Rich

Most welcome

FWIW: my past approach has been to loom a resistor with a value equal to a too high bias figure (tube or transistor) across the adjustment pot. Then, if the pot opens the bias will not go totally out of range.

In cases where readjustment is unlikely to be needed I simply place a fixed resistor at/near the desired value.

Bias pots are one of the most potentially troublesome areas -especially in consumer grad electronics. It is a shame that so many otherwise decent designs use the cheapest possible parts in this area. Those used in Seeburg amps where not the worst in this regard, but given the often filthy conditions they were exposed to a sealed pot, or even fixed resistor would have been preferable.

Rob

OK... I removed the preamp and driver pcb's and still had a short, so tonight I plan to test the 4 power supply diodes (probably just replace them with 1N4005 600v variants) , then remove the 4 2N3055A output transistors at Q17, Q18, Q27 & Q28 then test again. I figure I need to get the power supply humming again and outputting my +32V, -32V and +23V sources before I do anything else.

Looking at the schematics, a shorted Q17, Q18 ,Q27 or Q28 (or, obviously a bad rectifier diode) would drag the supply down regardless of weather or not the preamp or driver boards were installed or not, so I think it's a good bet that's where my problem lies.

Thoughts?

-The 23VDC will read much higher if no pre card is installed to provide load.

-I suggest using a 40 watt lamp as current limiting for test.

-Don't be misled by the fact that a short persists even without the driver bd. There may still be a shorted driver.

-You have probably figured out all the above so...carry-on.

Rob