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Author Topic: 1970 Standel SB30 troubleshooting/ death cap identification  (Read 473 times)

Ethernaut

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1970 Standel SB30 troubleshooting/ death cap identification
« on: October 23, 2021, 01:30:39 PM »
     Hello, all. Hoping someone here could assist w/ issues concerning a 1970 Standel Studio Bass 30. Some background: amp powered up and functioned upon purchase 3 mos. ago w/ two JBL K130s (non original, but 8 ohms ea., wired parallel so showing 4 Ohms). Has original 2 prong cord. I swapped two 8 Ohm Madison branded 15” in (JBls went to another rig), and powered it up via a lamp limiter which was plugged into a Variac at 120v. Bottom speaker sounded out, but not top. After about 15 seconds, it sputtered, produced no sound, power light flickered & died. I looked up to see that the bulb was bright & shut it down. Then I discovered a potential culprit:

-The top speaker was not connected

     I spoke w/ Danny McKinney, current owner of the Standel line, who is quite familiar w/ this amp and owns one himself. We sorted out that either:

A) Old caps finally gave way and it’s coincidental that this occurred w/ speaker change
B) The unattached speaker wires toutched & fried output transistors

I got the chassis out & without measuring any values (which I don’t know how to do- I have not progressed past basic safety, buying a variac, building a limiter and the ability to swap capacitors & upgrade to a 3 prong in an old Fender), I immediately saw a cap which appears shrunken & has white residue on it, which I’m hoping indicates that it has failed or is in the process of doing so, and that’s the true culprit. It is teal blue & connected to two tabs directly behind the power indicator bulb (I’d include a pic but can’t get one to upload). If I’m reading the schematic correctly (pdf link below), it’s a C14 .02 600V. Can anyone ID this component as the death cap? If it is, can I simply snip it out, remove the two pronger, solder in the leads of a grounded cord, ground to chassis & test again?

Another clue is that someone, at some point, wrote “short PC” on the side of the chassis- maybe this problem was known by a previous owner?

That was a large amount of words to pose a short question. All thoughts & advice greatly appreciated.

http://www.standelamps.com/support/schematics/documents/1970/custandstudiobass_service.pdf

Be good,
-Pete
« Last Edit: October 23, 2021, 08:03:15 PM by Ethernaut »

Loudthud

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #1 on: October 23, 2021, 10:00:53 PM »
The link you posted does not download completely for me. From what I can see, the death cap is C2, .02uF 600V. One side goes to ground, the other to one side of the power switch.

Enzo

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #2 on: October 24, 2021, 12:58:35 AM »
Link worked for me, four pages.

Ethernaut

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #3 on: October 24, 2021, 08:37:52 AM »
Well awright then. I’m gonna make a cap discharge tool, pluck that thing, change out the cord, test & report back.


g1

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #4 on: October 24, 2021, 01:35:25 PM »
You are correct in that C16 would be the death cap in units equipped with the polarity switch.
But according to the notes, only the PA6 had that switch, so it should be C2 (pg.3) as loudthud stated.

Attaching schematic as the link gave me a bit of trouble as well.
« Last Edit: October 24, 2021, 01:42:19 PM by g1 »

Ethernaut

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #5 on: October 26, 2021, 10:08:00 PM »
So.
Good news.
Death cap is out, three pronger in & testing for continuity. I went to test & everything functioned as normal. Then I realized that I’d mistakenly plugged directly into the variac, and not through the lamp limiter as intended. Shut down & powered up through limiter- amp did the loud hum, pass no other signal, no power light business again. Now, I’ve used this limiter with a ‘65 Super Reverb and a Univox U-1246 bass head with no problems. They each run just like they’re plugged into the wall. Not the Standel.

Is there any explaination for this? Does the limiter not allow enough current to flow to this amp in particular? If I hadn’t previously used it successfully with two other devices, I’d question my wiring.

Enzo

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Re: 1970 Standel SB30 troubleshooting/ death cap identification
« Reply #6 on: October 27, 2021, 12:02:32 AM »
The bulb is intended for testing.  It lets you apply power to the system while protecting it in case of excess current draw, which lights the bulb instead of burning up the amp.   This process does limit the mains voltage some and that is a problem for operation.   But the bulb unit is not intended for operating the amp.  Once we know the unit will work without blowing fuses, we ditch the bulb.

Then there is the variac,  if you dial it up slowly, the amp passes through unstable periods at the reduced voltage.   and with a speaker load, it won't always settle down.

You worked on the amp, fired it up, and it worked.   Then oddly you decided to fix it some more and put it back under test conditions.   The amp works, now is the time to go over it and make sure no adjustments remain to be made under real operating conditions.