SCA 35 trouble

Howdy from Roswell Ga, been registered for a bit , been spending my time here reading the sticky s and searching  for info
about a new to me SCA35. Its a factory wired unit that is in decent cosmetic shape and was operating when I got it.
So I had an idea to renew the filter caps and diodes which I did. So now it pops the fuse and in about 20 sec and the power
trans makes an odd sound like insects inside.

Based on info I got from this site, I started checking things with a multi-meter.  One of the diodes passes both ways and the other only measures .380 something one way and nothing the other way. I removed them to measure.

The power trans measures HV leads end to end 78 ohms. Nothing to ground. Center tap disconnected. If I test HV lead to
center tap, one wire is about 8 ohms and the other HV lead to center tap is 78 ohms.

I guess the near 60 year old power trans has bit the dust. I don't know if a bad diode did this or it was just bad timing.

Lucky for me I have on hand a new power transformer from Kevin at Dynakitparts, I am stealing it from another project
I haven't got around to yet. Bet you never heard of that before!! New diodes on the way.

Thanks for looking.

Junksleep wrote:Howdy from Roswell Ga, been registered for a bit , been spending my time here reading the sticky s and searching  for info
about a new to me SCA35. Its a factory wired unit that is in decent cosmetic shape and was operating when I got it.
So I had an idea to renew the filter caps and diodes which I did. So now it pops the fuse and in about 20 sec and the power
trans makes an odd sound like insects inside.

Based on info I got from this site, I started checking things with a multi-meter.  One of the diodes passes both ways and the other only measures .380 something one way and nothing the other way. I removed them to measure.

The power trans measures HV leads end to end 78 ohms. Nothing to ground. Center tap disconnected. If I test HV lead to
center tap, one wire is about 8 ohms and the other HV lead to center tap is 78 ohms.

I guess the near 60 year old power trans has bit the dust. I don't know if a bad diode did this or it was just bad timing.

Lucky for me I have on hand a new power transformer from Kevin at Dynakitparts, I am stealing it from another project
I haven't got around to yet. Bet you never heard of that before!! New diodes on the way.

Thanks for looking.

Remount the old diodes or a proper pair of 1n4007 or equivalent. Also replace the can cap anr recheck
the connections. Then restart with a current limiter or variac.

Going in this week with new power trans, Can caps, and diodes. Wish me luck!!

Junksleep wrote:Going in this week with new power trans, Can caps, and diodes. Wish me luck!!

I wish you find some kind of current limiter at the first start. This is to apply AC while observing voltages
and stop if anything suspect is found. A variac is best, a lamp in series could do.

I have ordered a variac!!

Hi Junksleep,

Sounds like you have a bad diode in the power supply. And a bad power transformer. Not sure which killed which...

I'm assuming that if you accurately measured the resistance of the diodes, with one giving you a .380 ohm resistance one way and nothing (reading open) the other way, this one is probably good. The second one reading the same resistance in both directions is shorted and thus is bad. I would replace both diodes. Likewise with the power transformer, reading an 8 ohm resistance from one HV lead to center tap on one side of the HV supply and 78 ohms on the other side says the power transformer is bad.

You indicated you had a new power transformer, so that is good ( even if you are stealing it from another project ) and I see later in the thread that you are also replacing the power supply capacitor, never a bad idea on an amp of this vintage.

Personally, I'd use a UF4007, rated at 1000 volts and 1 amp, the original diode was only rated at 500 ma, but you could get by with a 1N4007 rated at 1 amp and 1000 volts. The UF4007 would be just a little cleaner and the 1 amp rating gives just a little more reserve and possibly a little more punch on long bass passages.

While you are working on it, I would recommend adding a 2 amp rated NTC connected inline on the power cord, the part is called a CL-90 and it slows the initial inrush current momentarily, this will protect your tube heaters and extend your tube life. The CL-90 also drops the AC line voltage for a volt or two, never a bad idea when we have 122 or higher line voltage today and the SCA-35 was designed for either 117 VAC or 120 VAC

I have an SCA-35 that I started to rebuild, never finished it (  still have it ), also had an ST-35 that I rebuilt and really liked when I had high efficiency Klipsch speakers but sold it later after I sold the Klipsch and needed more power. It's a fine little design.

-Ed

Thanks for the reply, and I agree to all. The amp is working again and has played for an hour or so.
New Power transformer is running quietly and not hot. I have on hand the current inrush device you
described and will employ it. It has a mishmash of power tubes , no two alike , which I will replace
with a matched quad. I picked up a pair of nos RCA 7199 black plate tubes also. From what I
understand, you don't hear the preamp tubes unless using the phono section. In the future, I aim
to replace the phono board and driver boards with a modern offering but haven't decided which way
to go on that. I'll post a picture of the amp soon, its a factory wired unit in fair cosmetic condition,
and was in working condition when it arrived.  Yes my line voltage is 122.5.

The amp has been running really well. I have all of Dave's boards built up and ready to go in.
Looking forward to it.

these old amps do not have individual bias settings, so the four output tubes should be matched or the difference will kill some tubes eventually