HELP! SB 200 sunn bass amp

[Slam]

Hello all, I really need some expert knowledge to keep me from destroying this thing. I bought new filter caps and started to replace them by pulling one of the old ones without marking the polarity :duh I just assumed the board would be marked and it wasn't, I know i know. I tried to find a schematic but nothing anywhere because not many where made before Fender bought Sunn and they're pretty rare i guess.
   Can any one tell me for sure if the two filter caps should be in parallel or series??? I was sure it was series but it just pops fuses. Also there is a large ceramic cap (.02uf 1400vdc) located near the transformer that was disconnected by owner and i'm not sure where it fits in.
   This amp was originally a combo that was reboxed as a separate head by the current owner and couldn't get it to work. When i got it it just hummed what appeared to be a straight dc signal to the speaker. I am not really a tech but have fixed simple stuff in the past so i am trying to help a buddy out.
   If anybody can help it would be most appreciated and i would really like to learn more of the technical side of amp repair.

Thanks,
Mike.

[Slam]

Even a hand drawn diagram showing everything from the input side of the PT to the filter caps would be great. Somebody has got to have one of these amps. I heard they have a great sound, when working

[J M Fahey]

Hi Slam. Grab your multimeter with Buzzer/continuity tester.
In the power supplu you will have a bridge rectifier, eithes 4 sepatare diodes or a pack with 2 AC pins (the ones with a little wave symbol), a "+" pin and a "-" pin.
Now go to the pads where your filter capacitors were originally soldered. For each capacitor 1 pin will have continuity (beep) to ground, and the other either to "+" or "-".
You'll end with one capacitor connected to Gnd and + , and the other to Gnd and -.
Anyway, I don't think that was the only problem you had.
Good luck.

[Slam]

J M Fahey, thanks for the reply. I am sure also there are other issues but wanted to get it back together so all can be sorted out. I'll follow your steps after work and report back. Any thoughts on the 1400v ceramic? I am thinking it either goes across the 120vac or from the black to a ground.

Thank you much,
Mike.

[J M Fahey]

Hi.
That 1400V ceramic makes sense soldered across the transformer's primary taps, to reduce turn-off pops

[joecool85]

Hi.
That 1400V ceramic makes sense soldered across the transformer's primary taps, to reduce turn-off pops

How's that work?  I once built a stereo LM1875 that popped pretty bad when turned off.  Would that have solved it?  Also, am I right in reading that it seems you would connect the ceramic cap to the AC portion??

[J M Fahey]

Yes, the .02x1400V ceramic or polyester goes from one primary wire to the other as drawn to lower the "off" spark;  *or* across the switch contacts to it does not turn off instantly but some current flows through it a few extra milliseconds, also softening the spark. Really what *should* be used there is a capacitor rated "250V AC" by Underwriters Laboratories (that UL sticker you see in the back of most amps) which in practice translates into a capacitor with a *DC* rating of at least 3 or 4 times the AC outlet voltage.
630V (a common rating) is reasonable for a 120V line; for 220/240 you'll need at least 1200V, preferably 1500V if available.

[Brymus]

Is that why my big Crates have those .01 strapped across the mains before hitting the primaries?
To help prevent POP?

[Slam]

Yes, the .02x1400V ceramic or polyester goes from one primary wire to the other as drawn to lower the "off" spark;  *or* across the switch contacts to it does not turn off instantly but some current flows through it a few extra milliseconds, also softening the spark. Really what *should* be used there is a capacitor rated "250V AC" by Underwriters Laboratories (that UL sticker you see in the back of most amps) which in practice translates into a capacitor with a *DC* rating of at least 3 or 4 times the AC outlet voltage.
630V (a common rating) is reasonable for a 120V line; for 220/240 you'll need at least 1200V, preferably 1500V if available.

That right there is a big help and good information, i had some long work days this week but i will take a picture of the board and post it before this weeks over. Please stay tuned.

[Slam]

OK, i got a chance to dig back into this amp and i figured i would start from scratch. I checked the voltages across the secondary leads of the transformer. From center tap, each side showed 39vac and had 79vac across the outer two. I am guessing this is normal-ish. I plan on replacing all 4 bridge diodes but what value would i buy? the stock diodes have 56 and 6a4 written on them. They seem cheap enough so i want to purchase good ones. Can i buy them by voltage and amperage ratings? I will have to find a way to test the power transistors or just replace them also.
   I really don't want to give up on this.

thanks,
Mike.

[J M Fahey]

Hi. Don't "fix" it if it ain't broke.
6A4 are fat diodes, rated 6 Amperes, supporting 400 PIV (400 Volts, peak inverse).
If not burnt, leave them alone, i'm *sure* Sunn sourced much better quality ones than anything you (or me) can buy over the counter.
2x40VAC means a healthy, beefy supply, around +/- 60 VDC, which means close to 200W/8r . Loud!