The last couple days I’ve been working on a SHP3 amplifier. I recieved it for repair through the mail. The customer reported that the left channel didn’t work. It had massive damage. The amplifier is supposed to have a 3.2A fuse and really, a 2A is more than sufficient. This one had a 7.5A fuse.It was not blown and since it didn’t blow the amp just toasted all the transistors in the left channel. All of them. The drivers had the worst physical damage I have ever seen. What started this mess? The amp had all original capacitors. Let me climb on my soapbox….OK. Any Seeburg equipment with original capacitors is overdue to have them replaced. I mean the electrolytic capacitors. It’s over 40 years since Seeburg made anything. An amp may work but the electrolytic caps dry out and turn into resistors or just short out and cause a lot of damage. If the fuse doesn’t blow then real damage happens. Traces usually literally burn off the board and components burst into flame and this amp was no different.
I replaced all the transistors on the driver board for the left channel. All the electrolytic caps on both boards and chassis except for the filters. They are good. The left channel outputs were changed as well. While troubleshooting to see what all was bad I found that one of the right channel output transistors was bad. I replaced both of those so as to have a matched pair. The bias pots got thoroughly cleaned. Several of the “safety” resistors, the 100 ohm 1/2w’s in front of the drivers, were changed out. I replace those with 1w resistors to give less chance of burning up. They’ll blow but not burn. A trace on the back of the driver board was also repaired with a section of wire neatly inserted into the burned gap. After setting the bias I bench tested the amp and it worked well. I played it a while then transferred it to the STD4. Plenty of volume and excellent tone. I played one record, House Of The Rising Sun very loud. Who wouldn’t? The amp bias creeped up. After the record was over I let themech go to rest and kept an eye on the bias. It slowly came down as the amp cooled. It had gotten to 19ma on the left and 8ma on the right. I reset both to 5ma and played a few more records. The bias stayed rock steady. Mission accomplished.
My Seeburg Life 