Okay, back into this amp. problem is definately the amp as I was able to try this amp in another 1448 today, and it has the loud hum and 5U4 starts to glow bright. Im digging around the output transformer and a resistor that I cant find on the schematic. The resistor is riveted to the side and is about 2-3 inches long. like a long block type filter resistor. Has 3 terminals coming off it. Anyone know what the resistance should be on all of them? Also, what would be the correct readings for the primary of the output transformer? I have 3 wires, a blue, red and brown . 2 of the wires go to the 6l6 plates. What should my resistance look like there? My electrolytic filter caps all read ok. Only get this loud hum when the amp powers up to play a record or while scanning. Jim On Monday, July 31, 2023 at 09:53:29 PM EDT, Jay Hennigan via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 7/31/23 18:36, Jim McClave via Jukebox-list wrote:
Yes Jay, rebuilt amp and tubes checked. The schematic im using just shows the pin 5 to pin 2 ground connection. But since mine is reading 12 meg, I believe I see a connection to the field coil, Wouldnt that cause my indications?
Ground is pin 2. Field coil is between that and pin 3. A shorted field could cause the overload. There's also AC line voltage on the speaker jack between pins 1 and 6 as a form of interlock to prevent powering the high voltage without the speaker plugged in. Between that and the field voltage, that speaker plug is threatening to bite the unwary. Treat it with respect.
With the machine powered off, unplug the speaker. Measure the resistance between pins 2 and 3 of the speaker plug (not the amplifier socket), black and yellow wires. It should be in the vicinity of 6000 ohms or so. If so, the speaker field is good and your overload is elsewhere. If it's substantially below that, you have a shorted field coil.