Hi
This amp had its power supply dead. Upon checking, I realise that the thermal fuse between CP3 and CP5 (refer to shematic attached) had blown. So I connect the AC mains between CP5 and CP4, bypassing the thermal fuse. Now the transformer is working and there is supply to the amp. The problem now is the voltages across C33 has gone up. The voltage across C33 as shown on the schematic is 430V, but now it has gone up to 450V (up 20V). In fact the voltages across C31, C33, C35 and C36 have all gone up by approx 20V. Even the filament voltage has gone up from 3.3V AC to 3.6V AC. The amp works OK but the power transformer tends to get hot. I guess that if I can bring down the AC voltage going into the transformer slightly, then I can bring down all the secondary voltages. How can this be done or is it OK with the amp voltages up by 20V. The local power supply here is 240V AC, 50Hz. Can someone please advise.
Thanks
This amp had its power supply dead. Upon checking, I realise that the thermal fuse between CP3 and CP5 (refer to shematic attached) had blown. So I connect the AC mains between CP5 and CP4, bypassing the thermal fuse. Now the transformer is working and there is supply to the amp. The problem now is the voltages across C33 has gone up. The voltage across C33 as shown on the schematic is 430V, but now it has gone up to 450V (up 20V). In fact the voltages across C31, C33, C35 and C36 have all gone up by approx 20V. Even the filament voltage has gone up from 3.3V AC to 3.6V AC. The amp works OK but the power transformer tends to get hot. I guess that if I can bring down the AC voltage going into the transformer slightly, then I can bring down all the secondary voltages. How can this be done or is it OK with the amp voltages up by 20V. The local power supply here is 240V AC, 50Hz. Can someone please advise.
Thanks
Please post all guitar amplifier related threads/posts in the Instruments and Amps forum, not the Tubes/Valves forum.Read note number 4 on the schematic, it specifies all voltages may vary 20%.
Power transformers do get hot normally, but it may be excessive here. Disconnect all secondary loads from the transformer, and power it up alone. Does it still get hot? If so, you have a shorted turn somewhere, likely the cause of the fuse opening. Unloaded, the transformer ought not get hot.
Your local power is 240v nominal, but in your location is it also 240v actual, as measured? Here I have 120v power, but the actual voltage on a given day can be as low as 115v or up to 128v.
Power transformers do get hot normally, but it may be excessive here. Disconnect all secondary loads from the transformer, and power it up alone. Does it still get hot? If so, you have a shorted turn somewhere, likely the cause of the fuse opening. Unloaded, the transformer ought not get hot.
Your local power is 240v nominal, but in your location is it also 240v actual, as measured? Here I have 120v power, but the actual voltage on a given day can be as low as 115v or up to 128v.
The usual cause of a transformer overheating and taking the thermal protection is poor mains connection, sparking or loose connections in the cabling or mains sockets. Transformers do not like dodgy mains supplies.
Replace the thermal fuse with a 102℃ type, (EYP2BH101 | Panasonic 3 A@ 125 V ac, 3.5 A@ 50 V dc +102degC Non-Resettable Thermal Fuse, 50 V dc, 250 V ac | RS Components) and tuck it under the cardboard to get maximum thermal conductivity. Run up the amplifier after replacing the thermal fuse and monitor the secondary and primary voltages, checking temperature. If it runs hot, replace the transformer.
Replace the thermal fuse with a 102℃ type, (EYP2BH101 | Panasonic 3 A@ 125 V ac, 3.5 A@ 50 V dc +102degC Non-Resettable Thermal Fuse, 50 V dc, 250 V ac | RS Components) and tuck it under the cardboard to get maximum thermal conductivity. Run up the amplifier after replacing the thermal fuse and monitor the secondary and primary voltages, checking temperature. If it runs hot, replace the transformer.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.