Hi,
i recently bought a cheapo but working HK6500 amp from the early 90s.
When i took it apart i noticed it was somewhat badly repaired, one of the channels' power BJTs had most likely blown up and were replaced by low quality knockoffs, not original Toshiba transistors. I checked DC bias which was OK, but the idling current was way off and even after i tried to adjust it, i couldn't get the right values (pot maxed out). Power supply voltage was OK (more on that later), nothing was overheating or burned out. Speaker output was hissy, crackled when adjusting the volume and had constant (not changing in volume) mains hum.
Issue number one is the power. When first measuring the voltage, 49.2V rails were at 54V. The amplifier was set to 220VAC, so i set it to 240VAC and measured my wall, which read 235Vrms. Then the 49.2V rails read the correct voltage. Could anything be damaged from that higher supply voltage the amp ran with before?
The hum could point to bad PSU filtering capacitors, which are both 13000uF 63V. I tried looking online, but i can't find exact replacement locally. I would get 12000uF 80V cap and put 1200uF 63V cap in parallel on the bottom of the PCB, would that be a good solution? I think that would provide even bit better filtering, not sure. Also, the big capacitors are glued down to the PCB. What is the best way to get them off? Acetone?
For the spare parts, i ordered full set of 4 NOS Toshiba transistors for both channels and i listed out all of the electrolytic capacitors and ordered replacement ones (All Panasonic caps).
Is there anything else i should do?
Appreciate any help or advice.
i recently bought a cheapo but working HK6500 amp from the early 90s.
When i took it apart i noticed it was somewhat badly repaired, one of the channels' power BJTs had most likely blown up and were replaced by low quality knockoffs, not original Toshiba transistors. I checked DC bias which was OK, but the idling current was way off and even after i tried to adjust it, i couldn't get the right values (pot maxed out). Power supply voltage was OK (more on that later), nothing was overheating or burned out. Speaker output was hissy, crackled when adjusting the volume and had constant (not changing in volume) mains hum.
Issue number one is the power. When first measuring the voltage, 49.2V rails were at 54V. The amplifier was set to 220VAC, so i set it to 240VAC and measured my wall, which read 235Vrms. Then the 49.2V rails read the correct voltage. Could anything be damaged from that higher supply voltage the amp ran with before?
The hum could point to bad PSU filtering capacitors, which are both 13000uF 63V. I tried looking online, but i can't find exact replacement locally. I would get 12000uF 80V cap and put 1200uF 63V cap in parallel on the bottom of the PCB, would that be a good solution? I think that would provide even bit better filtering, not sure. Also, the big capacitors are glued down to the PCB. What is the best way to get them off? Acetone?
For the spare parts, i ordered full set of 4 NOS Toshiba transistors for both channels and i listed out all of the electrolytic capacitors and ordered replacement ones (All Panasonic caps).
Is there anything else i should do?
Appreciate any help or advice.