Short story -- reviving a 7100 for a friend, it sat for 20+ years and he tried to use it, but weak signal/crackling etc
I'm a modest electrical guy but have a decent scope DVM signal gen etc. I can read a schematic and spent a good deal of my youth building/fixing tube gear.
Anywho I got the service manual from hifiengine and started to troubleshoot, after blew dust/cobwebs/dirt etc out and did the ole contact cleaner gig.
First thing I checked was 27 and -27 V on the power amp board. No 27V, the 100 ohm resistor was burned up. Replaced it, checked the big elec caps, seemed OK so powered on, regulators seem OK and now I have 27 and -27. But still no gain in the amp to speak of, volume control works but even at full blast its clipped and sounds like a transisor radio volume-and-fidelity wise.
Protect relay working like it should and little DC offset on the output. More on that later
Then went to control board and found the 18 volt zener diode shorted! Hint here was when tone control bypass was engaged, no sound at all (not even shot noise!) Dead shorted diode. 220 ohm resistor apparently got hot enough to flow its solder joints as it was loose on the board! Never in my life seen a dead short in a diode-- I woulda thought it would burn up and go open --
anywho ordered a few 18v zeners and the 3300 uf/35 v caps and couple new 220 ohm metal film resistors. Once I get the control board power working I'll report back.
What I thought I would do is bring it back to life changing as little as possible to get a baseline, then go about the recap and relay driver xistor swaparooney, and maybe the driver xistors too.
Hints appreciated and helpful to me for this. I'm getting older and my visual acuity is not so great anymore but giving it a decent go!
I'm a modest electrical guy but have a decent scope DVM signal gen etc. I can read a schematic and spent a good deal of my youth building/fixing tube gear.
Anywho I got the service manual from hifiengine and started to troubleshoot, after blew dust/cobwebs/dirt etc out and did the ole contact cleaner gig.
First thing I checked was 27 and -27 V on the power amp board. No 27V, the 100 ohm resistor was burned up. Replaced it, checked the big elec caps, seemed OK so powered on, regulators seem OK and now I have 27 and -27. But still no gain in the amp to speak of, volume control works but even at full blast its clipped and sounds like a transisor radio volume-and-fidelity wise.
Protect relay working like it should and little DC offset on the output. More on that later
Then went to control board and found the 18 volt zener diode shorted! Hint here was when tone control bypass was engaged, no sound at all (not even shot noise!) Dead shorted diode. 220 ohm resistor apparently got hot enough to flow its solder joints as it was loose on the board! Never in my life seen a dead short in a diode-- I woulda thought it would burn up and go open --
anywho ordered a few 18v zeners and the 3300 uf/35 v caps and couple new 220 ohm metal film resistors. Once I get the control board power working I'll report back.
What I thought I would do is bring it back to life changing as little as possible to get a baseline, then go about the recap and relay driver xistor swaparooney, and maybe the driver xistors too.
Hints appreciated and helpful to me for this. I'm getting older and my visual acuity is not so great anymore but giving it a decent go!