Partial success story. Symptom: Hum, hiss, buzzing, & pops. This started in one channel from the phono section and eventually took over both channels, even from the line stage.
I was motivated. I inspected for open flame, or something obvious. Later, I tapped on each device with a chopstick. Everything seemed to be microphonic. Later (again) I aimed an infrared thermometer at the transformer: fine. I checked each of the larger rectification diodes, one at a time: fine, fine, fine, uh-oh. One of them was 40F hotter than the other three.
When I bought this pre I spent several enjoyable hours reading up on the Modulus history and repair tips. The large diodes are prone to heat, partly because Art mounted them right on the board. The consensus was to use the whole lead so the diodes stand above the board, giving room to cool. Years ago, on AudioKarma, John Curl (!) suggested fast recovery diodes. Who am I to argue?
I installed a pair of NOS, cryo tubes for the phono section. I added a 7 day timer, and an hour clock to the rack.
Plus duct tape between the chassis & the covers. And tube dampers.
Mr. Ferris did not leave much clearance underneath to allow cool air in. I intended to attach Vibrapods with #6 machine screws but... just before I picked up the drill I realized there was not enough clearance to the board! Close call! From a smartphone repair, I had leftover plastic adhesive, so I used that to secure the Vibrapods.
The result is the popping is gone but the hum lives on. (Yes, I eliminated the power amp. When I engage Mute, all sound stops; it's not the power amp.)
I was motivated. I inspected for open flame, or something obvious. Later, I tapped on each device with a chopstick. Everything seemed to be microphonic. Later (again) I aimed an infrared thermometer at the transformer: fine. I checked each of the larger rectification diodes, one at a time: fine, fine, fine, uh-oh. One of them was 40F hotter than the other three.
When I bought this pre I spent several enjoyable hours reading up on the Modulus history and repair tips. The large diodes are prone to heat, partly because Art mounted them right on the board. The consensus was to use the whole lead so the diodes stand above the board, giving room to cool. Years ago, on AudioKarma, John Curl (!) suggested fast recovery diodes. Who am I to argue?
I installed a pair of NOS, cryo tubes for the phono section. I added a 7 day timer, and an hour clock to the rack.
Plus duct tape between the chassis & the covers. And tube dampers.
Mr. Ferris did not leave much clearance underneath to allow cool air in. I intended to attach Vibrapods with #6 machine screws but... just before I picked up the drill I realized there was not enough clearance to the board! Close call! From a smartphone repair, I had leftover plastic adhesive, so I used that to secure the Vibrapods.
The result is the popping is gone but the hum lives on. (Yes, I eliminated the power amp. When I engage Mute, all sound stops; it's not the power amp.)
I'd look at the power supply, maybe some of the electrolytic capacitors are getting old.
Start with the one(s) closest to the rectifier diodes, since that's where the ripple current is highest,
and such capacitors usually fail first. The regulator may even be dropping out from the ripple voltage.
Start with the one(s) closest to the rectifier diodes, since that's where the ripple current is highest,
and such capacitors usually fail first. The regulator may even be dropping out from the ripple voltage.
Thanks! There is no schematic for this on the web but a blogger on AudioKarma said the 10,000 mF caps are supposed to be 25v. The previous owner paid a tech to replace the 'litics but he put in 10v caps. They fit and they measure 8,900 mF but I'm replacing them anyway. I added a dozen holes to the side panel to cool the 3A
diodes.
diodes.
It might be a trick of the lighting, but the far leg of CR5 and left leg of R42 doesn't look well soldered....
If they are 25V (or 10V) they are undoubtedly on the heater supply rather than the B+
At least some of those modulus preamps were hard on valves, so it might be that you have one starting to reach end of life. Not sure if your model is in that category though.
If they are 25V (or 10V) they are undoubtedly on the heater supply rather than the B+
At least some of those modulus preamps were hard on valves, so it might be that you have one starting to reach end of life. Not sure if your model is in that category though.
Thanks. I will touch up CR5 and look closely at the other diodes. The R42 looks that way because I took out the part to which it is attached.
Q3 (which appears to be in the high voltage power supply) is totally dead. According to a transistor checker. It is a TO-220 cast, attached to a double heatsink.
It's an old Motorola part which seems to be labelled 639. But that cannot be because a datasheet for 639 is a TO-92 case without a tab for bolting to a heatsink. I cannot attach a BC639 to a double heatsink.
Q3 (which appears to be in the high voltage power supply) is totally dead. According to a transistor checker. It is a TO-220 cast, attached to a double heatsink.
It's an old Motorola part which seems to be labelled 639. But that cannot be because a datasheet for 639 is a TO-92 case without a tab for bolting to a heatsink. I cannot attach a BC639 to a double heatsink.
The larger diodes are the FWBR for the heater and yes, they do generate heat and should be off the board.
When hum is present (in the 2 series - both channels) typically one or more transistors are in need of replacement. Shoot me an email offline if you'd like further info.
Good call. Yes, Q3 is bad but I found an NTE part locally, and hoping for the best.
I'm going to button it up tonight. I hope the IC is good.
The big caps were within spec. I replaced smaller ones where capacitance was 24% below the claimed value. ESR was fine on all of them.
Any indication of what transistors are suitable replacements would be great for future problem solvers!!