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across the bench: an electronic menagerie of diagnostic oddities plus photos and design analysis

cj mf2200 - 1.jpg
conrad johnson solid state amp needs to be verified plus basic inspection

cj mf2200 - 2.jpg
seems pretty basic and straight forward, lots of fuses
 
threshold 1s2 - 1.jpg
large and slightly scratched up threshold preamp, face plate silkscreen should have been a different color.

threshold 1s2 - 2.jpg
threshold model fet one preamp

threshold 1s2 - 3.jpg
series 2

www.usaudiomart.com

Threshold FET One, Series II

Offered here is a near-mint Threshold FET One Series II preamp, designed by the legendary Nelson Pass. The preamp has the option billet
www.usaudiomart.com www.usaudiomart.com
*example in a higher contrast color
 
threshold 1s2 - 4.jpg
original sticker survived, interesting to see the large service panel

threshold 1s2 - 5.jpg
testing functionality and doing the usual inspection. no tantalums unlike other threshold inspections.

threshold 1s2 - 6.jpg
mains voltage switch adds international versatility
 
threshold 1s2 - 7.jpg
thermals show minimal hot spots, nothing to be concerned about. at this distance the hottest spot is about 8-10f cooler vs being 6 inches away.

threshold 1s2 - 8.jpg
idle wattage at 5.5w confirms the overall low heat of the system. electrolytics should have a decent lifespan, nice to see for a change. measured 28v across each of the 75v rated filter caps, if i remember correctly, which should also help (lower leakage potential too). a recap would be a good idea for optimum performance.
 
denon avr1200 - 3.jpg
lots of small caps on multiple boards/cards. card slot connectors could be problematic.

denon avr1200 - 4.jpg
idles high even though there is more than just 2-ch under the hood. ended up dialing the bias down slightly for 35w idle and noticeably cooler heat sink idle temp.
 
sansui g5000 - 1.jpg
smaller sansui receiver, evaluating what it needs and if it still works

sansui g5000 - 2.jpg
sansui g-5000

sansui g5000 - 3.jpg
speaker terminals are still functional, no weak/broken springs or plastic tabs. antenna mount needs repair though.
 
onkyo grand m508 - 3.jpg
bottom panel removed

onkyo grand m508 - 4.jpg
rear pcb disrupted prior to photo when inspecting for issues, forgot to mount it back into place. overall nothing looks burned or broken inside.

when pushing the power button nothing lights up and both needles move slightly. no indication the system is powering up and no protection mode lighting. root cause ended up being something simple.
 
onkyo grand m508 - 5.jpg
removed the face plate since there are some bulbs to measure. one hint of the issue is sitting on the bench next to the amp chassis.
 
onkyo grand m508 - 7.jpg
closer look at the fuse socket that feeds some of the face plate circuit(s)

onkyo grand m508 - 6.jpg
gradual burn of the slow blow element might suggest consistent over-current from one or more bulbs. when a backlight bulb finally failed and created a momentary higher than average current flow event the fuse was pushed closer to failing.

open fuse stops the system from turning on. one of the backlight bulbs was open and needed a replacement. ideally all four bulbs per channel should be upgraded to LED.
 
mrehorst said:
The little meter light bulb was enough to blow the main power fuse? Somehow that doesn't seem right.
Click to expand...
That pictured slow blo fuse is not the amp`s main power fuse !
But protects some lower power level circuits that also includes the front panel bulbs, from my understanding.
 
onkyo grand m508 - 13.jpg
nice amount of space to relocate thermally sensitive components under the main board. there is a line or trench of fire behind the face plate area and the relocated caps above show where it is.

onkyo grand m508 - 14.jpg
once upon a time this was a 330uf electrolytic, from the fire zone thermally speaking
 
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