Marantz Model 7T repair

[budm]

One channel will intermittently makes low frequency thumb sound and the sound will go out. Narrow down the problem in the Tone amplify section, checking further when it is bad, the DC Voltage at Q404's Emitter will be around 12VDC when it is bad compared to the good channel, the cause of the problem is bad 3-diode module (D402A~C), the voltage drops went up to around 8V instead of around 1.8V (3 x Vf). I use three 1N4148 to replace the module, both channels are replaced since one channel already went bad).
This unit is hand built and from around 1965, it sounds really good, the customer has it for a long time. I suggested the customer to have new set of caps replaced but does not want to do it for now.

[kc8adu]

Re: Marantz Model 7T repair

not unusual to see these bias/thermal tracking diodes die.
opening up and turning transistor on hard.
lucky they didnt blow up!

[ben7]

Re: Marantz Model 7T repair

not unusual to see these bias/thermal tracking diodes die.
opening up and turning transistor on hard.
lucky they didnt blow up!

Indeed!

Also, I'm surprised the transistor sockets haven't caused issues either... or maybe they have in the past.

[budm]

Re: Marantz Model 7T repair

Open Diode on this board will not cause any damage since when the diode is open there will be no bias current to turn on Q450 or Q404 so those two transistors will be off. They are just simple 3-stage preamp with very low current not like the bias for the output amplifier section.