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4270 Power Supply Resistor Running Hot

Allegroconbrio

Allegroconbrio

Working the problem
Just finished restoring a Marantz 4270 with a full recap and overall restoration. I'm in the process of doing final tweaks before it goes out to a customer.

I noticed that power resistor R808 (150 ohms, 5W) on the power supply board is running very hot. I realize that by design these guys are supposed to dissipate heat, but you can give yourself a good burn if you touch it for more than a second. The current on the board measures .13 A, silkscreen on board specifies a .3 A fuse (interesting aside, there's a .5 A fuse in there now, don't know if that's stock or some replaced it).

The adjacent caps and diode are getting pretty warm too (not on their own, but by being near that resistor), and I'm concerned about long-term reliability. Has anyone had the same experience? Since the current is not excessive, I'm considering increasing the power rating of that resistor and extending its leads to get it away from the other components.

Thanks,

Rich W
 
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Was this the case before servicing the amp?
Were those resistors that are running hot mounted above the PCB surface (to increase airflow) or sitting on the board?
I would recheck my work since other components are getting hot.
 
jblmar said:
Was this the case before servicing the amp?
Were those resistors that are running hot mounted above the PCB surface (to increase airflow) or sitting on the board?
I would recheck my work since other components are getting hot.
Click to expand...

Hi jblmar. Resistor ran hot before the restore. It is mounted above the PCB to increase airflow. The other components are getting warm only by being near that resistor, not by their own internal heat generation.

Rich W
 
I have a working but unrestored one and went and measured the voltage on mine at 9.73Vdc which is about 65mA. That equates to about .63 watts which does feel a bit toasty. I recommend using 105C temperature rated caps that are located nearby.
 
rBuckner said:
I have a working but unrestored one and went and measured the voltage on mine at 9.73Vdc which is about 65mA. That equates to about .63 watts which does feel a bit toasty. I recommend using 105C temperature rated caps that are located nearby.
Click to expand...

rBuckner, thanks so much for that measurement. I'm running more than double that across R808, 21.9 V, 146mA. This will bear further investigation. I'm thinking H804 might be suspect.

Rich W
 
In that other 4270 thread, the guy was having problems with one or more of the H801 through 804 transistors, or something such. Replaced all four and that seems to have fixed the amp. So maybe this is a common issue on this model.
 
rBuckner said:
Any luck yet?

:lurk:
Click to expand...

H804 tested fine out of circuit, and zener H817 had 14 volts across it in circuit. But I know the H804 is prone to weakness, so I'm ordering a replacement from mouser 2SD331 ====> on semi MJE15032G. There's really not much to this leg of the power supply, whose sole purpose is to provide 14V to the FM boards (I measured 13.6V). H808 and H809 are common with the 35V amp board supply, no reason to suspect them.

Just a hot resistor passing too much current. All roads lead back to H804.

Whatever the solution, I'd like to see R808 a little higher on the board and away from the nearby components. So I'll be replacing it with a 7W (vs. the existing 5W) wirewound, and move it up with some porcelian wire-through insulators.

Mouser order goes out Monday. Will keep you posted.

Rich W
 
I have a feeling something is wrong which is drawing the extra current. Something the power supply is going to, not the power supply itself. Try this, measure the voltage drop on the "hottie" and switch the function knob through the various positions, AM and FM being key but do them all. The replacement being 7 watt won't hurt a thing!
 
rBuckner said:
I have a feeling something is wrong which is drawing the extra current. Something the power supply is going to, not the power supply itself. Try this, measure the voltage drop on the "hottie" and switch the function knob through the various positions, AM and FM being key but do them all. The replacement being 7 watt won't hurt a thing!
Click to expand...

Randy, you were right on about the power supply not being to blame. Checked voltage across R808, and consistently read 22V (high!) on all functions FM Phono, Aux, etc. except for AM, which was 11V. Then I realized the common denominator: All functions except AM caused the stereo light to turn on. Confirmed by tuning a mono FM and read 10V. Suspect someone replaced stereo bulb with a high-amperage substitute. Once again, confirmed - and not even a bi-pin bulb. Replaced with 8V 40ma bi-pin, read 17V across R808 in FM stereo, other functions 15.5V in stereo. In mono, depending on function, 8V to 11V, sounds like definitely in the ballpark. Randy, when you did your measurement, was the stereo light on?

Resistor still runs hot, but no longer capable of inflicting 3rd-degree burns. Sometimes the previous "visitors" to our vintage pieces lurk in hidden places!
Will probably still raise it off the board more and replace with the 7W version.

It's been a long road on this 4270 - from recapping through the most densely populated piece of vintage gear I've ever worked on, through replacing the relays (into the valley of death I rode . . .), and finally this latest episode. Ultimately, it is so gratifying (and addictive) to get these wonderful old chunks of metal running like new.

Thanks so much, Randy and AK.

Rich W
 
In short, I dunno on the stereo light. I just powered it up and measured it, and it's in AUX mode. I'm not sure if the stereo bulb works on it either for that matter! I think so though.

Great job in hunting it down and figuring it out, I'm impressed! :thmbsp::yes:
 
Bumping this old thread. I'm shooting that resistor at about 140F and my stereo light has been replaced with a new correct replacement from dgwojo. Is that good enough or should I dig in? This is an old unit that was on the shelf for years, between a local tech and myself we have recapped the BTL phase inverter board (me) relamped (me) troubleshot a fault in the preamp section that I didn't completely understand (him) and finally replaced the MPX board and realigned the tuner (him) it looks like he also replaced some caps on the PS board, but the resistor that the OP mentioned is still running hot as a cob; noticeably hotter than the output heatsinks, which doesn't seem right. Is this normal or do I need to dig deeper before I trust this unit?
 
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