Akai 1710w Help Needed

[DMcMahon]

A great shame the new Run capacitor did not help the hot resistor.

As per Post 4 the resistor is only in circuit in slow speed so presumably if you run it in Fast speed the resistor does not get hot, can you confirm that is the case ?

Have you measured the resistor, I assume it is a wire wound so its resistance is unlikely to change unless some wire wound turns get shorted.

David

[DMcMahon]

Is your mains voltage selection switch correctly set ?

[Billkrz]

Hi David,
I measured the resistor (in circuit) and it measures around 425 ohms so that seems ok.
I also check the voltage settings and they are set to 60hz and 110v which is where they have always been.
None of the caps in the circuitry under the tubes (valves) look swollen so I am at a lost.
Bill

[DMcMahon]

OK Bill, what about the Fast Speed question ?

David

[Billkrz]

Yes, thank you for reminding me!
In the high speed mode, after 10 minutes with the air temperature at 75 degrees F, the power resistor read 80 degrees F on the top and 83 degrees on the bottom!
So, Lawrence was correct that it should not be in circuit in the high speed mode and it did not really heat up, just warmed up with the components around it.
Next, I decided to take temperature readings of the tubes (valves) and here is what I measured after 10 minutes:
1) The two 6AR5 tubes that are in the furthest rear location: 350 and 275 degrees!
2) The 12AT7 that is just in front of the 6AR5 tubes: 192 degrees.
3) The 6AR5 that is to the left (looking from the rear of the unit) of the 12AT7: 197 degrees.
5) The 6CA4 that is to the left of the previous 6AR5: 187 degrees.
So...my questions are:
a) Why does the power resistor get so hot in low speed mode?
b) Are my vacuum tubes (valves) especially the two located at the rear of the unit too hot?
Thanks,
Bill

[DMcMahon]

Valves obviously can get very hot, I do not know if your valve temperatures are typical or abnormal.

I would not have thought that the valve temperature heat radiation would heat up the resistor sufficiently to make it run too hot.

Was there any actual evidence that the leaking wax did come out of the old 2uF Run capacitor ?

If you can do it safely would recommend you measure the voltage across the resistor in slow speed and measure the current flowing through it, then calculate the power to see if it is running above its 20 watt rating.

David

[Billkrz]

Hi David,
Regarding the wax, it was in the proximity of the cap. The cap also feels half empty, definitely heavier on one end. I do not see any other source.
I don't feel comfortable measuring the voltage.
I think I will box it up, try running it at the high speed (without the capstan) and see what happens.
Heading out on Holiday soon so I will wait until I return to try.
I did look up valve/tube temperatures and the tubes are rated for 450 degrees F, so I am below that, at least.
Best Regards,
Bill

[ms660]

If it's working there might be nothing wrong with it, the power resistor is rated at 20 watts according to a schematic, it's rated that high because it knows it's going to get hot, it would be interesting to know the voltage across the resistor in another tape recorder of the same type and do a comparison.

Lawrence.

[DMcMahon]

Does the resistor still strongly smell when it gets hot in low speed operation ?

[Billkrz]

Yes, it does. It was actually smelling a bit when I ran it at idle at the fast speed.
When I return from Holiday, I'm going to run the chassis in an open air location while playing in the fast speed and see what happens. Maybe it needs to air out (or blow up).
Bill

[DMcMahon]

As an interesting comparison (for the hot resistor issue) I have had a look at one of my Akai 4000DS Mk. 1s. It is not a direct comparison because mine is only a single speed motor but it does use a 450 ohm 20 watt resistor in series with the 2uf Run capacitor, like yours.

The voltage across my resistor is around 60 volts and the current through it around 134mA, so by simple Power (I x V) calculation, the power is around 8 watts (ignoring any power factor correction).

The top end of the resistor does get hot, after 3 or 4 minutes too hot to touch with a finger, approximate temperature around 200 degrees F (measured with DVM with thermocouple probe lead) at that time (so not as hot as yours), but no obvious smell.

The run capacitor does not get warm and the motor body gets a little warm.

My resistor is one of those white rectangular cement sealed wire wound resistors and has a metal clamp around its body, which is secured to the chassis metalwork (so that may help to dissipate a little of the generated heat).

The description of the resistor in the parts list includes "w/belt", which I guess means "with belt" and the belt it refers to I guess is the metal clamp ?

My resistor measures dead on at 450 ohms, yours at 425 ohms is right at the lower end of the +/- 5 % tolerance that is quite typical on wire wound resistors.

My motor is supplied with 100 volts from the transformer when switched to 240 volts mains, yours I am not sure what motor supply is used from the transformer. Because you are switched to 60Hz this picks up the 120v winding but because you are switched to 110 volts I am unsure what voltage ends up on the 120 volt tapping ?

My 100 volts motor supply actually gets boosted up to one side of the motor windings via the Run capacitor and resistor, for example in my case the 100 volts becomes 143V on the output of the capacitor and 116V on the output of the resistor, these voltages being measured wrt (with respect to) ground/0V.

Switching to 60Hz increases the voltages to 159V & 128V but the voltage measured across the resistor and the current through the resistor remain virtually constant.

Because I am switched for 50Hz the second run capacitor (0.5uF) which is in the dual run capacitor is connected in parallel with the 2uF so giving a total of 2.5uF

David