Ts4d Motorola

[timmy]

Originally Posted by jr_tech

1. Does the hum actually get louder as you turn the volume control down?

2. Can you measure the voltage of the hum at the speaker?



Sometimes increasing the capacitance value of of a filter cap too much can cause 60hz current spikes that will propagate throughout the entire ground system of the set and cause hum problems. More is not necessarily better.

jr

The hum just appears when the volume is lowered to minimum, gradually you can here it the lower it goes, then it's just a nasty hum. Volume at minimum IIRC the voltage was nearly nothing but my Dvom has a hz setting and with the grid off I'm getting a full 60 hz from the speaker. These big caps I used IIRC I used then in one of my other vt71 sets and don't have any problems with hum.

[timmy]

.012 ac voltage at speaker no hz, grid on, grid off voltage is 1.662 ac at 60hz . No DC voltage grid on or off.

[jr_tech]

Originally Posted by timmy

.012 ac voltage at speaker no hz, grid on, grid off voltage is 1.662 ac at 60hz . No DC voltage grid on or off.

1.662 volts with the grid of the 25L6 disconnected? That's quite high! Was there anything at all connected to the grid pin? For example, just a short clip lead will pick up a significant amount of hum.

Do you measure the same voltage with different 25L6s?

jr

[timmy]

Originally Posted by jr_tech

1.662 volts with the grid of the 25L6 disconnected? That's quite high! Was there anything at all connected to the grid pin? For example, just a short clip lead will pick up a significant amount of hum.

Do you measure the same voltage with different 25L6s?

jr

There was nothing on the grid pin when I measured at the speaker leads and got 1.662 volts ac.But I did use a clip wire to measure the grid with the wire on the grid. 25l6s the same. I've been going over this beast and can't find a thing out of place.

[timmy]

Well still cannot find the source of the hum but is it at all possible to get a hum like this from hv, the 1b3 gets its heater voltage from the hv coil so from there I don't know. The Motorola Manuel states that there is a 22 ohm resistor between the crt and the audio amp heaters to keep audio hum down but that resistor is there.

[dieseljeep]

Originally Posted by timmy

Well still cannot find the source of the hum but is it at all possible to get a hum like this from hv, the 1b3 gets its heater voltage from the hv coil so from there I don't know. The Motorola Manuel states that there is a 22 ohm resistor between the crt and the audio amp heaters to keep audio hum down but that resistor is there.

The 6SQ7 is a high mu tube. Try a 6SR7 tube, that has a lower mu.
The 22 ohm resistor in parallel with the 6SQ7 heater has to be in there, as the 6SQ7 tube has a heater rated at 300ma, directly in series with the CRT, which has a 600ma heater.
I'd be tempted to rewire the socket for a 6J5, which is a half of a 12SN7, but for now the 6SR7 is a plug and play replacement.
I have to look at the schematics again and do some comparisons with the various versions.
I never ran into this version, only the ones using the 12SN7.

[timmy]

Would 6sj7 or 6sk7 be of any use