Motorola ts907 low hv

[Yamamaya42]

way too much info on the subject!

http://www.geofex.com/ampdbug/coupling.htm

[timmy]

Pin 2 is showing 55mv and the tube is slightly red plating.

[Yamamaya42]

And it's quite normal for miniature output tubes to get hot enough to burn your skin off, but smoking the cathode resistor is NOT normal.

[timmy]

No it’s not normal but it was nice to see the voltages normal now I have to find the cap culprit.

[zeno]

Here is an example of a breaker with ratings
IIRC the hold is max amps that will not give accidental trips.
The break is for instant trips.
https://www.tedss.com/CBT400/

Thermistor seems just to prevent high cold set inrush current.
Should go to 0 ohms hot. You can jump it out to test.
When its cold you can put an ohm meter on it & heat it with a heat gun
etc & watch it drop in value.
BTW most thermistors that fall apart can be soldered back together to get you by.
Now if you leave out the 6GK6 you can work on everything else.

Zeno

[Yamamaya42]

https://frank.pocnet.net/sheets/049/6/6GK6.pdf
55mv (0.056 V) Is infinitesimal, not enough to be causing the problem, you have something else going on here.

[Yamamaya42]

check check and re-check

Primary of t5 (930 ohms)
C55/C56/C57/C58 ( All but c57 rated 1kv or higher)
6gk6 tubes, they may be bad even if tester says they are OK.
Make sure the output transformer is good and all caps are of good quality, you could be having a problem with parasitic oscillation, (high freq) that is overloading the output.

One thing to try is to ground the center pin ( wiper ) of the volume pot, this will force the grid to 0v, if you STILL red plate, then you know the problem is something else.

[timmy]

And I believe it’s the tube I have the chassis on the bench and tube in the resistor gets hot I pull the plug then plug back in and resistor stays cool it seems to be random and those 2 caps they only matter if the plug is in to complete the volume circuit so that really don’t leave much but the tube or a .005 pin 8 cap because it gets hot all plugged in and complete or on the bench.

[timmy]

1.4 ohms secondary primary .933 ohms comes up as k ohms. That’s good.

[Yamamaya42]

Do try what I mentioned, tie the center of the volume control to gnd and see if it stops getting hot.

[timmy]

The volume control is not plugged in this has become a random thing one moment the resistor and tube gets hot then it don’t even not plugged into the volume control so it don’t matter at this point if the chassis is in or out. Now it’s looking like a defective tube or as the schematic shows a .005 to ground with a 145v source on the other end but the sams don’t show that it has 2 .005 caps not one.

[Yamamaya42]

W/o that volume plugged in, there is nothing to keep that grid pulled low, and it will float high, causing the tube to bias on.

If it keeps acting up, what I would do is.

Replace BOTH c54/c56 with something like 667-ECW-H20103HV & c55 with something like 667-ECW-H8102RHV , and so on.
There is a tie to boost voltage via r82, but since there is no diode on this line, this means there is a strong horz pulse there, c54 is there to filter that, c56 is there as bypass and let only audio to the output stage, if ANY of these caps are faulty, DC and/or horz pulses can be passed on to the output.

[timmy]

What does the x z and w represent. Can a ceramic disc intermittently leak or short.

[Yamamaya42]

667-ECW-H20103HV & 667-ECW-H8102RHV are mouser part numbers
I did not wanna paste huge links here
tiny links
https://tinyurl.com/2p8apn29
https://tinyurl.com/yvbjsc9k
ceramic discs normally dont give problems, BUT, it does not mean the never do.

paper caps. ALWAYS BAD.

Vintage film caps and Mica, most of the time, don't act up, but I HAVE seen them cooked to death and bad before.
https://imgur.com/veMo2go
these are from my CTC-16XL and were causing color problems.

in other words, you must treat all old caps on a case by case basis by diagnosis ASIDE from the paper and electrolytic, which you will always assume to be bad.

[timmy]

My dvm has a capacitor mode to check them and it’s a great thing to have but even though they check good in circuit and under load I know is a different story. It can check uf and nf.