Fidelity HF35T (Transistor version).

[HRichard]

Looked at the volume control.
Pot is marked 100k ohm LG
Measuring from min to max from centre pin to left pin ranges from 18 ohms to 130k ohms
Readings are reversed if measuring from centre pin to right pin, ie 130k to 18 ohms.
This is with the cartridge disconnected.

[Sideband]

So the volume control is OK. So where did that 370 ohms come from? Try the buzz test with the cartridge disconnected completely

[Boulevardier]

Originally Posted by Sideband

So the volume control is OK. So where did that 370 ohms come from? Try the buzz test with the cartridge disconnected completely

Already tried (see posts 108 & 110). In my experience, high value pots don't always give zero ohms at minimum - 370 0hms isn't much out of 100K.

[Sideband]

That's true but in post 110 , the 370 ohms was measured ACROSS the volume control, I'm assuming that was the two outer connections which apparently varied with the setting? Maybe measured in the wrong place then. This is getting very confusing....

[Boulevardier]

Originally Posted by Sideband

That's true but in post 110 , the 370 ohms was measured ACROSS the volume control, I'm assuming that was the two outer connections which apparently varied with the setting? Maybe measured in the wrong place then. This is getting very confusing....

I wonder if the OP is mixing up his ohms and kilohms...
Mike

[HRichard]

Originally Posted by Sideband

So where did that 370 ohms come from?

The 2 terminals for the motor winding before I understood where the volume control was.

[Boulevardier]

Originally Posted by HRichard

The 2 terminals for the motor winding before I understood where the volume control was.

Ah, OK so it's post 121 that is correct.

[HRichard]

Originally Posted by Boulevardier

Ah, OK so it's post 121 that is correct.

Yes it is.
Sorry for the confusion. It's been 20 years since I was interested in this hobby previously so I am trying to relearn as have forgotten so much.

[HRichard]

Whilst measuring the resistance of the volume pot I was getting crackles through the speaker.
Don't know if that helps, just trying to give as much info as possible whilst fully appreciating how hard a remote diagnosis is.

[Boulevardier]

Originally Posted by HRichard

Whilst measuring the resistance of the volume pot I was getting crackles through the speaker.
Don't know if that helps, just trying to give as much info as possible whilst fully appreciating how hard a remote diagnosis is.

You're right, it can be very difficult!
But a tip - sounds like you were measuring resistance with the set powered (you wouldn't have got "crackles" otherwise). Don't try and measure resistance on a powered set - particularly on a mains set - could wreck your meter!
Mike

[HRichard]

OK, thanks Mike.
Yes I was but does that help in determining if the amp is working or not as there is nothing from doing the buzz test?

[Boulevardier]

Things are getting lost in the length of this thread. So, have you checked C4 (in circuit in post 78)? With set switched off, you should get very high resistance or infinity measuring across C4 with meter on resistance range. And there should be no voltage at all on centre tag of volume control.

[HRichard]

Originally Posted by Boulevardier

Things are getting lost in the length of this thread. So, have you checked C4 (in circuit in post 78)? With set switched off, you should get very high resistance or infinity measuring across C4 with meter on resistance range. And there should be no voltage at all on centre tag of volume control.

Not checked that. Will try to identify C4 and measure tomorrow.

[Boulevardier]

Sorry - didn't look carefully enough at schematic. You won't read infinity because both ends of C4 are grounded through the volume control and through R7 + R8. So tomorrow, try and unsolder one end (either end) of C4, and then check that you get very high resistance or infinity between its two leads.

[HRichard]

Can I replace C4 (if faulty) with a modern ceramic disc cap or does it need to be the metalised polyester type?

[Boulevardier]

Originally Posted by HRichard

Can I replace C4 (if faulty) with a modern ceramic disc cap or does it need to be the metalised polyester type?

Let us know what measurements you get on present one first. I'm no expert on use of different capacitor types, but for a test it probably won't matter which you use. C4 is the one that is connected at one end to centre tag of volume control, and at other end to junction of 120k (brown, red, yellow) and 330k (orange, orange, yellow) resistors .

[HRichard]

Q4 is dead.
Parts on order as only have 0.1uf ceramic and didn't want to risk them being wrong.

[Sideband]

How do you conclude that Q4 is dead when you have correct mid-point voltage at the emitters of the output transistors? If Q4 was dead, it would affect that voltage since the DC coupling to the output pair would be wrong.

[HRichard]

I meant C4 of course.
Edit: was correcting that as you posted above, sorry.

[Sideband]

OK fair enough. You could try your 0.1 capacitor just as a try-out. It won't do any harm as C4 is only signal coupling. It might prove one way or another that everything else is OK.