Hacker Cavalier with dimming output

[opus242]

Hi
I have gained a Hacker Cavalier and am keen to getting it up and working ( i should have the extension speaker by next week ) but it has an issue- the audio output fades after about 4 minutes.
I have repaired the record player itself and that is working well after reading a few threads on the forum.
I have also followed a few threads to get the amplifier working as it should, replacing the de-coupling caps and c4 c5 c9 and c10.
Also bought three new valves... but still after 4 or so minutes the out starts to fade to zero.

any help much appreciated.
Thanks

[Michael Maurice]

You must replace the coupling caps which are the two 47nF/400V capacitors which are mounted on the top tag strip. it is also advisable to replace the two 220K resistors also mounted on the tag strip.

You should also do the same with the extension speaker, though you will need a special tool to remove the amplifier chassis.

If you require help then please send me a PM. (private message)

[opus242]

Morning.
Coupling caps done. But not the resistors so will do them next.
Thanks
What tool do you need to work on the extension unit?
Cheers

[Techman]

Originally Posted by opus242

Morning.
Coupling caps done.
Cheers

Yes, but in your first post you said that you'd only done the "de-coupling" capacitors - they're two different things, or are the other capacitors you mention the ones, as I don't have the circuit in front of me?

[opus242]

Sorry over excited typing - I have replaced C6 and 7 as found in attached.

Mod edit: Copyright schematic removed. They are the coupling capacitors.

[snowman_al]

I do not have access to the diagram, but next step is to measure the HT voltage at the rectifier (pin 3), then the anodes of the output valves (pins 6) and the triode sections (pins 9).

Record them when you first get sound, then again as the sound fades. Let us know.
Alan

[opus242]

cheers Alan
will do

[opus242]

Hi all
ok

HT voltage at the rectifier (pin 3) 262v settling to 279v

23v and 24v then dropping to 12v after 3 mins- looking at the circuit diagram measured at the grid
the anodes stay around 250v

triode sections (pins 9) 223v ( settling at 214v) and 218v (settling to 212v )


voltage around the coupling caps looks to be the issue

[snowman_al]

OK the HT looks as if it is not falling over time, but
... ''23v and 24v then dropping to 12v after 3 mins- looking at the circuit diagram measured at the grid, the anodes stay around 250v''
Can you confirm which pin you measure the +24 volts on? Is it pin 8 (grid) or pin 7 (cathode)? Neither is correct or good.

As I said I do not have the circuit diagram, I have only found the layout so guessing a little now.
I assume C6 and C7 are the coupling caps, are these the ones you replaced?
Check R6 and R7 are 220K ohms (grid resistors).
Check R13 and R14 are 220 Ohms (cathode resistors).
You replaced C9 and C10 (?) are they the right way round?

If you did not measure the cathode voltage on pins 7 before, please do so and let us know. (Normal would be 7 to 10 volts perhaps?)

Ideas from anyone who has worked one before please.

[Nuvistor]

Snippet of the GP15 output stage.

[opus242]

Hi Alan
thanks for your time - sorry was trying to do this last thing with assistance from a cat
I was looking at pin8.
I double checked the caps - I have them in the right way around ( the issue was there before I started the trace the issue).
the resistors all tested ok cold but have replacements on order.

[snowman_al]

Thanks Nuvistor, that helps.
Obviously the 820K resistors (R11 and R12) are the grid resistors.
Therefore would it be correct to assume the 220K resistors (R6 and R7) are the triodes anode resistors? And the 1 meg resistor 'grounds' the grid of one triode section of the splitter?

opus242,
More measuring please.
First check the voltage on pins 7 (output section cathodes). Normal will be 7 to 10 volts.
Next check the voltage on pins 2 (triode cathodes). Normal around 1.5 to 2.5 volts? If you do not get this sort of result, check the resistance of the two 2.2K resistors (R4 and R8 on tag strip).
Then please measure the resistance of the 1 meg ohm resistor (R9?). The one shown in the extract posted by Nuvistor. I think it would go from pin 1 of one of the valves to ground/chassis.

Again let us know your results. Alan

[ms660]

If its of any use almost all of the schematic for the GP15 is shown in the link in Post#25 in this thread:

https://www.vintage-radio.net/forum/...t=23098&page=2

The GP42 circuit is almost identical (cathode, anode, grid resistors, mains transformer secondary voltages etc are the same as the GP15) but be aware that in that model all the load current flows through a 220 Ohm resistor in the HT line which is part of the CRC filter (CRC filter...50uF>>220 Ohm>>50uF) Below are the manufactures valve voltages given for the GP42 with 240V AC mains input.

Valve 1A pin 9 Anode 145V DC
Valve 2A pin 9 Anode 145V DC
Valve 1B pin 6 Anode 260V DC
valve 2B pin 6 Anode 260V DC
valve 1A pin 2 cathode 1.3V DC
valve 2A pin 2 cathode 1.2V DC
Valve 1B pin 7 cathode 7.8V DC
valve 2B pin 7 cathode 7.8V DC

Lawrence.

[opus242]

Hi all
Am not getting those voltages

Have a picture of circuit and have swapped out the two 220k resistors and the 18uf cap .still no joy.
the two large resistors (220 and 1k5 ) seems to out when measured at turn off so they are next . what wattage would be needed. 5 or 7?

[ms660]

I think you need to a full set of voltage measurements and post them here..

Lawrence.

[snowman_al]

Originally Posted by opus242

Hi all
Am not getting those voltages ... the two large resistors (220 and 1k5 ) seems to out when measured at turn off so they are next . what wattage would be needed. 5 or 7?

Hi opus, ignore those two at the moment. (You have 240 volts on pins 9 of the valves in the fault condition, so they are both working well enough for now.)

As Lawrence says; Please tell us the voltages you are getting reference the table in post no. 13. It will help us understand possible faults and give suggestions.

Alan

[Nuvistor]

I cannot find the GP15 manual in the Service Data, so this is the circuit and voltage readings.

[opus242]

Hi all
I have finally got the measurements ( I have finally read the bottom of the valve base the right way round)
9 gives. 25.8. and 26.8.
7 gives 4.9 on both
6 gives 246 and 247
But pin 2 gives millivolts and a couple times will measuring pin2 the house electrics go. Lol.( My wife wants me to stop trying right now lol )
Many thanks

[snowman_al]

Morning opus,
Just to be 100% sure about the pin numbers, count clockwise from the gap looking at the component side (underneath) the holder. See picture.

If that is the way you took the listed measurements, that does give a different slant on the fault... So lets start again, when you are allowed to...

Now the two resistors in your post no. 14 are important, as will be R10 (10K ohms) marked on second picture.
Best to start by measuring the voltage on them. Black meter lead on the chassis / ground, then measure the voltage both sides of each resistor (6 readings).
Then switch off and measure their resistances. (200 ohms, 1.5K and 10K ohms.)
Tell us what you get.

Alan

['LIVEWIRE?']

From the readings you provide, opus 242, it seems that neither V1a nor v2a are drawing any current, which would lead me to suspect the resistor, R10, or the connections to it, but why would connecting a meter probe to pins 2of either V1a or V2a cause the electrics to trip? Are you sure you have set your meter to a low DC Voltage range for this measurement? When making measurements on things which are powered up & switched on, the usual advice is to (a) ensure the meter is set correctly, in this case to DC Volts, and (b) start on a high range, and switch downwards to get sensible readings.