Taylor 45C Valve tester. - UK Vintage Radio Repair and Restoration Discussion Forum

Megatron · 16th Mar 2019, 10:11 am

My taylor 45 valve tester always reads high, for examle EL84 tests as 12gm
whereas the data says it should be 8. this seems to be true for other valves too.
I have used it like this for years, just having a golden sample of each valve type I regularly test as a reference. I thought it was time i did something about it.

I opened it up today and see someone has already been in there.

Given that the meter movement is free and all the cathode resistors are ww the only immediately obvious thing i can could see that may have a gross effect on both ranges is the rectifiers on top of the transformer are 1N4007.
Theres a single diode and a made up bridge. I guess these would have been selenium or copper oxide originally is this likely the issue?
If so how best to fix it? I guess it needs some added series R and Vf?
anyone done this before, do you think a zenner and resistor in series would work? if so what sort of voltage drop would I need to replicate the original rec? is this even the likely problem? There doesn't seem to be anything else in the GM test circuit that could drift that much.
I am thinking that if the voltages are all high then the cathode voltage must be high too, therefore when the resistor is shorted the voltage change is greater than it should be??
Should i start with a series ZD say 47 v and see what happens?

Megatron · 16th Mar 2019, 2:20 pm

Measured all the resistors and all the voltages and theyre not far off.
also the later testers used silicone diodes and seem to have no other changes......... mor investigation required.

Megatron · 16th Mar 2019, 4:29 pm

Replaced every resistor and capacitor in the thing and still reads high.
also now has developed another fault, when you switch from gm back to cathode leakage the ecc83 under test flashes over.
what i need is some close up photos of the top right hand selector switch to check its wired correctly and I havent got a resistor wrong...
its not a fault i have seen before whenever i have used it.
the colours on the resistors I replaced were well faded, but I measured them as well and were in the right ballpark figure though had drifted a bit.
replaced them one at a time too so everything should be right, but its a bit messy someone has been in there before.
circuit diagram i have has no resistor values on it.

sven_swe · 16th Mar 2019, 5:37 pm

Hi, below is a link and an attached PDF to schematics I used when trying to find the values for some resistors that were replaced in mine. The component values can be different depending on the time it was made it seems. Mine had different values and it was a mess. I am still not 100% done with it since something has come in between

Manual that includes schematic:
http://www.jogis-roehrenbude.de/Roeh...45C/manual.pdf

Megatron · 16th Mar 2019, 6:32 pm

Thanks for that thats quite helpful.
I stopped the wierd flashover changing between gm and leakage, seems the tower of wafers was a bit misaligned allowing a make before break or vice versa.

changed pretty everymuch resistor now and the caps too, but still measures high. or seems to. i have measured many el84 and ecc83 and all the good ones seem to measure EL84 measuring 12.5 as opposed to 8.

sven_swe · 16th Mar 2019, 8:07 pm

Last time I checked my manual had an error for EL84 and that is mentioned in this searchable valve chart that was posted on this forum(they differ often), not sure if that has any influence though:
https://www.vintage-radio.net/forum/...40&postcount=5

https://www.vintage-radio.net/forum/...ad.php?t=66896

Megatron · 16th Mar 2019, 8:24 pm

I just measured a load of 6v6 and they measure high as well.
should be 2.6 but all are over 3.

all the valves I have measured are far nearer the data sheet figure than the Taylor figures,
having studied the diagrams i cant see the type of rectifier making much difference as they provide adjustable supplies, I am begining to wonder if the meter has an issue or if theres a shunt I have missed thats a bit high.
maybe its a tolerance issue, 10% here and there will soon add up
if its all the same direction.

Freya · 16th Mar 2019, 8:35 pm

If it has silicon rectifiers in place of the metal oxide originals then it will be high on its GM readings, the recommended replacement was the GEX34 diode.
Are the original rectifiers still present, if so you could reinstate them, I have never known them to fail.

Megatron · 16th Mar 2019, 10:40 pm

No they're not there.
I will try adding extra diodes then to increase the volts drop and see what happens.....
is the bridge or single diode likely to have the biggest effect?

sven_swe · 16th Mar 2019, 11:56 pm

I read in this thread that they replaced the selenium rectifiers with 1N4001 with good results.

https://www.vintage-radio.net/forum/...ad.php?t=66896

Megatron · 17th Mar 2019, 12:19 am

I am pretty sure that's what's in there.
Don't the DC supplies just provide bucking voltage for zeroing the meter movement? So I wouldn't have thought they were that critical.

Megatron · 20th Mar 2019, 8:55 pm

Quote:

Originally Posted by Freya

If it has silicon rectifiers in place of the metal oxide originals then it will be high on its GM readings, the recommended replacement was the GEX34 diode.
Are the original rectifiers still present, if so you could reinstate them, I have never known them to fail.

Right then, I have rummaged through my huge tub of bridge rectifiers and diodes, in the bottom I found a GEC bag. In the bag are a large number of CV7176 CV7123 and GEX541.
these all have a low Vf and would all appear to be 6A germainum rectifier studs.
also i have some Erie 1200 028 50V 6735 that also seem to be low vf (0.048v) so probably germainum.

do we think this will fix it

? they should be plenty big enough

ill take before and after readings on a fairly high GM valve and see.

Megatron · 20th Mar 2019, 10:17 pm

Measures an EL84 as a golden sample. Gm 12 book says 8
changed all the rectifiers for germanium ones. re measured the golden sample.
Gm 12.5.

going to test the meter next i think, maybe its springs gone weak?

Megatron · 20th Mar 2019, 11:23 pm

getting somewhere now i think,
if i set the voltage across the cathode resistor to 1v by adjusting the grid voltage, when i press the test button i get for the EL84 a Gm of 8
repeating with the 6v6GT i get 2.5 (book says 2.6)
it must be something to do with the grid voltage?
anode and screen voltage are correct(AC)
Grid Potentiometer reads OK end to end. and ac transformer output is OK.
however the grid voltage set point to get the 1 volt across the cathode resistor is about double what you would expect.......

16th Mar 2019, 10:11 am	#1
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Taylor 45C Valve tester. My taylor 45 valve tester always reads high, for examle EL84 tests as 12gm whereas the data says it should be 8. this seems to be true for other valves too. I have used it like this for years, just having a golden sample of each valve type I regularly test as a reference. I thought it was time i did something about it. I opened it up today and see someone has already been in there. Given that the meter movement is free and all the cathode resistors are ww the only immediately obvious thing i can could see that may have a gross effect on both ranges is the rectifiers on top of the transformer are 1N4007. Theres a single diode and a made up bridge. I guess these would have been selenium or copper oxide originally is this likely the issue? If so how best to fix it? I guess it needs some added series R and Vf? anyone done this before, do you think a zenner and resistor in series would work? if so what sort of voltage drop would I need to replicate the original rec? is this even the likely problem? There doesn't seem to be anything else in the GM test circuit that could drift that much. I am thinking that if the voltages are all high then the cathode voltage must be high too, therefore when the resistor is shorted the voltage change is greater than it should be?? Should i start with a series ZD say 47 v and see what happens?

16th Mar 2019, 2:20 pm	#2
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. Measured all the resistors and all the voltages and theyre not far off. also the later testers used silicone diodes and seem to have no other changes......... mor investigation required.

16th Mar 2019, 4:29 pm	#3
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. Replaced every resistor and capacitor in the thing and still reads high. also now has developed another fault, when you switch from gm back to cathode leakage the ecc83 under test flashes over. what i need is some close up photos of the top right hand selector switch to check its wired correctly and I havent got a resistor wrong... its not a fault i have seen before whenever i have used it. the colours on the resistors I replaced were well faded, but I measured them as well and were in the right ballpark figure though had drifted a bit. replaced them one at a time too so everything should be right, but its a bit messy someone has been in there before. circuit diagram i have has no resistor values on it. Last edited by Megatron; 16th Mar 2019 at 4:50 pm.

16th Mar 2019, 6:32 pm	#5
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. Thanks for that thats quite helpful. I stopped the wierd flashover changing between gm and leakage, seems the tower of wafers was a bit misaligned allowing a make before break or vice versa. changed pretty everymuch resistor now and the caps too, but still measures high. or seems to. i have measured many el84 and ecc83 and all the good ones seem to measure EL84 measuring 12.5 as opposed to 8.

16th Mar 2019, 8:07 pm	#6
sven_swe Pentode Join Date: May 2018 Location: Cologne, Germany Posts: 104	Re: Taylor 45C Valve tester. Last time I checked my manual had an error for EL84 and that is mentioned in this searchable valve chart that was posted on this forum(they differ often), not sure if that has any influence though: https://www.vintage-radio.net/forum/...40&postcount=5 https://www.vintage-radio.net/forum/...ad.php?t=66896

16th Mar 2019, 8:24 pm	#7
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. I just measured a load of 6v6 and they measure high as well. should be 2.6 but all are over 3. all the valves I have measured are far nearer the data sheet figure than the Taylor figures, having studied the diagrams i cant see the type of rectifier making much difference as they provide adjustable supplies, I am begining to wonder if the meter has an issue or if theres a shunt I have missed thats a bit high. maybe its a tolerance issue, 10% here and there will soon add up if its all the same direction.

16th Mar 2019, 8:35 pm	#8
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Taylor 45C Valve tester. If it has silicon rectifiers in place of the metal oxide originals then it will be high on its GM readings, the recommended replacement was the GEX34 diode. Are the original rectifiers still present, if so you could reinstate them, I have never known them to fail. __________________ Stephen _________"It`s only an old telly" ___

16th Mar 2019, 10:40 pm	#9
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. No they're not there. I will try adding extra diodes then to increase the volts drop and see what happens..... is the bridge or single diode likely to have the biggest effect? Last edited by Megatron; 16th Mar 2019 at 10:47 pm.

16th Mar 2019, 11:56 pm	#10
sven_swe Pentode Join Date: May 2018 Location: Cologne, Germany Posts: 104	Re: Taylor 45C Valve tester. I read in this thread that they replaced the selenium rectifiers with 1N4001 with good results. https://www.vintage-radio.net/forum/...ad.php?t=66896

17th Mar 2019, 12:19 am	#11
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. I am pretty sure that's what's in there. Don't the DC supplies just provide bucking voltage for zeroing the meter movement? So I wouldn't have thought they were that critical.

20th Mar 2019, 10:17 pm	#13
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. Measures an EL84 as a golden sample. Gm 12 book says 8 changed all the rectifiers for germanium ones. re measured the golden sample. Gm 12.5. going to test the meter next i think, maybe its springs gone weak?

20th Mar 2019, 11:23 pm	#14
Megatron Pentode Join Date: Mar 2019 Location: Monmouth, Monmouthshire, Wales, UK. Posts: 166	Re: Taylor 45C Valve tester. getting somewhere now i think, if i set the voltage across the cathode resistor to 1v by adjusting the grid voltage, when i press the test button i get for the EL84 a Gm of 8 repeating with the 6v6GT i get 2.5 (book says 2.6) it must be something to do with the grid voltage? anode and screen voltage are correct(AC) Grid Potentiometer reads OK end to end. and ac transformer output is OK. however the grid voltage set point to get the 1 volt across the cathode resistor is about double what you would expect.......