Ferguson 621 U woes.

[poppydog]

I have a Ferguson 621 U here that I have quite a few issues with but the main thing I would like to start with is I have low voltage out of the rectifier. The sheet (Ferguson manufacturers) says the output of rectifier valve UY 85 (v6) should be AM 247V FM 242V, pin 3 to filter resistor r37 500r measures AM 231v fm229V, that’s a fair bit lower.

I have got a fair bit of hum with some rustling and whispering out of the speaker that’s the same regardless of volume level. Resistors r37(filter) and R38 (current limiter) have been replaced as they had gone high, r37 was flopping around on the board because the joints had failed completely and this has caused quite a bit of blackening of the board.

2 low value electrolytics c50 c59 have been replaced, the big electrolytic can with res/smoothing caps in it has been left and is cold to the touch.
Grid coupling cap has also been changed.

How can I work out if I have a bad rectifier valve, bad elec caps in the can or something somewhere else? I don’t want to go buying valves or changing caps only to find the faults are still there?
As this has a live chassis the less time I’m in there farting around the better.

Regards Poppydog

[GeoffK]

The live chassis must be connected to the neutral side of the mains. The chassis should then be safe to work on, with the usual precautions when working on any high voltage equipment. The UY85 rectifier valve might be low emission, or the resistance of the heater chain has changed if the valve filament is working at to lower voltage.

The U series 100mA heater valve line up is;

UY85 38V
UL84 45V
UABC80 28V
UF89 12.6V
UCH81 19V
UCC85 26V

Total 168.6V

The total A.C. voltage can be measured across pin5 V6 or pin5 end of R40 mains dropper and chassis. Or across pin5 and pin4 of each valve, the valves will have an overall high voltage higher up the series chain with respect to chassis so caution has to be exercised. Voltage measurement of the rectifier valve heater has to be done very carefully as pin9 V6 is at mains live potential and pin3 the rectified DC from the mains.

[jonnybear]

Those voltages you have of 247v and 242v will depend on the mains voltage and the quality of the reservoir capacitor, just because it is running cool does not mean it is good, as far as the hum is concerned I would disconnect reservoir/smoothing electrolytic and try known good one in its place, if you still have a hum short grid 1 of ul84 to chassis if you still have a hum possibly heater cathode leak.
John

[paulsherwin]

The HT is often lower than the service data suggests in AC/DC sets. The service data value is with a brand new rectifier and 240V mains, but the rectifier gradually loses efficiency and the mains is often closer to 220V than 240. I generally consider 200V to be an acceptable value for HT in these sets.

You can temporarily fit a 1N4007 and 200R resistor between the rectifier anode and cathode, but I'm not sure it'll tell you very much.

[Sideband]

Originally Posted by poppydog

measures AM 231v fm229V, that’s a fair bit lower.

...but probably not enough to make a lot of difference. I'd be more concerned about finding the hum and crackling issues before worrying about a few volts of HT missing. 20 volts or so will not stop the radio from working.

You can always temporarily substitute the smoothing caps for separate ones and see if there is any change. Similarly you can temporarily sub the rectifier for a 1N4007 and resistor as already suggested. These are standard tactics when tracking down these sorts of faults.

[poppydog]

Thanks for the replies.
I have shorted grid 1 of the ul84 to chassis and it cuts everything, there is no sound/hum at all. The big can with 3 elecs inside has been removed and put on Chinese tester and avo on ohms range and tests perfect, I don’t have anything here to test these high voltage caps properly, I have some caps on order that I can tack in when they arrive just to see if there is any change.
When the caps arrive I will try the diode and resistor, it might reveal something.
The concern I have is the blackening of the board, there are 1 or 2 threads about this on the forum, if it turns out that this is causing my problems, it will be game over.

regards poppydog

[poppydog]

I have now temporarily tacked 3 replacements in for C56 57 and C58 and voltages and hum are still the same.

AC voltage into mains dropper r38 is 238v ac and on pin9 of uy85 is 226v ac, pin 3 of uy85 230v dc (AM) 225V (FM)

The HT voltage at other end of r37/C57 is 175.4v (AM) 171.4v (FM).

I did think about trying the diode and series resistor but there would be a circus with sorting a resistor for the heater of the uy85 etc, unless someone can spot something else that’s not right is it worth try to find a good replacement uy85 to try?

The radio works on all wavebands, MW and LW is very distant on all stations but stable and FM is very unstable ie you move the mains lead or move around near radio and it hisses and distorts.

Regards
Poppydog

[Ed_Dinning]

Note that C56 is part of a filter network with the tap on the opt for hum cancellation, as such it's value can make a difference to the hum level

Ed

[poppydog]

C56 has been replaced with a 33uf.

poppydog

[poppydog]

Turns out this has a conductive board, this is not worth carrying on with so can this thread be closed.


poppydog

[Nuvistor]

How bad is it, can you drill the bad bits out?

[music-centre]

Is the hum very bad? I have had 2 or 3 of these over the years and maybe a slight background hum and the board a bit blackened in places around the output/rectifier valves but I always thought this model can work very well, sensitive on FM & AM and a good sound for a set that size.
Steve.

[poppydog]

The longer the radio is left on the louder the hum and the rustling starts to be constant.

I have cleaned the area of the board around the ul84 a bit more and its actually not bad at all, the area around the uy85 is quite bad, the whole of the board was covered in thick black crud and dust.
Measuring the voltage on the grid coupling cap (C54) is odd to me, I have 0v on 1 end of it and the other end that goes to grid 1 of the ul84 I have around 6v.

poppydog

[Nuvistor]

Either a leak in the UL84 to G1, PCB or valve base. Presume you have checked R28 and the voltage on the HT side of that resistor?

[poppydog]

I'll have a look in a bit Frank, I don't want to condemn the board if it may not be the problem. Substitution of valves is going to be expensive as i don't have any here i can try.

poppydog

[poppydog]

Ht end of r28 (120k) is 159.2v, meter on ohms says it measures 1.496 meg out of circuit, could this cause all my trouble?
I will look to see if i have a replacement tomorrow.

poppydog

[Nuvistor]

R28 requires replacing, 100k to 150k would work if you have no 120k. C54 will need to checked for leaks or it could make the voltage on UL84 G1 higher still. Do that after replacing the resistor and before applying power.
You don’t want to run the set too long with 6 volts on it’s G1, if there is too much anode current in the valve it could damage the output transformer.
Check r34, if that goes high it can cause problems.

[GeoffK]

The control grid is +6V wrc but negative of the cathode (+13V wrc) so the control grid is -7V wr to the cathode.

[Nuvistor]

Correct but G1 should be 0v in this circuit wrc to chassis i.e -13v wrc to cathode.
Suggested checking r34 grid leak resistor, if this went o/c the voltage on G1 could float.

[GeoffK]

It could be R34 open or gone high if the coupling capacitor is new and not leaky, or an internal fault in the valve. Though +6V didn't seem quite right.