I've had this set for a number of years and it's restored quite well although the cabinet mask is somewhat tatty but the CRT is a belter. There were quite a lot of faults and some interesting value changes around the video amp but I put it all back to original. I was however left with a niggling fault that I've not got to the bottom of and that is ripple (hum) on the picture. It's not there from cold but shows up after about 15 minutes to the point where it affects frame sync. Now I am aware of the strange (to me) bias of the frame output valve using the rectified heater supply via V4 and V8 heaters to provide a negative bias to the grid of the 30PL14. Just to clarify, all paper caps have been replaced where necessary. The main smoothing block (4 in one) has been replaced with a new exact replacement from stock, the heater rectifier has been replaced with a BY227, correct timebase valves have been fitted (it originally had a PY83 in place of the PY801 which took forever to warm up) and a PL81 in place of the PL81A (although I have no idea what the difference is, if any, between the two).
Since I thought that a cathode-heater leak in any one of the valves might cause a problem, I have systematically gone through every valve in the set by substitution but the hum still persists. The only thermionic device not checked is the CRT but I'm pretty sure that hasn't got a cathode-heater leak.
I haven't got technical with the scope yet since I need to lug the thing out from it's hiding place. Before I do, does anyone with more knowledge of Thorn sets have any pointers that I might have overlooked? I wonder if it might be leakage across the PCB since some areas are a little charred but I didn't think it was excessive.
Any pointers would be very helpful.
Thanks for any ideas
Rich.
@sideband
Are there any earthing points to chassis via chassis strengthening bars. I don’t know the chassis but have seen that sort of problem due to the solder breaking around the tab used to anchor the bar. Temperature changes causing the fault to appear, or disappear for that matter.
Frank
Hi Frank, it's worth checking but my experience of those sort of earthing problems is that tapping the chassis can make the fault come and go. I'll get the magnifier out though since it's an easy check. I'm tending to drift more towards PCB leakage which usually increases with heat. Someone had made a bodge of resistors near the centre of the PCB and there is some scorching there. I didn't actually replace the heater rectifier and I'm assuming that a BY227 is OK. I see no reason why not but I suppose I could try something else.
Some info for you Frank (and anyone else not so familiar with this chassis) the frame output valve 30PL14 uses grid bias rather than cathode bias which looks a bit odd to start with. The cathode is taken straight to chassis and a negative bias is applied to the grid derived from the rectified heater supply via V4 and V8 with a resistor and capacitor for smoothing. I can't help thinking this is a recipe for problems since any AC getting to the grid of the frame output valve is going to cause picture ripple. It was done like this so that in the event of the heater rectifier going short and overrunning the valves, it rendered the picture unwatchable because of the AC that would then be present at the frame output grid.
Rich.
I knew that I'd been around here before with the same problem. The mind gets foggy after a time....Thorn 981 chassis. Picture ripple. – Black & White Television – VRAT Forum (radios-tv.co.uk)
At that time I replaced the PY801 which seemed to cure the problem. Obviously a Red Herring. So back to the ripple....
The BY227 appears rated for 1250v 1.5A, assume RRMS and Peak current so it should be OK.
I don’t recall the frame bias type protection but other makes had similar sort of ideas, RBM feeding the Sync separator screen grid voltage from the rectifier heater supply.
A leak in the circuit board is definitely possible, the sets were not usually old enough when I fixed them to have that problem apart from the infamous Pye 697 chassis PSU PCB and CDA PCB.
Frank
I think it will have to be a scope on the grid of the frame output valve to see where all the hum is coming from.....
On the bench I used the oscilloscope more than a multimeter when faultfinding, using the meter when accurate voltage reading were needed.
I am interested were the hum is coming from, be a shame if it is the PCB, but hopefully repairable if it is.
Frank
I'm not sure whether it was intended or not but as with the 1400 series which employed a similar method of biasing a s/c heater rectifier produced a rolling mess on the screen. Unfortunately the customer seldom switched off and continued to listen to the programme sound, this lead to serious overunning of all the valves and CRT of course. The RBM A774 used a second protection rectifier reversed biased across the output of the heater diode. If it went s/c the both +ve and -ve half cycles appeared across the protection diode causing the fuse to blow.
Is the hum actually modulating the frame scan or just producing a darker bar?
John.
Is the hum actually modulating the frame scan or just producing a darker bar?
It's actually causing a visible ripple on the picture and gets to the point where the frame actually rolls once with each ripple. It doesn't appear to be there from cold so the plan is to monitor the frame drive and see what happens. I'm going to put it aside for a few days though since someone has just sent me a Pye 'Black Box' amplifier for repair....customers come first!!!
Hi.
It could be a leaking PCB around a valve base from heater to an adjacent G1. What you could try but this needs to be done quickly is to add in some smoothing into the negative heater line on the anode on the heater rectifier and see if this reduces or stops the issue. Remember adding a smoother effectively increases the heater volts so just quickly prod the cap on the diode anode (The heater supply is negative) if it clears you have a leaky PCB. Use a 47uf 350v or what ever you have.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
Hi Trevor,
I was beginning to think along the lines of a leaky PCB....I'll give your suggestion a try when I get back to the set....
BRC 980 series, is this the chassis in which the bias for the frame output valve is derived from the heater chain?
A BY114 supplies negative half cycles to the valve heaters, failure of the diode could cause serious damage so in the event of the diode going short circuit the bias to the frame output valve will be lost resulting in the frame timebase operating in an unstable manner.
Till Eulenspiegel.
BRC 980 series, is this the chassis in which the bias for the frame output valve is derived from the heater chain?
Yes
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Yes Till. See my post #3 where I explain about the heater rectifier and grid bias for the field output valve.
