Re: Bush BAC31 motorboating
 

There was continuity on all valve pins from the top of the pin to the end of the component leads connected to ensure any breakages would be picked up.

Finger on volume control brought in nothing.

On the DL94 I had 1.2 volts and on other valves one had 0.9 v and another had a few millivolts.
I then switched off due to my filament bypass bulging again...

I have also noticed a small buzz from the mains transformer so I hope that's ok....

Josh (who is not liking this radio much anymore!)

Re: Bush BAC31 motorboating
 

Only just coming back on line. A week away and back to the physio...

The filament voltage these takes is 7.5v. There should be 2.8v on the DL94 and 1.4v on the others. Check the heater smoothing cap and the other one as well as the resistors in the area. On the Bush sheet these are C23. C22, R13, R14, R15 and R16. Make sure these are OK and also don't overdrive these valves at all. HT is 90v. Try it on batteries first before playing with the Power Supply.

If the filament bypass is bulging or getting hot, then I would be inclined to suspect those capacitors listed above.

On the Bush Power Unit, check those resistors and the wiring.

Cheers,

Steve P.

Re: Bush BAC31 motorboating
 

Hi Steve, Glad your back with us :)

Are those part numbers the same as on the Trader sheet? That's what I'm working off.

Sadly I don't have enough batteries to make up the 90v and my eliminator is currently out of action.....

Cheers, Josh.

Re: Bush BAC31 motorboating
 

OK having fitted a new DVD writer in the PC I can now run the DVD and look at the circuit!

For test purposes only I would fit a higher voltage rating capacitor for the filament smoothing C28. Use 100V or higher...it can be put back to the lower voltage when the fault has been found. At least this will enable you to carry out some voltage checks without getting covered in capacitor goo!

OK chassis to point 8 (I'm using the Trader sheet), you should be measuring about 7 to 7.5 volts (may be a bit less if your reccy is low). If it's higher than this, then there must be an open circuit somewhere along the way. The voltage across C25 should not exceed 25V. If it does (I think you said about 50V) then it suggests and open-circuit on the lower end of R21. That could be wiring or perhaps S9(M) switch. As there is no load I would expect the HT to read high as well.

SB

Re: Bush BAC31 motorboating
 

Here's the filament circuit from the manufacturers manual. which is a little clearer than the trader sheet.

Check the voltage on pin 7 of the DL94. This should be 6.5v according to the Bush service sheet. If its significantly lower check the 700ohm resistor of the PSU (R21 on the trader sheet). If it's significantly higher check the filaments and the connections on the valve holders.

Keith

Re: Bush BAC31 motorboating
 

C28 has about 71 volts on the positive, no signs of heat or bulging yet though. It's a 50volt rated original capacitor.

I get 27vdc on Point 8

Pin 7 of DL94 measures 50vdc

All filaments and valve holders have continuity.

Looks as though it may be R21 then ???

Josh.

Re: Bush BAC31 motorboating
 

50V on pin 7 of the DL94 is not consistent with the filament string having continuity!

Re: Bush BAC31 motorboating
 

Everything tests ok though :shrug:

Re: Bush BAC31 motorboating
 

There looks to be a break in the filament chain

Check on each pin in the filament chain. At some point it will change from 50v to 0v. That's where the break is.

I had a faulty pin on a valve holder in the filament chain on one of mine. Fortunately there are unused pins which can be used as replacements.

Keith

Just a thought is the PSU chassis bolted to the receiver chassis as the ground connection is made through the bolt.

Re: Bush BAC31 motorboating
 

Now that is worrying. With the set on, quickly measure pins 1 and 7 on ALL valves. This should point us in the right direction.

Cheers,

Steve P.

Re: Bush BAC31 motorboating
 

That's a good point. I made that mistake when I first worked on mine.


SB

Re: Bush BAC31 motorboating
 

Yes it's screwed together. Will check valve filament voltages tomorrow.

Josh.

Re: Bush BAC31 motorboating
 

OK, here goes:
V1- DK92 Pin 1: mV Pin 7:mV
V2- 1T4 Pin 1: 46v Pin 7: 46v
V3- 1S5 Pin 1: mv Pin 7: mV
V4- DL94 Pin 1: 49v Pin 7: 49v

Re: Bush BAC31 motorboating
 

I'm scared stiff of 50V anywhere in the filament system!

There must be a defective joint / bad valveholder contact / whatever. but if you see this voltage, and start wiggling things till the joint makes good, then your 50V-charged electrolytic capacitor will discharge through the valves. Goodbye one or more!

Can you get a 10V Zener diode to tack across the filament supply to clamp the voltage, until you have found the fault?

Re: Bush BAC31 motorboating
 

Josh there has to be an open-circuit along the filament chain somewhere. When all the filaments add up to 7 volts then you can't have more than that on the DL94 end without something being open. There is only the filaments or the valveholders likely to cause this. The filament of V4 and V2 look good seeing as you have voltage on either side. However logical thinking should tell you that one filament pin of the 1T4 should also read the same as one filament pin the 1S5 as they are linked. However they don't....you only have mV apparently so logic tells me there is a break in the wiring somewhere.

Best thing is to disconnect it from the mains, switch you meter to ohms and put one prod on the DL94 (think pin 7), leave this on pin 7 and work your way through the filaments...so measure pin 1 of DL94 next, then measure pin 1 of 1T4 ( leaving the other prod on pin 7 DL94), then pin 7 1T4, then pin 1 of 1S4, then pin 7 1S4 and finally pin one and then pin 7 of DK92. This way you measure each part of the filament chain in turn and somewhere you should find a break.

Try it and come back. Whatever you do DO NOT try wiggling the valves in the hope of finding a loose connection....as Kalee said bye bye filaments!:o


SB

Re: Bush BAC31 motorboating
 

It looks as if there is a break in the wiring between V2 & V3. If the filaments were open circuit there should still be some continuity because of the resistors across the filaments.

Try removing all the valves with the set switched off and the measure the resisatnce between V4 pin 5 and ground. There should be approximately 1020ohms. If it's open circuit move down the filament chain. You should get the following result if the filament chain wiring is intact.

V4 pin 5 1020ohms
V4 pin 1 550ohms
V2 pin 7 550ohms
V2 pin 1 330ohms
V3 pin 7 330ohms
V3 pin 1 150ohms
V1 pin 7 150ohms
V1 pin 1 0ohms

I suspect there will be an open circuit between V2 pin 1 and V3 pin 7.

Keith

Re: Bush BAC31 motorboating
 

Ok, thankyou to you both :)

Will give this a shot either tomorrow or day after tomorrow.

Thinking logically? Me? :-)
:-D

Josh.

Re: Bush BAC31 motorboating
 

Here are some results (coursework is boring.... :-)):

With what Sideband suggested:
With one multimeter lead connected to pin 7 of DL94 I got:
DL94 Pin 1: 16.2 ohms
1T4 Pin 1: 25.5 ohms
1T4 Pin 7: 16.5 ohms
1S5 Pin 1: 4.5 Mohms, keeps growing
1S5 Pin 7: 5.5 Mohms, keeps growing
DK92 Pin 1: 5 Mohms, keeps growing
DK92 Pin 7: 4.72 Mohms, keeps growing

With what KeithsTV suggested:
DL94 Pin 5 to GND, should be 1020 ohms, measured 4.5M +
DL94 Pin 1 to GND, should be 550 ohms, measured 4.5M +
1T4 Pin 7 to GND, should be 550 ohms, measured 4.3M +
1T4 Pin 1 to GND, should be 330 ohms, measured 4.8M +
1S5 Pin 7 to GND, should be 330 ohms, measured 17.4 ohms
1S5 Pin 1 to GND, should be 150 ohms, measured 8.9 ohms
DK92 Pin 7 to GND, should be 150 ohms, measured 8.9 ohms
DK92 Pin 1 to GND, should be 0 ohms, measured 0.3 ohms

V1- DK92
V2- 1T4
V3- 1S5
V4- DL94

So a breakage somewhere between 1T4 and 1S5 I guess ??? (like what you both said ;))

Are there any components I should check as well as wires going between the two?

Josh.

Re: Bush BAC31 motorboating
 

Wires and valveholders. Even a piece of wire can hold a fault you know.

Please either put a zener in or use batteries for the 7.5v supply.

Cheers,

Steve P.

Re: Bush BAC31 motorboating
 

I don't think I've got any Zeners anywhere (I'm sure I had a couple somewhere :-))

Could someone send me a link to a suitable one on the RS site please? (Just had a look and there's loads....) Or failing that if anyone has one to spare.....

I will test wires etc for continuity tomorrow. Will also check all valve holders again.

Josh.