Another Bush VHF72
 

Given to me recently. It was working quietly on all bands so I replaced the coupling cap to UL84 ; no change. Voltage on grid 0.05v. Sound poor quality and volume but better on VHF , with some buzz across VHF band . It has been extended to get 101FM. bulbs were initially erratic with no sound when they were removed. Now using 6v 0.1A and just right . All voltages checked and about right compared to Trader Sheet. One relevant thing is that there is little response when a screwdriver is touched on tag of volume control- just a little crackle ,with vol. on max.
It looks almost original inside. I am not sure there is anything coming out of the electrostatic speaker, sound is quite sharp. Definitely out of main speaker. It has all its original wax caps so I will replace them next .There are several on the transformer . I have checked most of the resistors and seem to be OK.
The magic eye works well in dark room and gives full deflection on FM stations.
Am I on the right lines here?Given to me recently. It was working quietly on all bands so I replaced the coupling cap to UL84 ; no change. Voltage on grid 0.05v. Sound poor quality and volume but better on VHF , with some buzz across VHF band . It has been extended to get 101FM. bulbs were initially erratic with no sound when they were removed. Now using 6v 0.1A and just right . All voltages checked and about right compared to Trader Sheet. One relevant thing is that there is little response when a screwdriver is touched on tag of volume control- just a little crackle ,with vol. on max.
It looks almost original inside. I am not sure there is anything coming out of the electrostatic speaker, sound is quite sharp. Definitely out of main speaker. It has all its original wax caps so I will replace them next .There are several on the transformer . I have checked most of the resistors and seem to be OK.
The magic eye works well in dark room and gives full deflection on FM stations. Sound quality is much worse on AM.
Am I on the right lines here?
I have subbed all the valves into a good set and they all seem fine {Stella 239U} also squirted Servisol down valve sockets.

Re: Another Bush VHF72
 

Can you be more specific regarding the valve voltages measured on UABC80's triode and the UL84?

Lawrence.

Re: Another Bush VHF72
 

Electrostatic speaker is fixable, there are threads here.
Assuming you have around 190v on the anode, pin 7 of the UL84 & 70v on the UABC80 anode pin 9, then I would suggest checking C32, C38, C39, C40, C44, C45 & C46 first off.
All #s are from Bush manufacturers sheet, others may differ.
There are modifications to the pilot lamp wiring on this sheet too.

Re: Another Bush VHF72
 

I checked the voltages again last night and found that the 70v on UABC80 Anode was variable , anything from 75 to 60 v and dropped more after 15 mins . I was listening to VHF , not AM. The UL84 Anode was down but stable at 178V( not 190). I suspect a 270 PF cap and 180K resistor. I will test again soon on AM. I also noted that the pins 7 and 8 on the UL84 were both 178v and pin 8 is 'IC' with nothing connected to it.
I only have the Trader Sheet 1439.

Re: Another Bush VHF72
 

I have retested the voltages and realised the variable one on an anode was due to being tuned on a station.
UABC80 (on AM off station) -0.4,-1.0,-0.4,*,*,-0.5,0,0, 0,61 {pins 1 to 9}
UL84 ditto 0,0,12,*,*,0,185,185,165 .

I have replaced the suspect 180K and 270Pf items but no effect . Without a scope I am not sure that the voltages from the smoothing caps are smooth.
How useful is a Cap , ESR test on the 3 parts , out of circuit, on a little component tester? What would be good values for such a test?

Re: Another Bush VHF72
 

If its not humming like a top, the smoothing is OK unless they are getting hot.
ESRs in the 3 to 8 range should be good enough.
Check those paper capacitors, its the most likely fault.

Re: Another Bush VHF72
 

I have isolated the tweeter as nothing coming out, although there is 110v feeding it .It reads 91Pf on my meter in isolation and. the sound isn't improved by removing it. take it apart ?

Re: Another Bush VHF72
 

Inside the tweeter there is little to see. Is something missing?

Re: Another Bush VHF72
 

No, there is almost nothing to them.
There are threads on the forums that you can read for fixing them, basically its down to cleaning and careful reassembly.
They don't make a massive difference to the sound but you can tell when they work, more top colour in the notes.
You will need to sort out the audio problems first though.
How far have you got with the output fault?

UL84 connections are:-

1 2 3 4 5 6 7 8 9
ic g1 k,g3 h h ic a ic g2

So your voltages are about right, No + volts on the grid #1 is good, coupling cap must be OK, 12v on the cathode means the valve is drawing current (it may be a bit high, what value is the cathode resistor? I would have thought about 180 ohms), anode is above grid #2 volts, that's OK.

Pin 8 is ic, internal connection, don't know to what or whether Bush used it as a tag strip connection, anything connected to it?

Re: Another Bush VHF72
 

No nothing connected to pin 8 of UL 84.
I haven't found any specific faults yet but I will. Next is to replace relevant waxies and then take valves for testing, if no improvement .

Re: Another Bush VHF72
 

Thought, does the tone control have any effect.
Have you an aerial wire in the am aerial socket?

Re: Another Bush VHF72
 

Tone control works as expected with slightly more volume on max treble. Fitting a long wire gives a slight improvement on AM.I have checked aerial coils for continuity.

Re: Another Bush VHF72
 

I am now going to test the speaker and o/p trx by substitution as I am baffled>:( Could a speaker lose magnetic qualities?

Re: Another Bush VHF72
 

I would be tempted to replace this with a 3" cone tweeter. Even if repaired, the cone unit will sound better.

Re: Another Bush VHF72
 

..... Of course, a cone tweeter would be a low-impedance device. You would need to connect it to the secondary winding of output transformer, across the woofer, and add a series capacitor to block the low frequencies. (Start with 2.2 µF and adjust up or down for the subjectively-best sound; it must be a non-polarised type, but need not be rated more than 50 V. as it is on the low-voltage, high-current secondary side.)

But these Grundig electrostatic tweeters are eminently repairable devices. There are descriptions on here of how it is done (the same tweeter is also found in the Bush SRP31), and videos on YouTube showing it done.

Re: Another Bush VHF72
 

Not much progress. I have replaced any suspect wax caps and others in the output stage. No change. I have injected a signal from a CD player to the grid of UL84 output valve . ..quiet still .Also to the TRX primary ..the same . To the speaker direct and louder but still quiet. What does this tell me and where do I go now. Try another TRX , which I don't have ?.

Re: Another Bush VHF72
 

It tells you that the output stage UL84 is doing nothing to amplify the signal and that the output transformer is OK.

You have the pin outs for UL84, give us all the voltage readings. Pins 4 & 5 are AC (heater) the rest are DC. We can take it from there.

Check the cathode resistor.
You have replaced the grid 1 coupling capacitor I believe.
Disconnect the electrostatic speaker, just in case it is short circuit.
Check that the screened leads around the volume control are not shorting the inner to the outer grounded braiding.

There can't be much wrong now.

Re: Another Bush VHF72
 

I want to try another test .Disconnecting the input to the volume control and connecting it a mains powered computer speaker amp but concerned that this is an Ac/DC set with chassis being the common base for the input to amp. Will it trip the mains? I have had similar events in the past!

Re: Another Bush VHF72
 

Can you check those UL84 voltages? Don't go off at a tangent, proper fault finding will win out.
Connecting unknown computer amplifiers may cause more problems.

Re: Another Bush VHF72
 

I posted the UL84 voltages earlier in this topic and I doubt they have changed. I was wondering whether the voltages MAY be correct at the pins but not getting through to the valve .That is why I cleaned all the sockets and checked for continuity through all valve pins . Nothing spotted.???