Bush VHF90A - an unexpected 'fix'?

[Joe Ingle]

Dear all,

I've recently been working on a Bush VHF90A with generally unsatisfactory results. I have a Bush VHF90 which is one of my best performing VHF radios, so I was expecting a similar standard, using the same aerial setup (basic - a piece of wire running near a window)

I recapped all the 'waxies', plus the discriminator electrolytic (connected the right way round, +ve to chassis), but found that the reception just wasn't as good as the VHF90 - weak, hissy and a bit distorted.

There was sporadic crackling which I've encountered before, due to HT crossing the trimmers in the VHF head - I fixed this as per another thread on this forum - a 50pF or so capacitor in series, and that was cured.

I also checked all valve voltages against the Trader sheet and they were spot on.

I swapped over all the valves from my working VHF90, and it made no difference.

I read on another thread in this forum that the many .001uF decoupling capacitors can cause trouble, so using hookup leads I connected a fresh one across all of the existing ones in turn, and it made no difference. Whilst doing this, I was (a bit aimlessly I admit!) touching the capacitor hook-up probe onto other connections, and found that if I connected it from the anode of v5a to ground (this being the output from the 'top' diode in the discriminator, the one that goes to the -ve end of the discriminator electrolytic) the reception suddenly got louder and stronger, so I made the connection permanent, and the set is much better. Still not as good as the VHF90, but acceptable.

I also noticed that all of the 2.2MO resistors on what I'm gessing is the AGC line have gone high, though I'm not sure I totally trust my meter when it comes to measuring high resistances. Could this be damping down performance? I did't try replacing them...

Anyway, I thought I'd write this up as I have no idea at all why the 'extra' capacitor should have helped. I also wonder if someone with more knowledge than me will be able to use this info to tell me what might be wrong with the set at root - I can't believe I've stumbled on something the Bush designers didn't know - I'm guessing I've addressed the symptom of a problem rather than the cause.

Also - a simple side question - I used a 4.7uF discriminator electrolytic rated to 25v. The capacitance is a good match to the original (5uF), but is 25v enough?

Thanks for any help / ideas...

Joe

[Nuvistor]

If the 2.2M resistors are outs of spec then I would replace them, they will only get higher in value. Any paper capacitors I would replace whether wax or moulded. Have you checked the alignment, it could have been got at and distortion very often points to the last IF transformer off tune. I would not be altering it without some form of accurate signal source, the 001mfd is affecting the ratio detector or the last IF amp.

In the sets I have seen the AM rejection capacitor have had quite a high working voltage, the radios I recently repaired had 150vw ones fitted originally. I measured the voltage on a strong local station and it was about 35V across the capacitor, I fitted 63VW ones. Measure the voltage across the cap on your aerial with the strongest station, that should reveal is 25vw if enough for your application.
Frank

[crackle]

Hi Joe
So as I understand it, you in effect added another 0.001uF across the discriminator capacitor. It is quite common in KB VHF sets (and probably many others) to see a 0.001uF capacitor across the discriminator capacitor.
I dont understand why this would improve the working of the radio, but I would try another 4.7uF capacitor of say 150v working.

Mike

[Joe Ingle]

I've realised I gave some incorrect information above. The extra .001uf capacitor I connected was from v5b anode to ground (ie the uabc80 anode connected to the last I.F. Coil), so it's not across the discriminator capacitor after all.

I measured the voltage across the discriminator capacitor on a strong station and it only peaked at about 2v, which sounds rather low?

I'll order some 2.2m resistors for the agc line. I measured them today and all three are at least 50% high.

Joe.

[Nuvistor]

Again don't know why in that position the extra capacitor should help. It is possible that it is altering the tuning of the discriminator winding.

The voltage across the AM suppression capacitor depends on signal strength and the gain of the radio. Measure the voltage using the same aerial and station on your good radio and see what the difference is. It will probably be higher but at least it gives you a reference.

I suggest you recheck all the work you have done, quite easy to make a mistake and have all the paper capacitors been replaced?
Frank

[Nuvistor]

If you want to adjust the discriminator the instructions for adjusting IFT4 is in the manual, you will need to remove your extra capacitor first and I would follow Mikes advice on replacing the AM suppression cap with the correct voltage working one.
It is important that the signal generator is accurate, unless you can validate it you could make it worse. Take note of how much you alter the cores if you do try alignment so you can always put them back if no improvement.

Frank