Bush TR 82C repair advice wanted

[ducatiguzzi]

Thanks for your help

I am now getting a good range of channels so hopefully not too far away. I de-soldered the screen lead on the AF117s but it made no difference to the hiss, reception 'sounds' poor except for the strongest signals but always with the background hiss

I would imagine the audio side is working given the music and voice I'm hearing,

I'll fumble through the voltage checks just now and see what I get. I was considering taking the AF117's out and blasting them but due to the fact that the screen removal never helped the hiss an I correct in saying its probably not worth it?

[AlanBeckett]

Something I discovered today to my detriment is that WD40 contact cleaner melts the plastic, I still has the volume, tone and band knobs still on when I gave the chassis a good dowsing so they're looking pretty shabby now, bugger!

There but for the grace of God goes everybody. Step forward anybody who hasn't accidentally buggered something at one time or another

I've used WD40 Contact Cleaner without any problems although I usually stick to Servisol. However, I'll bear your experience im mind in the future.

Alan

[Mark Hennessy]

I was considering taking the AF117's out and blasting them but due to the fact that the screen removal never helped the hiss an I correct in saying its probably not worth it?

With these, a tin whisker making contact with one of the internal electrodes is normally enough to stop the set functioning. But once that's happened, the whiskers don't suddenly stop growing. Their growth is not fully understood by any means, but we are reasonably sure that the presence or otherwise of an electric field is not a significant factor. So if your set has been dormant for 20 years - say - since it originally stopped working, then in that time, other whiskers might well have made contact with other electrodes.

In that scenario, you have a short circuit between 2 or 3 electrodes of the transistor, and cutting the screen lead will not help. Indeed, once someone has cut a screen lead in the past to get the set going, you can be sure that sooner or later, the thing will fail again. I always advise against cutting screen leads for that reason.

The only way to know if you need to "blast" a transistor is to remove it and measure the resistance between the case and each of the legs in turn. If there is any hint of leakage - even 100s of k - then blast it. Indeed, as you've removed the transistor, you might as well do it anyway. It does sometimes change things, inexplicably (perhaps the physical disturbance while removing it is enough to temporarily displace the whiskers?).

In case you haven't seen it, I did write about this problem a while back: http://www.markhennessy.co.uk/articles/v...istors.htm

To the specifics of the set, it sounds like it's working, but with too much background hiss?

The first thing to establish is where is that hiss coming from. Is it the audio amp, or is it the tuner? To do that, simply turn the volume to minimum, and assess how much hiss is left.

(If that sounds like a case of stating the obvious, it's actually a common problem with many Hacker sets that use a transistor in the audio amplifier that is prone to failing to noise, but some folk think it's a problem with the tuner and report "poor tuning" or similar)

Next, is the hiss actually local interference? My workshop is in my cellar, which is poor for AM reception generally, but added to that there is a computer or two (my LCD monitor wipes out LW), some high-frequency light fittings (which run at 100kHz typically), plus a small rack full of IT gear and a load of Cat5 cables running there. I have an Ethernet switch mounted on the rear of my bench, and I have to turn that off to line up a radio (it especially affects FM). And interestingly, different radios respond to this "electronic fog" differently - so it's not enough to assume that because radio A is OK, radio B must have a fault. So, try taking the radio somewhere "quiet" - perhaps outdoors?

Finally, if you're sure the front end is definitely unwell, that's when things get complicated! Start with the DC conditions (essential in any fault-finding), and be suspicious of leaking capacitors that might change them (e.g. the AGC capacitor). Check that the resistors are not wildly off their value (note that carbon composition resistors drift high). Then, consider aligning the IF cores - not an easy job for someone who is new to all this, but not impossible either. Resist the temptation to randomly fiddle, but see if any of them improve the signal. If you turn them by no more than a couple of turns each way, and return them to the original position if no improvement can be had, then you're probably OK. Not making things worse, at least. Be very careful when adjusting, as the ferrite core inside is very brittle. Ideally, use the correct plastic tool.

Another possibly obvious one: is the ferrite aerial OK? Has it cracked and been repaired in the past? Does it look like the coils have ever been disturbed (their exact position on the rod affects sensitivity). Are there any broken wires (easy to miss)? I've had the windings fail just where they enter the solder tag - if held in place by tape or glue, it can be hard to spot that one! Finally, clean the wavechange switches. If you've already done so, do it again!

Have you got another example to compare with? Some early AM radios simply weren't very good! That said, by the time they'd got to AF117s, sets were generally pretty lively.

Anyway, hopefully a few ideas in all that - good luck!

Mark

[ducatiguzzi]

I only have a MK I service manual, will the voltages stated at the various test points be the same as my MK II?

Thanks again

Hi Mark

What is the AGC capacitor?
I turned the volume to minimum and the hiss remains
Outside there is less interference and noise but still the hiss
I reconnected TR1's screen lead just now to see what prevailed and it stopped the set working until some more blows were delivered, I'll blast it.
It seems the strongest stations are low in the spectrum or just where I live perhaps??

Cheers

[Mark Hennessy]

So if the hiss is present at minimum volume, then that's an amplifier problem. Forget about anything before the volume control.

The fault might be a faulty transistor, but it's impossible to say for sure. Check by substitution, starting with VT4 (OC71) first. Sometimes, carbon composition resistors go noisy. Again, check by substitution.

The AGC cap is C21 (8uF). If it is leaking, that could reduce the gain. If it is open-circuit, it could cause distortion.

[Refugee]

I would go for those red, black and yellow capacitors as they can and often do cause oscillation at high frequency.
That would also make a hissing noise.

[ducatiguzzi]

I've measured the voltages from the volume control onwards and they all seem about right give or take a bit, I'll post them if its of benefit, comparing it to the MKI service sheet. I have spotted at least one difference between the MKI & 2, I have an extra capacitor between TR4 and TR5. The only one that stuck out was 9.5v out TR6 and 7 push pull, a bit high?

The AGT (C21) measured 13 uF on a digital multimeter for the stated 8uF with one leg lifted but I didn't measure any DC voltage out of it.

I'll see if I can source the red black and yellow caps and the AGT cap, is there any particular characteristics I should know about?

Your help has been invaluble

[boater sam]

C21 is definitely shot, that's why it reads high, leaking.
These caps are just low voltage electrolytic capacitors. They have a wide tolerance so any near value and same or slightly higher voltage rating will be fine. You will find that new ones are smaller, if you can get the lead spacing the same it makes it easier to neatly install them into the pcb holes. 
Do you have a solder sucker to get the old ones out, or solder braid?

[Mark Hennessy]

No PCB in this set, Sam

The test is not conclusive. With a capacitor, you ideally need to measure 3 parameters - the basic value, the leakage, and the ESR. Here we have only measured value - which is within an acceptable tolerance. If it is leaking, any indicated deviation from the actual value depends very much on the capacitance meter - they are all different. I'm not sure what "I didn't measure and DC voltage" actually means, so I'd perhaps want to clarify that a bit. But in this set, the caps are known to be unreliable, and they are of no historical significance, so rather than think too deeply about it, I would just replace them. 10uF at any available voltage rating >9V would be absolutely fine for the AGC cap.

I do get a bit annoyed by people who do wholesale capacitor replacements in sets that don't need it - particularly the blue Mullard types used by Hacker, amongst others. But here, a blanket replacement of the electrolytic types would be a good idea.

[ducatiguzzi]

The comment "I didn't measure and DC voltage" was when the set was powered up I measured 0.016 Vdc downstream of C21 with 1.1Vdc upstream, I thought i might have measured more if the cap was leaking voltage, just out my head.

I woud also imagine the my DMM (Fluke 189) would typically read high on a 12V electrolytic given it is only charging the cap to 3V, again just a hunch. Any cap I have changed in the past (not on radios, industrial DC drive controllers and instrumentation equipment) it was visible or I had them changed anyway.

So I believe all your advice is spot on, don't think too much, just do it. I haven't got the test equipment for ESR anyway.

Oh, a new problem now the on/off switch is now sticking off following me squirting some contact cleaner in to clean the crackly pot track, WD40 type. I used to use Electrolube in the olden days, I'm sure it slightly lubricated as well as cleaned, the WD40 stuff is pure volatile solvent. New switch/pot?

I've got soldering equipment so shouldn't be a problem.

I'll do some cap shopping then, any preferred suppliers guys?

Thanks