UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 8th Oct 2015, 9:54 pm   #21
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hi Chris, yes i could try that as I dont have a larger 15R available, plenty of much nicer 4R and 8R though.

This is what I have done now:
I checked the output stage on a sine wave 8Khz with a resistor load (15R). Monitored the output on the scope.
The generator output was gradually increased until the output started to distort, that took place at an input of between 45mV and 50mV. At 45mV the output was at 6.5V undistorted. Im not sure quite what that proved but it gives a figure for the maximum input that does not cause the distortion.

I then turned to the RF side:
A 470Khz modulated signal was applied at R1 (switch side).
The audio signal produced at the top of the VC was monitored (scope Channel 1)
The audio output (Terminated 15R) was monitored (scope channel 2)

If the generator was adjusted to give more than say 46mV or more at the VC then the audio output was distorted. The signal at the top of the VC looks not to be distorted.
Full volume can be achieved if the signal at the top of the VC is no more than 45mV.
For this state the input from the signal generator was found to be 15mV.

I also did 2 quick checks using a radiated signal 1 at 300Khz and 1 at 1000Khz.
I found that it was quite easy to radiate sufficient signal to cause the overloading and distortion. I don't know what the value of the radiated signal was.

I now wonder if this problem is a direct result caused by the modified IFT2 as apparently the earlier version does not have this problem, (my friend has another late version with the same problem but also has early versions that don't)
I hope he sends it back to Bush for service ..

Dave.
davidgem1406 is offline  
Old 9th Oct 2015, 12:47 am   #22
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
Default Re: Bush TR130 distortion and overloading.

It does begin to sound like there's a mismatch between the detector output level and the audio level for max undistorted output.

What happens if you monitor the output from the detector as you increase the RF input level?

If it levels off with the detector output undistorted, then at least the AGC is working and you could reasonably just live with issue.

The audio gain change controlled by S2 is somewhat unusual and doesn't appear to be explained in the service data, but if C31 is removed, the gain on MW will be reduced.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 9th Oct 2015, 11:53 am   #23
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hi Chris,

I have been monitoring the level at the detector output but at the top of the VC (previous post paragraph 3), or are you meaning at the junction of the diode and R15?

S2 is actually SA on the service data im using. I just noticed that the early version shows the switch as closed and taking C32 to ground. The later version shows it as open so that C31 goes to ground via R19.
All switches are shown in the MW position in each version.

C15 you mentioned earlier (VC wiper to ground) makes no significant change, just alters the hf tone slightly.

Dave
davidgem1406 is offline  
Old 9th Oct 2015, 2:20 pm   #24
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
Default Re: Bush TR130 distortion and overloading.

What I meant was looking at the sinewave audio at the top of the VC as you increase the level of mod RF from the sig gen. If it rises with RF then levels off and stays undistorted, then the AGC is working and the problem is simply too much audio gain for level at which the AGC limits. Just turn down the VC to listening level and go with it.

I wonder if it's just a design compromise forced by limited AGC control range?
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 9th Oct 2015, 3:11 pm   #25
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

I thought I had done that but will do it again and post the result.
Thanks Chris.

Dave.
davidgem1406 is offline  
Old 9th Oct 2015, 8:44 pm   #26
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

I have now checked that one again and it would seem that the AGC is just about doing nothing.
Starting the signal generator at about zero and slowly increasing it the audio at the top of the VC just goes on increasing to the point of causing the output distortion.
I think there is just to much gain available.
It is actually quite difficult to determine how high the audio from the detector is rising as there is a lot of noise present that makes the waveform rather messy. I guess this is because of the amount of sensitivity in the RF circuits.

However I did notice in the information about realigning the IF's etc one could decrease the sensitivity by adding 8K2 from the junction of R11, C19, R7 to ground.

I added that and looked again at the waveforms, they were much clearer.
It certainly appears that the AGS does not do much at all and the audio from the detector just keeps rising with the generator input.

Then I connected the speaker and checked how it was with the generator removed and stations tuned in. The result was much better I could go to near full volume before the distortion started.
I'm looking at the audio out waveform right now and it's near to full volume and the peaks of the waveform are just starting to flatten.

Another thing I noticed was that the tone control has a dramatic affect on the detected audio waveform it increases and decreases considerably with the control. I think this is probably a very poor place to put the tone control.

I will check the other wavebands but I think the 8K2 stays in place and we live with the result.

Dave.
davidgem1406 is offline  
Old 9th Oct 2015, 10:20 pm   #27
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

I have now removed the switched connection from the junction of C31 and R19 so that it is now open on all bands, (service data shows it as open on MW but it was actually closed). Doing that has reduced the gain of the output stage.

I have had enough for today and will check the full results tomorrow, goodnight all.

Dave.
davidgem1406 is offline  
Old 9th Oct 2015, 11:31 pm   #28
TrevorG3VLF
Rest in Peace
 
TrevorG3VLF's Avatar
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
Default Re: Bush TR130 distortion and overloading.

The output of the IF stages should change little as the input signal is varied. The AGC line should increase as the input is increased. Does it do so?

The AGC response of a transistor is not very good and the Mullard reference manual of transistor circuits shows an addition to damp an IF transformer using a diode which conducts when the signal and AGC voltage are large. Does the Bush have such a circuit?

You need plenty of gain but it should be controlled.
TrevorG3VLF is offline  
Old 10th Oct 2015, 3:36 pm   #29
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hello Trevor,
The Bush does not have any form of diode, or any other as far as I can tell, damping in the IF.

I just checked it all again, the 8K2 resistor added earlier was lifted, this is what I found.
With increase or decrease in input signal (using a generator) the base and emitter voltages go up and down but not by much. No DC change can be seen at the collector, however there is no resistance other than from the IFT in the collector circuit.

With a scope on the collector the signal increases with generator input and then does reach a point were it starts to stop increasing, but this point is already to late as an increase in volume produces the distortion of the audio, the original problem.

If the generator output is increased further a point is reached where there is a sudden cut of, the volume then drops. I think this is nothing more than just overloading of the AGC transistor, thats my thought anyway.

Dave.
davidgem1406 is offline  
Old 10th Oct 2015, 7:41 pm   #30
TrevorG3VLF
Rest in Peace
 
TrevorG3VLF's Avatar
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
Default Re: Bush TR130 distortion and overloading.

I do not have a circuit diagram but I think that it was stated that you have an AF117. Germanium transistors can leak and still work. Perhaps the IF transistor will pass current even when the bias is low.
TrevorG3VLF is offline  
Old 11th Oct 2015, 3:36 pm   #31
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Yes AF117's were originals, they have been replace, my earlier posting #13

I searched out some circuits where there is a diode fitted between IFT1 and IFT2 so I fitted one in the bush.
Anode to IFT2 pin 1 and cathode to IFT1 pin 2.

That has improved things quite a lot but I feel that the diode is doing all the work as opposed to the AGC circuit.
On LW I can have full volume without distortion, the strongest signal being available is BBC2.

However on MW I still go into distortion if the wire at the junction of SA b is reconnected. If it remains disconnected then I get near full volume as the gain of the audio stage is reduced.

What I would like to do is to change the values of the components C31, R19 either 1 of them or both, so that the gain can be reduced to a point that clears the distortion. My guess is a change in C31 would be the one, as long as it does not affect the LW to much.

Having said that I don't see why it was deemed necessary to increase the audio gain for the MW only.

The circuit diagram is here: http://www.service-data.com/section.php/1089/1/tr130
Dave.

Last edited by AC/HL; 11th Oct 2015 at 4:49 pm. Reason: Forum rule B8
davidgem1406 is offline  
Old 11th Oct 2015, 5:19 pm   #32
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
Default Re: Bush TR130 distortion and overloading.

I could see some point in the gain increase for the Luxembourg band, if only to give the appearance there was some point to its existence, since the set should work just as well properly tuned to it on MW band. Maybe the switch wiring was a cockup from the word go?

At any rate, it does sound as if the AGC operating point on your set is wrong- whether from drifted components, dodgy transistors or marginal design, who knows?
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 12th Oct 2015, 11:34 am   #33
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hello Mods,
You removed my circuit from the posting and added a link to service data .com
The circuit from Service Data is not the correct version that is why I posted it.
There are 2 versions 66 to 67 (Service Data.com version) and 67 to 68 (My version from R & TV Servicing).
Regards Dave.
davidgem1406 is offline  
Old 12th Oct 2015, 11:41 am   #34
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hello Chris,

Yes, I could not see much point to it being that way.
My comment about the switch showing as being wrong was caused by bad drawing. It has been drawn with a large gap between the contacts, like a PB switch in fact it is slide, It just registered as wrong in my head, sorry another oopsy!

Dave
davidgem1406 is offline  
Old 12th Oct 2015, 3:42 pm   #35
AC/HL
Dekatron
 
AC/HL's Avatar
 
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
Default Re: Bush TR130 distortion and overloading.

Hi Dave,

Had you made it clear that the version posted had significant differences to the one on VRSD I wouldn't have edited it. In the event, we've decided it's acceptable if you would like to re-post it.
AC/HL is offline  
Old 13th Oct 2015, 8:55 pm   #36
davidgem1406
Heptode
 
davidgem1406's Avatar
 
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
Default Re: Bush TR130 distortion and overloading.

Hello AC/HL,

Thanks for the reply but its OK, I have solved the problem so the circuit is probably not wanted now, in the case of it being required I will upload it at that time.


Hello All,

The radio is now working quite well, with all the heavy distortion gone, all that is left is a small amount right at the top end of the VC.

Two things have been done first a mod installed putting a diode in between IFT1 and IFT2.
The 2 components R19 and C31 have been removed, this keeps the audio output gain much the same for LW and MW, much better.

The diode mod consisted of cutting the PCB track between IFT2-2 and the -Ve rail and fitting a 3K3 resistor decoupled by a capacitor 0.1uf to ground.
The diode, an OA81, was added from IFT1-2 to IFT2-1, anode to IFT1 and cathode to IFT2.

Things are much improved and im quite happy with the result now.

My grateful thanks to all,
Dave.
davidgem1406 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:37 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.