Bush TR82C - help request

[TinaRotator]

Hello all,
I am a newbie and I am trying to learn bits about electronics by trying to fix things up. I had successfully fixed some older record players by changing capacitors, soldering stray wires etc. I wanted to try my luck with an old radio, so I had recently purchased an untested Bush TR82C SN 346/152231 and I am hoping to get it going again. I will appreciate any guidance from the knowledgeable forum members  .

The cabinet itself was just dirty, no major chips/scratches/burns, all knobs and toggle buttons are in place and seem to be mechanically okay. This model has car aerial socket and an earphone jack socket. 

Upon opening of the cabinet, I noticed that there is no battery. I purchased a new PP9 and realised that the negative connector doesn't snap on, so I ordered a generic new one on eBay and I am planning to solder it on once it arrives.

I discovered that the chassis model is A177. The entrails weren't very dirty, I couldn't see any obvious dry joints or any wires that came loose. I cleaned the waveband switch so that there is continuity in both positions. The transistors present are made by Mullard, OC44, 2 x OC45, OC71, OC81D and  2 x OC81.

I powered it up with the battery and switched it on. I get a bit of hissing noise, but nothing that would remotely resemble of picking up a radio station. The hissing sound persists in both long and medium waves positions, as well as all positions of the tone and volume. Turning of the tuning dial has no impact whatsoever on the sound. The ferrite rod and the wiring on it appears to be intact. If I plug the earphones in 

How should I proceed? 
Some pictures below - please click the hyperlinks.
Cabinet: https://ibb.co/k0SjPSb
Front of the chassis: https://ibb.co/8PkdXCN
Back of the chassis: https://ibb.co/dgQtPzj

[Crackle]

Hi
It is always a good idea to add photos to your thread because in time your external linked images may disappear.
Apart from that I do not click on links I do not recognise and your links look to me just like the dodgy ones that appear in spam emails.
Mike

[FIXITNOW]

Red/Black with yellow sleeve on top Plessey Electrolytic often give trouble going low ohm in value rather than the capacitor they should be you can measure across them usually in circuit

[Crackle]

But apart from my comments above the next thing I would do is obtain the service information for the radio and let us know which one you got. (there are several publishers of service information, apart from the manufacturer the "Trader" service manuals are quite good.)
Your radio is one of the early ones and the transistors in it are normally very reliable. But with transistor sets of this age it is often the electrolytic capacitors that can cause problems.
Sometimes these electrolytics can reform and and the radio may start to work on its own after a few minutes or an hour or so.
It would seem that the final audio stages of the radio are working, so after checking some of the voltages and comparing with the information in the service manual, my first suspect to check would be C31 (Trader) a 100uF electrolytic, which is decoupling the RF and IF sections of the radio..

Mike

[ppppenguin]

Among other palces you should be able to get a free copy of the circuit diagram here: https://www.radiomuseum.org/r/bush_tr82ctr_82_2.html

The electrolytics with red and black ends should be replaced on sight. The other electrolytics will be dubious too. Even if they reform after a while, they are unlikely to stay good for long.

Electrolytics have very wide tolerances, often -20% to +50% so the replacement values are not at all critical. The voltage rating of the replacement is likewise uncritical provided it's at least equal to the original. So a 63V rated part will be just as good as a 10V part in this radio. Don't overdo this by using a 250V rated part though it will be OK in practice.

If replace non-electrolytic capacitors the voltage rating in a transistor set like this is completely non-critical.

[TinaRotator]

Many thanks for your replies and sorry about the format of the pictures - I cannot see the option to upload an attachment, and copy pasting the pictures creates a blob which blows up the length of the post above the permitted limits. Inserting it in a tag renders a placeholder image rather than the actual one. Maybe that's because I have just joined?

Anyway, my snap on connector arrived and I soldered it on. I just ordered the capacitors to replace Plessy's C27, C22, C23. Thanks for this advice, I will hopefully replace them tomorrow and come back with an update.

Penguin - thanks for the bit about replacing capacitors and the ratings, I had previously struggled to find replacements for capacitors in record players due to their unusual values, your pointers are very helpful.

Have a good evening all,
TinaRotator

[TinaRotator]

Hello,
I would appreciate your advice again.
After I changed the 3 Plessy electrolytic capacitors the radio started to pick up 1-2 stations on each band, although the sound is really faint, and the volume adjustment doesn't seem to do anything. I can clearly make out that this is a radio station and human speach, but it's to faint to make out which station etc. The tone adjustment changes the sound distinctively, so I would risk saying it works. In laymans terms I would say that the sound isn't amplified at all similarly to a record player generating sound purely from a needle as opposed to the desired loud output on the speakers. What I can only describe in laymans terms, I hope you will be able to help me translating into the techincal jargon and actions.

I managed to measure voltages across the transistors. My multimeter is that of a very basic variety, so just for comparison it reads 0.0000mV when probes are in contact, and it picks up around 9.6V from the new PP9 battery. The measurements are as follows (I wil go by the Bush's service Sheet naming, the correct measurements from there are in brackets).

Transistor         E                     B                      C
-------------------------------------------------------------------------
TR1 OC44        1.2V (1V)          1.3V (1.0V)       7.2V (6.4V)
-------------------------------------------------------------------------
TR2 OC45        0.18V (0.25V)    0.33V (0.35V)   8.0V (7.2V)
-------------------------------------------------------------------------
TR3 OC45        0.42V (0.45V)    0.6V (0.6V)       7.4V (7.0V)
-------------------------------------------------------------------------
TR4 OC71        0.12V (1.0V)     1.3V (1.1V)       1.7V (1.5V)
-------------------------------------------------------------------------
TR5 OC81D      1.7V (1.3V)       1.5V (1.5V)       8.8V (8.3V)
-------------------------------------------------------------------------
TR6 OC81        0.03V (0V)        0.18V (0.2V)     9.6V (9.0V)
-------------------------------------------------------------------------
TR7 OC81        0.02 V (0V)       0.2V (0.2V)       9.6V (9.0V)

Some of the readings are high, some are slightly low, I would really appreciate your comments on this one.

I also had a read of a simillar  thread in this forum and I found this post:

The first port of call for a gain fault is the red/black capacitor C22 and C10.
For distorted loud audio C27 first then C21.
For tinny or weak audio C23 and C25.

You will be able to see that C22 and C10 keep the power to the receiver part of the set clean of AC and leakage of the received signal. Any signal lost here does not get to the detector diode.

C21 will make the DC voltages on TR4 bad and part of the audio will get distorter off so that it never gets to the speaker and this will make the set sound awful. C27 will cause buzzing and screeching that can be intermittent or constant that gets worse as the battery ages.

C23 and C25 stop the audio from getting onto the resistors that have to be there to regulate the voltages on TR4 and TR5 the effect of one of these going faulty will start with poor bass and get worse. Faults with these capacitors will be amplified by the transistors.

Most of the buzzing will be from power cables close by and once the gain of the receiver part of the set is restored it should be drowned out by the stations you listen to.

Should I go with this advice and carry on replacing capacitors? I had replaced C23,C22 and C27, but I didn't touch C21, C25 or C10?

After reading through the thread I had also checked whether the Long/Medium switch operates any better with the contacts being pressed in lightly, but it doesn't change a thing.

Many thanks,
TinaRotator

[ppppenguin]

I only have the Trader circuit to hand and it's for the TR82B, not C but the designs are fairly similar.

The quoted voltages for TR1 must be an error in the service data. The base voltage must always be greater thant he emitter voltage for a transistor to conduct at all. I would expect to see about 0.2V to 0.3V for a germanium transistor such as those in this radio. Remember also that all voltages in this set are -ve wrt the chassis, even though they are expressed as +ve numbers.

I assume you understand that all voltages are measured wrt chassis or battery +ve.

Small variations from the "official" voltages are not important but 2 things stand out

TR4 OC71 0.12V (1.0V) 1.3V (1.1V) 1.7V (1.5V)
-------------------------------------------------------------------------
TR5 OC81D 1.7V (1.3V) 1.5V (1.5V) 8.8V (8.3V)

Assuming your measurements are correct, TR4 looks faulty. A working germanium transistor simply can't have about 1.2V between base and emiter. Also the readings for TR5 are odd. If the emitter voltage is higher (more -ve) than the base voltage it cannot be conducting at all.

These could be faulty transistors, though the ones used in this set are generally reliable. Or it could be measurement error (easily done even by experienced engineers) or other faulty components.

PS: Useful article about the various radios that Bush put in the this design of cabinet: https://www.aireradio.org/riviste_estere/BVWS/Bush.pdf The TR82B and C are the same internally but it look like there were slightly different types of transistors used even in sets of the same nominal model.

[TinaRotator]

Hi pppenguin,
Thanks for your reply. I can confirm that I measured from the test point from the service sheet which is I think synonymous to the chassis or +ve terminal of my battery. Anyways, I decided to spend my lunch break (the joys of homeworking!) double checking my results from yesterday and of course I have made mistakes! To be extra sure I measured also from chassis and battery.

The measurements for TR4 should read:

TR4 OC71 1.2V 0.12V (1.0V) 1.3V (1.1V) 1.7V (1.5V)

As for the TR5, I took extra time to ascertain which leg is which (it's probably dead obvious to while looking at the schematic, but it's a learning experience for me ). By freeing the TR5 from the wee bracket it sits, in I realised that I mixed up Emitter and Base, so that the measurement is:

TR5 OC81D 1.5V 1.7V (1.3V) 1.7V 1.5V (1.5V) 8.8V (8.3V)

so the measurements on both of them seem to be mostly 0.2V over the service sheet, which sounds like I shouldn't worry about the transistors?

Thank you for sharing the article, very interesting. Judging by the case - serial number starting with 346, and by the fact that it has the earphone plug, it's the later Mk I (1962), colour scheme is clearly that of a C type rather than B (blue rexine and greenish plastic). I realised that I am now able to add attachments in the acceptable way, so I am attaching the front and back of the chassis together with the cabinet.

   
   
   

Having ruled out the transistors, what should be my next port of call? Multimeter is quite limited, should I try to build a capacitor tester with my Arduino(ish) set? Any guidance or resources will be appreciated.

Best regards,
TinaRotator

[boater sam]

Switch on the earphone socket making proper contact?