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Radio Bush RG 34

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jonwarby
(@jonwarby)
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Hi everyone, I picked up a Bush RG 34 Radiogram last week with the aim to learn a bit about the technology and to hopefully restore it to a working condition.  I have been focusing on the mechanical aspect first and now have it to the point where all of the mechanisms are working.  Records can be stacked and it will drop them on at a time, play them, drop the next and play that until there are none left and then stop.  There wasn't a huge amount to do other than clean years of filth, replace one of the belts and degrease and oil the mechanisms but it felt great to have it all working again.

The next step is to tackle the sound side of it to improve how it sounds and also some preventative maintenance.  I am going to remove the amp and tuner at the weekend and clean the valve connections and have a look at the capacitors.  I am used to replacing capacitors in retro computers but nothing like what's in this machine.  Should I just stick to MKT1813 630v caps and is it best just to replace them all due to age?  Is there anything I should be focusing on?

Many thanks

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Topic starter Posted : 09/09/2021 11:52 pm
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crustytv
(@crustytv)
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Posts: 10793

Hi, and welcome aboard 👍

Posted by: @jonwarby

I am used to replacing capacitors in retro computers but nothing like what's in this machine.  Should I just stick to MKT1813 630v caps and is it best just to replace them all due to age?  Is there anything I should be focusing on?

Those caps should, for the most part, be suitable for non-electrolytic replacement. I'm sure you're aware the electrolytics will be of varying voltages and of course polarised.

Posted by: @jonwarby

Is there anything I should be focusing on?

Many things, it's worth taking time out to study the circuit (see below), I assume you are OK at following cct diagrams? Referring to the service data is essential to understanding each stage's function. It will also help you to be absolutely certain of all the component values. With regard to capacitors remembering to choose modern value equivs e.g old .05uF is now .047uF. Just a word of caution, and this may not apply to yourself. Just replacing capacitors is not a panacea to radio repair, there's no guarantee the radiogram will then work. I've lost count of the times folk have turned up here stating, “I've recapped the unit, but it still doesn't work”.

Official Bush RG34 Service Manual Circuit Extract

rg34cct

You may be lucky, and it will sort of work, then again it may not and good old-fashioned fault-finding will need to be employed. Start with getting the power supply working and all the voltage rails. Remember, this unit is nearly 70 years old, so some resistors will likely have gone high, especially the high ohm ones. For example, the AGC decoupling and load resistors.

The priority capacitor is the “Audio coupling” capacitor, which couples the anode of V3 (UBC41) to the grid of V4 (UL41), the output valve. Leakage (electrical, not physical) associated with this capacitor will put +ve volts on to the UL41 grid. Which will at best shorten the life of the UL41, at worst destroy it along with the audio output transformer (T1).

I'll stop there, and leave you in the capable hands of what remains of the radio boys at VRAT, to offer more advice. However, please be aware, this forum is not as active as it once was and is in decline. Most of the Radio folk have gone elsewhere, come to that a huge portion of the TV & Test equipment guys have also gone. I'm only mentioning this, so you don't get too disheartened by any lack of response to your post. What membership remains here is predominantly TV related, though as I say a couple do still dabble in radios. Though we're more than happy for fresh blood to join us and take up the reigns of the radio side.

Hopefully member @cathovisor Mike, will pop along and provide some input. Mike is a record deck aficionado, a Bush guru and runs the BushRadio website.

http://www.bushradio.co.uk/

Once again thank you for joining and good luck with your resto 👍

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Posted : 10/09/2021 7:58 am
Cathovisor
(@cathovisor)
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@jonwarby

Hi Jon and welcome aboard.

What deck is in this RG34? I'm intrigued because you said "you'd replaced a belt"?

The advice given by Chris is sound re: cap replacement and MKT1813s are the way to go however, do NOT use those in the positions of C33, 34 and 37 in the attached diagram - these really need to be 1000V rated caps with a polypropylene dielectric.The reason is that some very high voltages (in excess of the HT) appear across an audio transformer primary and the rate of change of this voltage means that caps better suited to dealing with pulse waveforms should be used in this position - which are polypropylene dielectric types.

I would also suggest that you look at replacing the main smoothing electrolytic C32/C36 sooner rather than later; the 50+50uF cap is easily sourced these days although you may find it'll still be okay. Myself, I prefer to err on the side of caution - unless you want to try reforming it first? Unlike the electrolytics that die so readily in older computers (and newer brown goods) you can make an attempt at reforming the dielectric layer in these first.

Plugging the set in straight after paper cap replacement and not at least checking that C32/C36 is in good order is inviting trouble!

Good luck with the work, keep us updated and if you run into any problems, we're here to help!

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Posted : 10/09/2021 10:29 am
crustytv
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Hi Mike, according to the service data in R&TS, the deck should be a Garrard RC.75 Automatic, assuming it's not been swapped at some point.

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Posted : 10/09/2021 10:36 am
jonwarby
(@jonwarby)
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Thank you everyone for the comments.  That's exactly what I was after to get me started.  I will take out the amp and tuner probably tonight and start to make a list of bits to replace.  The machine is working and playing it just doesn't sounds great and I'm sure it's only a matter of time before those caps start failing.

The deck is an RC75.  The belts connect the motor to different spindles depending on what speed is selected which in turn drives the idler wheel... apologies if the terminology isn't correct.   They are very short and fat rubber belts.  I had issue with torque originally and I think the idler wheel rubber could do with a refresh but after a light sand and a clean with some acetone it's doing the job now.

I started putting together a rough shopping list last night from farnell and was surprised at how expensive it was coming out at.  If anyone has a recommended supplier for the caps that would be appreciated

caps

I will update it based on the comments above and after a visual inspection and get a few pictures added as well.

Thanks again

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Topic starter Posted : 10/09/2021 10:57 am
crustytv
(@crustytv)
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Found an old VRAT thread covering the Bush RG34 from 9-years ago! Might prove to be a useful source of info.

https://www.radios-tv.co.uk/community/radio/bush-radiogram-or-making-a-big-ol-bush-look-pretty-again/

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Posted : 10/09/2021 1:30 pm
Nuvistor
(@nuvistor)
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Welcome,

A word of warning about the B8A valve bases. The socket very often has a circular spring clip to hold the valve in position, these rust and seize, trying to remove the valve can break off the glass pip that fits into the spring clip, the valve is then useless. If there are spring clips I suggest removing them.

Some valves have a metal base this also rusts and stick, careful removal is required.

The UL41 is known for developing internal leaks, pin 3 is an internal connection, there should be nothing connected to pin 3 on the valve base. Some manufacturers used this pin as a tag for other components. If the valve has a leakage isolating the components on Pin 3 may help.

Its interesting that the RG uses a fully isolated mains transformer yet uses series connected valves, the designers had a reason for this but I don’t know why. Perhaps easier and cheaper to make a transformer in this fashion.

Frank

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Posted : 10/09/2021 3:20 pm
Cathovisor
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Posted by: @nuvistor

Its interesting that the RG uses a fully isolated mains transformer yet uses series connected valves, the designers had a reason for this but I don’t know why. Perhaps easier and cheaper to make a transformer in this fashion.

Very easy to answer Frank - it simplifies production.

You only need keep one set of valves in stock for AC or DC receivers, you wire all the chassis all the same bar any isolating components. Just after the war, Bush made a point of saying that their AC sets could easily be converted if you moved to a DC district - no need to buy a new set but obviously the dealer would have to enhance the safety whilst fitting the dropper.

It's easily forgotten, but Bush were actually a small company in comparison to most of the others.

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Posted : 10/09/2021 4:04 pm
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Cathovisor
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@jonwarby

I know some people are very keen on using the caps supplied by Rapid - I'm not sure, but I think these are also supplied by the BVWS. Also, these days I tend to use RS rather than Farnell, who were my "go-to" for many years.

Polyester dielectric 0.047µF as sold by Rapid here: https://www.rapidonline.com/jb-capacitors-0-047uf-10-630v-axial-metallized-polyester-capacitor-08-1331

I don't know if you're aware, but the BVWS has a component shop for its members where capacitors can be purchased at fair prices.

Once I'd posted, I thought "oh hang on, the RC75 has those horrible little belts for the speed change!". A word of warning though - whilst 78rpm is at the right speed on these decks, 45 is (IIRC) actually fast. I have to say I'm not a fan of Garrard decks - they were often needlessly complicated and at one time they got it so badly wrong that they were forced into a major rebuild of one deck by the radio and TV manufacturers, so bad was it. Collaro (IMO) did a much better job but disappeared in the early 60s.

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Posted : 10/09/2021 4:15 pm
jonwarby
(@jonwarby)
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I removed the unit tonight.  Some of the caps do look wet.

Will start to try and map out what caps are what.  I was hoping they would be marked but it doesn't look like that is the case.

Posted by: @cathovisor

do NOT use those in the positions of C33, 34 and 37 in the attached diagram - these really need to be 1000V rated caps with a polypropylene dielectric

Thank you.  These look to be the ones next to the speaker so should be easy to identify

Posted by: @crustytv

The priority capacitor is the “Audio coupling” capacitor, which couples the anode of V3 (UBC41) to the grid of V4 (UL41), the output valve. Leakage (electrical, not physical) associated with this capacitor will put +ve volts on to the UL41 grid. Which will at best shorten the life of the UL41, at worst destroy it along with the audio output transformer (T1).

will get that identified and swapped out before it's powered on again

Posted by: @cathovisor

Polyester dielectric 0.047µF as sold by Rapid here: https://www.rapidonline.com/jb-capacitors-0-047uf-10-630v-axial-metallized-polyester-capacitor-08-1331

would that be instead of the 1813?

Posted by: @cathovisor

Once I'd posted, I thought "oh hang on, the RC75 has those horrible little belts for the speed change!". A word of warning though - whilst 78rpm is at the right speed on these decks, 45 is (IIRC) actually fast

I was impressed with the speed of the 45. I Played a 45 yesterday on the deck and synced the start with spotify and it kept time till the end.

Posted by: @cathovisor

I have to say I'm not a fan of Garrard decks

I've been impressed with how the auto mechanism works with my very limited experience.  It's a bit annoying that you seem to have to stack a record to play it.  You can't just put it on the deck and it plays.  But it's very clever for it's time and to have lasted so well.

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Topic starter Posted : 10/09/2021 9:28 pm
crustytv
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Posted by: @jonwarby

Will start to try and map out what caps are what.  I was hoping they would be marked but it doesn't look like that is the case.

To help you in that task, here is the plan view of the RG34 chassis with component markings

Official Bush RG34 Service Manual Plan View Extract

RG34plan

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Posted : 10/09/2021 9:57 pm
Cathovisor
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@crustytv

Quick caveat here - are the component designations used by the "Trader" the same as those in the circuit you provided?

@jonwarby - yes, the Rapid caps are suggested as an alternative to the MKT1813s.

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Posted : 10/09/2021 10:05 pm
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crustytv
(@crustytv)
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Good point Mike, I've now edited the post above and replaced the 'Trader' plan view with the actual Bush manual RG34 plan view, which is an exact match for the above cct 👍

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Posted : 10/09/2021 10:18 pm
jonwarby
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Posted by: @crustytv

To help you in that task, here is the plan view of the RG34 chassis with component markings

That is amazing. Thank you

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Topic starter Posted : 10/09/2021 10:24 pm
jonwarby
(@jonwarby)
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Thank you.  I've placed the order for most of the caps. Rapid came in a lot cheaper.  Will spend the time while I wait cleaning.  I wont remove the caps yet, I will just do it one by one so I don't lose track.

For the polypropylene dielectric caps I couldn't find where C37 was.

I had a look for the 50uf+50uf cap I wasn't able to find one that made sense with the pins.  I'll have to remove the one that's on there and see how the earth is wired.

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Topic starter Posted : 10/09/2021 11:37 pm
Boater Sam
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Welcome.

A word of caution, don't replace any of the mica caps in the tuner section unless they are proven to be faulty as swapping these will destroy the alignment of the radio section and I doubt you have the equipment necessary to realign.

Stick to testing the tubular wax caps and the electrolytics. If it is working at all, replace one at a time and retest that it still works, that way you will not introduce new faults.

Boater Sam
BVWS Member

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Posted : 11/09/2021 8:32 am
Cathovisor
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@jonwarby

I'll check the Bush manuals but C37 may only have been fitted to the radiogram, in which case a chassis layout for the table radio won't show it.

It won't be difficult to find though.

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Posted : 11/09/2021 9:15 am
crustytv
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The chassis layout above is taken from the official Bush RG34 manual, as is the cct diagram further up the thread (available in the data library)

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Posted : 11/09/2021 9:17 am
crustytv
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C37 (.01uF 350V 25%), is the tone corrector cap, it looks to be across T1 (Audio output TX) where is that TX? It does not look to be on the chassis, is it down on the Speaker? If so, that's where the cap will be found.

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Posted : 11/09/2021 9:20 am
Cathovisor
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@crustytv

Happy to be corrected, Chris.

Similarly, I'd have said the most likely location for C37 is on the output transformer, which I'd assume to be attached to either the baffle or the speaker itself.

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Posted : 11/09/2021 9:41 am
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