Bush VHF80 suddenly stopped working.

[benglossop]

Hi
I am new to this site, but thought I might ask if anyone has any ideas on this one. I have 2 Bush VHF80s. They always give such a good sound. They have both worked really well for years, apart from exploding mains capacitors!
However, one of them has suddenly decided to go completely dead. No sound, no dial lights when you switch on, nothing. Any ideas on what it could be? I've done the obvious and checked the fuse in the plug, which was fine.
Many thanks,
Ben

[Vicboduk]

Hi Ben, welcome to the forum. Firstly what level of experience do you have?
Are you familiar with reading circuit diagrams and the use of a multimeter? If yes to the last question, obtain a copy of the service data (see top right of this page) and check for continuity from the mains plug, on-off switch and all the way through the series heater chain.
If the last sentence meant nothing to you please let us know and don’t attempt to do anymore.
There is an inherent safety risk with this type of set, get back to us and we may be able to guide you through the process.

Vic

[benglossop]

Hi
I have collected valve radios since I was a child, always loved them. Started with a DAC90 rescued from the tip by my dad which I ve had 35 years, and it still works. When we moved house recently, my wife said most of them had to go, so I went down from 60-odd to I think 8. Just kept the ones that sounded best and were in best condition. Nothing special, just kept some because I much prefer older stuff!!
The last sentence you mentioned means 'a little' to me, as I do have a multimeter and I also know that the chassis / parts can be at mains voltage. When you mention the series heater chain, I know that valves have heaters to operate, and the lights are in series in the 'heater chain', but, to be honest that's about where my knowledge ends, apart from basic understanding of circuit diagrams and recognising components such as capacitors, resistors.
Look forward to hearing from you
Ben

[David G4EBT]

There are two versions of the Service Data which you can obtain at the top right of the page. The manufacturer's version runs to 14 pages, the Trader Sheet runs to four. (They use different component numbering).

The reference by Vic re inherent safety risk is that the set is 'live chassis' which means that depending on how the mains lead is wired, the chassis and any associated metalwork such as the tuning and volume control spindles could be live, but you may be able to safely locate the fault with the set unplugged from the mains. Given that the dial lights don't come on and the set is completely dead, I'd suggest that the two most likely possibilities are:

a) An open circuit mains switch, which could be either or both of the two poles. You could check that with a multi-meter on the low Ohms range with the set unplugged from the mains. If the switch is open circuit, it's ganged with the 1 Meg volume control so that would need replacement. ('RV1' on the Bush data sheet).

It's worth stating that though the bulbs are in series with the heater chain, blown bulbs would not stop the set working as they (and two thermistors) have shunt resistors across them.

b) If the mains switch isn't faulty, the next most likely cause could be an open circuit heater in one or more of the valves, of which there are seven.

You say the set 'suddenly went dead'. If by that you mean the set was switched on and working fine, then went dead, it's more likely to have been that a heater in one or more of the valves went open circuit. If on the other hand, the set was working fine until it was last switched off but when you switched it on again, it was dead, it could be either the switch or a heater.

You can test the valve heaters (with the set unplugged from the mains), by pulling out each valve in turn and checking across pins 4 & 5 of each valve with your multi-meter on the low Ohms range.

Obviously a blown heater is an easier fault to rectify - just need to plug in a replacement valve. A replacement volume control/mains switch is less straightforward.

As the set is now some 55 years old, two obvious candidates to replace are the X class cap across the mains (C61 on the Bush service data, located next to the volume control), and C49, the audio coupling cap (known as 'that cap'). If leaky (in the electrical sense), that puts the output valve and output transformer at risk of damage. Many a DAC90A puny output transformer has met its end that way.

Given your limited experience, if neither a) nor b) above locates the fault, it's going to need more investigation, and possibly some live testing, which you might be reluctant to embark upon.

A saving grace is that the chassis is well laid out and all of the components are clearly marked on the Bush data sheets, so it's not too difficult to work on.

Hope that helps a bit.

Good luck with it Ben.

[60 oldjohn]

I would like to add to what David has said. There is a possibility the speaker has gone open circuit. I use an analogue meter for this test across the Primary windings of the output transformer, You should hear a crackle in the speaker. Its tests two components at once and works for most sets. If a fault is suspected the speaker will have to be disconnected from the transformer for further measurements. This test works on most sets.

John.

[Station X]

An open circuit speaker wouldn't account for no dial lights, unless they and the speaker failed simultaneously. The OP implies that the dial lights were working.

We need to know whether the valves light up.

[60 oldjohn]

I agree. I was thinking sudden faults in general I should have read OP again.

John.