VHF90's and me!

[Bushbaby]

Hi All,
I've finally had some time to work on a radio (a rare weekend off) and yes, its another VHF90 (I must be a glutton for punishment).

I've put it on the lamp limiter, no glow, in fact nothing from the bulb. I've tested the voltage at the switch on the radio - zilch!

I thought it might be the lamp limiter but no, this worked perfectly when I put my soldering iron onto it.

I just hate to think that I've got yet another dodgy switch. Any suggestions?

Thanks

Rob

[Bushbaby]

To make it worse, my computers playing up and I've post this thread twice!

[Tedzed1]

No problem Rob, all sorted.

[Steve_P]

Use the Ohms range - safety!!

Plug to switch - Both legs. Should be virtally 0 Ohms.
Across switch - Off and On positions. Both legs of On-Off switch. Should lower to about 0 Ohms when in the On position.

Note: Looks dead does not mean 'is dead'. Check resistors on the voltage switch, CZ1, valve heaters. No heater current flowing means the set will appear dead.

Cheers,

Steve P.

[Bushbaby]

Hi Steve,
I assume with the power 'on' - just to double check.

Many thanks

Rob

[Bushbaby]

Across the switch (power on) - switched on and off, the reading fluctuates between 200 and 400R. Something is definitely not right.

[Yorkie]

I may have misunderstood you Rob, but I hope you didn’t apply the meter on the Ohms range with the set connected to the mains. Never apply a meter on the Ohms range to mains power (or indeed across any voltages). Unless the meter has a cut-out, you'll burn out the shunt resistor in the meter and ruin the meter for sure.

Many continuity tests can be done with the set disconnected from the mains, and that’s the safest and quickest way to proceed in the early stages of diagnostic tests.

I'd suggest that on the VHF90 with the set disconnected from the mains, you should check across each pole of the mains switch contacts and should of course get zero Ohms with the switch on, and infinity (open circuit) with the switch off. If not, the switch on the pot is faulty and you'll need to replace the pot.

Assuming the switch is OK, (and that the mains plug polarity is correctly wired), with the meter still on the Ohms range, if you apply one test lead to the live pin of the 3pin mains plug and the other prod to pin 9 of V7, you should see about 160 Ohms (R34 on the MANUFACTURE’S SHEET not the TRADER SHEET, which is naff).

If that's OK, apply one test prod to the neutral pin of the mains plug and the other prod to chassis (still on the Ohms range of course) and you should get a low Ohms reading if the dial light filaments are good, and even if they're open circuit, you'd expect to see 30 Ohms due to the dial light shunt resistor R37 being in circuit.

If these tests are good, you can then check the heater chain by applying one test prod to chassis, the other to pin 4 of V7. If you get an open circuit, pull each valve in turn and check across pins 4&5 of each valve to discover any duff valves. If just one is duff, in a series heater chain, none will light up.

If the heaters are ok, check the two 200R resistors in the heater chain, and the thermistor, the resistance of which will vary when cold compared to when it's hot, but shouldn't be open circuit.

Before you even think about applying mains to the set, clip out C60 (.05uF) and chuck it away. As has been said many times, the modulation hum capacitor - which Bush was fond of sticking across the mains (eg as in the DAC90A and VHF90) - must either be removed or replaced with a class X cap. I don't know how far you want to go with the set, but assuming you are going to replace the paper caps at some stage, at least replace 'that cap', before you power up the set, which in this set is C51 (.01uF), which goes from pin 9 of V5 to pin 2 (the grid) of V6 via the junction of the potential divider formed by R29 (4k7) and R30 (1meg). If you don't replace it and its leaky, it will let DC get onto the grid of the UL84 output valve you risk damaging the valve and possibly burning out the output transformer. (A common failing in DAC90As).

If the above tests are OK, you might wish to plug the set in via the lamp limiter and check the voltages as per the maker's sheet. They'll be a little low with the lamp limiter in, but not far off. For example, you should expect to see around 230 - 240V AC at pin 9 of V7 (UY85 rectifier), and at pin 3 of that valve, about 200 - 220V DC.

Really, unless and until the heaters light up and you have HT at pic 3 of V7, it's pointless poking around anywhere else.

I'd stress again don't use the manky four page Trader Sheet - use the excellent ten page Bush Service Sheet, which has far more detail and is copiously illustrated with voltages you should expect to see at many key points in the set.

Hope that helps a bit.

Best of luck with the restoration.

[Bushbaby]

Hi David,

Many thanks for you detailed post.

My mistake here and you are indeed quick to point out that I shouldn't have put my mutimeter in ohms setting with the mains on (lesson learn very quickly to an old dog like me!)

This set has been rewired, recapped and the appropriate resistors replaced as per the previous successful VHF90C that I restored. C60 has indeed been replaced by a class x cap.

I have just measured across the mains switch (power off of course) - its 15.5M R open and 12M R closed. This does indeed seem that the pot is kaputt! I shall replace this and see what happens next.

Regards

Rob

[pwdrive]

Across the switch (power on) - switched on and off, the reading fluctuates between 200 and 400R. Something is definitely not right.

Hi Rob, if as read then the meter might not be right ie: the meter might have gone puff...

Lawrence.

[Bushbaby]

Hi Lawrence,
The meters ok as I have just checked with a couple of resistors. Its a Maplin Precision Gold NF56 multiranging with usb computer interface. I cant say that I want to try the same again though.

Thanks

Rob