Bush VHF 61 Tuning Indicator - UK Vintage Radio Repair and Restoration Discussion Forum

Paul JD · 2nd Jun 2019, 10:19 am

I have just finished restoring a Bush VHF 61 but have a minor irritation with the tuning indicator. On VHF the indicator works well showing full deflection when a strong station is tuned but on MW or LW the indicator barely moves at all. The station is coming through strongly and there is little or no difference in output volume between VHF, MW & LW, just the tuning indicator doesn't seem to work well on MW or LW. Checking the grid voltage on the tuning indicator shows around -7 to -8v when a VHF station is tuned but only around -0.8v when a MW or LW station is tuned.

All the Hunts caps in the set have been replaced and all voltages on the valves are in spec so not really sure where to look next. Any suggestions appreciated!

ms660 · 2nd Jun 2019, 10:33 am

Trader sheet refs, check S18 contacts and the AGC feed resistor R22, if they're ok measure the AGC voltage at the junction of R21 and R22 when a strong AM station is tuned in.

Lawrence.

Paul JD · 2nd Jun 2019, 10:40 am

Quote:

Originally Posted by ms660

Trader sheet refs, check S18 contacts and the AGC feed resistor R22, if they're ok measure the AGC voltage at the junction of R21 and R22 when a strong AM station is tuned in.

Lawrence.

Thanks, I did look at the switch contacts and they seemed good but will check again and check the resistors.

Paul JD · 2nd Jun 2019, 2:52 pm

Checked the switch contacts and all OK, R21 & R22 were in spec but swapped them out just to be sure. With a strong MW station I'm getting around -1.5v at junction of R21/R22 which gives around -1.4v at the grid of the indicator and only a small deflection so no luck so far I'm afraid.

ronbryan · 2nd Jun 2019, 3:02 pm

I dimly recollect that this set's slider wavechange switch has HT+ wired to a contact adjacent to the AGC line contact. Leakage or tracking across the paxolin insulation can cause the AGC line to be biased more in the positive direction, thus limiting the eye deflection on AM.

Ron

Techman · 2nd Jun 2019, 3:55 pm

There isn't any fault with your radio, this is just how they are on AM.

Paul JD · 2nd Jun 2019, 6:01 pm

Quote:

Originally Posted by ronbryan

I dimly recollect that this set's slider wavechange switch has HT+ wired to a contact adjacent to the AGC line contact. Leakage or tracking across the paxolin insulation can cause the AGC line to be biased more in the positive direction, thus limiting the eye deflection on AM.

Ron

I think you may have solved it! I have had a good clean around the switch contacts and am now getting about half scale deflection on the indicator with a strong MW station, it was hardly moving at all before so much better.

@Techman Thanks, I had read that AM reception was not brilliant on these but there was definitely something not quite right as there was hardly any movement at all on the indicator.

Thanks everyone for the help.

2nd Jun 2019, 10:19 am	#1
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Bush VHF 61 Tuning Indicator I have just finished restoring a Bush VHF 61 but have a minor irritation with the tuning indicator. On VHF the indicator works well showing full deflection when a strong station is tuned but on MW or LW the indicator barely moves at all. The station is coming through strongly and there is little or no difference in output volume between VHF, MW & LW, just the tuning indicator doesn't seem to work well on MW or LW. Checking the grid voltage on the tuning indicator shows around -7 to -8v when a VHF station is tuned but only around -0.8v when a MW or LW station is tuned. All the Hunts caps in the set have been replaced and all voltages on the valves are in spec so not really sure where to look next. Any suggestions appreciated!

2nd Jun 2019, 10:33 am	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush VHF 61 Tuning Indicator Trader sheet refs, check S18 contacts and the AGC feed resistor R22, if they're ok measure the AGC voltage at the junction of R21 and R22 when a strong AM station is tuned in. Lawrence.

2nd Jun 2019, 2:52 pm	#4
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Re: Bush VHF 61 Tuning Indicator Checked the switch contacts and all OK, R21 & R22 were in spec but swapped them out just to be sure. With a strong MW station I'm getting around -1.5v at junction of R21/R22 which gives around -1.4v at the grid of the indicator and only a small deflection so no luck so far I'm afraid.

2nd Jun 2019, 3:02 pm	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Bush VHF 61 Tuning Indicator I dimly recollect that this set's slider wavechange switch has HT+ wired to a contact adjacent to the AGC line contact. Leakage or tracking across the paxolin insulation can cause the AGC line to be biased more in the positive direction, thus limiting the eye deflection on AM. Ron

2nd Jun 2019, 3:55 pm	#6
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Bush VHF 61 Tuning Indicator There isn't any fault with your radio, this is just how they are on AM.