HP8601A Output Level - UK Vintage Radio Repair and Restoration Discussion Forum

vosperd · 28th Jan 2014, 9:00 am

Hi,
Noticed that the output voltage is limiting at about 1.8v on the 3V range into a 50 Ohm load. Just wondering if this is normal for these sets. I gather that the output device in the video Amp is prone to failure and originals are unobtainable. I see someone is selling a replacement but I've a few other things on my wanted list at the moment.
Don m5aky

Radio Wrangler · 28th Jan 2014, 9:37 am

I've used them many times, but back in the '70s. I never thought of the output voltage, rather the output power, and it should work to the top of the scale.

The output level is controlled by a levelling loop with an amplitude detector controlling a variable gain amp, so that the output is fairly flat even on wide sweeps. It's part of an old 110MHz network analyser system.

David

vosperd · 28th Jan 2014, 11:02 am

Thanks for the reply. I understand about the power.
When I look at the output on a scope the sine wave tends to flatten off when I get to 1.8V on the 3v range. I guess it should be a perfect sine wave up to the top of the scale.
Don

Radio Wrangler · 28th Jan 2014, 3:38 pm

Power supplies OK?

David

vosperd · 28th Jan 2014, 4:42 pm

Power supplies are ok as far as I can see.
I'll do some more checks later. Having a browse through the manual for now.
Don

Radio Wrangler · 28th Jan 2014, 5:34 pm

The output amplifiers are thin-film hybrids, I'm afraid.

David

G0HZU_JMR · 28th Jan 2014, 6:48 pm

I've never used one of these generators but you might be able to (externally) diagnose if there is an issue with the loop gain in the levelling system by testing it in AM mode.

As it runs out of loop gain I expect that the peaks of the AM will flatten off as it goes open loop.

However, I'd expect the generator to derate the maximum carrier level allowed if you select AM because it needs 6dB headroom for the AM. Or maybe it is designed such that it can AM modulate even at max carrier level? (Many types of RF sig gens will back off the max level allowed by 6dB when you select AM mode)

If there does turn out to be a problem it might just be that there isn't enough loop gain to keep it in closed loop so the final amplifier might still be healthy and the problem is further back?

orbanp1 · 29th Jan 2014, 1:05 am

Hi Don,

I have the same unit, and I had similar problems.
I bought the equipment in semi/nonoperational condition.
I was lucky, I just had to re-seat the power supply board, and the output hybrid chip.
Having re-seated the output chip I got full output at all attenuator levels.

Good luck, Peter

Radio Wrangler · 29th Jan 2014, 9:40 am

They should be reliable, unless someone transmits into the output, and then it's usually the attenuator that cops it.

David

vosperd · 30th Jan 2014, 10:18 am

This one has been reliable and otherwise works well. I did have an accident with it some while ago and damaged the output device but managed to find a used replacement. Don't think I'm going to be lucky a second time if that is the problem.
I'll have a good look at the supplies and check connections etc. Might be a case of leaving well enough alone.
Many thanks for all the advice.
Regards
Don

28th Jan 2014, 9:00 am	#1
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 446	HP8601A Output Level Hi, Noticed that the output voltage is limiting at about 1.8v on the 3V range into a 50 Ohm load. Just wondering if this is normal for these sets. I gather that the output device in the video Amp is prone to failure and originals are unobtainable. I see someone is selling a replacement but I've a few other things on my wanted list at the moment. Don m5aky

28th Jan 2014, 9:37 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: HP8601A Output Level I've used them many times, but back in the '70s. I never thought of the output voltage, rather the output power, and it should work to the top of the scale. The output level is controlled by a levelling loop with an amplitude detector controlling a variable gain amp, so that the output is fairly flat even on wide sweeps. It's part of an old 110MHz network analyser system. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jan 2014, 11:02 am	#3
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 446	Re: HP8601A Output Level Thanks for the reply. I understand about the power. When I look at the output on a scope the sine wave tends to flatten off when I get to 1.8V on the 3v range. I guess it should be a perfect sine wave up to the top of the scale. Don

28th Jan 2014, 3:38 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: HP8601A Output Level Power supplies OK? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jan 2014, 4:42 pm	#5
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 446	Re: HP8601A Output Level Power supplies are ok as far as I can see. I'll do some more checks later. Having a browse through the manual for now. Don

28th Jan 2014, 5:34 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: HP8601A Output Level The output amplifiers are thin-film hybrids, I'm afraid. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jan 2014, 6:48 pm	#7
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: HP8601A Output Level I've never used one of these generators but you might be able to (externally) diagnose if there is an issue with the loop gain in the levelling system by testing it in AM mode. As it runs out of loop gain I expect that the peaks of the AM will flatten off as it goes open loop. However, I'd expect the generator to derate the maximum carrier level allowed if you select AM because it needs 6dB headroom for the AM. Or maybe it is designed such that it can AM modulate even at max carrier level? (Many types of RF sig gens will back off the max level allowed by 6dB when you select AM mode) If there does turn out to be a problem it might just be that there isn't enough loop gain to keep it in closed loop so the final amplifier might still be healthy and the problem is further back? __________________ Regards, Jeremy G0HZU

29th Jan 2014, 1:05 am	#8
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: HP8601A Output Level Hi Don, I have the same unit, and I had similar problems. I bought the equipment in semi/nonoperational condition. I was lucky, I just had to re-seat the power supply board, and the output hybrid chip. Having re-seated the output chip I got full output at all attenuator levels. Good luck, Peter

29th Jan 2014, 9:40 am	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: HP8601A Output Level They should be reliable, unless someone transmits into the output, and then it's usually the attenuator that cops it. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

30th Jan 2014, 10:18 am	#10
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 446	Re: HP8601A Output Level This one has been reliable and otherwise works well. I did have an accident with it some while ago and damaged the output device but managed to find a used replacement. Don't think I'm going to be lucky a second time if that is the problem. I'll have a good look at the supplies and check connections etc. Might be a case of leaving well enough alone. Many thanks for all the advice. Regards Don