Hewlett Packard 8601A sweep generator.

[agardiner]

Mods - sorry if this is in the wrong place, please move as needed.

I have picked up a Hewlett Packard 8601a sweep generator, which older threads suggest is popular among vintage radio hobbiests. Perhaps a bit overkill for radio alignment? Anyway, apart from replacing an internal fuse, it appears to work well although I haven't tested any of the video elements.

The frequency on CW is perfect across the ranges as measured on my frequency counter, and the modulation is perfect.

My question is concerning actual use of the sweep part. Can anyone help me understand how to use it? The manual is super complex. CW use is easy enough, but any king of sweep selection appears to sweep the entire frequency band. How would I for example use this to carry out IF S curve alignment in an FM receiver? Or is it totally the wrong instrument?

Many thanks.

[agardiner]

OK, might have figured it out. Been playing with it all day. It appears that I just need to use the SYM setting for sweeping, which used the set frequency as the centre point and then sweeps either side of it with adjustable width.

A complex machine! Seems good though. Does anyone else here use one?

[Station X]

Yes I use one, but only on comms receivers. You'll need a second SG to put in a marker at the Intermediate Frequency and of course it'll need synching with your 'scope.

[agardiner]

Incidentally, the only problem I appear to have with this unit is the output meter. Reads just below .2 at all times (when on). The output level control works and I have confirmed on a scope that the level adjusts correctly through the whole range. Any suggestions?

[Station X]

Do you have the operating and service manual? If not I can email it to you.

[agardiner]

Hi Graham. Yes I do. Been looking at the troubleshooting, following the flow charts, meter does peg when disconnecting A6 video amplifier DET OUT cable, as it should. Switching range and the meter falls as it should, so this points to the ALC troubleshooting tree.

[agardiner]

So, spent a few hours on this with service manual. Before I delve any further, please can someone confirm that the meter should read without anything connected to the RF output? IE, it doesn't require a load of the correct impedance for the meter to function?

RF appears to be working correctly, although I don't have a spectrum analyser, the the level control appears to be working just fine. Meter just reading very low level and not affected by the level control. The peak and ALC seem OK to me, so if there is a fault I suspect it will be in the video amplifier.

[frsimen]

The output meter is calibrated in RMS voltage into either a 50ohm load. Some variants work with a 75 ohm load, check the option number on the back of the unit. Options 8, 9 and 10 work with 75 ohms. With a high impedance load, the output will be double what is indicated on the meter.

If the RF output is working, the video amplifier board is unlikely to be the source of trouble. The RF detector for the meter is on the A9 board (circuit sheet 11). Do check that if you have a version with a MOD switch, the switch is in the position to measure RF.

Paula

[agardiner]

Thanks Paula. I don't have the mod switch version.

The fact that the meter does not change at all, regardless of what range is selected, is concerning. I have been studying the circuit of the RF detector, and it appears to be OK. If I disconnect the detector out cable as per the manual, then it does behave as expected.

[frsimen]

My reply in post #8 is wrong. The RF detector is in on the A6 board, not on A9 as I stated yesterday. The circuit on A9 is just the meter driver and, from your tests, that is working properly.

The RF detector is part of the output hybrid on the A6 board. Possibly the hybrid has failed in some way, if the input signal to it is correct. Check that the supply voltages are present at the output hybrid, the supplies are decoupled by some tantalum capacitors which are prone to going short circuit.

If they are correct, check the RF output. The detector won't work if there is no RF output, if the detector has failed there will be more RF output than expected.

There is some information about the hybrid here:

http://www.dasarodesigns.com/project...012-1820-0150/

That includes a reverse engineered circuit diagram, so you can see what is going on.

Paula

[Radio Wrangler]

There is the possibility that someone was using it to test a transceiver and sat on the PTT switch on the microphone.....

At some point you'll need to check all the output attenuators are OK.

Good little generator with excellent versatility.

The 110MHz range is perfect for FM broadcast receiver testing, and the 11MHz range for the IFs.

There are other boxes which extend this little unit into a full 110MHz vector network analyser.

David

[agardiner]

Thanks. There is definitely RF output, and the attenuater is working. Not sure about exact levels, as my old scope blew up a while ago. I have a new one on order though which should arrive next week. That will allow me to check the RF levels.

A failed tant might make sense. When I first got the unit, the 2A fuse for the 20V and -6.3V supplies was blown. I fitted a replacement, and it worked for around 1 min before the fuse blew again. I fitted another replacement so I could check the supply levels, but this has remained intact, despite running the unit for several hours. Will wait for my new scope and do some tests.

[Radio Wrangler]

That's good. It's possible the output level detector diode has been blown, it's also possible that one of the attenuator sections has been blown, though not all. You find the output vanishing at some output level settings, but not all. As you have output, then the hybrid amplifier must be OK. If the diode on it is also OK, then you're out of the woods as far as irreplaceable bits are concerned.

David

[agardiner]

Right. New scope arrived and I have been carrying out tests this afternoon as per suggestions.

I have come to the conclusion that the detector is not working. RF output is present and correct, but as suggested by Paula, the overall level is too high. The attenuation switch is working, but the vernier is not, and this confirms the ALC circuit is not operating. Previous tests though as per the manual, proved the ALC circuit itself. So my issue appears to be in the hybrid output device.

2 questions. 1) I have read about swapping the hybrids around. Are they actually all the same but used differently? 2) If not, could I possibly add an external detector, say a 1N4148 from the RF output to connect to the ALC circuit?

I did contact the chap who reverse engineered the hybrids, but sadly he is no longer making his replacement modules due to the transistors he was using being long discontinued.

[frsimen]

Hello Adrian,

The hybrids aren't the same, so you can't swap them over. I think the external detector approach ought to work, perhaps not quite as well as the original, but good enough. You could consider a Schottky diode rather than a 1N4148. Something like a BAT85 will work there, I think. A 1N4148 would be good enough to prove the point, however.

Paula

[agardiner]

Thanks Paula.

OK, point proven. Inserting a 1N4148 immediately causes RF to drop to a more expected level, the meter to rise and the vernier to work. So I have confirmed the detector is faulty.

Would love to find a replacement hybrid, but for now at least I will add an external solution.

Thanks everyone for your input!

[agardiner]

Well, that was unexpected. A Google search for the part number, (previously done with no result) actually brought up a company in America that has a small stock of the brand new recreated output hybrid. Not cheap at just over £200. However, oops; my finger slipped on the buy it now button.

Looking forward to getting this unit back into full working order. Really liking it.

[agardiner]

An operational question please from those who use one of these.

How exactly do you perform a sweep set up for FM alignment? I have set up a sym sweep OK, and connected the sweep output to the X input of my scope. However, the HP is not a wobbulator, and the sweep ramps rather than moving back and forth. I am using a 2nd CW generator for a marker, but am not getting anything coherent on my scope.

My scope also supports FFT and maths functions.

Thanks.

[agardiner]

And sorry, I have another question. Just set up a simple CW wave at approx 6Mhz, and also set up exactly the same frequency CW on another RF generator.

Both look correct on the scope; BUT, if I engage AM modulation on each generator, I can clearly see it on my other one, but the trace doesn't change at all on the HP. I don't get it, as modulation is working because it can be heard on a radio! Why? What I am missing?

[frsimen]

Hello Adrian,

I don't have an HP8601A but I have used a sweep generator to look at alignment. Some DSOs give poor results in X-Y mode compared with old CRT oscilloscopes.

Better results will be obtained if you connect the output of the FM detector to, say, channel 1 and the sweep output signal from the HP onto channel 2. Trigger the oscilloscope's timebase from channel 2 and adjust it so that the ramp is the full width of the screen. If you then turn the display of channel 2 off, channel 1 should be displaying the trace that you are hoping to see. Unbalanced ratio detectors, often used in valved radios, have rather a high DC offset compared with the wanted variation. This can make it difficult to obtain a good clear trace.

I don't think you will see the modulation envelope unless you are looking at the output of one of the generators directly and the oscilloscope is working in normal timebase mode. I think you are looking at the output of the FM detector, so limiting will strip most of the AM content in any case. Even an AM receiver won't show the modulation sidebands unless the bandwidth is very narrow (think a few 100Hz or less for a 1kHz modulation frequency).

Paula