Products > Test Equipment
Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
dazz1:
--- Quote from: dazz1 on November 15, 2024, 04:38:42 am ---
--- Quote from: tautech on November 15, 2024, 02:54:01 am ---
--- Quote from: dazz1 on November 15, 2024, 12:26:06 am ---My next problem is obtaining a new HV cap.
--- End quote ---
2m search:
https://www.aliexpress.com/item/1005006067184429.htm
--- End quote ---
I buy a lot of stuff of Aliexpress but I hadn't considered buying HV caps. I will look into that.
--- End quote ---
I have looked into the Aliexpress option for HV caps and decided not to go that path.
I can buy 5x HV caps from Aliexpress for the price of 1x HV cap from element14.
I only need 1x so buying 5x caps is not an advantage.
I would have greater confidence in the quality of parts from element14, so the plan is to buy from element14.
That won't stop me from buying a lot more stuff from Aliexpress.
tautech:
--- Quote from: dazz1 on November 16, 2024, 04:41:07 am ---
--- Quote from: dazz1 on November 15, 2024, 04:38:42 am ---
--- Quote from: tautech on November 15, 2024, 02:54:01 am ---
--- Quote from: dazz1 on November 15, 2024, 12:26:06 am ---My next problem is obtaining a new HV cap.
--- End quote ---
2m search:
https://www.aliexpress.com/item/1005006067184429.htm
--- End quote ---
I buy a lot of stuff of Aliexpress but I hadn't considered buying HV caps. I will look into that.
--- End quote ---
I have looked into the Aliexpress option for HV caps and decided not to go that path.
I can buy 5x HV caps from Aliexpress for the price of 1x HV cap from element14.
I only need 1x so buying 5x caps is not an advantage.
I would have greater confidence in the quality of parts from element14, so the plan is to buy from element14.
That won't stop me from buying a lot more stuff from Aliexpress.
--- End quote ---
Yet there are 5x 4700pF caps in that HV circuit......
Back when I repaired CRO's I sourced all the HV caps required from Ali....
YMMV
factory:
Trouble with parts from aliexpress is you never know if they will meet the spec (if there is a spec at all, can't view ali for some reason), they may send good parts one day, crap the next time.
If you haven't got a way of measuring 3kV, then I'm guessing you probably don't have a way of testing HV disc capacitors either. One thing to note with ceramic disc caps, is the capacitance changing with voltage applied, the capacitance drops with increasing voltage.
David
dazz1:
Hi
If there was a lot of $ at stake here, I would definitely give Aliexpress a go. The key to successful buying off Aliexpress is to buy the most expensive version, which is invariably heaps cheaper than a locally available version. With Aliexpress, more money = more quality. In this case, there is no cost saving compared to element14 because either way, I still have to pay out about the same amount of $.
There are multiple HV caps of the same value/voltage on my HP 3721A, but only one has failed. The failed one has led an arduous life because it has pulse current due to the 100 blips on each x deflection. The other HV caps basically sit at DC.
Another issue is the physical size. The existing caps have a pin spacing of about 15mm, but replacement options have spacing down to 5mm. I don't think 3kV across a 5mm gap is a recipe for long term reliability because of the risk of dirt leading to leakage, or even arc-over.
With no cash incentive to buy Aliexpress, the assumed quality and known physical dimensions of the element14 range win the day.
dazz1:
Hi
I figured it would be too difficult and too expensive to replace the missing and broken original late model HP knobs. I would have to buy from multiple suppliers and the total shipping costs would eye-watering. Also genuine HP knobs only seem to be over-priced. The broken and missing knobs were really annoying me. After a lot of research I found some close equivalents to the old model HP knobs. These work better than the late model knobs because the front panel art work was made for them. The late model knobs are a little too small.
So the new knobs were not cheap and cost me twice as much as I paid for the HP 3721A but I think they are worth it.
I had to modify knobs to fit the double decker control switch "Delay Offset" but the finished job almost looks factory. I am going to paint the pointer line and the brushed aluminium bits blue to match the panel.
I have attached a photo of an original HP with black knobs and my copy of the HP black knobs.
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