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Technical Assistance/Advice Thread


mwl168
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3 hours ago, spritzer said:

I thought some electrolytics looked off but can't tell with any certainty from the pics.  Make sure none of them are domed or have any leakage around them. 

Just checked all the electrolytics and none seem to be domed or leaking at all. 

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2 minutes ago, kevin gilmore said:

i'm assuming that all the meter numbers don't change when the channel cuts out? which kind of eliminates the front end tubes and circuitry?

 

The meter numbers only for the bias of the EL84 tubes, but no I don't notice any number differences. I can check again just to verify 100%

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On 11/2/2021 at 3:52 PM, kevin gilmore said:

a scope is a good thing to have. plenty of high voltage probes on ebay.

like this

Oscilloscope Scope Test Probe P4100 100:1 High Voltage Withstand 2KV 100MHz 1.2M | eBay

I have an oscilloscope and that test probe you linked coming. I have a SE to balanced cable arriving today and can try the BAL inputs to see if maybe can isolate the problem area.

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Best place to measure is the scope set to AC and tied to either side of the janzen caps.  the side next to the gain tubes is likely to be about +150v dc with music riding on it, probably 15 volts of peak to peak AC when volume is cranked.

There should be music on both sides of both janzen caps.

If there is still music on the caps when the sound disapears then the problem is the output tubes or output transformer

 

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Update:

Received the RCA->XLR cable and tested it out with BAL inputs. Problem persists, so it must be somewhere outside of the inputs. The Oscilloscope should be arriving in a week/week and half, will update with more information then/if I find test anything else out.

Thanks all for the assistance so far.

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  • 2 weeks later...

Hello everyone!

I thought I should build Dynahi from the parts I purchased more than 15 years ago.  Yes, original Dynahi.  I was kept busy in decade for job or for family.  Now my daughters are independent and I work less.   I should use my free time in something productive.   Before starting amplifier building, I made up a Tr/JFET measurement device and started measuring those devices.   Since I had so many transistors, Tr matching wasn't difficult.  An issue is JFET, K389 and J109, since I have only 4 pairs.   Luckily all JFETs have good match within a package, so I'm talking about matching between N ch and P ch.

I read old HeadFi thread about device matching, and found that people often measure only Idss for FET.   I understood that the FET's characteristic is defined by both Idss and Vgs.  They are usually correlated, but not well, right?   So I measured both Idss @ 10 V and Vgs @ 1mA because I read that 1mA is working condition of them in Dynahi.  Unfortunately I can't find even one pair with close enough for both parameters.   Some have very close Vgs such as 290 vs 300 mV, but Idss showed more than 2 mA difference.  One pair has good match in Idss, but Vgs are 390 vs 290 mV, totally different.  My question is which parameter should I prioritize for Dynahi input stage usage?  I tried to calculate the slope in V/A, but all K389 have ca 45 V/A of slope while J109 have ca. 35 V/A and don't fit.   I'm tempted to match Vgs and allow Idss gap of 2 mA...   Any advise?

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