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Old 28th May 2019, 3:17 pm   #40
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Hameg HM307-3 Cleaning and Repair

Hameg HM307.
Varying EHT. Usual culprit is the chain of resistors controlling the feedback - R313 toR309. They can change value under voltage stress. Change the 3 high value ones I suggest. Unlikely to be capacitors as the voltage has gone up.
Regulating transistor and 741 IC. Does the voltage (to chassis) on the base of the BF459 T301 vary as you alter the HV adjustment pot R318. If so, then 741 is likely to be OK.
Then do the same at the collector of T301. That will show both are working.
But drift could also be caused by the transistor failing in use as it heats up. Try a substitute, any medium power high voltage will do. Try raiding an old TV cathode drive panel. That will do for a quick test.
I always hesitate to unsolder components to test them and resoldering. I prefer voltage tests in situ.
Coming back to the FET input to the Y Amp. The current of the two FET's with zero gate-source voltage (Idss) has to be matched.
Put your Avo in series with the FET drain from 12v or so with a low resistor to limit current(100ohm) and gate+source together to ground.
Or see Wireless World Sept 1972 p443 as below. See also WW Dec 1971 p579 et seq for simple FET tester.
The FET match can be brought closer in circuit by R040 and then the balance control R042.
The importance with the FET is the matching for drain current with zero gate source voltage, Idss.
So don't waste time trying other types. Your BF256 will do as well as any. The circuit is quite uncritical as they are acting as source followers, provided they are matched and preferably low Idss. Get half a dozen of any FET and search closely similar Idss. It was difficult to manufacture FET with consistent characteristics, which is why they have such a wide range in data tables. The makers will match closely at a price.
For Wavey Dipole, I found a version of the HM307 manual on the web with the missing page of the CRT circuit. I think it was one described on Google as: HM307 Service user ID11526.
That page seems to be missing in Electrotanya, KO4BB, Rhode and Schwarz (amazing, yet they actually own Hameg and make the thing), Manuals.lib and All-about-circuits. Just shows how much these sites copy from each other.
I attach a scan of the missing page to save you searching.
I attach a Simple FET tester from Wireless World 1972 on measuring Idss with your Avo.
Oh yes, I suggest replace the focus pot 470k, rather than mess about repairing it. It lives at high voltage, and with a plastic shaft, I'd feel much happier using it. Value not critical 1/4M to 1M will do as it is shunted by the 180k. None of the values in that EHT resistor chain are critical, as long as the voltage ratio around the focus control is about right. Get your calculator out; the current is negligible down the chain.
wme_bill
Attached Files
File Type: pdf HM307 circuit4_CRT.pdf (498.6 KB, 92 views)
File Type: pdf SimpleFET tester.pdf (70.2 KB, 93 views)

Last edited by WME_bill; 28th May 2019 at 3:35 pm.
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