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Old 22nd May 2019, 11:45 pm   #18
WaveyDipole
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Join Date: Mar 2013
Location: Leicester, UK
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
R197 and R196 measure correctly. R190 and R191 (both 150Ω) measure lower at 135Ω.
What about the pot R223? Is that Ok?
Those resistors are about 10% off their nominal value. Even if their tolerance is 10% then the measurement (which I am assuming was made out of circuit?) is borderline and they are probably best replaced.

Quote:
Originally Posted by OldTechFan96 View Post
When pins 6+7 are removed from the CRT the voltages on TR123 and TR124 remain similar to what they were with pins 6+7 connected. I think this rules out the CRT having an affect on the circuit.
Good. No leakage via the CRT plates then.

Quote:
Originally Posted by OldTechFan96 View Post
Voltages on EY1 and EY2 (with the connector attached to the Y-preamp):

EY1: 21.7V
EY2: 7.8V

Voltages on BF199s T003-T006 (All wrong!)

I believe they should be: B-2.7V, E-2V, C-15.6V

T003: B-5.9V, E-3.6V, C-8.2V
T004: B-5.9V, E-5.1V, C-22.6V
T005: B-22V, E-21V, C-22V
T006: B-8.9V, E-21V, C-23V
T003 and T004 should have different voltages around it than T005 and T006:

T003 & T004: B~+2.7v E=+2v C=15.6v
T005 & T006: B=+15.6v, E=+15v, C= <+24v

The base voltages of T003 and T004 will depend on the output from the IC. I am a bit puzzled by your reading though. For example, T003 collector is connected directly to T005 base and should have the same voltage reading. You show 8.2v on one and 22v on the other? That cannot be unless the track is broken?

While the voltage at the bases of TR003 and TR004 looks a little high, it is exactly the same on both outputs from the IC which is a good sign. What's the voltage at the inputs, i.e. pins 1 and 14?

Quote:
Originally Posted by OldTechFan96 View Post
T005 looks to have had it. I pulled it and tested it with my component tester and it showed a diode between C and E. Using diode check on my DMM there is a 0.7V forward voltage between C and E. I believe it should read O/L. I decided to pull the rest and test them.

Three of the four BF199s give these strange readings and show a forward voltage between C and E. All defective, no doubt.

The forth one is open and tests as a diode on my component tester.
All of the above will need replacing.
Although one sometimes gets odd readings from those cheap testers, these are "ordinary" NPN transistors so should measure as such. Those anomalies, as you surmise and which appear to be backed up by your DMM readings, would suggest that these transistors are at least leaky and you have identified at least one that appears to have one junction that is o/c. I would agree they need replacing. Check the low value resistors such as R027, R028, R025, R026 as well. There should not be any significant current across R023 and R024 so I think they should be OK. While you are checking, also have a look at T007 and R036. These will probably be OK but it doesn't hurt to check while investigating this part of the circuit.

Quote:
Originally Posted by OldTechFan96 View Post
Is this amp DC coupled? I heard that if you miss a malfunctioning part it will destroy any replaced parts upon being turned on.
Looking at the circuit, there are no capacitors decoupling the various stages of the amplifier. Each stage is indeed DC coupled to the next either directly or via various bias resistors. A leakage or short within a component will usually cause an unexpected current flow somewhere. If that current flow exceeds design limitations, it can indeed damage adjacent components along its path within the circuit, sometimes in a cascade fashion. The amount of damage will depend on which component fails, its mode of failure and how well the circuit is designed and built among other factors. Perhaps someone much cleverer than me can comment in more detail.

Last edited by WaveyDipole; 23rd May 2019 at 12:05 am.
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